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From: bill@videovax.UUCP (William K. McFadden)
Newsgroups: net.ham-radio
Subject: Re: Schematics for stereo FM modulator needed
Message-ID: <1448@videovax.UUCP>
Date: Tue, 17-Sep-85 12:09:02 EDT
Article-I.D.: videovax.1448
Posted: Tue Sep 17 12:09:02 1985
Date-Received: Thu, 19-Sep-85 06:26:51 EDT
References: <1237@brl-tgr.ARPA>
Reply-To: bill@videovax.UUCP (William K. McFadden)
Organization: Tektronix, Wilsonville OR
Lines: 75


I tried to send this via mail but it failed.  My apologies to the net for
any inconvenience.
--------------------

I have had some experience designing cheap FM stereo
transmitters.  (I designed and built one as a college senior project.)
Your project sounds interesting, so I hope some of this will be useful.

First, the technique used to modulate FM stereo onto videotape is
different than the one used in broadcasting.  The Beta/VHS Hi-fi systems
use two separate FM carriers, one for each channel.  On the other hand,
FM broadcasting modulates the baseband with mono (L+R) and a 38 KHz
subcarrier with L-R information.  The subcarrier is AM double sideband,
suppresed carrier (DSB-SC), and a 19 KHz pilot tone is needed to
properly demodulate this signal (also provides a way for your receiver
to tell if an incoming signal is in stereo).  To regenerate the L and R
channels, they are mixed in the following way.

	(L+R) + (L-R) = 2L
	(L+R) - (L-R) = 2R

Thus, a simple matrix will decode the signals.  Mono receivers demodulate
only the baseband signal (L+R) and ignore the subcarriers.  Thus, the FM stereo
signal is compatible with both types of receivers. (You may already know
all of this, but I thought I'd include it for completeness.)

You are right that PLL is the way to go.  As part of my project I looked at
several ways to generate a stable carrier and digital frequency synthesis
was by far the best and cheapest.  My design uses a transistor oscillator
with varactor diode tuning to generate the FM carrier.  This is divided
down to ~1 MHz with digital counters and fed into a CD4059 modulo-N counter.
The CD4059 can divide by any number between 3 and 9999 inclusive and is
programmed with with a 16-bit input consisting of four BCD digits.  This
made it easy to use dip switches or thumbwheel switches for tuning.
I am not aware of any versastile divide-by-N counters that operate up to
100 MHz, but there are high speed decade counters that run this fast.
Therefore it was easy to divide the carrier frequency to the slower rate
needed by the 4059.

The phase comparator was a CD4046 PLL chip with the internal VCO disabled
(it only goes to 1 MHz).  The frequency out of the 4059 was 1 KHz, thus
a 1 MHz crystal oscillator followed by a divide-by-1000 counter (MC14559)
generated the reference frequency.

I should stop here before I get carried away.  If you would like to know
more about my design (e.g., schematics), let me know.

There are a couple of companies that offer plans, boards, and kits for
FM stereo transmitters, some of which use PLL synthesis and dipswitch
programming.  I have included their addresses below.

	Panaxis Productions
	P.O. Box 130
	Paradise, CA  95969
	(916) 534-0417
	Contact: Ernie Wilson

	Stellatron     	(This company may not still be in business)
	4942 Whitsett Av. #205
	N. Hollywood, CA 91607
	(213) 506-0415
	Contact: Wayne Slater


					Bill McFadden
					Tektronix, Inc.
					P.O. Box 500
					MS 58-594
					Beaverton, OR  97077
					(503) 627-6920

	Net address:
	...{ucbvax,ihnp4,uw-beaver,decvax}!tektronix!bill@videovax.UUCP