Power supply noise frequency

Feb 14, 2011 3 Replies
M

Is there a way to estimate/measure power supply noise frequency? or measuring it with an oscilloscope is the only way?



For example, the processor in the board I am looking at has an external 24MHz crystal, on-chip 400MHz PLL driving CPU clock, 480 mbps USB bus, extenal memory interface running at 133mhz, SPI bus running at 25MHz and several GPIOs. I would like to know the possible digital power supply noise frequencies for this board.



or is it greatly dependent on switching of internal gates, bypass capacitors and board layout and hence cannot be estimated with mere clock frequencies.



Thanks in advance. mjnk


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F

Mjnk-

There may be noise associated with logic switching, but a power supply can have its own switching frequency with harmonics across the spectrum.

Fred

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R

"HF" noise from power lines is theoretically 3rd, 5th, 7th harmonics with capacitive and corona-based coupling putting emphasis on the higher frequencies to maybe 100KHz. Now, it seems you are interested in noise from _internal_ device supplies, as well as any other bazz-fazz (extremely technical term) from "stuff" on the board. Well, a small loop antenna or capacitive antenna to a spectrum analyzer would seem to be a better "fit" for seeing the junk,but a scope is OK as well. If the board app is having problems, it is possible that it is generating enough RFI to goof itself up. If this is the case, and nearby radios, etc are going nuts, the PCB layout is piss poor to say the least and it is a wonder that it works at all.

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T

If you're looking at frequencies, a spectrum analyzer it the tool.

Tauno Voipio
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