Products > Test Equipment
Affordable <200MHz PDN analysis / impedance measurement hardware?
joeqsmith:
--- Quote from: tszaboo on November 13, 2024, 01:04:20 pm ---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 wanted to get some data before responding to this comment. Shown is my NanoVNA H4 Rev 4.2. Again, it was damaged and repaired. During that time, I had added 5 capacitors which greatly improved the high frequency performance but not the low. I suspect it would be on par with another virgin H4.
Data was taken with the current released firmware which supports a lower limit of 1.6kHz. VNA was swept from 1.6k to 10kHz, 401 points, 100Hz IFBW.
Looking at S21, after normalize and terminated, H4_1 showing dynamic range. It's at least 70dB below 5kHz (not K or Kelvin BTW).
I purchased a cheap 40dB attenuator for another test. With a 70dB dynamic range we should be able to measure it without any problem. Results can be seen in H4_2.
Also showing the setup.
joeqsmith:
--- Quote from: tszaboo on November 13, 2024, 01:04:20 pm --- nano/liteVNA
--- End quote ---
Over the years, I have attempted to explain that below 300MHz, the original NanoVNA can often out perform the V2Plus/4 and LiteVNA/64. See my previous post:
https://www.eevblog.com/forum/testgear/affordable-lt200mhz-pdn-analysis-hardware/msg5670833/#msg5670833
I have seen people post about their disappointment after they purchased one of these higher frequency VNAs and attempt to use them for narrow band measurements. Even if you crank down the IFBW, take a day per sweep rather than seconds, it will still produce artifacts that we can't address. I've always said the fix, get the original NanoVNA.
OP was using an H4. The video I linked used my original NanoVNA. These are both much better suited for PDN measurements. You bring up the Lite though. Using the latest 3.2 hardware and released firmware, 100Hz IFBW, 801 points, normalized. Swept from 1.6kHz to 50kHz with ports terminated. Note the poor dynamic range compared with my H4. All of my LiteVNA64s are also very non-linear once you get below about 25kHz. I never use them this low so it's not been a problem for me.
Hopefully this clears up some of the misconceptions about these low cost VNAs.
mawyatt:
--- Quote from: inevitableavoidance on October 28, 2024, 02:57:41 pm ---I ended up designing a little adapter board for the Analog Discovery series!
Here's the board measuring a low impedance capacitor array:
(Attachment Link)
(Attachment Link)
Here's a bunch of verification resistors (with varying inductances):
(Attachment Link)
I realized I didn't need to measure higher than ~25MHz - once I see the inductance slope at the end I'll known enough about what happens afterwards. The board has a 200mA output current buffer, and two fully differential opamps. This gives me both more current and enough resolution for all the low stuff. The bottom limit is about 100uΩ. :)
The flat tinned bars are spaceholders for tantalum capacitors, to be able to measure with a DC Bias as well. Works great so far.
Thanks for all the tips along the way!
--- End quote ---
Took a better look at your schematic and have a question.
The buffer amp U1 is powered from +-5V and the sense amps U2 and U3 are from +-2.5V. How does the current sense differential amp U3 deal with the voltage across current sense resistor R2 which is directly driven by the output from U1?
Seems this might exceed the Common Mode range for the inputs to U3 which the Data Sheet shows +Vs -1.2V and -Vs -0.2V which would indicate an input range of +1.3 to -2.7V with +- 2.5V supplies?
Best
rhb:
--- Quote from: joeqsmith on November 13, 2024, 02:14:02 am ---He seems to be asking why you need the bridge to measure S21. I took it that you feel you can make these measurements with S11, possible not understanding the errors that go along with it for this particular application.
--- End quote ---
I was only responding to "my VNA won't go to 10 Hz and I can't afford a 4395A".
I got super lucky as shown by the screen shot. The seller didn't know what it was and wanted it gone. They were in the business computer business. So far as I can tell it works flawlessly. And I've used it a lot as a scalar network analyzer evaluating combinations of EMI filters.
VNAs have directional bridges in them. I was simply pointing out that you don't need a VNA. A signal generator, DSO and understanding the problem will solve it.
To the frequency limits of the DSO, analog log amplifier probes and some software are capable of providing full VNA capability from DC to the limit of the DSO you currently have. Any that will write data to an SD card will do.
I bought a cheap 12 bit 100 MHz Owon because it has very deep memory, 20 MPt IIRC. Sucks as a scope but is great for data capture. A perfect square wave with fast rise times will generate harmonics of very high order. Choose the fundamental based on the sample interval in frequency and use a sub ns rise time 7400 series divide by two output. Record it open circuit and then measure at various points. The processing understanding is a one time effort and valuable skill to boot.
That said, my favorite tool for impedance discontinuity tests is a Tek 11801 and SD-24, but that is rather exotic and hard to come by. But it does show the precise (<0.1 mm) relative location of *every* impedance change. Just not well suited to a use case where an active device is at the other end of the line.
As should be clear from the various links, this is NOT a simple problem to solve even with a large sack of money to apply as a hammer.
After rereading the OP a few times I should start with a DSO and a very small DIY H field probe to isolate which segments of an actual PDN have issues as a first step. Small loop of the smallest coax you can find. Preferably semi-rigid. You'll be able to directly detect what parts of the PCB have problems.
Then use an FFT to get a frequency domain view to give a sense of scale to the parasitics of concern. Build the fixtures in the Keysight App note and measure them.
A differential DC log amp probe with a high input impedance should overcome the DSO dynamic range limitation. In principle, one should be able to probe two nearby points on a trace to measure the current flow changes. We all know that there is a voltage change as the current varies, but whether that's actually practical as a measurement technique for this use case is another story. I suspect that careful construction would make it viable as a punch through the solder mask probe. But someone will need to try that out to know for sure. Almost anything else has ground loop issues to be addressed.
Have Fun!
Reg
inevitableavoidance:
--- Quote from: mawyatt on November 15, 2024, 02:45:17 pm ---
--- Quote from: inevitableavoidance on October 28, 2024, 02:57:41 pm ---I ended up designing a little adapter board for the Analog Discovery series!
Here's the board measuring a low impedance capacitor array:
(Attachment Link)
(Attachment Link)
Here's a bunch of verification resistors (with varying inductances):
(Attachment Link)
I realized I didn't need to measure higher than ~25MHz - once I see the inductance slope at the end I'll known enough about what happens afterwards. The board has a 200mA output current buffer, and two fully differential opamps. This gives me both more current and enough resolution for all the low stuff. The bottom limit is about 100uΩ. :)
The flat tinned bars are spaceholders for tantalum capacitors, to be able to measure with a DC Bias as well. Works great so far.
Thanks for all the tips along the way!
--- End quote ---
Took a better look at your schematic and have a question.
The buffer amp U1 is powered from +-5V and the sense amps U2 and U3 are from +-2.5V. How does the current sense differential amp U3 deal with the voltage across current sense resistor R2 which is directly driven by the output from U1?
Seems this might exceed the Common Mode range for the inputs to U3 which the Data Sheet shows +Vs -1.2V and -Vs -0.2V which would indicate an input range of +1.3 to -2.7V with +- 2.5V supplies?
Best
--- End quote ---
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. :)
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