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DC coupled 2.7 GHz Active Probe Project - Now Available!
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2N3055:
I agree with Tggzzz, nice work on the probe...

I also have great respect for Picotech...
And 3 Picoscopes (actually 4, with retired one I don't use anymore)
lasmux:
The noise calculation was a bit tricky as the scope noise was much worse than the probe noise level. For subtracting noise, I found the following equation :
𝑃𝑟𝑜𝑏𝑒𝑅𝑀𝑆= √(𝑆𝑐𝑜𝑝𝑒𝐴𝑛𝑑𝑃𝑟𝑜𝑏𝑒𝑅𝑀𝑆2 − 𝑆𝑐𝑜𝑝𝑒𝑅𝑀𝑆2 )
So I'd measure the AC RMS noise of the scope without the probe plugged in, and then measure the AC RMS noise with the probe plugged in and tips grounded, and then perform the above calculation.

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

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

Probe noise without probe attached:


Probe noise with probe attached:


jmw:
Nice work!

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




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

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

I suppose more attenuation makes it easier to get lower input capacitance; is this the reason you went with 26 dB attenuation?
lasmux:
Hey, nice probe! Nice flat response :)

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


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