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.
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.
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 havean 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:
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.
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 havean 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 just need to figure out how to set up a store correctly on my website
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 just need to figure out how to set up a store correctly on my websiteYou 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.
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 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.
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.
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.
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.
QuoteI 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.