PDN Measurements Video

fre jun 24, 2011 (Axcon)

How do you do validate bypass capacitor selection and effectiveness on the mounted PC board?

This is very easy to do, and this video was created to illustrate how this is done in the lab with a Rohde & Schwarz network analyzer. It used to be a picture was worth a thousand words, but these days it’s a video.

The two boards used in the video are actual designs, one mounted with capacitors only and one bare. For the purpose of demonstrations both boards have been equipped with SMA connectors, so the measurements can be done easily with both hands free.

Simpler methods for probing exists, and that may be a topic for another post. Usually probing it not a big issue, but obviously it’s easier when the boards have been prepared for probing.

In reality the measurements work just as well on a board with all parts mounted, where you have the benefit of also measuring the capacitance on chip and package.



The frequency range used in this demonstration is 9 kHz to 1 GHz, with as low a measurement bandwidth as you can bear waiting for (lower bandwidth, longer sweep). A nice thing about this analyzer is that it can do a logarithmic sweep and display the results that way as well. This is exactly how we want to see the results.

When measuring the S21 insertion loss, this comes out in dB. As shown, this is converted to Ohms by the simple formula:

|Z| = 25 \Omega \times 10 ^{\frac{dB}{20}}

Was this useful? The focus here is on content rather than video/sound quality – is this the right trade-off? Please use the comment field below.

Kommentarer (2)

  1. henning larsen added on 18. juli 2011

    Video’en er fin kvalitet til at illustrere netop dette setup og måleresultat, samt hvor meget afkobling faktisk betyder for impedansen, derimod mangler jeg noget mere information om
    .) hvor proberne er koblet til PDN (lille diagram)
    .) hvorfor det er netop 25ohm i formlen (lille diagram)
    .) hvor man skal tilkoble prober for at få den bedste måling.

    vh henning

  2. Rolf added on 18. juli 2011

    Gode spørgsmål – det kan måske blive til et par videoer mere :-)

    1) Det er ikke så afgørende hvor proberne er koblet til – så længe vi taler om de lavere frekvenser, hvor de diskrete afkoblingskondensatorer dominerer.

    2) De 25 ohm i formlen overlades som en øvelse til den interesserede læser :-) Eller duk op til et af mine kurser i SI…

    3) Hvor man tilkobler proberne er som nævnt ikke så afgørende, men man kan med fordel forberede printet så man ikke behøver at lodde.

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