[SI-LIST] Mixed digital/RF board with switching power supplies

  • From: "Mikhail Matusov" <matusov@xxxxxxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 6 Jul 2007 10:27:50 -0400

Dear experts,

I have designed a mixed digital/RF board, which performs rather well, but we 
want it to perform even better in the next revision. The problem we are 
having is spurious components in the RF chain caused by the high harmonics 
of the DC/DC switching regulators. The regulators run at about 1.2 MHz and 
the band of our interest is roughly 50 to 90 MHz, although obviously, since 
it is a sampling system, much higher harmonics can be aliased into the band 
of interest... The absolute level of the offending components is -94 dBm and 
below, so it is in microvolts, which probably makes it more of an EMI sort 
of problem than a SI one. I believe, based on some experiments we did, that 
the problem is caused by the switching currents travelling in the ground 
planes. The board is quite dense and there is no easy way to divide it into 
an analog and digital sections... I was wondering if someone here has any 
experience in resolving this sort of problems? Also, what would be the best 
book to pick up on the subject? I looked at a few EMI/PCB design books on 
Amazon, but couldn't make my choice as their "look inside" feature is very 
limiting and wouldn't let me see the chapters I wanted... The books I am 
considering are Introduction to Electromagnetic Compatibility by Paul 
Clayton,  Noise Reduction Techniques in Electronic Systems by Henry Ott, EMC 
& and the Printed Circuit Board by Mark Montrose...

Mikhail Matusov
Senior Hardware Design Engineer
Square Peg Communications
Tel.: +1 (613) 271-0044 ext.231
Fax: +1 (613) 271-3007

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