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Affordable <200MHz PDN analysis / impedance measurement hardware?

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nctnico:

--- Quote from: joeqsmith on November 13, 2024, 12:33:14 pm ---
--- Quote from: nctnico on November 13, 2024, 11:11:05 am ---
--- Quote from: rhb on November 13, 2024, 12:46:38 am ---The directional bridge is the sum of a pair of sinusoids measured at different points.  If you are looking for impedance mismatch it’s handy.  As I understood the question it was directed at locating power supply impedance mismatches.  I may well have misunderstood the question.

--- End quote ---
PDN measurement is not for impedance matching but for measuring the impedance of a power supply to chips (like high speed processors, FPGA and memory) over a wide frequency range. From DC to tens or even hundreds of MHz. High speed logic chips typically have a specification in the design manuals for the required impedance of the power supply versus frequency. So this is something you'll want to verify.

--- End quote ---

Impedance Matching Is Critical,  Steve Sandler

--- End quote ---
Still, using a directional bridge is not the right tool for the job as the impedances are far too low. For those interested, I think this page explains the goals of PDN design pretty well: https://www.signalintegrityjournal.com/blogs/12-fundamentals/post/2108-power-integrity-fundamentals-impedance-vs-frequency In real designs you won't get the curve perfectly flat. And it doesn't have to be; just as long as the impedance stays below the maximum for the circuit to work correctly.

tszaboo:
Let's see, this should explain it.

joeqsmith:

--- Quote from: tszaboo on November 13, 2024, 01:04:20 pm ---
--- Quote from: joeqsmith on November 13, 2024, 01:00:17 pm ---
--- Quote from: tszaboo on November 13, 2024, 10:50:17 am ---VNAs can measure PDN with it powered up. It's a measurement that I never did myself, and not sure what's the setup to do. I can also say, that with the typical low cost VNAs we have access to, it's not useful to make the measurements powered up. The loop response of a power supply is going to be in the KHz region, where these VNAs are not measuring, or the measurements are not accurate enough to measure mOhm.

--- End quote ---

Ok, I think these two statements help me understand.   It seems like you have one particular problem in mind, or a narrow view of what problems  to look for.   There are some very good presentations on-line about proper design.


--- Quote from: tszaboo on November 13, 2024, 10:50:17 am ---I quickly looked at the available info about PDN measurements.
https://www.keysight.com/zz/en/assets/7018-08474/application-notes/5989-5935.pdf

(might need registration to download)
In this document they publish ON, OFF and modelled impedance-frequency graphs for practical PDNs. For example Figure 28. The ON and OFF state measurements only differ in any meaningful way up to 5kHz.
Additionally, for the nano/liteVNA any noise present on the power supply would enter the ADC. From what I recall, mixers have frequency response that passes low frequency noise, and these VNAs don't have any bandpass filters on port 2.
So yeah, I wasn't very precise when suggesting VNAs only test OFF.

--- End quote ---

We are interested in impedance far beyond 5kHz.

--- End quote ---
I don't think you understand (with respect). Measuring a PDN with a VNA, it's going to produce the same graph whether it's ON or OFF above ~5KHz. You cannot measure with a nano/liteVNA below 5KHz. So why would you measure it when it's ON, it's just extra risk for your VNA, plus noise, without any benefits.

--- End quote ---

I refer you to my previous data where the on/off states converge at 400kHz.   
https://www.eevblog.com/forum/testgear/affordable-lt200mhz-pdn-analysis-hardware/msg5712085/#msg5712085

tszaboo:

--- Quote from: joeqsmith on November 13, 2024, 03:21:00 pm ---I refer you to my previous data where the on/off states converge at 400kHz.   
https://www.eevblog.com/forum/testgear/affordable-lt200mhz-pdn-analysis-hardware/msg5712085/#msg5712085

--- End quote ---
Interesting. You are right. I totally missed that.
Do you know what is the DC-DC converter used on that board? It would make sense that Keysight's generic DC-DC module would have a slow control loop, and an FPGA board would have a fast transient response converter.

joeqsmith:

--- Quote from: nctnico on November 13, 2024, 01:09:19 pm ---
--- Quote from: joeqsmith on November 13, 2024, 12:33:14 pm ---
--- Quote from: nctnico on November 13, 2024, 11:11:05 am ---
--- Quote from: rhb on November 13, 2024, 12:46:38 am ---The directional bridge is the sum of a pair of sinusoids measured at different points.  If you are looking for impedance mismatch it’s handy.  As I understood the question it was directed at locating power supply impedance mismatches.  I may well have misunderstood the question.

--- End quote ---
PDN measurement is not for impedance matching but for measuring the impedance of a power supply to chips (like high speed processors, FPGA and memory) over a wide frequency range. From DC to tens or even hundreds of MHz. High speed logic chips typically have a specification in the design manuals for the required impedance of the power supply versus frequency. So this is something you'll want to verify.

--- End quote ---

Impedance Matching Is Critical,  Steve Sandler

--- End quote ---
Still, using a directional bridge is not the right tool for the job as the impedances are far too low. For those interested, I think this page explains the goals of PDN design pretty well: https://www.signalintegrityjournal.com/blogs/12-fundamentals/post/2108-power-integrity-fundamentals-impedance-vs-frequency In real designs you won't get the curve perfectly flat. And it doesn't have to be; just as long as the impedance stays below the maximum for the circuit to work correctly.

--- End quote ---

Yes,  S21 is the better choice.   Having the lowest impedance may not be the best choice.  Discontinuities can cause unwanted radiation.   I had posted a few video links on PDN design and testing back when I was working on the software to measure them with the low cost VNAs.  You can find that here:
 
https://www.eevblog.com/forum/rf-microwave/impedance-measurement-with-vna-using-series-shuntseries-through-methods/msg3556953/#msg3556953

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