RE: [SI-LIST] : Copper pour beneath oscillators,

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From: Ken Cantrell ([email protected])
Date: Wed Jun 06 2001 - 08:00:52 PDT

I didn't mention signal trace isolation (thanks for reminding me Larry), the
area underneath the top pour contains no traces between itself and the
ground plane it is stitched to. The "island" itself is dimensioned slightly
larger than the oscillator(s) to defeat fringe capacitance coupling, and is
not resonant in its dimensions. The oscillator trace(s) are striplined,
with extra spacing, to their destination(s), which are made as short as
possible. This differs slightly from the classic moat or bridge, single
point ground method. I tried it because multi-point grounding provides a
lower inductance path for the RF noise.
-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Larry Miller
Sent: Wednesday, June 06, 2001 8:01 AM
To: 'Shoaib'; [email protected]
Subject: RE: [SI-LIST] : Copper pour beneath oscillators,

Your designer probably wants a moat (gap) around the copper under the
oscillator. This is often recommended by the oscillator manufacturers.

The idea is to attach the moat to the surrounding ground plane only at the
point where the power and signal traces cross over into the "outside" world.
This gives a controlled ground reference for the oscillator (which usually
has a substantial metal enclosure) which is free from stray currents caused
by adjacent circuitry (your designer has it right).

If you leave the copper off altogether you have to keep other traces a long
distance away to avoid affecting the oscillator.

Note that the copper island under the oscillator is not floating! It is
attached to ground at only one point so that no currents can flow in or
across the island due to outside circuit activity.

As usual, Howard Johnson's book discusses this, I believe.

Larry Miller

  -----Original Message-----
  From: Shoaib [mailto:[email protected]]
  Sent: Monday, June 04, 2001 10:24 PM
  To: [email protected]
  Subject: [SI-LIST] : Copper pour beneath oscillators,

       I'm a novice and my question might sound stupid but nontheless its a
question to me. I'm desiging a board with 20 Mhz clock oscillators on it, i
have put copper pour on the vacant areas of the board and connected the
copper to the ground (common to all components), this copper also extends
beneath the oscillators. The designer of the circuit is not happy with it
and says that it will inflict all the noise generated by other components
onto the oscillators output and rather he suggests that copper beneath the
oscillators should remain unconnected this way any noise inflicted on or by
the oscillator will remain isolated and not cause any problems. Can anyone
please explain as it is a little confusing for me.
   Thanx in advance,

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