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Affordable <200MHz PDN analysis / impedance measurement hardware?

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tszaboo:

--- Quote from: joeqsmith on November 13, 2024, 02:34:53 am ---
--- Quote from: joeqsmith on November 12, 2024, 11:29:52 pm ---
--- Quote from: tszaboo on November 12, 2024, 10:43:45 pm ---Ah, you want to measure it while on, not like with the VNAs.

--- End quote ---

?

--- End quote ---
You seem to infer that when using a VNA to measure a PDN, you would never power it up.  Obviously this is not correct. 

Attached plots are using the original NanoVNA to look at a PDN with the populated board both unpowered and powered.  Also shown is the same bare board. 

Of course, there are risks.  The blocking caps I made for these measurements will support these low frequencies.  They could pack a punch to the VNA.  I have not had damaged my original NanoVNA when making these measurements but it certainly could happen.    I did however unknowingly damage my H4 with what appeared to be an ESD event last time I used it.  I sacrificed my original NanoVNA to bring it back to life.  These VNAs do not have a lot of protection.

--- End quote ---
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.
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.

nctnico:

--- 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.

joeqsmith:

--- 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

joeqsmith:

--- 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.   

tszaboo:

--- 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.

mod: Note to others, read below, this is not correct.

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