[SI-LIST] Re: Decoupling of Oscillator

Martin,

One of the worst EMI problems I ever worked on resulted from 
circuit two.  The BEAD in the ground really caused problems.  
When it was replaced with a zero ohm jumper things were not 
too much better.

A nice solid ground for the return currents helps so much.

dav0
David Lieby, SI Engineering
Acuson Corporation, a Siemens Company
1230 Shorebird Way
Mountain View, CA 94043

Building E
P.O. Box 7393
Mountain View, Ca 94039-7393
(650)694-5030


Martin Euredjian wrote:
> 
> From: Philip Ross Wellington
> 
> > Some provided Pi filters for the
> > ground but that didn't work well logically because of the return path
> > inductance and Signal Integrity (not even coined back then) induced
> > problems.
> 
> On datasheets I've seen recommended isolation/filtering networks that look
> like this:
> (values chosen at random)
> 
> RAW_VCC -----------BEAD-------------- VCC'
>          |      |          |     |
>          |      |          |     |
>         22uF  0.1uF      0.1uF  22uF
>          |      |          |     |
>          |      |          |     |
> RAW_GND -----------BEAD-------------- GND'
> 
> versus:
> 
> RAW_VCC -----------BEAD-------------- VCC'
>          |      |          |     |
>          |      |          |     |
>         22uF  0.1uF      0.1uF  22uF
>          |      |          |     |
>          |      |          |     |
> RAW_GND ----------------------------- GND'
> 
> Is there any merit to the first approach, say, for a clock, or a device with
> a PLL, or a sensitive analog sub-section?  Both in terms of circuit
> operation and EMI/RFI concerns.
> 
> I've also seen a couple where the beads are replaced with low or 0 ohm
> resistors.
> 
> Thanks,
> 
> ===============================
>  Martin Euredjian
>   eCinema Systems, Inc.
>        voice: 661-305-9320
>        fax:   661-775-4876
>   martin@xxxxxxxxxxxxxx
>   www.ecinemasys.com
> ===============================
> 
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