Measuring noise

Thread Starter

kubeek

Joined Sep 20, 2005
5,794
I recently came across an article written by @Mark Hughes which talks a bit about measuring noise on a PCB. https://www.allaboutcircuits.com/te...ization-custom-PCB-inclinometer-data-capture/
This article inspired a few questions about how to properly measure noise.

When I was measuring noise of a 10V SMPS, I used a piece of coax soldered directly across the output cap, and straight into the 1Meg input of oscilloscope. I was interested in mainly low frequency noise, so I didn´t bother with reflections etc.
However if I were to measure the case in the article, where the noisy blip is in the 10s of MHz frequency range, what would be the proper way to do it? I would think that a 50ohm resistor in series with the coax, and 50 ohm termination in the scope would be the way to go? But the voltage cannot be too high, as the scope has 8Vrms limit on the input. So I would need an DC blocking capacitor in series.

Now if I wanted to measure the whole range with a spectrum analyser, say 0.1Hz to 10MHz, I would need a rather huge capacitor to get the low end response. Is this the normal way such measurements are done?
 

Mark Hughes

Joined Jun 14, 2016
409
@kubeek,
Coax soldered to the board is a time-honored tradition in labs across the world. It would of course be better to have a proper tap so the impedance is constant, but sometimes you have to make due.
What is the expected amplitude of the noise? I'm not entirely certain that I understand the problem at play. Are you concerned that your test setup is affecting the readings? What is it you're trying to determine?
Thanks!
Mark
 
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