DC coupled 2.7 GHz Active Probe Project - Now Available!

[lasmux]

UPDATE, 17/04/2024: These are now available at http://www.lasmux.com/product/single-ended-active-probes/. 2.7GHz probe.

I've been working on an active probe design for around a year. The goal started off as creating a DC-coupled active probe to support a photon counting sensor I am also working on, but it was a very fun project and I spent so much time on it that now the plan is to sell it. Could I have some feedback on the probe/performance, and on the contents of the datasheet, before I start buying the first batch of parts... which will be quite expensive.

I have another post that I'm putting together where I'll go into the development process a bit more.

I'm making two versions, a 1GHz version, and a 2GHz version.

The datasheet is here: https://www.lasmux.com/wp-content/uploads/2023/07/LD-ASP-1G_2G.pdf



Quick specs:
Bandwidth: DC-1GHz, DC-2GHz
Input capacitance (measured at 1GHz): 0.7pF
Attenuation: 20x
DC input resistance: 1Mohm

1GHz version frequency response (linear and log axis):


2GHz version frequency response (linear and log axis):


Tip input impedance of both probes, depending on which ground lead is fitted:


The resistive ground lead can be used to stop a resonance developing on the ground connection, which reduced the input impedance above 1.5GHz. I talk about this a bit more in the datasheet.



In terms of step response for the system, I've 'only' got a 500MHz oscilloscope, so can't properly test the rising edge speed unfortunately. This is the probe measuring a 50 ohm terminated 100MHz signal, with a <100ps rise time. This greatly exceeds the bandwidth of the scope so there's some ringing. The trace looks basically identical if I measure the signal directly by the oscilloscope.

Currently I'm aiming for around £150 for the 1GHz version, and £185 for the 2GHz version.

[lasmux]

There have been... quite a lot of iterations of the hardware design...



I started off with a BNC output on the probe itself, and the batteries mounted to the probe also, but then went for an SMA connector and an external 9V battery box. It does mean more cables, and the coax cable needs to be custom to go from SMA-BNC, but it is a much easier device to use because of it.

I went through a lot of iterations on the amplifier and signal conditioning circuitry, and got very used to soldering 0402's, which made my life miserable. Also given the extremely small capacitances on the board, every single time I made a change to the passive networks, I'd have to clean the board in IPA to remove flux residues.

[djsb]

Hi,
When will this be back in stock? Or is it initially for evaluation purposes only (as you mention hardware changes)? Thanks.

[lasmux]

I've not made the first batch yet. I'd expect maybe six to eight weeks, assuming my PCB assembler doesn't mess me about.

If the response from this thread is positive, I'll make an initial batch of 25 and start the process immediately.

[lasmux]

Here is a simplified schematic of the probe input to give you a gist of how it works.



C1 balances the parasitic capacitance across R2, to get a resistor/capacitor divider between R2/R3. As you could imagine, the parasitic capacitance across R2 is extremely low, given it's an 0402 passive. By varying C1 and observing the probe frequency response, I measured it at less than 0.1pF! This is not a lot of capacitance.

The capacitance of the probe tip is a lot more than 0.1pF (0.7pF) because there is some coupling between the probe tip and ground itself. This is not so difficult to avoid by just isolating that resistor from ground completely. Unfortunately, the more you pull ground away from R2, the worse the noise pick-up on the probe is. The net which connects R2 to the non-inverting amp input is high impedance, and if you pull the ground away from it, it picks up a lot of noise. One of my design iterations removed a lot of the ground planes around this net to really try to keep the tip capacitance down as low as possible, and the noise pickup was really bad. So you kinda have to choose your demon. The noise level is quite a lot better than other active probes (only 650uV), but because it's a x20 probe, I needed to keep the noise level to a minimum.

One thing which I could do in future is reduce the R2 to 200k or something, so I would have to correspondingly reduce R3, which would make that node much lower impedance and less succeptible to noise. I could then pull the ground a bit further back to try to further reduce the tip capacitance. Not sure.

The 1Mohm resistor R2 also protects the non-inverting input of the high speed FET input op amp from ESD damage. It does have ESD protection diodes built in, but the resistor helps keep them safe. A lot safer than just having discrete FETs on the inputs.

[nctnico]

It does mean more cables, and the coax cable needs to be custom to go from SMA-BNC, but it is a much easier device to use because of it.

You can buy these cables off-the-shelve from China.

[lasmux]

It does mean more cables, and the coax cable needs to be custom to go from SMA-BNC, but it is a much easier device to use because of it.

You can buy these off-the-shelve from China.

lol. I've just looked and they're everywhere on ebay etc. I have no idea how I missed that  . Thanks.

[Weston]

Really cool work! It looks like the plastic for the probe tip is injection molded? Thats a lot of commitment.

Have you considered selling this through crowd supply? There might be a bit of product testing required (CE certification), but Crowd Supply provides a direct path to distribution though Mouser. It's also a lot easier to not have to deal with fulfillment yourself and the crowdfunding model removes any monetary risks with fronting money for parts. Its now discontinued, but I sold the Little Bee current probe through Crowd Supply and it was a great experience https://www.crowdsupply.com/weston-braun/little-bee

Is the probe stable with an AC coupled 50 ohm termination? A lot of equipment, such as VNAs/Spectrum Analyzers have an AC coupled 50 ohm termination. You obviously don't need a probe that works down to DC for that, but this would be a cost effective high impedance probe.

[lasmux]

Really cool work! It looks like the plastic for the probe tip is injection molded? Thats a lot of commitment.

It's not injection moulded, it's MJF 3D printed, and then vibro polished to get a smooth finish. Really quite remarkable how comparable it is to injection moulding. I should do a post on the mechanical side of things also.

Have you considered selling this through crowd supply? There might be a bit of product testing required (CE certification), but Crowd Supply provides a direct path to distribution though Mouser. It's also a lot easier to not have to deal with fulfillment yourself and the crowdfunding model removes any monetary risks with fronting money for parts. Its now discontinued, but I sold the Little Bee current probe through Crowd Supply and it was a great experience.

I'd not known about this. Very cool that they distribute through Mouser. I'll have to look into this. Did you have to promote the crowd funding event? Getting 30k in funding is very impressive! I wonder if they'd accept a self certification CE process.

Is the probe stable with an AC coupled 50 ohm termination? A lot of equipment, such as VNAs/Spectrum Analyzers have an AC coupled 50 ohm termination. You obviously don't need a probe that works down to DC for that, but this would be a cost effective high impedance probe.

Yes it works with AC coupling on the input or output. So long as the instrument it's plugged into can operate with small DC offsets on the input, as obviously the output is not AC coupled. My VNA didn't have any trouble.

[lasmux]

Modelling was completed in the venerable Fusion 360. I'm not the best at mechanical design so this was quite a learning curve to create an ergonomic (ish) design.



I then went through a number of different 3D printing technologies till I found one I was happy with. The PCB is a very close fit with the enclosure so I needed really good dimensional accuracy, and I didn't want lots of layer lines visible either, which would be more apparent on such a small part.


From left to right we have:
MJF nylon 12, vibro-polished. This was the best result which combined a smooth finish with excellent dimensional accuracy.
SLS nylon 12. Good enough dimensional accuracy, but the finish wasn't smooth enough. I tried melting/smoothing the surface by passing a hot air gun over it lightly. This did work, and ended up with a nice gloss, but was quite risky as a little too much heat and the thin walls of the part would start to distort.
SLA resin "tough 2k". Blotchy finish and not enough dimensional accuracy. Also the support structure was more apparent where it was broken off.
SLS nylon 12. In grey, same issues as before.
SLA resin "rigid 10k". A glass filled resin which gives a really smooth finish, unfortunately the dimensional accuracy wasn't quite good enough. The support structures here weren't as visible.
SLA resin "clear". Blotchy finish, and not enough dimensional accuracy. Had quite a lot of issues with the support structures not having broken off cleanly with this one.

[KungFuJosh]

I want one. I don't ever measure anything that I really need an active probe for, but that looks cool. 😉

[lasmux]

Thanks

It's quite good as a general purpose high-speed probe, of if you want to measure a moderately fast or high impedance signal... but yes, it's not for everyone.

I was also wondering if it could be used as a sensor preamp, perhaps for a high-speed photodiode or something.

[mawyatt]

Really cool work! It looks like the plastic for the probe tip is injection molded? Thats a lot of commitment.

Have you considered selling this through crowd supply? There might be a bit of product testing required (CE certification), but Crowd Supply provides a direct path to distribution though Mouser. It's also a lot easier to not have to deal with fulfillment yourself and the crowdfunding model removes any monetary risks with fronting money for parts. Its now discontinued, but I sold the Little Bee current probe through Crowd Supply and it was a great experience https://www.crowdsupply.com/weston-braun/little-bee

Is the probe stable with an AC coupled 50 ohm termination? A lot of equipment, such as VNAs/Spectrum Analyzers have an AC coupled 50 ohm termination. You obviously don't need a probe that works down to DC for that, but this would be a cost effective high impedance probe.

We have the Little Bee B1, don't use it much, but when we need to measure smaller currents it's served well 

Thanks for the product

Best,

[mawyatt]

@ lasmux,

Nice project, well done

We didn't know about the MJF printing, that case looks very nice indeed!!

You certainly picked up on the Fusion 360, and produced a nice rendering as well!!

A question, why the selection of 26dB rather than 20dB attenuation?? Guessing limits of the +-2.5V supply rails?

Best,

[nctnico]

1:10 for the probe and another 1:2 for the 50 Ohm termination makes 26 dB.

[lasmux]

@ lasmux,
Nice project, well done
We didn't know about the MJF printing, that case looks very nice indeed!!
You certainly picked up on the Fusion 360, and produced a nice rendering as well!!
A question, why the selection of 26dB rather than 20dB attenuation?? Guessing limits of the +-2.5V supply rails?

Thanks for the encouraging words

Yes, MJF 3D printing is amazing. Combined with vibro-polishing I feel it sets a new standard for low-volume projects.

I could have made it x10 into the 50 ohm, but I would have had to double the gain of the op amp which would have reduced the bandwidth somewhat. Also it would have reduced the measurable voltage range to +/-7V due to the limited rails, as you alluded to.

[lasmux]

To add. I also couldn't reduce the input voltage divider as it would have also meant reducing C1 by half (see schematic above) to maintain the capacitor divider ratio. C1 is a combination of an 0402 capacitor, and parasitic capacitance to ground. The 0402 capacitor part of C1 would have likely needed to be a smaller value than the smallest 0402 capacitor available.

[lasmux]

Yes, theoretically, it's a 10:1 probe if you put it into a 1Mohm oscilloscope, but then with a 1 metre coax cable, you're going to get reflection artefacts causing issues over 20MHz or so. Typically for 50 ohm terminated active probes, they say the attenuation as measured across a 50 ohm terminated scope.

[JohnG]

This looks like a really nice and affordable probe.

I have a question regarding large signal performance. The datasheet alludes to something that sounds like slew rate limiting that may affect the large-signal bandwidth. This is important to me because my primary application would be a scope probe. Have you done anything to look as this, or can you find someone with a high-bandwidth scope and source to look at it? I'm happy to volunteer, but I bet there are folks who would do the same and are much closer to you.

Thanks,
John

[lasmux]

This looks like a really nice and affordable probe.
I have a question regarding large signal performance. The datasheet alludes to something that sounds like slew rate limiting that may affect the large-signal bandwidth. This is important to me because my primary application would be a scope probe. Have you done anything to look as this, or can you find someone with a high-bandwidth scope and source to look at it? I'm happy to volunteer, but I bet there are folks who would do the same and are much closer to you.

Thanks!
The slew rate of the output is limited to 2V/ns, so for example a 10V rising edge input would be divided by 10x due to the probe attenuation to 1V (before 50 ohm termination), so due to the slew rate limitation would have an additional 500ps rise time (I think) (edit. around a 600ps rise, see post below). You'd have to have quite the unusual signal to be generating this kind of signal though. In my spice simulations this kind of input starts to make step responses/square waves look a little trapezoid. I don't have a high amplitidue pulse generator (or fast enough oscilloscope) to test this properly. In general, I don't know what the rise time is for more sensible signals as my oscilloscope just isn't fast enough (500MHz).

Note though that other active probes such as the Keysight N2796A 2GHz probe also limit their dynamic range at higher frequencies. This is a screenshot from their datasheet:

[lasmux]

Also if you (or anyone else) do want to verify the probes performance, I'd be happy to loan you my prototype. Especially if you have a decent VNA to check the bandwidth, and fast scope to check the rise time.
I don't mind shipping it internationally.

[lasmux]

I just ran this through my spice simulation again, and the rise time went from around 250ps to around 610ps, for a 1V rising edge or a 10V rising edge. These are just indications as my spice model isn't that similar to the actual performance of the probe. The spice version only has a bandwidth of 1.4GHz.

Edited with updated numbers.

[hpw]

Well,

to measure the slew rate, consider Bodnar Fast Pulse and a DSO may with 10Gs and RIS...

I started with an active Probe from CalTest as CT4121 (now recognized that the ground to the probe tip is missing),
have also a 2.5G HPF and now a differential AP034.

All active probe have a certain connection pins, not fully side by side as yours, but love now my single AP034

While you may simple connect an 2.54mm dual pin header as they are parallel and an additional GND pin on the side.

So easy to use 2..4 of them (as for I2S), leave them on the table along, without having the hands full of them. And turn with 2 hands on the DSO.

So my 2 cents on real measurement tasks

hp

[lasmux]

to measure the slew rate, consider Bodnar Fast Pulse and a DSO may with 10Gs and RIS...

I started with an active Probe from CalTest as CT4121 (now recognized that the ground to the probe tip is missing),
have also a 2.5G HPF and now a differential AP034.
All active probe have a certain connection pins, not fully side by side as yours, but love now my single AP034
While you may simple connect an 2.54mm dual pin header as they are parallel and an additional GND pin on the side.

That's a very fast rising edge from that pulse generator! If I manage to find a faster scope, I'll have a look at that, thanks.

[tszaboo]

Looks cool. Those socket connectors at the end, are they the same size as the Agilent active probes?
For me it's a bit strange that you choose to have the 1M to be in series with the signal, those probes have a relatively small series resistance, and the 1M is used to shunt the 1pF. Any reason for that? Not judging, I'm genuinely curious, as I haven't built high speed scope probe yet, only high gain ones for shunt resistors.

[lasmux]

Looks cool. Those socket connectors at the end, are they the same size as the Agilent active probes?
For me it's a bit strange that you choose to have the 1M to be in series with the signal, those probes have a relatively small series resistance, and the 1M is used to shunt the 1pF. Any reason for that? Not judging, I'm genuinely curious, as I haven't built high speed scope probe yet, only high gain ones for shunt resistors.

I'm not sure the size of the keysight/agilent probes. I think they might be 25mil (0.635mm), the ones I selected are 1mm, so a bit larger maybe. Would that be a problem for you? I will include solderable adapters that you could maybe connect up to those probes, but that's not as seamless.
I could theoretically replace the socket with this which I think would be compatible if you think it would be a deal breaker, although I'd have to change the enclosure which would be a bit of a faff
https://www.mill-max.com/products/pin-receptacle/wire-termination-receptacle-soldercup-type/1134


The 1M is to make the probe a bit more general purpose for low speed signals. Above a few MHz the input resistor divider between R2 and R3 has almost no bearing on the signal attenuation. It's mostly down to the capacitor divider between the parasitic capacitance across the 1M, and C1. To get a linear frequency response from low frequencies to high frequencies, you need to closely match the resistor divider and the capacitor divider attenuation. So in that sense it doesn't matter whether it's 1M or 10k, it just pushes the frequency out further at which point the capacitor divider starts to dominate. But the capacitor divider still needs to be correct due to the high bandwidth of the measurement.

[tszaboo]

Looks cool. Those socket connectors at the end, are they the same size as the Agilent active probes?
For me it's a bit strange that you choose to have the 1M to be in series with the signal, those probes have a relatively small series resistance, and the 1M is used to shunt the 1pF. Any reason for that? Not judging, I'm genuinely curious, as I haven't built high speed scope probe yet, only high gain ones for shunt resistors.

I'm not sure the size of the keysight/agilent probes. I think they might be 25mil (0.635mm), the ones I selected are 1mm, so a bit larger maybe. Would that be a problem for you? I will include solderable adapters that you could maybe connect up to those probes, but that's not as seamless.
I could theoretically replace the socket with this which I think would be compatible if you think it would be a deal breaker, although I'd have to change the enclosure which would be a bit of a faff
https://www.mill-max.com/products/pin-receptacle/wire-termination-receptacle-soldercup-type/1134


The 1M is to make the probe a bit more general purpose for low speed signals. Above a few MHz the input resistor divider between R2 and R3 has almost no bearing on the signal attenuation. It's mostly down to the capacitor divider between the parasitic capacitance across the 1M, and C1. To get a linear frequency response from low frequencies to high frequencies, you need to closely match the resistor divider and the capacitor divider attenuation. So in that sense it doesn't matter whether it's 1M or 10k, it just pushes the frequency out further at which point the capacitor divider starts to dominate. But the capacitor divider still needs to be correct due to the high bandwidth of the measurement.

Problem? No, I have those 1GHz active probes at work (N2795), and saw how versatile  the accessory package is. According to my caliper, the pins are 0.7mm. I don't know if they sell the probe pins separate.
The equivalent input on these are 120Ohm series, 1pF 1Mohm parallel to ground.

[JohnG]

Thanks!
The slew rate of the output is limited to 2V/ns, so for example a 10V rising edge input would be divided by 10x due to the probe attenuation to 1V (before 50 ohm termination), so due to the slew rate limitation would have an additional 500ps rise time (I think) (edit. around a 600ps rise, see post below). You'd have to have quite the unusual signal to be generating this kind of signal though. In my spice simulations this kind of input starts to make step responses/square waves look a little trapezoid. I don't have a high amplitidue pulse generator (or fast enough oscilloscope) to test this properly. In general, I don't know what the rise time is for more sensible signals as my oscilloscope just isn't fast enough (500MHz).

Note though that other active probes such as the Keysight N2796A 2GHz probe also limit their dynamic range at higher frequencies. This is a screenshot from their datasheet:

I measure such signals on a routine basis, so it doesn't seem so unusual to me (maybe I am an outlier, though).

Regarding the Agilent/Keysight probe example, usually such a graph shows probe derating due to some reliability or safety limitation. The same rolloff is typical for completely passive probes which will not exhibit slew rate limiting. The data sheet was not very clear on this, but from the impedance plot, an 8V AC amplitude signal at 1 GHz will give you about 50ish mA AC RMS of probe tip current, which is not insignificant. These probes have an adjustable offset of +/-12V, which is how they arrive at the 20V max input. This is not slew rate limiting, this is a way to get around the AC voltage limitations of the probe tip amplifier. It's useful for looking at noise on a voltage bus, for example, by adjusting the offset to the bus voltage so you have the full AC dynamic range centered around the bus voltage.

As I mentioned, I'd be happy to test the probe. I have a Bodnar pulser and an HP8131 pulse generator, and an old, but very functional, 6 GHz scope. I should warn you ahead of time that I'm not real quick about getting to these things, so if you find someone locally, you are likely to get a quicker turnaround. Also, if it is really slew rate limited to 1V/ns at the output, it's not something I can use.

John

[lasmux]

Looks cool. Those socket connectors at the end, are they the same size as the Agilent active probes?
For me it's a bit strange that you choose to have the 1M to be in series with the signal, those probes have a relatively small series resistance, and the 1M is used to shunt the 1pF. Any reason for that? Not judging, I'm genuinely curious, as I haven't built high speed scope probe yet, only high gain ones for shunt resistors.

I'm not sure the size of the keysight/agilent probes. I think they might be 25mil (0.635mm), the ones I selected are 1mm, so a bit larger maybe. Would that be a problem for you? I will include solderable adapters that you could maybe connect up to those probes, but that's not as seamless.
I could theoretically replace the socket with this which I think would be compatible if you think it would be a deal breaker, although I'd have to change the enclosure which would be a bit of a faff
https://www.mill-max.com/products/pin-receptacle/wire-termination-receptacle-soldercup-type/1134


The 1M is to make the probe a bit more general purpose for low speed signals. Above a few MHz the input resistor divider between R2 and R3 has almost no bearing on the signal attenuation. It's mostly down to the capacitor divider between the parasitic capacitance across the 1M, and C1. To get a linear frequency response from low frequencies to high frequencies, you need to closely match the resistor divider and the capacitor divider attenuation. So in that sense it doesn't matter whether it's 1M or 10k, it just pushes the frequency out further at which point the capacitor divider starts to dominate. But the capacitor divider still needs to be correct due to the high bandwidth of the measurement.

Problem? No, I have those 1GHz active probes at work (N2795), and saw how versatile  the accessory package is. According to my caliper, the pins are 0.7mm. I don't know if they sell the probe pins separate.
The equivalent input on these are 120Ohm series, 1pF 1Mohm parallel to ground.

My probe divides the signal prior to the amplifier, and then re-amplifies it. The N2795 doesn't attenuate at all to begin with, and then I guess has the amplifier gain configured to divide by 10 perhaps. I'd guess that that could also be why their ESD damage tolerance is so low also, the input is only protected by the 120R resistor.

[lasmux]

Thanks!
The slew rate of the output is limited to 2V/ns, so for example a 10V rising edge input would be divided by 10x due to the probe attenuation to 1V (before 50 ohm termination), so due to the slew rate limitation would have an additional 500ps rise time (I think) (edit. around a 600ps rise, see post below). You'd have to have quite the unusual signal to be generating this kind of signal though. In my spice simulations this kind of input starts to make step responses/square waves look a little trapezoid. I don't have a high amplitidue pulse generator (or fast enough oscilloscope) to test this properly. In general, I don't know what the rise time is for more sensible signals as my oscilloscope just isn't fast enough (500MHz).

Note though that other active probes such as the Keysight N2796A 2GHz probe also limit their dynamic range at higher frequencies. This is a screenshot from their datasheet:

I measure such signals on a routine basis, so it doesn't seem so unusual to me (maybe I am an outlier, though).

Regarding the Agilent/Keysight probe example, usually such a graph shows probe derating due to some reliability or safety limitation. The same rolloff is typical for completely passive probes which will not exhibit slew rate limiting. The data sheet was not very clear on this, but from the impedance plot, an 8V AC amplitude signal at 1 GHz will give you about 50ish mA AC RMS of probe tip current, which is not insignificant. These probes have an adjustable offset of +/-12V, which is how they arrive at the 20V max input. This is not slew rate limiting, this is a way to get around the AC voltage limitations of the probe tip amplifier. It's useful for looking at noise on a voltage bus, for example, by adjusting the offset to the bus voltage so you have the full AC dynamic range centered around the bus voltage.

As I mentioned, I'd be happy to test the probe. I have a Bodnar pulser and an HP8131 pulse generator, and an old, but very functional, 6 GHz scope. I should warn you ahead of time that I'm not real quick about getting to these things, so if you find someone locally, you are likely to get a quicker turnaround. Also, if it is really slew rate limited to 1V/ns at the output, it's not something I can use.
John

I think because of how I divide the voltage prior to the amplifier, I am less succeptible to damage induced by high amplitude/high frequency signals. Last time I checked (edit. in spice) for a 28V p-p signal at 2GHz into the probe, there would be less than 1mA RMS going into the op amp, with most of the power dissipated within the input resistor network, which can handle it. I do need to run some more simulations on this though. But thanks for the clarification, I had misunderstood that the dynamic range line on that graph was not referring to the probe dynamic range in general, but more focusing on the voltage derating.

The slew rate is limited to 2V/ns at the output (remember the 10x output attenuation). For example, your HP8131 has a maximum p-p voltage of 5V, with a 200ps transition time. The slew rate limited rise time on that signal would be just slew rate limited to 250ps on my probe (theoretically). So only slightly longer, maybe here the bandwidth is reduced to around 1.3-1.4GHz for such signals. For the Bodnar pulser, the maximum p-p voltage is only 1.2V, so the slew rate limitation of the probe wouldn't affect the reading, it would be bandwidth limited here and wouldnt achieve the 40ps rise time.

If no-one else volunteers, I would be very grateful if you'd have a look at the probe. It'll be really interesting to see how it performs on a fast oscilloscope!

[joeqsmith]

Looks like you have done a nice job with it.   Any idea on the selling cost yet?   With the 50 ohm drive, I can see using it for applications outside of a scope probe.   You mentioned using it with your VNA.  There was a thread some time back about someone wanting to make a buffer to drive their 50 ohm inputs.  I had posted a commercial one and some of my own but they are capped at a bit over 100MHz.   

If you are looking for people to evaluate them, I may be able to help as well.

Thanks for posting and keep us updated.   

[lasmux]

Hi Joe,
I've appreciated a lot of your forum posts on VNAs this past year I've been working on this project!

Currently I'm aiming for around £150 for the 1GHz version, and £180 for the 2GHz version on ebay. I've not gone through my BOM thoroughly yet though, so things may shift, potentially down. I can do it a bit cheaper for the forumites as 12% ebay fees wouldn't apply, I just need to figure out how to set up a store correctly on my website  .

And yeah, I think some kind of sensor preamp, or an input buffer for a VNA are very valid use cases.

[joeqsmith]

Very reasonable.   Just looking at used prices for the LeCroy 2.5GHz single ended HFP2500, about $400 - $500 USD.   Even then, you don't know if they work or how they were treated.  It's a total crap shoot.   

For the most part, when looking at single ended low voltage signals, I tend to stay with resistive dividers.  Cheap and minimalish loading.    I have a few LeCroy 4ish GHz diff probes for my scope.   I thought about trying to build a DC -  >>GHz differential probe for logic signals but at the time, I wasn't able to source popcorn parts that would do it.   I haven't checked in several years and would guess there are much higher performance parts available today.   

Having that DC-100MHz buffer around has been helpful.  I've used it to drive my spectrum analyzers as well.  Nice thing I can use what ever 1X // 10X  probe with it.  Downside, 100MHz.   

Thanks.  Yes, I've had a lot of fun playing with these low cost VNAs.  I'm very impressed with what they have been able to achieve in such a small package.  Especially with that LiteVNA and Dislords firmware.   Hopefully your scope probes will be as successful.   

[KungFuJosh]

I just need to figure out how to set up a store correctly on my website 

You already have woocommerce installed, you just need to make sure it's setup correctly, and setup the products, payments, and shipping options. I can help in exchange for a couple 2G probes. 😉

[lasmux]

Very reasonable.   Just looking at used prices for the LeCroy 2.5GHz single ended HFP2500, about $400 - $500 USD.   Even then, you don't know if they work or how they were treated.  It's a total crap shoot.   
For the most part, when looking at single ended low voltage signals, I tend to stay with resistive dividers.  Cheap and minimalish loading.    I have a few LeCroy 4ish GHz diff probes for my scope.   I thought about trying to build a DC -  >>GHz differential probe for logic signals but at the time, I wasn't able to source popcorn parts that would do it.   I haven't checked in several years and would guess there are much higher performance parts available today.   
Having that DC-100MHz buffer around has been helpful.  I've used it to drive my spectrum analyzers as well.  Nice thing I can use what ever 1X // 10X  probe with it.  Downside, 100MHz.   
Thanks.  Yes, I've had a lot of fun playing with these low cost VNAs.  I'm very impressed with what they have been able to achieve in such a small package.  Especially with that LiteVNA and Dislords firmware.   Hopefully your scope probes will be as successful.   

I'm glad you think the pricing is reasonable. In general, probes are in a weird space in terms of pricing. They don't feel like they should be so expensive, but they are quite difficult to get right, and contain expensive and low volume custom components. That said, I feel commercial active probes are unreasonably expensive. High speed passive probes aren't much better.

Yeah, DIY Zo probes are definitely a reasonable (and dirt cheap) alternative. That said, their linearity can be a bit questionable, and I think if you measured the input impedance on one of them (I've not done this), I suspect it may be a bit variable. The inductance of the ground connection really screws up everything, which is why the expensive active probes often provide ground blade/leaf connections. In my probe I can counter the issue with a resistive ground connection. I don't think that would work in a Zo probe.

I'll send you a PM tomorrow regarding sending you a probe for you to have a play with. Thanks for offering to help out

I just need to figure out how to set up a store correctly on my website 

You already have woocommerce installed, you just need to make sure it's setup correctly, and setup the products, payments, and shipping options. I can help in exchange for a couple 2G probes. 😉

I really hated setting up that website lol. The woocommerce plugin is just rammed full of features that make figuring out how to do something simple, very difficult. I really just need to watch a few tutorials on youtube and I'm sure it'll all be easy.

[KungFuJosh]

I really hated setting up that website lol. The woocommerce plugin is just rammed full of features that make figuring out how to do something simple, very difficult. I really just need to watch a few tutorials on youtube and I'm sure it'll all be easy.

If you're only selling 2 products, you can skip woocommerce and just use PayPal buttons. If you plan on expanding later, that's a different story.

[lasmux]

I had another look around, there's a little bit of testing of Zo probes done by... you!
https://www.eevblog.com/forum/testgear/fifty-ohm-probes/msg607659/#msg607659
And some others:
https://www.eevblog.com/forum/projects/lo-z-probe/msg801477/#msg801477
There's also this one:
http://jahonen.kapsi.fi/Electronics/DIY%201k%20probe/

Everyone seems to get very different results. Not quite sure what's going on. I think my active probes are probably better performing up to their stated bandwidth, but lower bandwidth potential than a well built Zo probe. Although getting a linear response across the bandwidth seems quite tricky on a Zo probe.

I really hated setting up that website lol. The woocommerce plugin is just rammed full of features that make figuring out how to do something simple, very difficult. I really just need to watch a few tutorials on youtube and I'm sure it'll all be easy.

If you're only selling 2 products, you can skip woocommerce and just use PayPal buttons. If you plan on expanding later, that's a different story.

I might go with this. Thanks for the suggestion.

[joeqsmith]

I had another look around, there's a little bit of testing of Zo probes done by... you!
..
Everyone seems to get very different results. Not quite sure what's going on. I think my active probes are probably better performing up to their stated bandwidth, but lower bandwidth potential than a well built Zo probe. Although getting a linear response across the bandwidth seems quite tricky on a Zo probe.

Indeed, I have tried various configurations when making resistive probes.  Some better than others.   You can image that component selection and construction is going to have a major effect as you move beyond a GHz. 

Another from 2017 where I was playing with a cheap demo board.  The resistive elements were placed onto a board. 

https://www.eevblog.com/forum/microcontrollers/typical-speed-of-fpgas/msg1279898/#msg1279898

Even my LeCroy PP061 probes I mentioned are not good for these higher frequencies.  The diff probes I have are good for about 4GHz.  I also have the PP066 which are a 7.5GHz resistive probe.     

https://cdn.teledynelecroy.com/files/manuals/pp066-rp4030-user-manual.pdf

[joeqsmith]

It would be nice if probes did not load down the circuit you are wanting to test.  I started out building one of Bob Pease's active FET probes.  From memory, maybe 100MHz BW.   It saved me a few times as a young engineer when the 10X probes we had were presenting too much load for my circuits.   For home, I eventually bought a Tektronix P6202 active probe.  This served me well until I started playing with faster circuits.   

Capture1 lists the higher BW probes I have for my hobby use along with the basic specs.  I included yours as a reference. 

I have a LeCroy coplanar waveguide test board and used this as the thru for the LiteVNA.   I then attached both a LeCroy PP061 and PP005 probe to a scope and used them to probe the board.   I was never able to locate a manual or data for the PP061.

Capture2 is looking at S21.  The old 10x probe presents less of a load than the resistive probe until the cross over point at 70MHz. 

Capture3  is looking at S11.  The impedance the VNA sees with the probe attached.  This is not the probe, but the probe in parallel with port 2.   Typically I would have some driver and load and want to probe between them. 

I think to run this sort of test I would want to use my old PNA.  Some of my probes are limited to about -4dBm.  And I would want to use the unknown thru model for that waveguide.   

***
Added photo of my old Tektronix active probe along with with the probes for my old WaveMaster.   

[lasmux]

You have a lot of probes!

Thanks for this data. Interesting. I guess with a lot of the active probes, their manufacturer specific interface makes it very difficult to actually measure their response. Obviously you're able to measure the impedance. Regarding capture 2... it looks like it's true that passive probes don't help signal integrity  The same is also true of certain Ukrainian active probes that can be found on Ebay these days. I bought one because it was cheap, and the signal loading was not pretty. Response linearity was apalling too. Not bad for $25, but not that useful either.

I personally have used three methods to measure the impedance.
1. I use a SMA T piece on the VNA port 1, with a 50 ohm termination. I then place the probe in the other side of the T piece. This is better than using the port 2 as termination as I think there will be worse reflection artefacts by not measuring near the termination point.
2. I use a custom built 50 ohm load which has had it's resistors exposed, and measure directly across those.
3. A custom 'open' SMA termination which I can measure directly across.

1 and 2 tend to give fairly close results. 3 tends to give lower impedance values at lower frequencies for some reason. I'm not sure if this is because it's trying to measure impedances much much higher than 50 ohms. Maybe it gets less accurate. I'm not sure.

For bandwidth measurements, I use the same setup method 1/2, but then have the probe output measured by the VNA port 2.

[joeqsmith]

LeCroy does or did offer an accessory that allows their probes to be used with a standard scope.  Keysight and Tektronix also offers this for some of their probes.   I have yet to find a used one for mine. 

I would think to measure a probes impedance you want to have a fixture that you can remove all the errors right to where the probe attaches.   No added 50 ohm termination, no Ts, no extra length...   Your option 3 with an open that had the reference plane matched to the SOL standards, then just measure S11.   For low frequencies (GHz ish) I would just use those coplanar waveguide boards.  Then attach the probe in place of where the two 100 ohm resistors attach.   Beyond about 1.5GHz, I wouldn't trust it.   

Where my low cost VNAs don't have good return loss for port 2, the old PNA is another story.   Still just to see the loading effects between those two probes, good enough to use the Lite. 

***
I should add that when you measure the impedance, you need to choose the proper method for best accuracy.  Depending on the range, you would use the shunt, shunt thru, or series method.    In your case, you may want to use two different methods to cover the range.   


For BW,  I would still stick with the terminated coplanar waveguide, but in this case, I have one from LeCroy.   The T adds error.  Same with the custom terminator.  Consider cal'ing at the end of the waveguide, then leaving the cal standard attached when making the measurement.  You just want to know the probe.  Hope this makes some sense.   

The other test would be to just measure some basic digital signals and compare results.

I had told a friend about your project and projected selling price.  I looked up the current cost of the PP005 on Digikey and even that probe would cost more.  So yes, I think your target price is still very fair, assuming the probe works.   Let me know once you have these first set built up.   

***
Reminds me of my attempt to replicate someones experiment where they attempted to measure the capacitance of some resistors.  Far away from 50 ohms....
 
https://www.eevblog.com/forum/rf-microwave/shunt-capacitance-of-1206-smd-resistors-jeroen-belleman-december-2010/

[joeqsmith]

The PP005A attached to the home made coplanar waveguide with the terminators not populated.   The probe wasn't terminated.  Tip capacitance seems to measure around 6p rather than 11.  Note too that rather than 10M, it measures 100k (300 shown).   If I have time tonight, I will try and measure it in series with my low frequency network analyzer to give you some idea of the difference.   That NA can go up to 150MHz. 

[joeqsmith]

Using a PECL buffer that feeds a splitter.  One leg which goes to one of the scopes inputs (50ohms).  The other goes to the LeCroy coplanar waveguide test board which was then terminated to 50 ohms.   Scope probe when then attached to second channel on scope and then used to probe the waveguide. 

No load:  No second probe attached.  This scope is only spec'ed for 600MHz (approx 600ps), and that PECL driver is about 350ps.  Cable adds a fair bit of loading.  Idea is to show the effects of the probe loading.

PP002:  This is a 350MHz 10X probe.   Causes a fair bit of loading.  Think about that PECL signal going off to some other part of the circuit rather than the scope.   Also, with the splitter, things are somewhat isolated.   

PP061:  The resistive probe still has some loading (not a great probe)  but much better than the 10X probe.   

Once you get a bit closer,  I'll bust out my faster scope and active probes.     

[joeqsmith]

Next, I removed the splitter and connected the LeCroy waveguide directly to the PECL driver.  The opposite end of the waveguide connected to the scopes 50 ohm input.  Basically, how you would probe a typical signal on a board but the scope is again the target load.     

145:  The PP061 resistive probe pink.  M1 is the unloaded waveguide (nothing attached).  C1 is the loaded waveguide.  Again, some loading.  Scopes BW is limiting the edge to about 600ps. 

146:  The PP005 500MHz 10X probe.   It really messes with the signal.  I did not clear the traces so ignore the averaged data.

147: The P6202A 500MHz active probe.   Has very little loading effects compared with the 10X passive probe.   According to the link below, the probe cost about $1300 in 1998.  The cables are thick and heavy making the probe difficult to use.   You can start to see why the resistive probes were a better choice in most cases.   

http://www.barrytech.com/tektronix/probes/tekp6202a.html

[lasmux]

I think your second set of measurements will be better? With the splitter you will have two 50 ohm terminations (to ground, not VCC-2V) on the PECL driver, which it might not like? I've not used PECL before though to be honest, only a bit in simulation.

Very interesting how the PP005 really does a lot of damage to the signal. The PP061 lowers the PECL voltage levels slightly with it's loading? But the shape of the loaded waveform is more similar. The P6202A waveform has almost no loading (at these frequencies), but the output waveform maybe has slightly more peaking on the rising edge? It's still qualitatively very similar.

[joeqsmith]

I think your second set of measurements will be better? With the splitter you will have two 50 ohm terminations (to ground, not VCC-2V) on the PECL driver, which it might not like? I've not used PECL before though to be honest, only a bit in simulation.

I had used a resistive splitter, so with the two ports loaded with 50 ohms, the PECL driver sees 50 ohms.  The signal the scope sees is 6dB lower than the original.  The reason I thought about running this test was to somewhat isolate the two ports.   

https://www.microwaves101.com/encyclopedias/resistive-power-splitters

Very interesting how the PP005 really does a lot of damage to the signal. The PP061 lowers the PECL voltage levels slightly with it's loading? But the shape of the loaded waveform is more similar. The P6202A waveform has almost no loading (at these frequencies), but the output waveform maybe has slightly more peaking on the rising edge? It's still qualitatively very similar.

From my previous posts where I provided some basic metrics for my probes, you can see the PP061 is a 10X probe, so the DCR was 500 ohms (which includes the scope's input).   The waveguide was loaded with 50 ohms by the scopes other channel.  Now we added another 500 ohms.   Your comment about the P6202A "at these frequencies" is key.   It's not a perfect 500 ohms and the response is not going to be flat as we start to look at faster signals.   

Currently the scope is doing a good job of masking what the actual signal looks like.       

***
Thinking about your previous comment, I am guessing you thought I was using a "T" which is not the same thing as a splitter.

[joeqsmith]

They claim 1GHz, for $250 and it is not DC coupled.  I would have no use for something like this.   
https://www.ebay.com/itm/254162548019?hash=item3b2d44b533

$29 and again, not DC coupled. 
https://www.ebay.com/itm/175751864133?hash=item28eba09f45

There doesn't appear to be a low cost 2GHz product that would compete with you.  The only choice for the hobbyist is the used market...

[joeqsmith]

I had another look around, there's a little bit of testing of Zo probes done by... you!
....
Everyone seems to get very different results. Not quite sure what's going on.

Because much of what I had shown here by attempting to embed resistors in the cable and such, isn't really how I typically would make a probe, I wanted to show what something a bit more common.   This is a 20X (953 ohm) probe.   Zoomed in 1ns/div.  Again, fairly low frequencies and the scope is hiding the details but hopefully it helps show that the results shouldn't vary by much.    The cable (Teflon) and connector came from Pasternak.   Surface mount resistor is stabilized with a bit of shrink tube.  Not a whole lot invested.  Maybe a half hour labor and $15.   

Maybe we can run some of these on the VNA and my faster scope later on to help paint a clearer picture.   

[lasmux]

Currently the scope is doing a good job of masking what the actual signal looks like.       
[...]
Thinking about your previous comment, I am guessing you thought I was using a "T" which is not the same thing as a splitter.

Ah thanks about clarifying about the splitter. I had indeed misunderstood, and yes, I have the same problem with my oscilloscope. The true signal form is hidden behind the insufficient bandwidth.

With regards to those other probes, I have seen those. They're not great tbh. The first ones construction looks quite good, but the input capacitance is 3.5pF. This is really quite bad, not far off a good passive probe. As you said, not DC coupled either. The second one looks very similar to other active probes based on the BF-998 that are on ebay. I have a similar version from Ukraine. The frequency response linearity is awful (swings +/- 8db or so across the bandwidth), and the input capacitance is in reality much higher than the quoted 0.5pF. I measured the tip loading drop to 20 ohms at around 500MHz, which again, is really bad.

I had another look around, there's a little bit of testing of Zo probes done by... you!
....
Everyone seems to get very different results. Not quite sure what's going on.

Because much of what I had shown here by attempting to embed resistors in the cable and such, isn't really how I typically would make a probe, I wanted to show what something a bit more common.   This is a 20X (953 ohm) probe.   Zoomed in 1ns/div.  Again, fairly low frequencies and the scope is hiding the details but hopefully it helps show that the results shouldn't vary by much.    The cable (Teflon) and connector came from Pasternak.   Surface mount resistor is stabilized with a bit of shrink tube.  Not a whole lot invested.  Maybe a half hour labor and $15.   
Maybe we can run some of these on the VNA and my faster scope later on to help paint a clearer picture.   

Indeed, there is a very low loading here! Would definitely be interesting to see how it performs on the VNA/faster scope.

[joeqsmith]

Before doing this, another option may be to make a new one that uses a high grade SMA rather than the BNC.  I should have some cable and connectors on-hand.   The tip loop is also fairly large on this one.   I wouldn't want to give you the wrong impression about these  homemade probes once we start speeding things up.   

To look at the BW with the VNA, I am thinking we stay with the same waveguide.  Terminate the backside to 50.  Then attach the probe to the second port.  Using my homemade probe, we should see 26dB.   I would need to use my old Agilent for this.   Maybe then check the tip capacitance as well.  I could run your probe as well as my resistive probes but none of my active or standard 10X probes. 

[joeqsmith]

M2 is no load, C2 is the loaded waveguide.  PP061 resistive probe blue.   Rise time is roughly 500ps for the probe, or around 700MHz.   Edge without the probe attached measures about 200ps.   

With a better scope and lower transitions the probe doesn't  look so impressive.      

[joeqsmith]

But if you think that's bad here is that same homemade resistive probe.      That's s mighty big loop, lots of inductance.  Not too surprised it peaks up.   I would expect your probe would do a much better job with this 200ps edge.   

Scope is an old WM8500A 5GHz BW. 

[joeqsmith]

Using a different driver with my Tektronix P6202A probe.  It makes a decent filter. 

[MathWizard]

Cool project, I'd buy this probe down the road.

How much worse would it be if you used all the same chips, but used all standard size through hole parts ? What if you didn't pay attention to spacing or layout ? I'm just trying to get a feel for how sensitive these circuits really are. If 2GHz is ~15cm waves, thats getting down to circuit size.

1 of the next things I need to buy, is cables, terminators, and splitters, for using between my scopes and signal gen. And I want to try making 1 of those passive probes that's just coax and a 1k resistor, and has very low capacitance. Dave showed 1 in a video on probes. I can't remember their downside, besides low series resistance. But even mucking about with BJT amplifiers lately, I realize how big 15pF of 10x probes can be.

[joeqsmith]

Cool project, I'd buy this probe down the road.

How much worse would it be if you used all the same chips, but used all standard size through hole parts ? What if you didn't pay attention to spacing or layout ? I'm just trying to get a feel for how sensitive these circuits really are. If 2GHz is ~15cm waves, thats getting down to circuit size.

1 of the next things I need to buy, is cables, terminators, and splitters, for using between my scopes and signal gen. And I want to try making 1 of those passive probes that's just coax and a 1k resistor, and has very low capacitance. Dave showed 1 in a video on probes. I can't remember their downside, besides low series resistance. But even mucking about with BJT amplifiers lately, I realize how big 15pF of 10x probes can be.

MathWizard, for a 20:1 ratio, you want to use a 950 ohm resistor, not a 1k.   The scope's input is 50, so the circuit you are probing sees 1000ohms (assuming a perfect world).

The following was taken from this paper:
https://people.ece.ubc.ca/robertor/Links_files/Files/TEK-Understanding-Scope-BW-tr-Fidelity.pdf

Analog bandwidth is a measurement specification that simply defines the frequency at which the measured amplitude of a sinewave is 3 dB lower than the actual sinewave amplitude (see IEEE-1057). Figure 1 shows an idealized amplitude roll off error as a sinewave signal approaches the specified bandwidth frequency of a measurement device having a first order or single pole Gaussian response. At the rated bandwidth, the measurement error approaches 30%! If you want to make a measurement on a sinewave that has only 3% error, you would only want to measure sinewaves much lower in frequency than the rated bandwidth of the oscilloscope, about 0.3 times the rated instrument bandwidth. Because most signals are more complex than sinewaves, it is a general rule of thumb is use a measurement device, like an oscilloscope, that has 5 times the bandwidth of the signal you intend to measure (explained later and shown in Figure 5).

***
In case it wasn't clear, the following was also taken from the same paper: 

In fact, every probe manufacturer assumes that at the maximum specified bandwidth, the probe’s frequency response is down 3 dB.

[lasmux]

But if you think that's bad here is that same homemade resistive probe.      That's s mighty big loop, lots of inductance.  Not too surprised it peaks up.   I would expect your probe would do a much better job with this 200ps edge.   

Scope is an old WM8500A 5GHz BW. 

Yeah, that's a lot of ringing! Very neat how little it's loaded the original signal though.

[lasmux]

Cool project, I'd buy this probe down the road.
How much worse would it be if you used all the same chips, but used all standard size through hole parts ? What if you didn't pay attention to spacing or layout ? I'm just trying to get a feel for how sensitive these circuits really are. If 2GHz is ~15cm waves, thats getting down to circuit size.

Thanks, I'll let you know when it's ready

The probe amplifier input and feedback network on the probe is all 0402 passives. Even with that I have to have various snubbing resistors etc as stray inductances/capacitances cause havoc with the frequency response. You might be able to get something working with through hole components, but it would take much more effort to get it working correctly. And you probably wouldn't be able to reach 2GHz bandwidth.
Compared to the DIY 1k resistive probes, it is more general purpose due to the 1M input resistance, and in theory will have a more precisely tuned frequency response, but has lower bandwidth potential.

[joeqsmith]

But if you think that's bad here is that same homemade resistive probe.      That's s mighty big loop, lots of inductance.  Not too surprised it peaks up.   I would expect your probe would do a much better job with this 200ps edge.   

Scope is an old WM8500A 5GHz BW. 

Yeah, that's a lot of ringing! Very neat how little it's loaded the original signal though.

To give you some idea of the effects of lowering the inductance, these are two poor man's 10X probes.  The resistors are butted up against the coax center pin with a much shorter probe tip.  All to reduce that big loop.   

As before, M2 is the unloaded waveguide. C2 is loaded waveguide with probe #2 attached.  C3 is probe #2.  M3 is probe #1.    You mentioned the amount of variance seen with homemade probes but these two seem fairly well matched.   

[joeqsmith]

Just for completeness, I reattached the homemade 20X probe to compare it with the 10X probe #1.   Construction is everything and as you pointed out, I did try a few ways to improve it but they all failed to exceed what I could achieve with the simple techniques shown. 

[joeqsmith]

I have thought about trying to add a blade for the ground rather than the wire to help reduce the inductance.   One thing I have done in the past was when I wrap the wire around the coax to form the ground, I use both ends rather than one.  I normally solder this coiled wire.  The problem is getting them attached to the board.   Picotech offers a probe like that today.   Looks like a 20X 6GHz probe would cost about $1000 USD.   

https://www.picotech.com/accessories/gigabit-digital-passive-test-probes

[tggzzz]

I have thought about trying to add a blade for the ground rather than the wire to help reduce the inductance. 

Like this HPAK N2878A?

[Mechatrommer]

I've been working on an active probe design for around a year. The goal started off as creating a DC-coupled active probe to support a photon counting sensor I am also working on, but it was a very fun project and I spent so much time on it that now the plan is to sell it. Could I have some feedback on the probe/performance, and on the contents of the datasheet, before I start buying the first batch of parts... which will be quite expensive.

I have another post that I'm putting together where I'll go into the development process a bit more.

I'm making two versions, a 1GHz version, and a 2GHz version.

The datasheet is here: https://www.lasmux.com/wp-content/uploads/2023/07/LD-ASP-1G_2G.pdf



Quick specs:
Bandwidth: DC-1GHz, DC-2GHz
Input capacitance (measured at 1GHz): 0.7pF
Attenuation: 20x
DC input resistance: 1Mohm

1GHz version frequency response (linear and log axis):


2GHz version frequency response (linear and log axis):


Tip input impedance of both probes, depending on which ground lead is fitted:


The resistive ground lead can be used to stop a resonance developing on the ground connection, which reduced the input impedance above 1.5GHz. I talk about this a bit more in the datasheet.



In terms of step response for the system, I've 'only' got a 500MHz oscilloscope, so can't properly test the rising edge speed unfortunately. This is the probe measuring a 50 ohm terminated 100MHz signal, with a <100ps rise time. This greatly exceeds the bandwidth of the scope so there's some ringing. The trace looks basically identical if I measure the signal directly by the oscilloscope.

Currently I'm aiming for around £150 for the 1GHz version, and £185 for the 2GHz version.

good work!

[joeqsmith]

I have thought about trying to add a blade for the ground rather than the wire to help reduce the inductance. 

Like this HPAK N2878A?

Yes, but with a smaller loop. 

[joeqsmith]

I used some copper spring material to make an "L" shape for the blade.  The tab was formed into a cylinder and soldered to the coax. 

Again, M2 is the unloaded waveguide (probe not attached).  C2 is with the probe loading the waveguide.   C3 is the measured signal from the blade probe.   Of course there are some errors with gain and I can compensate for the phase.  A bit too much gain in the plot but works fairly well compared with the others.   

***
The blob used to mechanically stabilize the resistor is Devcon 5 minute epoxy.   It doesn't appear to hurt the performance of the probe at least at these frequencies.   

[joeqsmith]

Making similar measurement with an old LeCroy 7200.  The only option for saving data is with a floppy drive which I can read with my USB floppy.  For plotting, I use CERN's viewer.   

Shown looking at the 10X blade probe on channel B2.   Most of the error is due to the how the scope works.   It has a step generator built-in and can loopback that signal to the display.   It's is not the same as probing at the end of the waveguide like I have been showing.   I need one more channel for that.   

The scope doesn't have a lot of storage and I don't remember it having a way to adjust the trigger independently for each channel.  Even at 1nS/div, I can't align the two waveforms.    Consider we are using 30+ year old scope (the 7200 was released in 1989),  results seem fair.   

[lasmux]

I used some copper spring material to make an "L" shape for the blade.  The tab was formed into a cylinder and soldered to the coax. 

Again, M2 is the unloaded waveguide (probe not attached).  C2 is with the probe loading the waveguide.   C3 is the measured signal from the blade probe.   Of course there are some errors with gain and I can compensate for the phase.  A bit too much gain in the plot but works fairly well compared with the others.   

***
The blob used to mechanically stabilize the resistor is Devcon 5 minute epoxy.   It doesn't appear to hurt the performance of the probe at least at these frequencies.

This is really nice. I love the short L ground blade design. The probe response matches the source waveform really well! Even the ripple is very close.

Making similar measurement with an old LeCroy 7200.  The only option for saving data is with a floppy drive which I can read with my USB floppy.  For plotting, I use CERN's viewer.   

Shown looking at the 10X blade probe on channel B2.   Most of the error is due to the how the scope works.   It has a step generator built-in and can loopback that signal to the display.   It's is not the same as probing at the end of the waveguide like I have been showing.   I need one more channel for that.   

The scope doesn't have a lot of storage and I don't remember it having a way to adjust the trigger independently for each channel.  Even at 1nS/div, I can't align the two waveforms.    Consider we are using 30+ year old scope (the 7200 was released in 1989),  results seem fair.   

Can you save waveforms on the screen? This is how I sometimes do it. I save and display a reference waveform, and then use the delay on the trigger to overlay the live view of another signal.

[joeqsmith]

I believe it has the freedom to do what you suggest, if the scope was 100% working.  It throws the BRAM (Battery RAM) fault on start as the internal battery is dead.  I'm not sure if that is the problem but when I select the internal memory, the scope resets.  The scope shows the memory is garbage and selecting clear will also cause a reset.   I am using a version of firmware that is known to have problems but it supports the plug-ins that I am using.  It's possible that the memory problem could be tied to firmware.  30 years old, maybe even a hardware failure.  This thing is an old VME chassis.    


***
This was a kickstarter resistive probe.  While a try to reduce the inductance, if you read their comments,  "the Vishay FC0402 resistors I use in the attenuator are rated for 30V each. I have a series string of 100-75-75-75-75-50 ohms so with 50V at the tip you shouldn't be over 30 on any single resistor."   Hard to believe they would get any level of performance out of it.

Note how they terminated the test board so we can't see the loading effects.   

https://www.kickstarter.com/projects/azonenberg/akl-pt1-2-ghz-passive-oscilloscope-probe

[joeqsmith]

While we wait for your probes,  attempted to low format the drive, which failed.  The drive I have been using is an old Seagate ST125.  Note the jumper wire is not factory but a mod I sorted out that makes the drive act like the old Mini-scribe drives that came from the factory.  These scopes only supported a few different drives. The KALOK KL320 was another one it supported.  I inserted my backup drive and again, fails out.  That drive has been in storage for maybe 15 years or so.   

I had converted this scope to use rechargeable batteries for the backup.  I had forgot that the low format would fail if they were dead.  So after a full charge,  low format, reinstall, the scope now works.  I went through the same process with the second drive and it works as well.     Putting it back together,  it no longer resets when the internal memory is selected and does not show garbage when you try and display the contents.     

So I tried your suggestion of using the storage and delayed trigger using about a foot or so of coax for a delay.   Works like a charm, well, for now.    

[joeqsmith]

For fun, I took the ECL output into a DC block, into a Mini-Circuits 8,1GHz tripler and then into a wideand amplifier.   The hope was to get a faster edge.  Loaded waveguide is C2 and the bladed probe, C3.   The WM8500A has a BW of 5GHz, or roughly 70ps.   Signal level is a bit low but the homemade probe seems to do a fair job tracking it. 

Granted, you can't use it for high voltages and at low frequencies, even a basic 10X probe would present less loading but if you work with high speed digital, it may be difficult to match the price performance of a resistor and some coax.  Well, and apparently a really good ground.   

[lasmux]

I've seen that probe from Andrew Zonenberg before. He has some really nice other projects also. I think the main issue with that probe is the tip capacitance is actually quite high for a resistive probe, and the bandwidth stops at 2GHz. Not sure why it was necessary to use those Vishay FC0402 resistors. Under 2GHz I don't think a normal 0402 resistor would have been an issue, and they are miles cheaper.

Glad you managed to fix your scope I've never been good at fixing old oscilloscopes. Last time I took apart my HP 54111D, I managed to achieve nothing but waste time and stress out about killing myself on the CRT.
Nice tuning of that delay haha. Very nice match between the source/measured signal. But yeah, damn dude, that resistive probe is working a charm. Really nice. As always, I think there is a place for active probes above resistive probes. Where you want reduced signal loading, probably sub 2GHz, and for frequency response stability.
My original application was my actively quenched APD as a photon counting module, where I wanted to measure across a 1k ballast resistor restricting current through an APD. A passive probe had way too much capacitance and would have slowed down the response of the signal, a resistive probe would have made the ballast resistor ineffective and probably damaged the APD. An active probe was the only way that I could think of.

On another topic, I was considering a differential version of the probe. I've been playing around in LTSPICE, and I could probably get over 1GHz of bandwidth, with the same 0.7pF tip capacitance, +/- 30V input dynamic range, but I suspect my CMRR wouldn't be great without a bit of work on matching the two inputs closely.

[joeqsmith]

I agree, active probes certainly have their place.  I would like to see your diff probe if you pull it off.  When I investigated it, I could not locate parts to do the job.  That has been a long time ago and I suspect based on your goals, you will be able to pull it off.   That would be a handy probe to have on hand.  Much of the high speed digital has been differential for some time.   

Searching ebay, even junk 1GHz diff probes have an asking price of over $200.   Not suggesting anyone would pay that.  Looking for my D300, this one was about the cheapest I could find which at least appears complete and they offer returns. 

https://www.ebay.com/itm/175614100115

Of course, that's about 4X the BW you are targeting but still, I think there is money to be made if you can keep the costs down.

When my wife and I were first married, I was needing a higher speed scope than we had at work.  I rented a scope similar to that and had it shipped to our house so I could sort out how to run it before taking it to work.  My wife was worried that something would happen.  I think the cost was more than our house at the time.    

[hpw]

On another topic, I was considering a differential version of the probe. I've been playing around in LTSPICE, and I could probably get over 1GHz of bandwidth, with the same 0.7pF tip capacitance, +/- 30V input dynamic range, but I suspect my CMRR wouldn't be great without a bit of work on matching the two inputs closely.

Yeah, as I currently use the AP034 diff probe (see picture) as nice using 0.1" pin headers for fixed or tips connections over the active HFP2500 or the crappy active Cal Test CT4121. Even the CT4121 as shielded front header still picks up RF waves.

Hp

[tszaboo]

I've seen that probe from Andrew Zonenberg before. He has some really nice other projects also. I think the main issue with that probe is the tip capacitance is actually quite high for a resistive probe, and the bandwidth stops at 2GHz. Not sure why it was necessary to use those Vishay FC0402 resistors. Under 2GHz I don't think a normal 0402 resistor would have been an issue, and they are miles cheaper.

I just looked at those FC0402 resistors and they are 5 USD in single pieces.
I really don't get it. 0402 SMD thin film inductor is going to set you back 2c, but if you want a resistor with the same technology, it's going to cost 300 times more.

[joeqsmith]

S11 & S21 for the blade probe using the PNA.  My homemade PCB waveguides are going to be useless.    Starts out at 500 ohms as and drops off rather quickly reaching 100 ohms at 6 GHz.   Mag isn't very stable  (when holding all the parts in my hands, moving them around...).  Then there is de-embedding the interconnect.   Consider it all just a gross measurement to give us some idea how it behaves.

[JohnG]

My original application was my actively quenched APD as a photon counting module, where I wanted to measure across a 1k ballast resistor restricting current through an APD. A passive probe had way too much capacitance and would have slowed down the response of the signal, a resistive probe would have made the ballast resistor ineffective and probably damaged the APD. An active probe was the only way that I could think of.

There is another way that might work. You make the 1k ballast part of your probe by putting a 953 ohm resistor in series with 50 ohm microstrip or similar transmission line right on the same PCB. Just don't forget to terminate the other end of the line if it is not connected to your scope. You get your intended loading and measurement at the same time. Note that this does not work as well if the ballast is connected to the positive bus, but sometimes it is possible to do something clever and make it work anyway, especially if you can put in a series dc blocking cap (if you don't need a dc response).

John

[tszaboo]

I've seen that probe from Andrew Zonenberg before. He has some really nice other projects also. I think the main issue with that probe is the tip capacitance is actually quite high for a resistive probe, and the bandwidth stops at 2GHz. Not sure why it was necessary to use those Vishay FC0402 resistors. Under 2GHz I don't think a normal 0402 resistor would have been an issue, and they are miles cheaper.

You made me do this. 
But yeah, you piqued my interest, I designed this small board to be able to measure passives with the VNA. I was always disappointed by the inductors in RF matching, and wanted a way to measure them, this gave me the extra push to make this.
My issue now, how do I calibrate the VNA to 50 Ohm on a resistor, and then measure the DUT which is a resistor.

[joeqsmith]

Why have components mounted on the thru, open and short?  For the load, why wouldn't you use a standard?  Are you concerned the length of the load will made a big difference?

[tszaboo]

Why have components mounted on the thru, open and short?  For the load, why wouldn't you use a standard?  Are you concerned the length of the load will made a big difference?

I don't intend to mount those components, but I placed the ~3mm long transmission line and the component footprint anyway.
I guess I could use an FC0402 50Ohm as the load. Normally the length shouldn't make a difference, but from what I've seen, the closer you can make the calibration look like the real world condition, the better.
Plus, I only have a female cal kit for for SMA (at work).
But you know what, it's an interesting question as well. I'll calibrate it without the components on it and mount some 0402s and see if it makes a difference.

[nctnico]

Why have components mounted on the thru, open and short?  For the load, why wouldn't you use a standard?  Are you concerned the length of the load will made a big difference?

I don't intend to mount those components, but I placed the ~3mm long transmission line and the component footprint anyway.
I guess I could use an FC0402 50Ohm as the load. Normally the length shouldn't make a difference, but from what I've seen, the closer you can make the calibration look like the real world condition, the better.
Plus, I only have a female cal kit for for SMA (at work).
But you know what, it's an interesting question as well. I'll calibrate it without the components on it and mount some 0402s and see if it makes a difference.

With an OSL you will be calibrating at the connector. So whatever is different (like the trace not having a perfect 50 Ohm impedance and FR4 loss factor) for the real measurement will show up. Then again, I'm missing the DUT position for a reflection based measurement so how useful are the OSL parts on your board anyway?

But it does make sense to calibrate the board from an OSL calibration in order to determine whether the error is as expected c.q. up to what frequency your board is useable. But the through measurement will be the most useful anyway because the error is low over a much wider frequency range.

[joeqsmith]

Board would be taken out and reference plane would be at the end of the transmission line.   I assumed they were making multiple boards and populated the test parts on the second one.   

[JohnG]

FC resistors may be overkill, but they are nice and accurate. If you get them, you should look at the datasheet and consider the difference between the "wrap-around" termination and the "flip-chip" termination, shown in the figure labeled "Internal Impedance." (Good god, I hate this graphic designer trend of eliminating section and figure numbers. I curse them...). It shows a few % increase in impedance at 2 GHz, and a lot more at 6 GHz, for the "wrap-around" versus the "flip-chip".

https://www.vishay.com/docs/60162/fchpseries.pdf

If you use a more cost-effective thin film resistor, you can get the same benefit by mounting the resistor upside-down, i.e. with the resistive element adjacent to the PCB. It can save a few hundred pH. If you are measuring your boards and feel so inclined, you can measure this difference pretty easily at a few GHz.

Hope you find this useful.

John

[tszaboo]

Why have components mounted on the thru, open and short?  For the load, why wouldn't you use a standard?  Are you concerned the length of the load will made a big difference?

I don't intend to mount those components, but I placed the ~3mm long transmission line and the component footprint anyway.
I guess I could use an FC0402 50Ohm as the load. Normally the length shouldn't make a difference, but from what I've seen, the closer you can make the calibration look like the real world condition, the better.
Plus, I only have a female cal kit for for SMA (at work).
But you know what, it's an interesting question as well. I'll calibrate it without the components on it and mount some 0402s and see if it makes a difference.

With an OSL you will be calibrating at the connector. So whatever is different (like the trace not having a perfect 50 Ohm impedance and FR4 loss factor) for the real measurement will show up. Then again, I'm missing the DUT position for a reflection based measurement so how useful are the OSL parts on your board anyway?

But it does make sense to calibrate the board from an OSL calibration in order to determine whether the error is as expected c.q. up to what frequency your board is useable. But the through measurement will be the most useful anyway because the error is low over a much wider frequency range.

I'm planning OSLT, the measurement is not a reflection based it's through.
https://coppermountaintech.com/measurement-of-electronic-component-impedance-using-a-vector-network-analyzer/

According to this article, measuring 100 Ohm and above, I should use the through method, and around 50 Ohm, I can use the reflection
based on a S11 measurement.

Board would be taken out and reference plane would be at the end of the transmission line.   I assumed they were making multiple boards and populated the test parts on the second one.   

Exactly.
I'm going to open another thread for this, so we don't pollute lasmux's excellent design with off topic stuff.
https://www.eevblog.com/forum/rf-microwave/measuring-passive-parts-with-vna/new/#new

[RFKev]

S11 & S21 for the blade probe using the PNA.  My homemade PCB waveguides are going to be useless.    Starts out at 500 ohms as and drops off rather quickly reaching 100 ohms at 6 GHz.   Mag isn't very stable  (when holding all the parts in my hands, moving them around...).  Then there is de-embedding the interconnect.   Consider it all just a gross measurement to give us some idea how it behaves.

What was the measurement setup and METAS VNA Data Explorer configuration to get the probe impedance with frequency?

[joeqsmith]

S11 & S21 for the blade probe using the PNA.  My homemade PCB waveguides are going to be useless.    Starts out at 500 ohms as and drops off rather quickly reaching 100 ohms at 6 GHz.   Mag isn't very stable  (when holding all the parts in my hands, moving them around...).  Then there is de-embedding the interconnect.   Consider it all just a gross measurement to give us some idea how it behaves.

What was the measurement setup and METAS VNA Data Explorer configuration to get the probe impedance with frequency?

I showed the screen shot of the METAS setup and used S11 to measure the impedance.  Guessing that is not what you are asking.  Maybe consider posting a more detailed question. 

[RFKev]

S11 & S21 for the blade probe using the PNA.  My homemade PCB waveguides are going to be useless.    Starts out at 500 ohms as and drops off rather quickly reaching 100 ohms at 6 GHz.   Mag isn't very stable  (when holding all the parts in my hands, moving them around...).  Then there is de-embedding the interconnect.   Consider it all just a gross measurement to give us some idea how it behaves.

What was the measurement setup and METAS VNA Data Explorer configuration to get the probe impedance with frequency?

I showed the screen shot of the METAS setup and used S11 to measure the impedance.  Guessing that is not what you are asking.  Maybe consider posting a more detailed question.

I can see the settings along the top of the METAS screenshot. Was there anything configured in the "Setup" dropdown?  My main question is how was the probe interfacing with the VNA.  Did you touch the probe on the port 1 connector, or probe a transmission line?  If you probed a line connected to port 1, was it terminated with a 50ohm load?

[joeqsmith]

S11 & S21 for the blade probe using the PNA.  My homemade PCB waveguides are going to be useless.    Starts out at 500 ohms as and drops off rather quickly reaching 100 ohms at 6 GHz.   Mag isn't very stable  (when holding all the parts in my hands, moving them around...).  Then there is de-embedding the interconnect.   Consider it all just a gross measurement to give us some idea how it behaves.

What was the measurement setup and METAS VNA Data Explorer configuration to get the probe impedance with frequency?

I showed the screen shot of the METAS setup and used S11 to measure the impedance.  Guessing that is not what you are asking.  Maybe consider posting a more detailed question.

I can see the settings along the top of the METAS screenshot. Was there anything configured in the "Setup" dropdown?  My main question is how was the probe interfacing with the VNA.  Did you touch the probe on the port 1 connector, or probe a transmission line?  If you probed a line connected to port 1, was it terminated with a 50ohm load?

No, just S11. Everything is defaults except the top.   My comment about de-embedding the interconnect was the main point.  Ideally, I would have a custom jig for this.   You can't... well, er... you could probe an RF connector directly but nah....   I have a connector that matches with my home made standards and touched off on it.  This gets up roughly the correct reference plane.  Still, I expect some error.  There is nothing attached to the test connector besides the probe (which is what I am wanting to measure).   Think of it this way.  If the test connector were terminated to 50, then I  place the probe in parallel with that, we are not going to measure 500 ohms but rather 500 in parallel with 50, product over sum....

[schratterulrich]

Your probe project is very impressive. It certainly took a lot of effort to achieve this high bandwidth.
I came across this post because I probably took a similar approach to building an active probe.
My intention was to get a probe with very high input impedance and low capacitance to test sensitive nets like oscillator signals and to get an high impedance amplifier for 50 Ohm instruments.
I chose the ADA4817 operational amplifier and got the following characteristics for the active probe. (With direct SMA connection)
500 MHz bandwith
660 ps Rise Time
1,3 pF input capacitance
95 MOhm Input impedance

[lasmux]

Your probe project is very impressive. It certainly took a lot of effort to achieve this high bandwidth.
I came across this post because I probably took a similar approach to building an active probe.
My intention was to get a probe with very high input impedance and low capacitance to test sensitive nets like oscillator signals and to get an high impedance amplifier for 50 Ohm instruments.
I chose the ADA4817 operational amplifier and got the following characteristics for the active probe. (With direct SMA connection)
500 MHz bandwith
660 ps Rise Time
1,3 pF input capacitance
95 MOhm Input impedance

Hey, cool probe there Nice linear response out to 300MHz or so!

Thanks for the feedback. It definitely did take a lot of effort, and even more learning. I am currently waiting for the 9th revision PCB to arrive (!!!). Fortunately active probe PCBs are small and relatively cheap.

[hpw]

Fortunately active probe PCBs are small and relatively cheap.

And please do not forget any 2GHz differential mode(s) 

[LooseJunkHater]

What is the estimated price for the probe?

[lasmux]

What is the estimated price for the probe?

It'll be around £160/$200, excl shipping. I'm thinking of just sticking with a 2GHz version, and not bothering with an artifically worse/cheaper 1GHz model. I'm building a (hopefully) final iteration at the moment which will likely improve most of the specifications, then go into production.

[LooseJunkHater]

What is the estimated price for the probe?

It'll be around £160/$200, excl shipping. I'm thinking of just sticking with a 2GHz version, and not bothering with an artifically worse/cheaper 1GHz model. I'm building a (hopefully) final iteration at the moment which will likely improve most of the specifications, then go into production.

I'm not personally interested in the single-ended probe, but I'd absolutely love if you came out with a variable attenuation dual-ended probe. $200USD is a pretty great price! I'd imagine it would be difficult however for it to reach 1GHz. I wonder if you'd be able to make minor changes to your circuit to allow for it.

[lasmux]

After a bit of radio silence, I have a large update with big improvements to the probe:

I have worked through another two iterations of the probe as there were various specifications that I thought could be improved upon. We're at V10 now.

I also was in contact with Picotech, who generously offered to lend me a fast oscilloscope (Picoscope 9404-16, 16GHz sampling scope) for proper time-domain measurements of the probe. This has been a huge pleasure to use, and I am very grateful to them for lending me such a high specification oscilloscope. I unfortunately have to ship the scope back this week. So yes, big thanks to them! Buy their oscilloscopes!

I now no longer use the optional resistive ground lead to reduce the DUT loading, instead the signal pin is now always resistive. As such there's no longer a trade-off between probe bandwidth and reduced DUT loading. You get both high bandwidth and reduced loading

So the main new specifications are:
Bandwidth: 2.7GHz
Rise time: 130ps
DUT loading: 120Ohms worst case (at 1.6GHz).
Tip capacitance: 0.7pF
Return loss on a 50 ohm transmission line -15.4db max
Noise: 350uV RMS output, or 7mV RMS referred to input (20x probe)

Now for the performance plots.


Showing -3db at just over 2.8GHz. Linearity is still very good.


Tip loading is now minimum 120Ohms, and stable out to 4GHz at 200 ohms. My VNA is unstable beyond this.


Step response test from signal generator with 350ps rise time. Probe response is very close to the signal generator, and probe loading has not modified the signal at all. Note red and green lines are almost perfectly overlapping. Note that red and green traces are saved waveforms so they can be overlaid with the probe signal. The probe has a 5.4ns latency.


Rise time measurement using fast edge (70ps) from fast signal generator. This edge has a bit of ringing, so I'm uncertain how much of the probe ringing is because of the source signal ringing vs ringing because the source rise time is so much faster than the probe rise time. Probe rise time around 130ps which corresponds to a 2.7GHz bandwidth. Note probe has only a minor loading on this very fast signal. The rise time is slowed by around 6ps and there is some reduction to the peaking.


Nice flat-topped low-frequency square waves (1khz).


Test setup for oscilloscope measurements. Signal generator routed into oscilloscope (channel 4) via intermediate coplanar waveguide board. Active probe measures signal on coplanar waveguide and is read by oscilloscope channel 2.

I'm pretty sure I've said this before, but hopefully for real this time. I am looking at starting production of these. If anyone wants one, send me a PM to get yourself to the front of the queue
The cost will be between £180 and £200, depending on how much of my time I end up spending on each probe. Hopefully more on the lower end of that. The probe tips are hand-made, and there may need to be some tuning on each probe if the frequency response isn't as good as it should be when I test them.

[tggzzz]

Looks good Kudos to you.

Kudos also to PicoTech. Their attitude is like Bill and Dave, and that was instrumental (ho ho) in building HP into the great company it was.

If I was younger, it sounds like I would want to work for PicoTech

[lasmux]

Looks good Kudos to you.

Kudos also to PicoTech. Their attitude is like Bill and Dave, and that was instrumental (ho ho) in building HP into the great company it was.

If I was younger, it sounds like I would want to work for PicoTech

Yes exactly. They do seem like an excellent company in all respects! Really cool vibes from them.
I do also have a picoscope 2206B (50MHz bandwidth) which I use for all my embedded and lower speed development. It and its software are seriously good.

[2N3055]

I agree with Tggzzz, nice work on the probe...

I also have great respect for Picotech...
And 3 Picoscopes (actually 4, with retired one I don't use anymore)

[lasmux]

The noise calculation was a bit tricky as the scope noise was much worse than the probe noise level. For subtracting noise, I found the following equation :
𝑃𝑟𝑜𝑏𝑒_𝑅𝑀𝑆= √(𝑆𝑐𝑜𝑝𝑒𝐴𝑛𝑑𝑃𝑟𝑜𝑏𝑒_𝑅𝑀𝑆² − 𝑆𝑐𝑜𝑝𝑒_𝑅𝑀𝑆² )
So I'd measure the AC RMS noise of the scope without the probe plugged in, and then measure the AC RMS noise with the probe plugged in and tips grounded, and then perform the above calculation.

The noise level on the oscilloscope with nothing plugged in was 1.986mV RMS, with the probe plugged in and the tips grounded, this rose to 2.0258mV RMS, meaning the probe had a noise output of about 0.35mV RMS.

I think I had the scope set up right... I used the oscilloscope channel with the lowest baseline noise, which was channel 4. The scope was configured to operate with random equivalent time sampling, free-running mode, 5us per division, and 250000 points - for an effective sample rate of 5GS/s. When I selected shorter or longer time bases, it didn't make much difference to the final calculated noise level.

Probe noise without probe attached:


Probe noise with probe attached:

[jmw]

Nice work!

I've been slowly working on a probe based on the BUF802, with some similar specs: 10x attenuation, input impedance of 1 MΩ || 1 pF, Zmin ≅ 100 Ω. I've been simulating and testing one piece at at a time, and put together the first complete prototype this weekend.




It's hitting about 2.3 GHz, and there's no big resonant peak in the frequency response, so I'm probably even leaving some bandwidth on the table. I used a test jig PCB that fits against the front of the probe, but had to "cheat" a little by making another ground connection to lower the ground lead inductance. When held by hand as pictured, the bandwidth is closer to 1.6 GHz.

I'm trying to think of practical ways of keeping the right ergonomics and pushing inductance lower. Unlike the input capacitance that is more or less fixed by the probe design, ground inductance is mostly determined by the user's probing setup. A handheld probe needs to have a pointy shape to get into tight spaces, so there's always going to be a little bit of a loop. Most commercial probes have similar input impedance, so they must have some way to get super low inductance to hit their bandwidth number. And even Keysight mentioned their probe bandwidth numbers are measured using jigs that are not representative of most real world use. 

I suppose more attenuation makes it easier to get lower input capacitance; is this the reason you went with 26 dB attenuation?

[lasmux]

Hey, nice probe! Nice flat response

I've found that dealing with stray inductances and capacitances is quite difficult, and quite design/layout specific. Some designs were more sensitive to it than others. It's madeningly easy to set up some LC resonance somewhere which causes a big dip or peak in the response at some frequency. Maybe you have something similar at 1.6GHz and 1.9GHz? Those dips are just single data points however so it could just be noise?
Dealing with ground lead inductance is difficult as a lot of things you can try either make the probe more difficult to use, or damage the response linearity/bandwidth. I stuck with using a spring pin probe for the ground lead because it makes probing real circuits much much easier if one of the connections is sprung, even though this ground lead has quite a bit of inductance. You don't have to get the probe angle perfect to get a good connection.


I chose a 20x attenuation for a number of reasons. As you say, it makes it easier to reduce the input capacitance, also, the slew rate on the op amp I'm using is a bit slower so by increasing the attenuation it increases the bandwidth at which the probe becomes slew-rate limited for very large AC-amplitude input signals.

[joeqsmith]

Any progress with the latest revision?   

[lasmux]

I'm organising an initial manufacturing run of a batch of 25 PCB assemblies now. Going through some normal back-and-forth with the assembly house over some of the details.

[joeqsmith]

Nice.  Just let me know when you are ready. 

[points2]

I'm organising an initial manufacturing run of a batch of 25 PCB assemblies now. Going through some normal back-and-forth with the assembly house over some of the details.

Hi lasmux,
any info about when the probes will be available ?

[lasmux]

I'm organising an initial manufacturing run of a batch of 25 PCB assemblies now. Going through some normal back-and-forth with the assembly house over some of the details.

Hi lasmux,
any info about when the probes will be available ?

Soon. Did I say that last time though 

[lasmux]

In other news, I have the first batch of 25 PCBs here (at long last) and the first couple have tested fine!!


I've also spent what felt like an eternity soldering together a mountain of probe tips. Each signal tip is in three parts. There's a pin to go into the probe, a series resistor, and then a sharp tip, all cut down and then soldered together. This is then carefully glued for rigidity, and finally tidied up with some semi-rigid heatshrink. 10 minutes assembly per tip. 50 tips. I've managed to burn through every decent podcast I know of. Fortunately the ground tip sockets are a bit simpler to assemble.

[lasmux]

I'm super proud to say that at long last, these are available at the site:
http://www.lasmux.com/product/single-ended-active-probes/

The datasheet has also been extensively updated, so worth checking that out!

It's surprising how much time and effort it is to get something like this up and running. Production issues, delays, customs and shipping, fighting insurance companies... the list just goes on and on. My hourly wage for this first batch, even if I exclude the 1.5 year long design work, is very, very low. Hopefully the next batch will be smoother.

[mawyatt]

Nice effort & probe, well done 

If/When we need an Active Probe we'll order one

Best,

[lasmux]

I think one of the biggest things I've learnt from this project is that manufacturing (more than one-offs) things which aren't just straight PCBs is really hard, despite having some R&D experience in many of these aspects. It's also been surprising how long seemingly trivial tasks take. I had a bunch of estimates on how long various tasks would take for each probe. Pin assembly, testing, customs paperwork, etc. For almost everything I've underestimated the time requirements, despite having some experience in all of these things in the past.

I have been thinking on ways to improve my workflow for subsequent batches, including test jigs, and assembly jigs for the pins. It's definitely tricky as these kinds of tooling jobs take time to set up, and the time-saving payoff will take a while to reach.