Products > Test Equipment
Affordable <200MHz PDN analysis / impedance measurement hardware?
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.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version