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Affordable <200MHz PDN analysis / impedance measurement hardware?

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inevitableavoidance:

--- Quote from: nctnico on October 10, 2024, 09:08:00 am ---
--- Quote from: inevitableavoidance on October 10, 2024, 08:26:36 am ---@nctnico I've seen your DC to daylight differential amplifier on here - would it be possible to somehow spin that circuit to have a 50 ohm input impedance, and make the ground loop breaker that way?

--- End quote ---
For PDN measurement purposes, the 26dB dampening of my DIP1400 probe is not what you want. Using a transformer is a better option.

--- End quote ---

The LMH3401 by itself has a positive gain right? Wouldn't an alternative input resistor scheme allow for 50 ohm input impedance while still offering some gain at the end?

nctnico:

--- Quote from: inevitableavoidance on October 10, 2024, 10:15:44 am ---
--- Quote from: nctnico on October 10, 2024, 09:08:00 am ---
--- Quote from: inevitableavoidance on October 10, 2024, 08:26:36 am ---@nctnico I've seen your DC to daylight differential amplifier on here - would it be possible to somehow spin that circuit to have a 50 ohm input impedance, and make the ground loop breaker that way?

--- End quote ---
For PDN measurement purposes, the 26dB dampening of my DIP1400 probe is not what you want. Using a transformer is a better option.

--- End quote ---

The LMH3401 by itself has a positive gain right? Wouldn't an alternative input resistor scheme allow for 50 ohm input impedance while still offering some gain at the end?

--- End quote ---
If the resistors are changed, then you could turn it into a buffer or even a pre-amplifier for sure but the bandwidth will be much lower ofcourse. I'm not sure what the effect of the added noise will be.

inevitableavoidance:

--- Quote from: nctnico on October 10, 2024, 10:24:47 am ---If the resistors are changed, then you could turn it into a buffer or even a pre-amplifier for sure but the bandwidth will be much lower ofcourse.

--- End quote ---

Enough for my desired 200MHz range? Do you happen to have a simulation I could play with?

nctnico:
If you use the LMH3401 model, you should be able to get a decent idea of how it would perform. I don't expect surprises at 200MHz.

joeqsmith:

--- Quote from: ch_scr on October 10, 2024, 08:42:25 am ---
--- Quote from: inevitableavoidance on October 10, 2024, 08:26:36 am ---@joeqsmith I've seen the coaxes on the breadboard - how'd you do it for an actual circuit board? Solder two together, run the through calibration, and then solder both to a pin pair you'd like to measure? How'd you do the other calibrations?

--- End quote ---
The trick I've learned is to have a box of (semi-rigid) coaxes, terminated to e.g. SMA on one side, cut to precisely the same length on the other.
Then you can e.g. solder two together as a cal rig, solder the other two to the circuit, have a few spare for other stuff - but still share the cal rig...

--- End quote ---

Add a cal standard to the board, maybe on a rail   
Solder them together and then to the board   
Insert a thru adapter and call it good 
In the case the breadboard, left the board unpopulated and called that good

I am not sure what "other calibrations" would be.

Solving the ground loop could certainly be done without a transformer using a diff amp.  You could easily out perform my transformers at lower frequencies going with solid state.  However, the one transformer I use has 25db@10k and 37db@100k.  The 3dB point is around 100Hz.  Consider that these low cost VNAs can't run down this low, it may be good enough for your needs.   Yes, I know they have all sorts of lower limits.  I think I had seen one allowing 1Khz operation.  That doesn't suggest you can measure anything useful that low.   

Another thing with the transformer, it's very robust.  You may want to consider investing in some very low capacitance TVSs and attach them to the VNA.   

***
Another option would be to purchase the parts you need.  Maybe have a look at what Pico offers. 
https://www.picotest.com/solutions/pdn-measurement-forpower-integrity/

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