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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: DC motor noise in audio circuit
Summary: Wire routing is critical
Message-ID: <6706@milton.u.washington.edu>
Date: 24 Aug 90 21:17:55 GMT
References: <15857@s.ms.uky.edu>
Distribution: na
Organization: University of Washington, Seattle
Lines: 24

In article <15857@s.ms.uky.edu> tindle@ms.uky.edu (Ken Tindle) writes:
>I'm working on an audio project and have run into a nasty problem.
>
>I'm using some cassette mechanisms with a circuit of my design in a
>language learning system, and have magnetic coupling of noise to the
>ground plane.
>
	First, a sleeve around the motor will reduce the magnetic
emissions considerably (most hi-fi cassette motors have a couple
of deep-drawn soft steel covers that accomplish this).  Secondly,
the position of wires and the orientation of circuit boards can be changed.
It is not unusual for an audio component to go back to the manufacturer's
bench a few times before the wiring is dressed so that the stray
hum pickup meets the noise specification.
	Lastly, DON'T use a ground plane for low frequency signals (and
if you're not in the hundreds of megahertz, it's low frequency.)
Instead, try to use single-point grounding as much as possible.
The energy in the magnetic field is MAXIMALLY coupled into a low 
impedance path (like a thin ground plane).  Keep any shielded
covers connected to ground at EXACTLY ONE point.  It's only at 
high frequencies that wires fail to conduct and a ground plane
becomes the only way to distribute your ground.

	John Whitmore