From: John Higdon <apple!zygot!john@decwrl.dec.com>
Subject: Re: Praise the Lord and Pass the RF Filters
Date: 27 Jun 89 08:34:27 GMT
Organization: ATI Wares Team

In article <telecom-v09i0211m07@vector.dallas.tx.us>, goldstein@delni.enet.
dec.com (Fred R. Goldstein dtn226-7388) writes:
> [description of FM station being heard in all kinds of devices]
> But an FM station?  Not so fast.  FM transmitters maintain a constant
> amplitude, so if you "detect" them with a simple rectifier, you won't
> get anything but perhaps a steady DC voltage.  (It'll light a
> fluorescent tube, though, if you're close enough.)  Telephones won't
> detect FM the way they detect AM.  I've noticed a faint hum on phone
> lines caused by FM blanketing, but not the actual modulation.

FM can easily be heard on non-FM receiving devices by one of two
methods. The first is called "slope detection" and depends on the
device doing the detecting to have a slight frequency dependent
characterisic. As the frequency changes with modulation, so does the
voltage produced by detection and voila! you have audio detection of
the signal.

A more common occurance involves "synchronous amplitude modulation". If
an FM transmitter is not perfectly tuned, its output (amplitude) will
vary with normal modulation. The FCC requires broadcasters to maintain
this AM component at least 50 db below carrier level, but even at that,
given sufficient signal you will easily hear the sermon. From what I've
read here, these people might have more synchronous AM than the FCC
allows.

In any event, I have frequently had to exorcise FM-induced audio out of
telephone lines. Not uncommon at all.
-- 
        John Higdon         |   P. O. Box 7648   |   +1 408 723 1395
      john@zygot.uucp       | San Jose, CA 95150 |       M o o !