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DC coupled 2.7 GHz Active Probe Project - Now Available!

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joeqsmith:
LeCroy does or did offer an accessory that allows their probes to be used with a standard scope.  Keysight and Tektronix also offers this for some of their probes.   I have yet to find a used one for mine. 

I would think to measure a probes impedance you want to have a fixture that you can remove all the errors right to where the probe attaches.   No added 50 ohm termination, no Ts, no extra length...   Your option 3 with an open that had the reference plane matched to the SOL standards, then just measure S11.   For low frequencies (GHz ish) I would just use those coplanar waveguide boards.  Then attach the probe in place of where the two 100 ohm resistors attach.   Beyond about 1.5GHz, I wouldn't trust it.   

Where my low cost VNAs don't have good return loss for port 2, the old PNA is another story.   Still just to see the loading effects between those two probes, good enough to use the Lite. 

***
I should add that when you measure the impedance, you need to choose the proper method for best accuracy.  Depending on the range, you would use the shunt, shunt thru, or series method.    In your case, you may want to use two different methods to cover the range.   


For BW,  I would still stick with the terminated coplanar waveguide, but in this case, I have one from LeCroy.   The T adds error.  Same with the custom terminator.  Consider cal'ing at the end of the waveguide, then leaving the cal standard attached when making the measurement.  You just want to know the probe.  Hope this makes some sense.   

The other test would be to just measure some basic digital signals and compare results.

I had told a friend about your project and projected selling price.  I looked up the current cost of the PP005 on Digikey and even that probe would cost more.  So yes, I think your target price is still very fair, assuming the probe works.   Let me know once you have these first set built up.   

***
Reminds me of my attempt to replicate someones experiment where they attempted to measure the capacitance of some resistors.  Far away from 50 ohms....
 
https://www.eevblog.com/forum/rf-microwave/shunt-capacitance-of-1206-smd-resistors-jeroen-belleman-december-2010/

joeqsmith:
The PP005A attached to the home made coplanar waveguide with the terminators not populated.   The probe wasn't terminated.  Tip capacitance seems to measure around 6p rather than 11.  Note too that rather than 10M, it measures 100k (300 shown).   If I have time tonight, I will try and measure it in series with my low frequency network analyzer to give you some idea of the difference.   That NA can go up to 150MHz. 

joeqsmith:
Using a PECL buffer that feeds a splitter.  One leg which goes to one of the scopes inputs (50ohms).  The other goes to the LeCroy coplanar waveguide test board which was then terminated to 50 ohms.   Scope probe when then attached to second channel on scope and then used to probe the waveguide. 

No load:  No second probe attached.  This scope is only spec'ed for 600MHz (approx 600ps), and that PECL driver is about 350ps.  Cable adds a fair bit of loading.  Idea is to show the effects of the probe loading.

PP002:  This is a 350MHz 10X probe.   Causes a fair bit of loading.  Think about that PECL signal going off to some other part of the circuit rather than the scope.   Also, with the splitter, things are somewhat isolated.   

PP061:  The resistive probe still has some loading (not a great probe)  but much better than the 10X probe.   

Once you get a bit closer,  I'll bust out my faster scope and active probes.     

joeqsmith:
Next, I removed the splitter and connected the LeCroy waveguide directly to the PECL driver.  The opposite end of the waveguide connected to the scopes 50 ohm input.  Basically, how you would probe a typical signal on a board but the scope is again the target load.     

145:  The PP061 resistive probe pink.  M1 is the unloaded waveguide (nothing attached).  C1 is the loaded waveguide.  Again, some loading.  Scopes BW is limiting the edge to about 600ps. 

146:  The PP005 500MHz 10X probe.   It really messes with the signal.  I did not clear the traces so ignore the averaged data.

147: The P6202A 500MHz active probe.   Has very little loading effects compared with the 10X passive probe.   According to the link below, the probe cost about $1300 in 1998.  The cables are thick and heavy making the probe difficult to use.   You can start to see why the resistive probes were a better choice in most cases.   

http://www.barrytech.com/tektronix/probes/tekp6202a.html

lasmux:
I think your second set of measurements will be better? With the splitter you will have two 50 ohm terminations (to ground, not VCC-2V) on the PECL driver, which it might not like? I've not used PECL before though to be honest, only a bit in simulation.

Very interesting how the PP005 really does a lot of damage to the signal. The PP061 lowers the PECL voltage levels slightly with it's loading? But the shape of the loaded waveform is more similar. The P6202A waveform has almost no loading (at these frequencies), but the output waveform maybe has slightly more peaking on the rising edge? It's still qualitatively very similar.

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