What would change if you move the PDN measurement points when you measure the PDN using a network analyzer? Ever since I did the video on how to use a network analyzer to measure the resulting impedance of all the bypass capacitors on a board, this question keeps coming up.
One could speculate that two locations close to each other would yield a much different result than two locations far from each other. But what better is there than to test this out?
To recap the method used to measure a power distribution network (PDN) on a board with fairly solid power and ground planes, what we do is basically an S21 measurement. The signal is injected from the 50R generator in the network analyzer at one point and measured at another point. By doing this as a 4-point measurement, the effect of the connection resistance is minimized.
First PDN measurement points
Here is a client board prepared for the measurement with the two coax leads far away from each other on the board:
SecondPDN measurement points
And here is the reference measurement setup where one of the coax leads has been moved so the two points now are very close to each other:
Notice how the one coax cable is moved. If you look really carefully, you may also notice a little change I had to do on the board since this board was not prepared for my probes. Two bigger caps were moved to sit on top of some other similar caps, so their footprints could be used to attach the cables. And when the board was changed from one placement of the coax cables to another placement, such two caps were moved as well. You can see how that looks here (sorry for my sloppy soldering):
So how much of a difference did this make on the measurement results?
Well not much really if you look at the frequencies of interest up to around 100 MHz, where other effects like board plane resonances start to dominate. Here is the plot to show this:
Notice that not only did the PDN measurement points change – two big capacitors were moved as well.
Be aware that this result would have been much different if the board was not designed with fairly solid power and ground planes. The ground plane covers most of the board, and there is a nice good area of the board for the power plane as well. More on what happens if this is not the case later…
If you want to learn more about PDN design, there are plenty of opportunities in the courses we arrange. See the course calendar to find the best option for you or contact us to arrange an on-site course.
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