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EMI from SigmaDSP processors

Question asked by KJBob on Jul 25, 2012
Latest reply on May 17, 2013 by KJBob

     Our  project is telephone-related so it needs FCC part 15 and 68 approval.  We have received approval on other products that use only  a PIC microcontroller, but 50 MIPS processors are new to us. Thus I did some very informal EMI measurements taken with a 6" wire draped on top of the circuitry in question, connected to a spectrum analyzer.  All photos are 100 kHz -- 100 MHz.  In this one, the wire is near a PIC microcontroller on our prototype board. As shown, there isn't much going on above the noise floor (about -85 dBm)

 

PICProto.jpg

 

The next one shows when the wire is moved near the ADAU1701 on the same prototype.  As seen, the largest spike is at the MIPS frequency (48 kHz x 1024).:

 

1701Proto.jpg

     This prototype is a double-sided PCB with generous ground planes, however not necessarily following everything in the SigmaDSP PCB Layout Best Practices advice.  For example, we placed the crystal right at the -1701 as we would do with a PIC, which of course is good -- but doing so traded off the closeness of perhaps more critical components.  This and using 0805 caps for easier manufacturing, somewhat compromised our layout and resulting EMI performance.  Compare this photo to the next one, showing the -1701 on its evaluation board as supplied by AD:

 

1701Eval.jpg

     This shows something at the crystal frequency (~12 MHz), but is mostly quieter otherwise.  Our next spin of this board will have 0603 caps; it will be interesting to see how this new layout will affect its EMI results.  BTW -- In my opinion there's far too much EMI in the world.  As a teenager I listened to all kinds of broadcast and amateur transmissions on a three-tube regenerative receiver.  Now there's only buzz, regardless of gear.

 

 

    ***   Does anyone  have experience with gaining FCC Part 15 Class B approval of a product containing Sigma DSP(s)?  How did it go?

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