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> 1 GHz DIY differential probes

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nctnico:

--- Quote from: lukier on August 15, 2016, 12:18:37 am ---
--- Quote from: nctnico 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.

--- End quote ---

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?

--- End quote ---
I have not looked into the technical details. I'm just pointing towards an existing solution which could be hacked.

JohnG:
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

tggzzz:

--- Quote from: JohnG on August 15, 2016, 12:46:13 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.

--- End quote ---

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.

Marco:

--- Quote from: JohnG on August 15, 2016, 12:46:13 pm ---So unless you are sure about the design, it may be premature to say it is not a differential probe.

--- End quote ---

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.

joeqsmith:
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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