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DC coupled 2.7 GHz Active Probe Project - Now Available!

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lasmux:

--- Quote from: joeqsmith on August 09, 2023, 12:31:09 am ---But if you think that's bad here is that same homemade resistive probe.   :-DD   That's s mighty big loop, lots of inductance.  Not too surprised it peaks up.   I would expect your probe would do a much better job with this 200ps edge.   

Scope is an old WM8500A 5GHz BW. 

--- End quote ---
Yeah, that's a lot of ringing! Very neat how little it's loaded the original signal though.

lasmux:

--- Quote from: MathWizard on August 09, 2023, 09:56:33 am ---Cool project, I'd buy this probe down the road.
How much worse would it be if you used all the same chips, but used all standard size through hole parts ? What if you didn't pay attention to spacing or layout ? I'm just trying to get a feel for how sensitive these circuits really are. If 2GHz is ~15cm waves, thats getting down to circuit size.

--- End quote ---
Thanks, I'll let you know when it's ready :)

The probe amplifier input and feedback network on the probe is all 0402 passives. Even with that I have to have various snubbing resistors etc as stray inductances/capacitances cause havoc with the frequency response. You might be able to get something working with through hole components, but it would take much more effort to get it working correctly. And you probably wouldn't be able to reach 2GHz bandwidth.
Compared to the DIY 1k resistive probes, it is more general purpose due to the 1M input resistance, and in theory will have a more precisely tuned frequency response, but has lower bandwidth potential.

joeqsmith:

--- Quote from: lasmux on August 09, 2023, 03:08:45 pm ---
--- Quote from: joeqsmith on August 09, 2023, 12:31:09 am ---But if you think that's bad here is that same homemade resistive probe.   :-DD   That's s mighty big loop, lots of inductance.  Not too surprised it peaks up.   I would expect your probe would do a much better job with this 200ps edge.   

Scope is an old WM8500A 5GHz BW. 

--- End quote ---
Yeah, that's a lot of ringing! Very neat how little it's loaded the original signal though.

--- End quote ---

To give you some idea of the effects of lowering the inductance, these are two poor man's 10X probes.  The resistors are butted up against the coax center pin with a much shorter probe tip.  All to reduce that big loop.   

As before, M2 is the unloaded waveguide. C2 is loaded waveguide with probe #2 attached.  C3 is probe #2.  M3 is probe #1.    You mentioned the amount of variance seen with homemade probes but these two seem fairly well matched.   

joeqsmith:
Just for completeness, I reattached the homemade 20X probe to compare it with the 10X probe #1.   Construction is everything and as you pointed out, I did try a few ways to improve it but they all failed to exceed what I could achieve with the simple techniques shown. 

joeqsmith:
I have thought about trying to add a blade for the ground rather than the wire to help reduce the inductance.   One thing I have done in the past was when I wrap the wire around the coax to form the ground, I use both ends rather than one.  I normally solder this coiled wire.  The problem is getting them attached to the board.   Picotech offers a probe like that today.   Looks like a 20X 6GHz probe would cost about $1000 USD.   

https://www.picotech.com/accessories/gigabit-digital-passive-test-probes

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