After a bit of radio silence, I have a large update with big improvements to the probe:
I have worked through another two iterations of the probe as there were various specifications that I thought could be improved upon. We're at V10 now.
I also was in contact with Picotech, who generously offered to lend me a fast oscilloscope (Picoscope 9404-16, 16GHz sampling scope) for proper time-domain measurements of the probe. This has been a huge pleasure to use, and I am very grateful to them for lending me such a high specification oscilloscope. I unfortunately have to ship the scope back this week. So yes, big thanks to them! Buy their oscilloscopes!
I now no longer use the optional resistive ground lead to reduce the DUT loading, instead the signal pin is now always resistive. As such there's no longer a trade-off between probe bandwidth and reduced DUT loading. You get both high bandwidth and reduced loading
So the main new specifications are:
Bandwidth: 2.7GHz
Rise time: 130ps
DUT loading: 120Ohms worst case (at 1.6GHz).
Tip capacitance: 0.7pF
Return loss on a 50 ohm transmission line -15.4db max
Noise: 350uV RMS output, or 7mV RMS referred to input (20x probe)
Now for the performance plots.
Showing -3db at just over 2.8GHz. Linearity is still very good.
Tip loading is now minimum 120Ohms, and stable out to 4GHz at 200 ohms. My VNA is unstable beyond this.
Step response test from signal generator with 350ps rise time. Probe response is very close to the signal generator, and probe loading has not modified the signal at all. Note red and green lines are almost perfectly overlapping. Note that red and green traces are saved waveforms so they can be overlaid with the probe signal. The probe has a 5.4ns latency.
Rise time measurement using fast edge (70ps) from fast signal generator. This edge has a bit of ringing, so I'm uncertain how much of the probe ringing is because of the source signal ringing vs ringing because the source rise time is so much faster than the probe rise time. Probe rise time around 130ps which corresponds to a 2.7GHz bandwidth. Note probe has only a minor loading on this very fast signal. The rise time is slowed by around 6ps and there is some reduction to the peaking.
Nice flat-topped low-frequency square waves (1khz).
Test setup for oscilloscope measurements. Signal generator routed into oscilloscope (channel 4) via intermediate coplanar waveguide board. Active probe measures signal on coplanar waveguide and is read by oscilloscope channel 2.
I'm pretty sure I've said this before, but hopefully for real this time. I am looking at starting production of these. If anyone wants one, send me a PM to get yourself to the front of the queue
The cost will be between £180 and £200, depending on how much of my time I end up spending on each probe. Hopefully more on the lower end of that. The probe tips are hand-made, and there may need to be some tuning on each probe if the frequency response isn't as good as it should be when I test them.