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Affordable <200MHz PDN analysis / impedance measurement hardware?

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joeqsmith:

--- Quote from: inevitableavoidance on October 08, 2024, 07:27:23 am ---I just bought a NanoVNA-H4, with a dynamic range of 70dB from 50kHz-300MHz, specified with a minimum frequency of 10khz.
--- End quote ---

After my friend flipper saw the low frequency data from my repaired/modified H4,  they sent me a brand new H4.   It is revision 4.3_MS with the MS synthesizer where mine has the Si.  The plan was to modify his H4 with the same capacitors and TVSs that I used on mine.  He also wants to use that VNA for measuring PDNs.  However, when I did a baseline check the low frequency performance is really poor.

Please do me a favor.  Provide the revision of your H4 and synthesizer type.  Set it to 401 points, 1kHz IFBW, with a freq range of 1.6k to 10kHz.  Short port 1&2 and normalize.  Open the ports and terminate.  Measure S21.  I want to know if you get anywhere near that 75-80dB I measure with mine.   

I mentioned I doubted the caps I added had anything to do with the low frequency performance, but I never took a baseline this low.   I wonder if that synthesizer is going to be a problem.   

They have their original NanoVNA which was purchased from the same source, same order as mine.   I am wondering if I will need to swap the that old Si part to get the H4 to perform down this low.   

mawyatt:

--- Quote from: inevitableavoidance on November 16, 2024, 10:07:06 am ---The board is developed around a maximum excitation voltage of 100mV. Besides the 10x gain on the PCB I have modified my AD2 for an additional 5x less attenuation internally, so am limited to an input voltage of ±140mV either way. :)

--- End quote ---

Keeping the excitation at 100mV with a 1/2 Ω current sense places the DUT current at 200mA seems reasonable for the direct USB Powered AD2 and the OPA891 Buffer Amp limitation. With an external Power Supply the AD2 specs indicates a max of 700ma, but you would need a higher current capable buffer and the later voltage limit with the 10X gain, both of which increase complexity.

Reducing the AD2 attenuation by shorting the input resistor seems a good idea, but exposes the ADG612 to external signals without any series current limit resistor. Maybe a 10K parallel resistor might be better and provide some level of protection and could be compensated with the Scale Value.

Anyway, are the Gerber files available? We are considered building this adapter since our AD2 doesn't get utilized for anything now.

Best

inevitableavoidance:

--- Quote from: mawyatt on November 16, 2024, 06:02:01 pm ---Keeping the excitation at 100mV with a 1/2 Ω current sense places the DUT current at 200mA seems reasonable for the direct USB Powered AD2 and the OPA891 Buffer Amp limitation. With an external Power Supply the AD2 specs indicates a max of 700ma, but you would need a higher current capable buffer and the later voltage limit with the 10X gain, both of which increase complexity.

Reducing the AD2 attenuation by shorting the input resistor seems a good idea, but exposes the ADG612 to external signals without any series current limit resistor. Maybe a 10K parallel resistor might be better and provide some level of protection and could be compensated with the Scale Value.

Anyway, are the Gerber files available? We are considered building this adapter since our AD2 doesn't get utilized for anything now.

Best

--- End quote ---

The gerber files, BOM, and pick an place files are in the output folder in the github repository: https://github.com/remidavidkoster/Redako-PDNAnalyzer/tree/master/Project%20Outputs%20for%20PDN%20Analyzer

I'm not sure what DC bias caps are in the BOM - I ended up assembling 5 different versions of those with the best I could find for each voltage range. It doesn't show on the pictures, but I paralleled / stacked two capacitors together for the excite signal current path.

I agree some current limiting with indeed be best, and simple soldering another resistor over the current 820k ones is easy enough.

Please keep me posted if you end up making some - happy to see someone else get a use of the design. :)

inevitableavoidance:

--- Quote from: joeqsmith on November 16, 2024, 05:04:11 pm ---After my friend flipper saw the low frequency data from my repaired/modified H4,  they sent me a brand new H4.   It is revision 4.3_MS with the MS synthesizer where mine has the Si.  The plan was to modify his H4 with the same capacitors and TVSs that I used on mine.  He also wants to use that VNA for measuring PDNs.  However, when I did a baseline check the low frequency performance is really poor.

Please do me a favor.  Provide the revision of your H4 and synthesizer type.  Set it to 401 points, 1kHz IFBW, with a freq range of 1.6k to 10kHz.  Short port 1&2 and normalize.  Open the ports and terminate.  Measure S21.  I want to know if you get anywhere near that 75-80dB I measure with mine.   

I mentioned I doubted the caps I added had anything to do with the low frequency performance, but I never took a baseline this low.   I wonder if that synthesizer is going to be a problem.   

They have their original NanoVNA which was purchased from the same source, same order as mine.   I am wondering if I will need to swap the that old Si part to get the H4 to perform down this low.

--- End quote ---

I'm getting nowhere near that figure - 10dB per division:



Where do I find the revision and synthesizer?

joeqsmith:

--- Quote from: inevitableavoidance on November 17, 2024, 11:22:40 am ---I'm getting nowhere near that figure - 10dB per division:
...
Where do I find the revision and synthesizer?

--- End quote ---

The back of the unit should have the hardware revision and possibly an _MS.   You can also select Config, Expert Settings, More.  At the top is should show the synthesizer selected (MS5351 or Si5351).   

You can try to set the IFBW to 30Hz which should at least remove the pattern you are seeing. 

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