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Offline JohnG

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> 1 GHz DIY differential probes
« on: August 13, 2016, 03:37:19 pm »
I'm wondering if anyone has a working design or knows of one for a DIY wideband (DC to > 1 GHz) DIY differential probe. I've search for this on and off, and found some attempts, but have not found a successful design. I have come across references to a 2 GHz design published in Elektor July/Aug 2015, but I don't have the article so I don't know if this is a working design or not. I'd buy the issue if I had some indication that this was indeed a working design.

I've used these types of probes before, so I understand the voltage and impedance limitations they present.

There are a few reasons I'd like to have a DIY design. First, to learn from. Second, to have a design that is not tied to a particular brand scope  >:(. Third, to extend or modify for different applications. In theory, one could have a few different probes for different voltage ranges, or similar.

Any help appreciated!

John
"Those who learn the lessons of history are doomed to know when they are repeating the mistakes of the past." Putt's Law of History
 

Offline joeqsmith

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Re: > 1 GHz DIY differential probes
« Reply #1 on: August 14, 2016, 02:09:15 am »
I attempted to design a DC coupled wide band differential probe last year.  Got as far as coming up with requirements.  In the end I could not find parts suitable for the job.   

I had seen a few homemade probes people had came up with but nothing DC coupled and nothing as fast as I was looking to build.     

Beyond the DC-1GHz diff, what are your requirements? 

https://xellers.wordpress.com/electronics/1ghz-active-differential-probe/
https://xellers.wordpress.com/2014/09/28/diy-active-differential-probe-characterization-round-2/
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Offline JohnG

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Re: > 1 GHz DIY differential probes
« Reply #2 on: August 14, 2016, 02:53:41 pm »
Since you asked about requirements:

Bandwidth: > 1GHZ (risetime < ~300 ps) or so, faster is better
Input: >= 1k resistive, <= 2pF capacitive (higher resistance would be preferred
Attentuation: 10:1
INput voltage range: >10 V DM, >5V CM
CMRR: >20 db at 1 GHz, if possible

If I could do a 100:1 with a variation on the same circuit, that would be great. I could take some hit on speed at the higher voltage, but the input resistance would have to go up to at least 10k. It would not need to meet 100V continuous operation, but the ability to get capture short pulses (up to 1-2us wide) at 100V or close would be a big plus. Not many commercial probes can do this at this kind of BW.

Here is what I really want, but it will only work with a Tek Scope, which I don't have. It will probably cost as much as the scope, or close: http://www.tek.com/dl/51W-60485-0%2520IsoVu%2520White%2520Paper%2520%2520TN%25203-25-16.pdf


All the CMRR you could ever want. I have seen it in action, and it works. But, I won't be getting one anytime soon :(.

John
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Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #3 on: August 14, 2016, 05:36:04 pm »
Here is what I really want

Not really. That's a high frequency 50 Ohm input isolation amplifier, not a high input impedance differential probe. You can simply add two resistors for a differential version of the passive attenuated probe, but just as with the passive attenuated probe that's not optimal as far as noise is concerned.

The Elektor design uses a high frequency fully differential amplifier (ADA4927-1) as a differential to single ended converter, witch matched resistors to provide attenuation. Boring :) Also they didn't actually measure CMRR, you can't rely on datasheet CMRR because that's for differential->differential and common mode errors for the two outputs could very well cancel out in differential mode.
« Last Edit: August 14, 2016, 05:37:51 pm by Marco »
 

Offline joeqsmith

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Re: > 1 GHz DIY differential probes
« Reply #4 on: August 14, 2016, 05:47:04 pm »
When I saw 1GHz to DC I was thinking digital, not something high voltage dual purpose.  I was hoping to keep loading above a couple hundred ohms with  maybe +/-2.5V differential and same for common mode.  Maybe 0.2pf at the tip sort of thing, couple of GHz BW.  I was thinking that with modern parts I could pull it off but I spent a few weeks looking and gave up.  I was also having problems trying to figure out how I was going to tackle the DC part.   My plan was to use 2 AC paths and two DC to something paths.  Then somehow combine the whole mess and not ruin signal.  The circuitry would have been fairly complex, at least for me.   

Keep us posted if you try it.   

On the bright side, at least for digital work the used probe market seems to be growing.  Prices are all over the place.   Thousands for broken probes with missing parts, to under a K for this baby!

http://www.ebay.com/itm/Teledyne-LeCroy-D13000PS-13GHz-Differential-Probe-System-/131895698595?hash=item1eb598a4a3:g:4WEAAOSw~otWce6k

I could see this for my hobby use.
http://www.ebay.com/itm/LeCroy-D600-7-5-GHz-WaveLink-Probe/162154103823?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20131003132420%26meid%3D1b91768d4ae64145a92f0a3a455d4605%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D311133558055

 
That Tektronix probe looks slick.  Having a 2KV CM standoff or galvanic isolation is not something I would need at home.  I did find another article on it.  Sounds like it may not yet be available. 

http://electronicdesign.com/blog/1-ghz-isovu-leads-tektronix-probe-performanceusability-charge-apec

And a video on their facebook page
https://www.facebook.com/tektronix/videos/vb.50990632804/10153395327767805/?type=2&theater
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Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #5 on: August 14, 2016, 06:26:23 pm »
Teledyne-LeCroy-D13000PS-13GHz-Differential-Probe-System

How do these actually work? From the pics and the impedance specs I would guess each signal is probed by a separate passive divider probe (with 500 Ohm tip resistance). Because it seems to have two separate transmission lines coming out.
 

Offline joeqsmith

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Re: > 1 GHz DIY differential probes
« Reply #6 on: August 14, 2016, 07:15:58 pm »
Teledyne-LeCroy-D13000PS-13GHz-Differential-Probe-System

How do these actually work? From the pics and the impedance specs I would guess each signal is probed by a separate passive divider probe (with 500 Ohm tip resistance). Because it seems to have two separate transmission lines coming out.

I have not idea but am guessing the changeable interconnect to a custom ASIC to the scope.   :-DD   Spent some time to see if they had a whitepaper or some other notes.  May be of interest:

http://cdn.teledynelecroy.com/files/whitepapers/wp_differential_measurements.pdf
http://cdn.teledynelecroy.com/files/whitepapers/probing_technology.pdf
http://cdn.teledynelecroy.com/files/whitepapers/technologies-for-very-high-bandwidth-real-time-oscilloscopes.pdf
http://cdn.teledynelecroy.com/files/manuals/d13000ps-om-e.pdf
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Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #7 on: August 14, 2016, 10:26:02 pm »
That's the Lecroy, the IsoVu was at 80 dB at 1 GHz. As I said though, it's not a differential probe, it's a 50 Ohm input isolation amplifier.
« Last Edit: August 14, 2016, 10:28:24 pm by Marco »
 

Offline tggzzz

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Re: > 1 GHz DIY differential probes
« Reply #8 on: August 14, 2016, 10:36:26 pm »
A novel very simple and robust differential probe: http://emcesd.com/pdf/cd94scr.pdf

Not DC, the article measures refers to 500MHz.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #9 on: August 14, 2016, 10:54:11 pm »
Simple, but not cheap. It should be possible to make a combiner out of coax and ferrite beads.
 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #10 on: August 14, 2016, 11:16:54 pm »
Does it need to be DIY from scratch? Tektronix P6860 logic analyser probes can be found cheap (say $50) and they have a differential amplifier for the clock / qualifier inputs and regular FET probe inputs for the other signals. All in all one probe gives you 2 differential and 16 regular fet probes with >2GHz (probably 3Ghz) bandwidth.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline lukier

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Re: > 1 GHz DIY differential probes
« Reply #11 on: August 15, 2016, 12:18:37 am »
Does it need to be DIY from scratch? Tektronix P6860 logic analyser probes can be found cheap (say $50) and they have a differential amplifier for the clock / qualifier inputs and regular FET probe inputs for the other signals. All in all one probe gives you 2 differential and 16 regular fet probes with >2GHz (probably 3Ghz) bandwidth.

Interesting. Tektronix doesn't share much detail on the inner workings of their probes (but some companies seem to know more - e.g. http://www.movingpixel.com/SQUIRE.html). How do you power this probe (without TLA card/mainframe I assume in this thread) and terminate it for the scope (50 Ohm). Did you build an adapter from the probe high density connector (the side that goes to the TLA7AA4) to BNCs?

 

Offline Cerebus

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Re: > 1 GHz DIY differential probes
« Reply #12 on: August 15, 2016, 12:38:16 am »
Please ignore this, just marking thus to come back to.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 

Offline Someone

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Re: > 1 GHz DIY differential probes
« Reply #13 on: August 15, 2016, 01:20:55 am »
I'm wondering if anyone has a working design or knows of one for a DIY wideband (DC to > 1 GHz) DIY differential probe. I've search for this on and off, and found some attempts, but have not found a successful design. I have come across references to a 2 GHz design published in Elektor July/Aug 2015, but I don't have the article so I don't know if this is a working design or not. I'd buy the issue if I had some indication that this was indeed a working design.

I've used these types of probes before, so I understand the voltage and impedance limitations they present.

There are a few reasons I'd like to have a DIY design. First, to learn from. Second, to have a design that is not tied to a particular brand scope  >:(. Third, to extend or modify for different applications. In theory, one could have a few different probes for different voltage ranges, or similar.
You'll have challenges simply driving the cable and scope input capacitance at 1GHz, you could start with a simple implementation of the THS3217 to get a feel for the project but it won't hit 1GHz. Cheap probes without manufacturer dependence are possible: https://www.eevblog.com/forum/projects/diy-100mhz-differential-probe/
 

Offline joeqsmith

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Re: > 1 GHz DIY differential probes
« Reply #14 on: August 15, 2016, 02:26:57 am »
A novel very simple and robust differential probe: http://emcesd.com/pdf/cd94scr.pdf

Not DC, the article measures refers to 500MHz.

Interesting idea.  Looks like the Mini-circuits 1-500MHz w/SMA cost about $65.  Isolation is worse on their surface mount parts.  I wonder why he did not have a resistive probe in his comparison.
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Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #15 on: August 15, 2016, 10:51:04 am »
Does it need to be DIY from scratch? Tektronix P6860 logic analyser probes can be found cheap (say $50) and they have a differential amplifier for the clock / qualifier inputs and regular FET probe inputs for the other signals. All in all one probe gives you 2 differential and 16 regular fet probes with >2GHz (probably 3Ghz) bandwidth.

Interesting. Tektronix doesn't share much detail on the inner workings of their probes (but some companies seem to know more - e.g. http://www.movingpixel.com/SQUIRE.html). How do you power this probe (without TLA card/mainframe I assume in this thread) and terminate it for the scope (50 Ohm). Did you build an adapter from the probe high density connector (the side that goes to the TLA7AA4) to BNCs?
I have not looked into the technical details. I'm just pointing towards an existing solution which could be hacked.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline JohnG

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Re: > 1 GHz DIY differential probes
« Reply #16 on: August 15, 2016, 12:46:13 pm »
Thanks for the feedback, everyone.

1 GHz is a hard requirement, and is probably barely enough for my application. I've seen the link to the design with the transformer, and it looks intriguing, but the bandwidth is not there and I would really like the ability to go to DC.

I've looked at buying used probes and hacking at them, but this is for work, and my priority is not to design a probe, but get measurements. At some point it becomes cheaper to buy, just not there yet. I would like the flexibility of having a design - recently I have built transmission line probes into some designs and they work surprisingly well.

The IsoVue may not look like a differential probe, but it sure acts like one. Yes, it looks like a 50 ohm isolation amplifier, but it is a high resolution analog one, and they are not exaggerating the CMRR. Because the CM impedance is so high, there is very little conversion of CM to DM. I'm not sure about the innards, except that I know they use an electro-optic modulator. These can be designed to be inherently balanced, so it may be that the only imbalanced part is the input coax/attenuator. So unless you are sure about the design, it may be premature to say it is not a differential probe. Here is a link with some interesting information: https://www.google.com/patents/US7310455


John
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Offline tggzzz

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Re: > 1 GHz DIY differential probes
« Reply #17 on: August 15, 2016, 12:50:01 pm »
I've looked at buying used probes and hacking at them, but this is for work, and my priority is not to design a probe, but get measurements. At some point it becomes cheaper to buy, just not there yet.

Have you considered renting a probe? Last time I looked at renting, a long time ago, the monthly rental charge was 10% the purchase price.
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Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #18 on: August 15, 2016, 01:59:11 pm »
So unless you are sure about the design, it may be premature to say it is not a differential probe.

I thought that because they used MMCX connectors all the input impedances of the probes were 50 Ohm, but apparently they just ignore the mismatch (not big deal at 1 GHz) and the attenuated probes have higher input impedance. They seem to be simply using simple passive dividers for attenuation, with no termination at the probe end.

So yeah, it's a high impedance differential probe ... but a noisy one, because of the attenuation. 25x for 1.25k input impedance. I guess they want to sell an active probe separately in the near future, undoubtedly for a ton of money.
« Last Edit: August 15, 2016, 02:06:41 pm by Marco »
 

Offline joeqsmith

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Re: > 1 GHz DIY differential probes
« Reply #19 on: August 15, 2016, 10:34:04 pm »
Rental may not be a bad way to go but I too have not done this in many years.   I wonder if the cost of a used one would be close to the rental fees.   Something to consider if its a one off measurement.     

I looked around to see if I could find a better 180 degree combiner but did not find anything.  May have been fun to try and replicate his tests. 
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Offline David Hess

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Re: > 1 GHz DIY differential probes
« Reply #20 on: August 17, 2016, 09:53:14 pm »
Source impedance is a major problem with wide bandwidth differential probes and a major limitation of performance.  If you are probing a differential transmission line, then it may be better to build a test point into the circuit with 50 ohm outputs to connect directly to the oscilloscope which can then do the differential measurement itself.
 

Offline joeqsmith

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Re: > 1 GHz DIY differential probes
« Reply #21 on: August 17, 2016, 11:37:02 pm »
Source impedance is a major problem with wide bandwidth differential probes and a major limitation of performance.  If you are probing a differential transmission line, then it may be better to build a test point into the circuit with 50 ohm outputs to connect directly to the oscilloscope which can then do the differential measurement itself.


 :-// I guess that depends on what you consider a major problem.   My old DC coupled diff probes are 300 ohms at 4GHz.  Tip capacitance is a 1/10th of a pf. 
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Offline David Hess

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Re: > 1 GHz DIY differential probes
« Reply #22 on: August 18, 2016, 06:31:04 am »
:-// I guess that depends on what you consider a major problem.   My old DC coupled diff probes are 300 ohms at 4GHz.  Tip capacitance is a 1/10th of a pf.

How much source impedance mismatch are you dealing with though?  A 300 ohm differential probe is not intended for general purpose measurements; it requires closely matched source impedances to maintain common mode rejection ratio.

This Tektronix article, Making Single-ended Measurements with a Differential Probe, refers to the problem on page 11:

A 50 ohm source impedance mismatch for a single-ended signal measurement has only a minor effect on the DC CMRR of a probe with a 50 Kohm probe input impedance.  As the signal frequency increases however, the probe input impedance begins to decrease and eventually this 50 ohm source impedance mismatch for a single-ended measurement can become quite significant. High frequency AC common mode voltage transients that may be largely rejected by the relatively high CMRR in a differential measurement may show a noticeable effect due to degraded CMRR in a single-ended measurement.
 

Offline tronde

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Re: > 1 GHz DIY differential probes
« Reply #23 on: August 19, 2016, 12:51:39 am »
I'm wondering if anyone has a working design or knows of one for a DIY wideband (DC to > 1 GHz) DIY differential probe. I've search for this on and off, and found some attempts, but have not found a successful design. I have come across references to a 2 GHz design published in Elektor July/Aug 2015, but I don't have the article so I don't know if this is a working design or not. I'd buy the issue if I had some indication that this was indeed a working design.



I don't want to break Elektor's copyright, but this is the specs:

Technical specification
• Attenuation: 10:1 with a differential signal and 50 ? termination in the ‘scope
• Differential input resistance: 5 k?
• Single-ended input resistance: 2.5 k?
• Output resistance: 50 ?
• Bandwidth: 1.9 GHz (–3 dB)
• Rise/fall time: 300 ps
• Power supply: ±8 to 12 V DC.

They also write:

For readers who are interested the author
offers ready-to-use and tested PCB modules, also a kit consisting of case, RF cable
with BNC connector and power supply
lead with plug. Further information from:
alfred_rosenkraenzer@gmx.de.
 

Offline joeqsmith

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Re: > 1 GHz DIY differential probes
« Reply #24 on: August 19, 2016, 03:07:58 am »
How much source impedance mismatch are you dealing with though?  A 300 ohm differential probe is not intended for general purpose measurements; it requires closely matched source impedances to maintain common mode rejection ratio.

Differential digital, LVDS etc.
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Offline joeqsmith

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Re: > 1 GHz DIY differential probes
« Reply #25 on: August 19, 2016, 03:19:41 am »
I don't want to break Elektor's copyright, but this is the specs:

Technical specification
• Attenuation: 10:1 with a differential signal and 50 ? termination in the ‘scope
• Differential input resistance: 5 k?
• Single-ended input resistance: 2.5 k?
• Output resistance: 50 ?
• Bandwidth: 1.9 GHz (–3 dB)
• Rise/fall time: 300 ps
• Power supply: ±8 to 12 V DC.

They also write:

For readers who are interested the author
offers ready-to-use and tested PCB modules, also a kit consisting of case, RF cable
with BNC connector and power supply
lead with plug. Further information from:
alfred_rosenkraenzer@gmx.de.

Looked to see if they had a website or something with more info.  Do you have a link besides the email address?
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Offline Cerebus

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Re: > 1 GHz DIY differential probes
« Reply #26 on: August 19, 2016, 12:48:57 pm »
I don't want to break Elektor's copyright, but this is the specs:

Technical specification
• Attenuation: 10:1 with a differential signal and 50 ? termination in the ‘scope
• Differential input resistance: 5 k?
• Single-ended input resistance: 2.5 k?
• Output resistance: 50 ?
• Bandwidth: 1.9 GHz (–3 dB)
• Rise/fall time: 300 ps
• Power supply: ±8 to 12 V DC.

They also write:

For readers who are interested the author
offers ready-to-use and tested PCB modules, also a kit consisting of case, RF cable
with BNC connector and power supply
lead with plug. Further information from:
alfred_rosenkraenzer@gmx.de.

Looked to see if they had a website or something with more info.  Do you have a link besides the email address?

I finally managed to find a copy of this to look at. There's very little to this probe. An ADA4927-1 op amp, input and feedback resistors for it, two 7xL05 regulators and associated decoupling. That is pretty much all. The amp only has an  input common mode range of +/- 3.5V. The most concerning thing from my perspective is that there is not even a nod to input protection - the author relies on 2297 ohms of input resistance and whatever the chip has on board as input protection. Given there is a differential output already available from the amplifier so some sort of bootstrapped protection network wouldn't have been too taxing to design.

I'm pretty comfortable that posting just the schematic for study and discussion falls under fair use, so here it is:

Anybody got a syringe I can use to squeeze the magic smoke back into this?
 
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Offline JohnG

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Re: > 1 GHz DIY differential probes
« Reply #27 on: August 19, 2016, 01:14:17 pm »
Interesting. I would not have guess that you could do the differential to single-ended conversion that way.

Also, a 300 ps rise and fall time does not correspond to a 1.9 GHz BW. That is more what I would expect from a ~1.2 GHz probe. Either the frequency response is not approximately single-pole near the BW, or 1.9 GHz is for small-signal only. I'll look up the specs on the chip.

John
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Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #28 on: August 19, 2016, 07:39:26 pm »
Interesting. I would not have guess that you could do the differential to single-ended conversion that way.

I'd like to see some CMRR measurements. As I said before the datasheet CMRR might be a result of errors in the differential output canceling out.
 

Offline alfredr

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Re: > 1 GHz DIY differential probes
« Reply #29 on: September 05, 2018, 06:53:58 am »
Hi, my name is Alfred Rosenkraenzer.
I am the designer of this active high speed probe described in Elektor 2016. This version is no longer available, but there is a new one described in ELEKTOR 3/2017(DE) using a USB power supply. The analog specs are similiar.
I am selling loaded and tested pcb and finished probes (no blank boards). More info under alfred_rosenkraenzer@gmx.de
 
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Offline KE5FX

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Re: > 1 GHz DIY differential probes
« Reply #30 on: September 05, 2018, 07:13:15 am »
Hi, my name is Alfred Rosenkraenzer.
I am the designer of this active high speed probe described in Elektor 2016. This version is no longer available, but there is a new one described in ELEKTOR 3/2017(DE) using a USB power supply. The analog specs are similiar.
I am selling loaded and tested pcb and finished probes (no blank boards). More info under alfred_rosenkraenzer@gmx.de

Nice.  Is that an LMH5401?
 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #31 on: September 05, 2018, 07:39:09 am »
Hi, my name is Alfred Rosenkraenzer.
I am the designer of this active high speed probe described in Elektor 2016. This version is no longer available, but there is a new one described in ELEKTOR 3/2017(DE) using a USB power supply. The analog specs are similiar.
I am selling loaded and tested pcb and finished probes (no blank boards). More info under alfred_rosenkraenzer@gmx.de
Very interesting. Do you have some kind of datasheet? Also, why are you using RG316? In my experience this is already quite lossy at 'only' 1 GHz.
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Offline alfredr

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Re: > 1 GHz DIY differential probes
« Reply #32 on: September 05, 2018, 07:48:06 am »
No, it is an Analog Devices ADA4927-1. It gives you about 2 GHz bandwidth (-3dB). The differential input impedance is about 5.1 KOhm
The LMH5401 gives you even more bandwidth (maybe up to 4 GHz) but it lowers the input impedance even more.
I use the LMH5401 in another application, but I did not test it for a probe.
The higher bandwidth might not be usefull for most people since you need a expensive scope.
There is a description with specs attached.
I use the RG316 to stay at an affordable price.
 
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Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #33 on: September 05, 2018, 08:03:13 am »
And what would the price be when using better coax? 1 meter of RG316 gives a loss of 1.2dB at 1GHz which is an amplitude error of around 15%. I could use this probe but I would like to have the best possible frequency response.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline alfredr

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Re: > 1 GHz DIY differential probes
« Reply #34 on: September 05, 2018, 09:51:02 am »
I would propose you buy the loaded board and the housing (drilled holes and added connectors if you like) and install whatever cable you like.
The USB cable should not be aproblem
 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #35 on: October 14, 2018, 08:41:46 pm »
I got a bit intrigued and rebuild the Elektor design but this time using 0402 capacitors & resistors to get better HF performance and an improved board layout which should minimise the HF peaking. This posting is an update of where I'm at now. Unfortunately my design also showed some hefty peaking so I didn't got very far. A 400MHz square wave for example:




Things get even worse for a 200MHz square wave. Time to pull out the spectrum analyser and do some sweeps. Now a differential probe should show the same signal amplitude regardless the polarity of the input signal. Interestingly reversing the polarity of the input shows two entirely different traces. In one situation it peaks and in the other it drops. The effect seems to be the strongest between 700MHz and 800 MHz. A CMMR (common mode rejection ratio) measurement also shows poor performance in the same frequency range.



This turned into a bit of a head scratcher. The simulation (using the ADA4927 model from Analog Devices) doesn't show this behaviour at all and I did not manage to find the cause in the circuit board layout. It has to be something in the amplifier which isn't in the pspice model!

At one point this graph in the ADA4927 datasheet the  caught my eye:



This shows the common mode amplification with peaking in the 750MHz region! And since the ADA4927 is not used differentially but as a common-mode amplifier this graph just explains everything. Some further research showed me that all so called fully differential amplifiers will suffer from this behaviour because the amplifier for the common mode has a poor frequency response. IMHO the fully differential amplifier are simply not suitable for use in a differential probe if the output is used in a single-ended way.
« Last Edit: October 14, 2018, 09:10:58 pm by nctnico »
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Offline Cerebus

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Re: > 1 GHz DIY differential probes
« Reply #36 on: October 14, 2018, 09:17:55 pm »
I find pretty much all of nctnico's findings unsurprising. Every commercial differential probe design I've seen has frequency response trimming between both halves of the differential input (one or both sides), CMMR trim, etc. The assumption that this design makes in omitting them and expecting to get acceptable results is, IMHO, a foolish one and seems to be bourne out by measurement.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 

Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #37 on: October 15, 2018, 09:18:07 am »
IMHO the fully differential amplifier are simply not suitable for use in a differential probe if the output is used in a single-ended way.
Need a GHz version of the AD8129.
 

Online dietert1

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Re: > 1 GHz DIY differential probes
« Reply #38 on: October 15, 2018, 09:49:54 am »
Happened to find this thread after completing various measurements with a probe i bought recently from A. Rosenkränzer. In my opinion the probe is useful to record a LVDS data stream, but i would not call it a 1.9 GHz differential probe. My measurements with our HP 8560A show that ADA4927 common mode is limited to about 600 MHz if you require 3 dB accuracy. The measurement shows gain on direct (faint curve) and inverting input (bright curve).
I would consider the design incomplete. For example, he writes the probe is 1:10 while it really is 1:20. 400 mV difference on LVDS results in 20 mV in the scope (with 50 Ohm terminated input).
Anyway the concept should be valid, when elaborated carefully, e.g. with metal enclosure. Maybe a LMH3401 serves better. It is specified with a common mode bandwidth of 3.3 GHz.
Regards,
Dieter
« Last Edit: October 15, 2018, 09:56:27 am by dietert1 »
 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #39 on: October 15, 2018, 10:17:24 am »
The LMH3401 won't work because the minimum stable gain is >6.3 which together with the GBW product throws you back to little over 1GHz of bandwidth in an actual design. Most (if not all) of these fully differential amplifiers are designed as a front-end for fast AD converters which need a 1Vpp or 2Vpp (-ish) input signal. Also I don't think a metal enclosure will help much. All nodes in the circuit are low impedance anyway.

@Cerebus: the problem isn't in the mismatch between the inputs. I've used a symmetric PCB design and 0.1% resistors. There is no amplification so any mismatch between the inputs doesn't get multiplied. I'm convinced the problem is in the behaviour of the common mode amplifier of the ADA4927.
« Last Edit: October 15, 2018, 10:30:22 am by nctnico »
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Online dietert1

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Re: > 1 GHz DIY differential probes
« Reply #40 on: October 15, 2018, 10:37:09 am »
Where did you get that number "6.3"? Can't find that in the LMH3401 datasheet.
The evaluation board manual describes a 50/200 Ohm configuration (gain = 4), so i unterstood that was supposed to be stable.
 

Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #41 on: October 15, 2018, 11:40:12 am »
Why not build an AC differential amplifier from a pair of RF transistors?
 

Offline tggzzz

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Re: > 1 GHz DIY differential probes
« Reply #42 on: October 15, 2018, 01:39:31 pm »
Why not build an AC differential amplifier from a pair of RF transistors?

Or from a few bits of coax and a combiner, e.g. http://emcesd.com/pdf/cd94scr.pdf

Note: I haven't tried such a probe!
There are lies, damned lies, statistics - and ADC/DAC specs.
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Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #43 on: October 15, 2018, 01:46:30 pm »
Where did you get that number "6.3"? Can't find that in the LMH3401 datasheet.
The evaluation board manual describes a 50/200 Ohm configuration (gain = 4), so i unterstood that was supposed to be stable.
The number 6.3 is from the specs on the website. That corresponds with a gain of 16dB. I just checked the evaluation kit manual but I can't find any mention of configuring the chip for a gain of 4x. Creating an opamp with a high bandwidth which is stable at a gain of 1 is very hard because it needs compensation to be stable but at the same time the compensation eats into the bandwidth.
« Last Edit: October 15, 2018, 01:55:42 pm by nctnico »
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Online dietert1

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Re: > 1 GHz DIY differential probes
« Reply #44 on: October 15, 2018, 02:35:35 pm »
Yes, they use 16 dB as a basic configuration of the LMH3401 in the specs. But that is not a stability limit, see for example Figure 2 in the datasheet, which is for 12 dB and extends well up to 5 GHz. Schematic is in Figure 51. The 12 dB configuration is also mentioned in the evaluation board user manual, paragraph 4.4.
As far as i understand one advantage of the LMH3401 over the LMH5401 mentioned before is the internal feedback path which helps for stability.
 

Offline snoopy

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Re: > 1 GHz DIY differential probes
« Reply #45 on: October 15, 2018, 11:11:24 pm »
The probe heads on the commercial probes such as Keysight and Tektronix are usually built using strip-line techniques on special pcb materials with consistent dielectrics used for rf design so there is quite a bit of R&D in them to get them right. I just picked up a differential probe off ebay for a good price but they are getting hard to find at good prices. Considering the new ones cost megabucks your best bet is to look for used ones that are in working order.





 

Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #46 on: October 16, 2018, 06:22:41 am »
Those aren't high impedance though, not unless you count a couple hundred Ohms as high impedance.

GHz is in that sweet-spot where you can still have something relatively high impedance and general purpose.
 

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Re: > 1 GHz DIY differential probes
« Reply #47 on: October 16, 2018, 08:10:23 am »
The offers of Keysight may be well competitive, but they forgot who are their customers: engineers and scientists. Sorry, they cannot address me with a TV spot talking about a spiral inductor and ESD protection. They don't offer a solution compatible with various makes of scopes.
The calling price of a generic 1.5 GHz differential probe seems to be $ 1500, see http://www.significantdevices.com.au. Any hands on experience?
 

Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #48 on: October 16, 2018, 08:24:17 am »
Or from a few bits of coax and a combiner, e.g. http://emcesd.com/pdf/cd94scr.pdf

More expensive than a bunch of BFU660F's though. How about something simple like this? Is the simulator really giving me that bad an impression of how it would work with a tight layout and some =<0204 components? R1=R2=0 is only useful for say 10s of mV of input, probably want to bias the bases to ~4V with opamps instead of resistor dividers, might want to clamp R5-R8 with Schottkys.

PS. that Deltasense probe has nice specs, large voltage range and input impedance.
« Last Edit: October 16, 2018, 08:37:22 am by Marco »
 

Offline Cerebus

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Re: > 1 GHz DIY differential probes
« Reply #49 on: October 16, 2018, 09:55:13 am »
Those aren't high impedance though, not unless you count a couple hundred Ohms as high impedance.

GHz is in that sweet-spot where you can still have something relatively high impedance and general purpose.

What are you calling "high impedance" in this context, bearing in mind that 1pF across an input @ 1GHz would equate to 159 ohms, and that the stray capacitance across an 0603 part is around 0.04pF and an 0201 part 0.02pF (the latter being ~8k @ 1 GHz)?
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Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #50 on: October 16, 2018, 10:14:08 am »
High resistance at low frequency, high enough impedance to still be useful at high frequency. So still useful for general purpose probing in a pinch.

The really high frequency differential probes all try to be constant impedance, just high enough to not fuck a 50 Ohm line too much ... and that's all they are useful for.
 

Offline BravoV

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Re: > 1 GHz DIY differential probes
« Reply #51 on: October 16, 2018, 11:32:57 am »
can we make 100fF?

Femto farad ? I'm guessing its not that easy, example of a 3 decades old, HP 54701A 2.5GHz active probe (not a differential probe) has 0.6pF (typical) with input resistance of 100K Ohm.


Online Rerouter

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Re: > 1 GHz DIY differential probes
« Reply #52 on: October 16, 2018, 11:44:20 am »
If you want crazy low input capacitance, things get complex, but I would guess we can reduce what is currently there and iterate towards the goal, anything under 0.5pF already appears to be crazy level money,

Most of the schematics I have glimpsed in the past from reverse engineered crazy high bandwidth probes seemed to use an input network to cancel out the apparent capacitance. and pretty much no protections at all.
 

Offline Gerhard_dk4xp

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Re: > 1 GHz DIY differential probes
« Reply #53 on: October 16, 2018, 11:51:31 am »
I absolutely LOVE these HP probes, also the 1152A successor!
The 0.6pF make the series resonance of the ground lead much less important
and you can probe around without fear of burning sth. @ less than 50V.

regards,
Gerhard
« Last Edit: October 16, 2018, 11:56:20 am by Gerhard_dk4xp »
 

Offline Mechatrommer

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Re: > 1 GHz DIY differential probes
« Reply #54 on: October 16, 2018, 12:12:28 pm »
can we make 100fF?
Femto farad ? I'm guessing its not that easy, example of a 3 decades old, HP 54701A 2.5GHz active probe (not a differential probe) has 0.6pF (typical) with input resistance of 100K Ohm.
sorry i screwed up (msg deleted) i didnt see the low-f respond (1st attachment). so 1pF seems feasible? i got 0806 smd 1pF here ;D (2nd attachment) they both load the 500 ohm node by a bit anyway @ 1GHz ???

anything under 0.5pF already appears to be crazy level money,
how about pcb printed capacitor? or diy like 2 plates separated by a piece of paper? is it wrong to be unacademical?
« Last Edit: October 16, 2018, 12:15:37 pm by Mechatrommer »
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Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #55 on: October 16, 2018, 05:22:05 pm »
Or from a few bits of coax and a combiner, e.g. http://emcesd.com/pdf/cd94scr.pdf

More expensive than a bunch of BFU660F's though. How about something simple like this? Is the simulator really giving me that bad an impression of how it would work with a tight layout and some =<0204 components? R1=R2=0 is only useful for say 10s of mV of input, probably want to bias the bases to ~4V with opamps instead of resistor dividers, might want to clamp R5-R8 with Schottkys.

PS. that Deltasense probe has nice specs, large voltage range and input impedance.
I don't think this configuration will work very well (probably due to the miller effect). I did find a fast opamp and I'm going to try a classic difference amplifier approach. After all the reason to use a differential (instrumentation) amplifier is to have extremely high input impedances on both inputs but in this case a few kilo Ohms is already OK. Perhaps a discrete solution specifically designed to act as a differential probe can be pushed to a higher bandwidth especially if it doesn't need to amplify. DC offset and temperature stability will be challenging nuts to crack though.
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Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #56 on: October 16, 2018, 05:37:55 pm »
There's not a whole lot of amplification in that circuit, 1.5 or so since the emitter resistance of RF transistors is rather high, so Miller capacitance isn't a big deal. The simulator says the circuit has >400 Ohm of input impedance at 1 GHz. When you increase R1&R2 the Miller capacitance is reduced further still.
« Last Edit: October 16, 2018, 05:43:39 pm by Marco »
 

Online dietert1

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Re: > 1 GHz DIY differential probes
« Reply #57 on: October 18, 2018, 07:14:15 am »
Your difference stage (and whatever amplifier you make) will have an input capacitance of about 0.5 to 1 pF, which is much better than the usual passive probe and may be good enough for a 1 GHz probe.
Yet if you want the 0.04 pF or 0.02 pF that cerebus proposed above and that Mr. Rosenkränzer implements in his probe, there will be a capacitive divider in front of the amplifier. This is also mentioned in the Keysight video. The difference stage always looses a factor 4, because you drop one of two output ports and you want to drive the line to the scope with 50 Ohm output impedance. In total you will have a factor 1:50 or 1:100.
I think you really want some gain. If you put emitter followers in front of your difference stage this will get you roughly a factor 5 (BFU66F Cbe/ Ccb). Those who want DC to GHz might use THS4302 gain blocks.
By the way, i received a LMH3401 evaluation board. That is an excellent difference stage, like +/- 1 dB up to 2.9 GHz. Image is boring.
 

Offline dzseki

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Re: > 1 GHz DIY differential probes
« Reply #58 on: October 18, 2018, 08:43:38 am »
can we make 100fF?

Femto farad ? I'm guessing its not that easy, example of a 3 decades old, HP 54701A 2.5GHz active probe (not a differential probe) has 0.6pF (typical) with input resistance of 100K Ohm.



I have the HP 1120A which is only 500MHz active probe (not differential), it has 100kOhm||3pF input impedance in 1:1 mode and 1MOhm|| <1pF when using a divider (10:1 or 100:1).
The point is this is a quite old stuff, appeared in one HP Journal from 1969 I think, the service manual do actually contains all the schematics, even the probe's hybrid, it might worth a look.
HP 1720A scope with HP 1120A probe, EMG 12563 pulse generator, EMG 1257 function generator, MEV TR-1660C bench multimeter
 

Online dietert1

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Re: > 1 GHz DIY differential probes
« Reply #59 on: October 18, 2018, 09:29:22 am »
In my opinion that old technology is completely obsolete and its analysis a waste of time. With the THS4302  gain block i mentioned you can make a high impedance 1:1 probe with 0.4 pF input capacitance and a bandwidth of 2.4 GHz.
« Last Edit: October 18, 2018, 09:33:27 am by dietert1 »
 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #60 on: October 18, 2018, 10:04:18 am »
By the way, i received a LMH3401 evaluation board. That is an excellent difference stage, like +/- 1 dB up to 2.9 GHz. Image is boring.
That sounds promising. Did you test single-ended in and single-ended out? I did some testing with an LMH6703 (dead-bug style) and that looks promising as well albeit with less bandwidth. At least there is no difference between reversing the inputs.
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Online dietert1

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Re: > 1 GHz DIY differential probes
« Reply #61 on: October 18, 2018, 11:25:27 am »
Yes, the LMH3401 was tested with the HP 8560A again, so it is single ended in (TG) and single ended out. The curves from both inputs to the same output appear flat +/- 1 dB up to at least 2.9 GHz. I would not trust my setup to better than a dB.

In the meantime i experimented a little with the ADA4927 probe from Mr. Rosenkränzer. I modified the difference stage to the standard setup with 4 301R resistors (inputs = Gnd). The 2K5 0603 input resistors were replaced each by a 2x 620R 0402 resistor pair. I also removed two pads and brought the coax cable closer to the amp.
Now it works better, but the asymmetry of the ADA4927 still exists. With our WR64Xi and a Lattice FPGA measured risetimes are 345 psec on indirect input, 306 psec on direct input. The direct trace still shows more overshoot/ringing at 1.4 GHz .
 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #62 on: October 18, 2018, 12:04:35 pm »
OK. I think I'm going to re-design my board with an LMH3401 then instead of the LMH6703.
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Offline David Hess

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Re: > 1 GHz DIY differential probes
« Reply #63 on: October 20, 2018, 02:35:21 am »
Bob Pease's probe with 0.29 picofarads of input capacitance is shown below.  The printed circuit board stiffener adds another 0.08 picofarads but drilling holes in it reduces this to 0.06 picofarads of added capacitance.

Old designs like the Tektronix P6202 (which is what I would start with if I wanted to design a general purpose probe) use an input divider to reduce the capacitance of the input device, 2.0 picofarads in this case.
« Last Edit: October 20, 2018, 02:39:01 am by David Hess »
 

Offline Cerebus

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Re: > 1 GHz DIY differential probes
« Reply #64 on: October 20, 2018, 08:07:01 am »
Both designs are well and good in themselves, but don't forget that the title contains >1GHz and differential in its title.

Both are single ended, the JFETs in Bob's design run out of steam at ~200MHz and the P6202 is a 500MHz design.
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Online dietert1

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Re: > 1 GHz DIY differential probes
« Reply #65 on: October 20, 2018, 08:44:31 am »
Thanks for posting the probe schematics. We cannot emphasize enough the advantage of using a low capacitance active probe, even if its bandwidth is "only" 500 MHz instead of 1 GHz and even if it's not differential. When you try to make a probe for time domain measurements, you want the gain curve to level off smoothly at the upper bandwidth limit. Otherwise you will see ringing, like with audio brickwall filters. That may be a reason to prefer a discrete design since in general it will have less components. For example the proposed transistor difference stage levels off smoothly.
Anyway i would recommend to have a look at the "contemporary" TI proposals for a 2 GHz scope frontend with LMH5401 and LMH6401. Using that kind of building block is roughly how Mr. Rosenkränzer arrived at a working DIY type GHz differential probe.
Really, in 2018 we should have DSOs with builtin probe deembedding including some handy calibration scheme.
 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #66 on: October 20, 2018, 12:03:03 pm »
At some point you just can't get away with a discrete solution especially on readily available PCB materials. The parasitics will kill the circuit. Meanwhile I've updated my design with the LMH3401 to see how that works. The datasheet for the LMH3401 says that TI choose to have the feedback resistors on the chip itself to avoid the parasitic capacitance of the circuit board. Unfortunately these are 200 Ohm and for a 1:20 differential attenuation this means a differential input impedance of only 2k Ohm. It is still OK-ish because on a typical differential pair terminated with 100 Ohm it will result in an amplitude error of 5% due to loading the signal. Still seeing is believing. The simulation shows some peaking at higher frequencies (several GHz) so hopefully the loss of FR4 takes care of it.
« Last Edit: October 20, 2018, 12:44:21 pm by nctnico »
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Online dietert1

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Re: > 1 GHz DIY differential probes
« Reply #67 on: October 21, 2018, 09:17:32 am »
I bought some ATF-35143 FETs to make buffer input stages. That works +/- 1 dB at least up to 2.9 GHz with a 51K/10K input divider made from 0805 SMD resistors. Then the parasitic capacitance of the 0805 resistor roughly compensates the FET input capacitance. Low frequency noise is less then a mV. Attached schematic describes a test setup. With smaller parts and a careful layout a 10:1 divider should compensate. I will try to drive the LMH3401EVM inputs with two of these input stages.
« Last Edit: October 21, 2018, 09:22:40 am by dietert1 »
 
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Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #68 on: October 21, 2018, 10:37:40 am »
That is interesting! Are these FETs available as a matched pair? I think that will greatly improve the CMMR and DC offset matching.
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Offline Gerhard_dk4xp

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Re: > 1 GHz DIY differential probes
« Reply #69 on: October 21, 2018, 10:57:20 am »
Discontinued.
No matched pairs, if that word is allowed at all in the context of FETs.
Maybe one could enforce equal currents with a mirror.

possible replacements:
https://www.digikey.de/product-detail/de/CE3521M4-C2/CE3521M4-C2CT-ND/6165474/?itemSeq=265991607     >

https://www.digikey.de/product-detail/de/CE3520K3-C1/CE3520K3-C1CT-ND/6165473/?itemSeq=265991735     >

<   https://www.digikey.de/product-detail/de/cel/CE3514M4-C2/CE3514M4-C2CT-ND/6165472     >


Also interesting:

<    https://www.digikey.de/product-detail/de/analog-devices-inc/ADL5565ACPZ-R7/ADL5565ACPZ-R7TR-ND/2773701     >

regards,
Gerhard


« Last Edit: October 21, 2018, 11:19:21 am by Gerhard_dk4xp »
 

Offline Carrington

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Re: > 1 GHz DIY differential probes
« Reply #70 on: October 21, 2018, 11:07:01 am »
I just wanted to say that this is getting more and more interesting, and I think that something pretty good is going to come out of here.  :-+
My English can be pretty bad, so suggestions are welcome. ;)
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Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #71 on: October 21, 2018, 07:38:56 pm »
That is interesting! Are these FETs available as a matched pair? I think that will greatly improve the CMMR and DC offset matching.
How close can you match the capacitance of the resistors though? Seems to me the solder blobs would throw it off.
 

Online 0xdeadbeef

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Re: > 1 GHz DIY differential probes
« Reply #72 on: October 21, 2018, 10:56:24 pm »
I'm aware it's neither differential nor >1GHz, and well, it's only available in German, but the most elaborate take on a DIY active probe I stumbled over in the last years was this one:
http://welecw2000a.sourceforge.net/docs/Hardware/Aktiver_Tastkopf_mit_OPA659.pdf
The probe uses an OPA659 with a trimmable capacitive divider. The main developer and author of this PDF "branadic" is also active in this forum btw.
Building some of these is one of multiple projects on my "stack".

As a side note: I also bought some of the probes by Alfred Rosenkraenzer (PCB version). When I bought the first one, I was well aware that the bandwidth sounded a bit optimistic. Still, my main intention was to measure LVDS signals in the 20-40MHz range and the probes worked excellently for that. I.e. where the passive 500MHz probes just showed a sine shaped signal, I got a clear rectangular signal with steep edges. I already used them for 5V signals as well though (e.g. 100ns CS pulses) where they are also far superior to 500MHz passive probes. Admittedly, the scoped used was a 600MHz Lecroy so I don't really care if the probes really have a much higher bandwidth or not. And yes, looking at the differential amplitude, the probes show a 20:1 attenuation, I guess the 10:1 value was meant for each input. In the meantime, the description in the eBay offers was also changed as far as I can tell.
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Online dietert1

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Re: > 1 GHz DIY differential probes
« Reply #73 on: October 22, 2018, 09:20:56 am »
Today i received a THS4302 evaluation module. Compared to the OPA659 the THS4302 has more bandwidth and some gain.
The EVM gain curve  measured by our HP 8560A shows 7.8 dB at low frequencies (G=5 gives 14 dB - 6 dB output line attenuation). 3 dB bandwidth is roughly 1.8 GHz.  Up to 2.9 GHz it levels off smoothly, so this will result in low ringing.
The evaluation module has a terminated strip line as input. A probe needs a different design. THS4302 input capacitance is specified as < 1 pF, so if you put a 10:1 divider in front, you may get a 0.2 pF high impedance probe.
 

Offline Hydron

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Re: > 1 GHz DIY differential probes
« Reply #74 on: October 22, 2018, 09:39:30 am »
Looks like there's also a gain-of-10 version of the THS4302 too (the THS4303) if it's of interest.

Thanks for all the work being done in this thread - sounds like the final result could be pretty handy.
 

Online dietert1

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Re: > 1 GHz DIY differential probes
« Reply #75 on: October 22, 2018, 06:57:17 pm »
Yes, definitely 10x gain is  better than 5x. Also very interesting: The THS4303 datasheet contains a schematic diagram of the amplifier.
When we compare the large signal gain curves (Figure 3), both amplifiers show the -3 dB point at about 1.4 or 1.5 GHz. The THS3402 has a minor bandwidth advantage only with small signals.
My own 1.8 GHz bandwidth estimate above was "almost large signal": -10 dBm into 50 Ohm = 200 mVpp, 5x => 1 Vpp.
 

Offline David Hess

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Re: > 1 GHz DIY differential probes
« Reply #76 on: October 24, 2018, 09:08:57 pm »
Both designs are well and good in themselves, but don't forget that the title contains >1GHz and differential in its title.

Both are single ended, the JFETs in Bob's design run out of steam at ~200MHz and the P6202 is a 500MHz design.

I linked them as examples of what is involved.  Both serve as benchmarks for what is possible without resorting to hybrid construction.

I think a >1GHz differential probe is unrealistic without hybrid construction but performance better than the famous and very long lived Tektronix 100MHz P6046 differential probe should be feasible.  I would take a very close look at maximum performance design using the AD8130 difference amplifier as the differential to single ended conversion stage to get better than P6046 performance.

If there is a better part than the AD8130 for differential probes, I have not found it yet.
 

Online dietert1

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Re: > 1 GHz DIY differential probes
« Reply #77 on: October 25, 2018, 07:25:45 am »
The amplifier testkits i wrote about are no hybrids but standard PC boards. The LMH3401 board has four layers with the amplifier side made from RF material. Its common mode is near perfect up to 3 GHz, its differential mode extends up to 7 GHz (datasheet spec). The HEMT fet buffer i tested should be good for 5 or 10 GHz as well. The wiggles in my measurements are reflections and mean nothing. Some days ago i found better patch cables and some SMA attenuators which make the measurements smooth to about +/- 0.2 dB.
If you prefer AD over TI, the differential amplifier ADL5565 mentioned above seems to be very similar to the LMH3401. Or the ADL5569 with 2x 20 dB gain on a single chip and good common mode suppression up to 3 GHz.
If you want to make hybrids, please search for "agilent ghz die" at ebay. Wellcome to 2018!
« Last Edit: October 25, 2018, 09:03:58 am by dietert1 »
 

Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #78 on: October 25, 2018, 02:42:42 pm »
AFAICS it's impossible to guess what the CMRR is for the ADs for single ended output. Only the LMH3401 has anything in the datasheet about it (Fig. 42, Common-mode input, common-mode output transfer function).
 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #79 on: October 25, 2018, 04:42:56 pm »
Both designs are well and good in themselves, but don't forget that the title contains >1GHz and differential in its title.

Both are single ended, the JFETs in Bob's design run out of steam at ~200MHz and the P6202 is a 500MHz design.

I linked them as examples of what is involved.  Both serve as benchmarks for what is possible without resorting to hybrid construction.

I think a >1GHz differential probe is unrealistic without hybrid construction but performance better than the famous and very long lived Tektronix 100MHz P6046 differential probe should be feasible.  I would take a very close look at maximum performance design using the AD8130 difference amplifier as the differential to single ended conversion stage to get better than P6046 performance.

If there is a better part than the AD8130 for differential probes, I have not found it yet.
One way to get in the ball park of 1GHz is to use 2 closed loop buffers (from TI for example) and drive a regular opamp or differential amplifier with these to get the differential signal. It should be possible to get over 20k Ohm of differential input impedance and more if you make it a 1:10 probe but I think the capacitive divider will be tricky to get right.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Online dietert1

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Re: > 1 GHz DIY differential probes
« Reply #80 on: October 26, 2018, 09:38:20 am »
If we are still considering DIY, a 3 mm "antenna" of thin wire soldered vertical to one end of a 0603 resistor helps. Then you can adjust compensation by bending the loose end in one direction or the other. Also "solder blobs" were mentioned above.
 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #81 on: October 26, 2018, 11:55:11 am »
The question is how low do you want to get the input capacitance. On one of my prototypes I measured less than 0.5pf between the tips (and that is with some uncertainties). The actual capacitance between the tips may even be lower. This also implies that aiming for a very high differential input impedance isn't going to do much good because at several GHz the input capacitance will dominate the input impedance anyway.
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Online 0xdeadbeef

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Re: > 1 GHz DIY differential probes
« Reply #82 on: October 26, 2018, 06:26:03 pm »
This also implies that aiming for a very high differential input impedance isn't going to do much good because at several GHz the input capacitance will dominate the input impedance anyway.
While it's true of course that a capacitance becomes low ohmic at high frequencies (e.g. 1pF at 1GHz equals ~159Ohm), your signal typically consists of a wide range of frequencies where the (real part) resistance will matter the more, the more low frequency components you have.
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Online dietert1

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Re: > 1 GHz DIY differential probes
« Reply #83 on: October 26, 2018, 10:28:18 pm »
This will be similar to any other scope frontend.  Once you know how to DIY you can solve the DC impedance problem using AC coupling or using an adjustable DC offset source or by adapting the division ratio. Depends on the application.
For myself i want a high impedance 10:1 probe, so i designed a small 4-layer board with 2x HEMT buffers plus DC servos plus a LMH3401 difference stage plus low noise +/- 2.5 V regulators. A HEMT easily oscillates above 10 GHz and the probe will have two of them next to each other, so i already know i will suffer. Probably i should first try a THS4303 single ended 10:1 probe, will be much easier.
 

Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #84 on: October 27, 2018, 12:13:26 am »
The quality to which Baluns can be made is quite impressive by the way. Marki has one which does 40 dB CMRR out to 6 GHz ... they of course go higher, but for those the CMRR is reduced across the board.

What's inside these things? I suspect ferrite on coax.
 

Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #85 on: October 29, 2018, 12:58:06 pm »
Discontinued.
Such a shame too. Some of those Broadcomm pHEMT's go up to a transconductance of 2000 mS, there's nothing else like them in the discrete world.
 

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Re: > 1 GHz DIY differential probes
« Reply #86 on: October 29, 2018, 02:14:55 pm »
Discontinued.
Such a shame too. Some of those Broadcomm pHEMT's go up to a transconductance of 2000 mS, there's nothing else like them in the discrete world.
Off topic: IMHO it is a major PITA that Broadcom is killing all the nice RF parts from HP/Agilent/Avago. Also a lot of opto stuff is being discontinued.  :wtf:
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Offline LapTop006

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Re: > 1 GHz DIY differential probes
« Reply #87 on: October 29, 2018, 02:53:08 pm »
Off topic: IMHO it is a major PITA that Broadcom is killing all the nice RF parts from HP/Agilent/Avago. Also a lot of opto stuff is being discontinued.  :wtf:

Given how hard it is to be able to purchase or even just get real datasheets for their parts that might actually be the better option, at least now a competitor knows they have a good chance at picking up customers.
 

Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #88 on: October 29, 2018, 04:31:02 pm »
High GHz discrete RF is such a niche that probably no one will pick them up. CEL and Skyworks will probably keep producing their parts for a while, but those just don't compare to Broadcomm's as far as transconductance is concerned. I don't see them developing ones to rival Broadcomm's ... gone with the wind now.
« Last Edit: October 29, 2018, 08:07:05 pm by Marco »
 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #89 on: October 31, 2018, 09:32:13 pm »
Today I had some time to put a new board with the LMH3401 together. The differential impedance is 2k Ohm with a 1:20 attenuation. I'm contemplating having a 1:50 version with 5k Ohm differential input impedance but that is for later. Some initial testing shows promising results:



This is with the input normal and reversed so it should give an idea on how good the common mode amplification is. I'll need to do a whole lot more testing but for that I need to finish the rest of the board first. This is just the amplifier (and 0402 decoupling capacitors) powered from a bench PSU.
« Last Edit: October 31, 2018, 09:35:45 pm by nctnico »
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Offline David Hess

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Re: > 1 GHz DIY differential probes
« Reply #90 on: November 01, 2018, 12:08:25 am »
High GHz discrete RF is such a niche that probably no one will pick them up. CEL and Skyworks will probably keep producing their parts for a while, but those just don't compare to Broadcomm's as far as transconductance is concerned. I don't see them developing ones to rival Broadcomm's ... gone with the wind now.

NXP has discontinued their 4GHz PNP RF transistors as well and unlike some of the Avago/Broadcom parts, there are no substitutes for them or at least I have not found any.

 

Offline Gerhard_dk4xp

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Re: > 1 GHz DIY differential probes
« Reply #91 on: November 01, 2018, 12:57:59 am »
https://www.digikey.de/products/de?keywords=hfa3096bz-nd   >

but yes, it's a dying species.

regards, Gerhard
 

Online Marco

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Re: > 1 GHz DIY differential probes
« Reply #92 on: November 01, 2018, 01:12:50 am »
There are so many legacy BFT92 designs, they could charge 2$ a piece and they'd still sell them.
 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #93 on: November 15, 2018, 11:58:49 pm »
Time for an update on my design using an LMH3401. The differential impedance is 2k Ohm and the capacitance is less than 0.5pf (I can't measure any lower). The maximum differential input voltage is +/-20V RMS (based on dissipation). The maximum common-mode voltage is +/-13V. These ranges are wide enough to measure low-side current sense resistors and the gates of MOSFETs in the low side of a switching power supply as well.

First some graphs from the spectrum analyser up to 3GHz.
Normal and reverse connected bandwidth to show where the common mode amplification starts to deteriorate:


Common mode rejection:


It looks reasonably flat up to 2.2GHz and the CMMR is -40dB at the worst point. All measurements where made using a levelled RF generator set to 0dBm and 50cm of Huber+Suhner coax (and a DC blocker) connected between the probe and the spectrum analyser.

On to the practical part. On the differential probes I've seen there are moveably probe tips so points with a variety of distances can be probeb. I wanted to achieve something similar. After some thinking I came up with the idea of using thin stainless spring steel rods inserted into sockets. The end result is this:


I didn't expect the offset voltage to be as bad as it is (in the prototype it is 30mV) so I had to bodge a potmeter onto it. The ground lead is to equalise the ground of the probe with the ground of the DUT. The power is supplied using a micro-USB connector. A switching capacitor chip with linear post-regulators provides a positive and negative suply voltage.

Let's measure some signals. First a 1.25Gbit ethernet stream:




Secondly a 250MHz square wave:


The stainless steel spring rods seem to work fine. Probing a 0402 component is doable as well:


All in all I'm quite impressed. It took 3 board spins to come up with something decent. The results from the initial version with the ADA4927 made me sceptical about using a fully differential amplifier but I'm happy dietert1 held a strong argument in favour of using the LM3401.

I still need to add the potmeter to the final PCB design. Perhaps I'll add a slit between the input divider resistors to reduce the input capacitance as far as possible. If there is some interest I could have a small batch made. For now I have used heat-shrink tubing as a casing but I have created some indents in the board so it can be held in place in a 3D printed housing.
« Last Edit: November 16, 2018, 12:47:03 am by nctnico »
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Offline Hydron

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Re: > 1 GHz DIY differential probes
« Reply #94 on: November 16, 2018, 11:41:36 am »
Looks great, definitely interested in more details/design info.

RE the offset issue - I assume that you're terminating the inverting output in 100R, could you potentially use this output to sense the offset when inputs are shorted and use a micro to calibrate it out using a digital pot/dac when a button is pressed? If the offset doesn't drift then this is probably overkill and a pot is the best.
 
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Offline Carrington

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Re: > 1 GHz DIY differential probes
« Reply #95 on: November 16, 2018, 12:21:14 pm »
Cool!  :-+

... If there is some interest I could have a small batch made ...

I'm interested, and I think I'm not going to be the only one.  :)
My English can be pretty bad, so suggestions are welcome. ;)
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Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #96 on: November 16, 2018, 03:11:17 pm »
Looks great, definitely interested in more details/design info.

RE the offset issue - I assume that you're terminating the inverting output in 100R, could you potentially use this output to sense the offset when inputs are shorted and use a micro to calibrate it out using a digital pot/dac when a button is pressed? If the offset doesn't drift then this is probably overkill and a pot is the best.
That could be an option. The datasheet from TI actually suggests to use an extra opamp to correct the offset error of the LMH3401. I'm wary to do that because it complicates the PCB design and some extra load needs to be connected to the output of the LMH3401. I suppose the offset doesn't drift that much but it is something I could test to see if it depends a lot on the operating temperature.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline JohnG

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Re: > 1 GHz DIY differential probes
« Reply #97 on: November 19, 2018, 02:34:27 pm »
Cool!  :-+

... If there is some interest I could have a small batch made ...

I'm interested, and I think I'm not going to be the only one.  :)

I would also be interested.

Thanks,
John
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Offline dpenev

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Re: &gt; 1 GHz DIY differential probes
« Reply #98 on: November 19, 2018, 04:18:34 pm »
I am interested too.

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Offline rx8pilot

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Re: &gt; 1 GHz DIY differential probes
« Reply #99 on: November 19, 2018, 05:16:42 pm »
This is a very interesting and practical project. Very impressive results.

Digging through the rest of the thread looking to see how you measured impedance and capacitance - delicate measurements.

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Online 2N3055

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Re: > 1 GHz DIY differential probes
« Reply #100 on: November 19, 2018, 09:57:48 pm »
I would be interested too.
Nice work.
 

Offline mk_

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Re: > 1 GHz DIY differential probes
« Reply #101 on: November 25, 2018, 09:05:41 pm »
still working on my 400Mhz-diffprobe... I`m interested too

 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #102 on: November 25, 2018, 09:24:36 pm »
I still have to update the PCB design to include the offset pot. I hope I can squeeze this in this week and then get a quotation from an assembler to have a couple of these probes produced professionally. It is not going to be really cheap (likely >100 euro) but still extremely competitive compared to 'the real deal'. I want to sell the probe as a kit complete with SMA to BNC cable, USB power cable and stainless steel wire to make probe tips.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 
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Offline mk_

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Re: > 1 GHz DIY differential probes
« Reply #103 on: November 26, 2018, 10:17:43 am »
I still have to update the PCB design to include the offset pot. I hope I can squeeze this in this week and then get a quotation from an assembler to have a couple of these probes produced professionally. It is not going to be really cheap (likely >100 euro) but still extremely competitive compared to 'the real deal'. I want to sell the probe as a kit complete with SMA to BNC cable, USB power cable and stainless steel wire to make probe tips.

At least here there are enough SMA>BNC-Cables aviable.

I use for my 400MHz-Probe something like this:
https://www.schukat.com/schukat/pdf.nsf/index/D796B48DFC770906C1257933002CFFBC/$file/PTR_Serie_1012_E.pdf

Anyway, 100€ are fine and if you need an assembler mail me PM, I`m working "inside" an assember....



 
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Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #104 on: December 05, 2018, 09:33:22 pm »
I still have to update the PCB design to include the offset pot. I hope I can squeeze this in this week and then get a quotation from an assembler to have a couple of these probes produced professionally. It is not going to be really cheap (likely >100 euro) but still extremely competitive compared to 'the real deal'. I want to sell the probe as a kit complete with SMA to BNC cable, USB power cable and stainless steel wire to make probe tips.
I just ordered a first batch of 10 pieces. The price for the complete kit is going to be 140 Euro including VAT. I'll create a user manual / specification sheet as well.
« Last Edit: December 05, 2018, 09:56:56 pm by nctnico »
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Offline bitbanger

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Re: > 1 GHz DIY differential probes
« Reply #105 on: December 28, 2018, 05:50:35 am »
Expressing interest for one of these.
 

Offline BFX

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Re: > 1 GHz DIY differential probes
« Reply #106 on: December 28, 2018, 09:28:02 pm »
This is really great  :-+
But isn't possible to do that in little bit better formfactor?
Slim to be able to put inside some nice aluminum cover?  :box:
 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #107 on: December 28, 2018, 10:22:42 pm »
This is really great  :-+
But isn't possible to do that in little bit better formfactor?
Slim to be able to put inside some nice aluminum cover?  :box:
I tried to make the board as narrow as possible however the width is dictated by the width of the micro-USB and SMA connectors. IMHO the design isn't lumpy compared to differential probes you can buy from R&S, Keysight or Lecroy but yes, it definitely is more lumpy than a passive probe. BTW I'm having a plastic case designed & manufactured because the heat-shrink sleeving just didn't sit right with me.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline mk_

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Re: > 1 GHz DIY differential probes
« Reply #108 on: December 28, 2018, 10:39:06 pm »
Slim to be able to put inside some nice aluminum cover?  :box:

I buildt a 400MHz-Diffprobe some time ago and decided against a nice aluminiumcover because if it sits somewere on the circuit the chance for a destructive short is high.   
 

Offline BFX

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Re: > 1 GHz DIY differential probes
« Reply #109 on: December 30, 2018, 04:13:53 pm »
Slim to be able to put inside some nice aluminum cover?  :box:

I buildt a 400MHz-Diffprobe some time ago and decided against a nice aluminiumcover because if it sits somewere on the circuit the chance for a destructive short is high.
It's all depends on design. If it's done properly there is no chance for some shorts. But yes it's necessary to have access to some CNC machine.
Here is for example my noise source.

 
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Offline joeqsmith

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Re: > 1 GHz DIY differential probes
« Reply #110 on: December 30, 2018, 04:17:39 pm »
It's all depends on design. If it's done properly there is no chance for some shorts. But yes it's necessary to have access to some CNC machine.
Here is for example my noise source.

That's some nice work.
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 

Offline mk_

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Re: > 1 GHz DIY differential probes
« Reply #111 on: December 30, 2018, 04:49:54 pm »
Quote
It's all depends on design. If it's done properly there is no chance for some shorts. But yes it's necessary to have access to some CNC machine.
Here is for example my noise source.

I didn`t mean short in the diffprobe itself but that the diffprobe, housed in an aluminiumcover and lying around can easily create shorts in the circuit it is probing below.
 
 

Offline David Hess

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Re: > 1 GHz DIY differential probes
« Reply #112 on: December 31, 2018, 06:19:46 pm »
A conductive case is actually desired to prevent degradation of the AC common mode rejection from variation in the probe's environment.  Plastic enclosures should have a conductive film on the inside.
 

Offline BravoV

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Re: > 1 GHz DIY differential probes
« Reply #113 on: December 31, 2018, 06:40:16 pm »
A conductive case is actually desired to prevent degradation of the AC common mode rejection from variation in the probe's environment.  Plastic enclosures should have a conductive film on the inside.

Will something like the old conductive carbon spray for coating the CRT TV case internal or the CRT tube work ?

Example :
« Last Edit: December 31, 2018, 06:42:02 pm by BravoV »
 

Offline luma

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Re: > 1 GHz DIY differential probes
« Reply #114 on: December 31, 2018, 09:06:50 pm »
I’m definitely interested in placing an order when you think you’re ready for testers.  I have a fair bit of experience designing 3D printable enclosures for OSHW projects and would get to work on that immediately upon receipt of an assembled board.
 

Offline bitbanger

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Re: > 1 GHz DIY differential probes
« Reply #115 on: December 31, 2018, 10:56:52 pm »
^ with him/her. I'd be perfectly happy with as-is stated performance and 3D printing an enclosure.
 

Offline David Hess

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Re: > 1 GHz DIY differential probes
« Reply #116 on: January 01, 2019, 02:57:36 am »
A conductive case is actually desired to prevent degradation of the AC common mode rejection from variation in the probe's environment.  Plastic enclosures should have a conductive film on the inside.

Will something like the old conductive carbon spray for coating the CRT TV case internal or the CRT tube work ?

Example :


If it will work for EMI shielding then it will probably work.  The cases on the Apple 2 computer used something like that on the inside.
 

Offline _Wim_

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Re: > 1 GHz DIY differential probes
« Reply #117 on: January 01, 2019, 08:45:27 am »
A conductive case is actually desired to prevent degradation of the AC common mode rejection from variation in the probe's environment.  Plastic enclosures should have a conductive film on the inside.

Will something like the old conductive carbon spray for coating the CRT TV case internal or the CRT tube work ?

Example :


If it will work for EMI shielding then it will probably work.  The cases on the Apple 2 computer used something like that on the inside.

I think for EMI it is better to use this one on a plastic enclosure (kontakt chemie EMV 35 or EMI 35): https://uk.farnell.com/kontakt-chemie/emi-35-200ml/coating-conductive-emi-35-200ml/dp/2142398

 

Offline the Willows

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Re: > 1 GHz DIY differential probes
« Reply #118 on: January 04, 2019, 10:52:19 am »
Very interesting, was on the way to order from the German guy, but this seems to be a much better design.
Please write me up for 2 pcs.

We own (me and Daniel from Xtreemtec) since last december a LeCroy WavePro 7300, but no probes yes. Hard to find item or huge prices.
So this is a good alternative.
 

Offline paul67

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Re: > 1 GHz DIY differential probes
« Reply #119 on: February 19, 2019, 07:19:29 pm »
Are there any probe kits remaining? I'm interested in one. Thanks.
 

Offline paul67

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Re: > 1 GHz DIY differential probes
« Reply #120 on: February 19, 2019, 07:23:29 pm »
I still have to update the PCB design to include the offset pot. I hope I can squeeze this in this week and then get a quotation from an assembler to have a couple of these probes produced professionally. It is not going to be really cheap (likely >100 euro) but still extremely competitive compared to 'the real deal'. I want to sell the probe as a kit complete with SMA to BNC cable, USB power cable and stainless steel wire to make probe tips.
I just ordered a first batch of 10 pieces. The price for the complete kit is going to be 140 Euro including VAT. I'll create a user manual / specification sheet as well.

Meant to 'quote' that. I'm interested in one of those probe kits...
 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #121 on: February 19, 2019, 09:45:18 pm »
I'm almost ready to put the first batch together. Give me a few more days to make the official announcement.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline technogeeky

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Re: > 1 GHz DIY differential probes
« Reply #122 on: February 20, 2019, 12:14:18 am »
I'm almost ready to put the first batch together. Give me a few more days to make the official announcement.

I'm also interested.
 

Offline the Willows

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Re: > 1 GHz DIY differential probes
« Reply #123 on: February 20, 2019, 08:32:59 am »
Tonnie Wittenaar wrote on the 4th of January:
Quote
Very interesting, was on the way to order from the German guy, but this seems to be a much better design.
Please write me up for 2 pcs.

So if Designer can give us an update how things are going, or project has died, please let us now.
 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #124 on: February 20, 2019, 09:42:11 am »
Nearly ready! Everything except the SMA to BNC cables has been delivered. The boards have been tested and adjusted. I'm going to assemble the first batch in the next couple of days. The datasheet is also nearly ready. I want to do a few more measurements regarding spurs and noise.
« Last Edit: February 20, 2019, 09:44:14 am by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline the Willows

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Re: > 1 GHz DIY differential probes
« Reply #125 on: February 20, 2019, 09:44:24 am »
Your are the best.
Thanks!

 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #126 on: March 02, 2019, 07:45:58 pm »
The first batch of 9 probes is ready to go. The product name has become 'DIP1400' (DIfferential Probe 1400MHz). The bandwidth is +/-3dB up to 1.4GHz.



Datasheet:
http://www.nctdev.nl/NCT%20Instruments%20DIP1400%20V1.0.pdf

The final price is 125 euro ex. VAT ex. Paypal / wire transfer fees. Shipping in Europe is 16 euro, worldwide 27 euro including insurance.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 
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Offline paul67

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Re: > 1 GHz DIY differential probes
« Reply #127 on: March 03, 2019, 09:51:27 pm »
Looks great nctnico :-)
PM Sent with purchase request...
 

Offline rsjsouza

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Re: > 1 GHz DIY differential probes
« Reply #128 on: March 04, 2019, 01:32:57 am »
Great work, Nico! It will be on my wish list as my finances for hobbies are quite depleted at the time.

I read the datasheet and spotted a couple of typos:
Page 3: pads cannot be torn off very easily

Page 6: A more coarse measurement of the bandwidth

One question:
Page 4: do you have a characterization for the worst case scenario on the table?

Again, great work!
Vbe - vídeo blog eletrônico http://videos.vbeletronico.com

Oh, the "whys" of the datasheets... The information is there not to be an axiomatic truth, but instead each speck of data must be slowly inhaled while carefully performing a deep search inside oneself to find the true metaphysical sense...
 

Offline paul67

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Re: > 1 GHz DIY differential probes
« Reply #129 on: March 14, 2019, 10:29:26 pm »
Thanks for the delivery of the probe, Nico. It works really nicely! I'm very happy with it. I wondered, what do you do with the LMH3401 inverting output... connect directly to 0V, or...?
I looked at the datasheet but couldn't see guidance for using in single-ended output mode.
 
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Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #130 on: March 14, 2019, 10:35:13 pm »
Thanks for the delivery of the probe, Nico. It works really nicely! I'm very happy with it. I wondered, what do you do with the LMH3401 inverting output... connect directly to 0V, or...?
I looked at the datasheet but couldn't see guidance for using in single-ended output mode.
The inverted output is terminated to ground with a (in total) 50 Ohm resistor so both output see (about) the same load.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Evodad

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Re: > 1 GHz DIY differential probes
« Reply #131 on: November 01, 2019, 06:04:13 pm »
Hi ntcnico,

Any chance you have any probes left ?
This was an interesting thread. Why did it not survive longer ? Any spin-offs ?

// Per
 

Online nctnico

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Re: > 1 GHz DIY differential probes
« Reply #132 on: November 01, 2019, 06:07:36 pm »
Hi ntcnico,

Any chance you have any probes left ?
This was an interesting thread. Why did it not survive longer ? Any spin-offs ?

// Per
Actually I have 20 new probes fresh from the assembler.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Evodad

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Re: > 1 GHz DIY differential probes
« Reply #133 on: November 03, 2019, 07:04:20 pm »
That was good news  :)

Any modifications since the last batch ?

May I buy 2 of you ?

)) Per
 


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