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Affordable <200MHz PDN analysis / impedance measurement hardware?

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Kean:
Following along as I have a new Zeenko NanoVNA-H4 on the way

inevitableavoidance:

--- Quote from: joeqsmith on November 18, 2024, 10:50:31 pm ---Thanks for checking.  That's on par with my friends.   I should have his original NanoVNA in a few days.  I'll post the results once I start swapping parts. 

If it is the clock generator, it doesn't really help you other than maybe someone you know might be willing to swap you their older unit to get a brand new one.

--- End quote ---

Awesome, curious to know. Would be dope to get some more low end performance out of it. I do indeed see the series capacitors on the SMA in and outputs on the PCB here. Can’t imagine that to pass a whole lot of low end.

Seeing ferrite beads without bulk capacitors also tends to worry me - they usually resonate like crazy in the lower kHz’es. Might use the AD2 to measure its PDN impedances. ;)

joeqsmith:

--- Quote from: Kean on November 19, 2024, 12:50:46 am ---Following along as I have a new Zeenko NanoVNA-H4 on the way

--- End quote ---

Once you receive it, please run the same test and post your results along with the hardware revision and type of synthesizer.   


--- Quote from: inevitableavoidance on November 19, 2024, 10:23:08 am ---Awesome, curious to know. Would be dope to get some more low end performance out of it. I do indeed see the series capacitors on the SMA in and outputs on the PCB here. Can’t imagine that to pass a whole lot of low end.

Seeing ferrite beads without bulk capacitors also tends to worry me - they usually resonate like crazy in the lower kHz’es. Might use the AD2 to measure its PDN impedances. ;)

--- End quote ---

My only guess is there was a fairly large group of users applying DC to their VNAs and smoking the input network and then claiming the units were defective under warranty. 

FYI, looking at my H4, I had also added another cap near the synthesizer. 

***
I had connected a step attenuator to H4 after repairs and swept from 0 to 80dB and 50k to 2GHz.  This may also give you a good comparison.  My guess is your high frequency performance is not as good as I saw some improvements by adding those caps to mine.  If you want to know values and placement, let me know and I can provide them but I would start by running a baseline.   Maybe with a working unit, the gains would be less dramatic. 

https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg5698991/#msg5698991


inevitableavoidance:

--- Quote from: joeqsmith on November 19, 2024, 01:18:43 pm ---My only guess is there was a fairly large group of users applying DC to their VNAs and smoking the input network and then claiming the units were defective under warranty. 

--- End quote ---

I removed both the in- and output capacitors to see how much extra low end would get through:


More than I expected, but at a lower frequency than I thought. This is the difference in noise floor in the 1k6-50k range:


Before this I also took at look at the power delivery networks on the board - there's barely any decoupling in any of them, and the traces used are ridiculously thin. This is the difference in full range noise floor after adding a handful of 2.2uF 0603 capacitors:


Makes me feel like a first year EE student again, adding capacitors hoping it'll solve a problem, though with more knowledge about the difference it makes, and apparently it's still a valid approach.

Quite surprised to be honest that even VNA designing EE's don't seem to know how to properly apply decoupling capacitors.

The NanoVNA-H4 is specified like this:
70dB | 50kHz-300MHz
60dB | 300MHz-900MHz
40dB | 0,9GHz-1,5GHz

Before I was getting:
70dB | 10kHz-436MHz
60dB | 436MHz-740MHz
40dB | 740MHz-1,7GHz

Now I'm getting nearly getting:
70dB | 10kHz-661MHz
60dB | 661MHz-1446MHz
40dB | 1446MHz-1,85GHz

How did you improve the decoupling department over there? What did you add where?

joeqsmith:

--- Quote from: inevitableavoidance on November 19, 2024, 04:48:44 pm ---Makes me feel like a first year EE student again, adding capacitors hoping it'll solve a problem, though with more knowledge about the difference it makes, and apparently it's still a valid approach.

--- End quote ---

 :-DD :-DD  I am near retirement and still feel like a first year student.   :-DD


--- Quote from: inevitableavoidance on November 19, 2024, 04:48:44 pm ---How did you improve the decoupling department over there? What did you add where?

--- End quote ---

A couple of things.  There was no attempt what so ever to look at this from anything beyond an empirical test.  I am not suggesting these changes will help or hurt the performance.  My only goal in adding them was to see if I could detect a change in the performance of the VNA after I had damaged it.  It was part of my hunting down the root problem.   I just never backed out the changes and in my case noticed an improvement at the higher frequencies.   

Lets start with the same schematic for the H4.  See attached.  This will not match your H4.  Note the lack of the DC blocking caps.   

I placed a 4.7uF 0603 cer. across C10, C39, C41, C43.   I placed a 10uF 1206 cer across C61 & C62 (doubt these made any difference, I was shotgunning).    I placed a TVS on each port to ground, at the connector.  The parts I had on-hand are about a half puff.  While they did not effect the performance,  I would look for a better part.  Digikey has some with much lower capacitance. 

I'm still surprised I damaged mine and my only goal was not to loose it to ESD a second time.   If you go ahead and add those first four caps, I am interested in seeing if you find any difference, and what mixers were installed in your H4.   

****
When I state across, I turned the caps on-end and solder them right to the pads along side the original caps.   For the TVSs,  I scraped a bit of mask and soldered them right to the ground plane, near the SMAs. 

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