Ramsey Electronics Model No. SCA1
• Tunes entire SCA subcarrier band 50 - 100 KHz
• Excellent performance - uses PLL decoder
• Feature both speaker and line-level audio outputs
• Connects to any FM receiver or simply plugs into our
FR-1 FM broadcast receiver
• Band-pass filtered input for excellent fidelity
• Front panel volume and tuning controls
• Informative manual answers questions on
theory, hook-ups, and uses - enhances resale
value , too!
• Add our case set for a finished ‘Pro’
look. Cases match all Ramsey products
• Clear, concise assembly instructions carefully guide you
to a finished kit that works the FIRST time!
SCA FM SUBCARRIER
DECODER KIT
Commercial-free music, stock quotes, news - all available
around-the-clock on your local FM station? Sure, buried in a
special subcarrier that you’ll be able to tune with the SCA-1.
This is how most restaurants, department stores, and hotels
pick up their music.
SCA1 • 2
RAMSEY TRANSMITTER KITS
• FM100B Professional FM Stereo Transmitter
• FM25B Synthesized Stereo Transmitter
• AM1, AM25 AM Transmitters
• TV6 Television Transmitter
RAMSEY RECEIVER KITS
• FR1 FM Broadcast Receiver
• AR1 Aircraft Band Receiver
• SR2 Shortwave Receiver
• AA7 Active Antenna
• SC1 Shortwave Converter
RAMSEY HOBBY KITS
• SG7 Personal Speed Radar
• SS70A Speech Scrambler
• SP1 Speakerphone
• WCT20 Wizard Cable Tracer
• PH10 Peak hold Meter
• LC1 Inductance-Capacitance Meter
RAMSEY AMATEUR RADIO KITS
• DDF1 Doppler Direction Finder
• HR Series HF All Mode Receivers
• QRP Series HF CW Transmitters
• CW7 CW Keyer
• CPO3 Code Practice Oscillator
• QRP Power Amplifiers
RAMSEY MINI-KITS
Many other kits are available for hobby, school, Scouts and just plain FUN. New
kits are always under development. Write or call for our free Ramsey catalog.
SCA1 FM AUDIO SUBCARRIER DEMODULATOR KIT INSTRUCTION MANUAL
Ramsey Electronics publication No. MSCA1 Revision 1.2a
COPYRIGHT
1994 by Ramsey Electronics, Inc. 590 Fishers Station Drive, Victor, New York
14564. All rights reserved. No portion of this publication may be copied or duplicated without the
written permission of Ramsey Electronics, Inc. Printed in the United States of America.
SCA1 • 3
Ramsey Publication No. MSCA1
Price $5.00
TABLE OF CONTENTS
Introduction to the SCA .................. 4
FM and subcarrier background ...... 4
Circuit description .......................... 6
Parts list ......................................... 7
Parts layout diagram ...................... 9
Schematic diagram ...................... 10
Assembly instructions .................. 11
Set-up and testing ........................ 15
Hook-up details ............................ 16
Troubleshooting guide ................. 17
Enclosure ideas ........................... 18
Ramsey kit warranty .................... 19
KIT ASSEMBLY
AND INSTRUCTION MANUAL FOR
SCA1 FM AUDIO
SUBCARRIER
DEMODULATOR
SCA1 • 4
INTRODUCTION
The Ramsey Subcarrier Adapter kit allows you to hear the commercial free
background music that is broadcast along with some commercial FM signals.
This is accomplished by connecting the SCA1 to a standard FM broadcast
receiver tuned to the local FM station which is providing the subcarrier
service. The SCA1 then acts as a second receiver tuning in subcarrier
signals that are modulated along with the regular FM broadcast
programming. Be aware that not all cities have FM stations that transmit
SCA subcarrier signals, but if any are there, the SCA1 will find them!
While this kit can be easily assembled using our step by step instructions,
the actual connection to a FM receiver can be quite complicated and should
only be attempted by a qualified technician. This interconnection will be
described later in more detail. If you decide not to delve into your existing FM
receiver, we offer an FM broadcast band receiver with SCA baseband
demodulator output (model FR-1) which allows an easy, one step hookup for
the adapter.
Due to the myriad of different receivers and possible combinations that can
be encountered, Ramsey regrets that it cannot assume any responsibility for
connection to your FM receiver, or to provide any more specific information
than that which is provided in this manual.
BUILDING YOUR SCA1 KIT
There are over 100 solder connections on the SCA1 printed circuit board.
That means your work could be 99% perfect and you could still have 1 or 2
poor solder joints or solder bridges. Please take us seriously when we say
that good soldering technique is essential to the proper operation of your
subcarrier demodulator. Here are a few tips for a trouble-free assembly:
• Use a 25-50 watt soldering pencil with a clean tip.
• Use only rosin core solder intended for electronic use.
• Use bright lighting. A magnifying lamp or bench-style magnifier
may be helpful.
• Do your work in stages, taking breaks to check your work.
• Carefully brush away wire cuttings so they don't lodge between
solder connections.
SCA1 • 5
FM BROADCASTING AND SUBCARRIERS - SOME BACKGROUND
Before we explain how the SCA decoder works, we first need to know what it is
"looking" for in the received FM signal. (A note to newcomers: don't let this
description scare you. Rest assured that your SCA decoder will perform the
desired task even if you don't fully grasp all the theory that is thrown at you
in the following passages).
Let's discuss what's happening in the Frequency Modulated signal coming from
a commercial transmitter. Look at the diagram above (referred to in
engineering circles as a spectral diagram) to help you understand what we're
about to describe. The signal contains a Left+Right FM modulated carrier which is the standard monaural audio signal - and a Left-Right signal which is
used to provide the stereo signal. Follow closely to see how we get separate
Left and Right stereo audio channels, and its not done with mirrors - only
simple algebra!
First, let's see what we have to deal with:
1. A Left+Right signal (called the mono audio channel)
2. A Left-Right signal (called the difference audio channel).
Now let's apply a little algebraic addition:
(L+R) + (L-R) = L+L+R-R = 2L (the R terms cancel)
The Left audio channel.
Now, how about a little algebraic subtraction:
(L+R) - (L-R) = L-L+R-(-R) = 2R (the L terms cancel)
The Right audio channel.
Now, the number "2" in 2R and 2L simply means that the signal is twice the
amplitude (twice as loud) and should not be confused with interpreting it as two
left or right channels.
SCA1 • 6
Well, you may say, that works out nicely, but why not just transmit a separate
Left and separate Right channel, and avoid the math? There are a couple of
reasons why: One, to receive on a Mono receiver, the receiver would have to
detect and demodulate both Left and Right signals and combine them for the
L+R mono channel. The extra cost of adding another demodulator would
unnecessarily raise the cost of simple mono receivers. And, two: signal
bandwidth is limited; there is not enough "room" for an independent (Left Right) carrier. In fact, to conserve bandwidth even more, the stereo subcarrier
is transmitted in Double-Side-Band-Suppressed-Carrier (DSBSC) mode. This
signal is located 38 KHz away from the center frequency that the receiver is
tuned to. To recover this audio, a 19 KHz subcarrier (called the "pilot") is
transmitted. In the receiver, this pilot signal is doubled to 38 KHz and is used to
properly demodulate the Left - Right signal.
Have you ever noticed that no matter how expensive the FM receiver is, the top
end of the frequency response is always less than 19 KHz? If it wasn't, while
sitting back enjoying your Hi-Fi, you'd be able to smell your tweeters burning up
as they tried to reproduce the constant 19KHz pilot signal! In addition to all this
activity, there may be another subcarrier, usually 67 KHz away from the center
frequency, containing completely independent audio! Normally your receiver
would effectively filter out this signal, but this is the subcarrier that we are
interested in, and is what our kit is designed to tune in and demodulate.
Why all this talk about stereo and its subcarriers? To give you a little familiarity
with subcarriers and to let you know that there's more out there than expected!
SCA1 CIRCUIT DESCRIPTION
The input is bandpass filtered using L1, L2, and associated components to
isolate the higher frequency subcarriers. The filtered signal is then amplified by
Q1 and applied to the PLL (Phase-Locked-Loop) detector IC chip. The LM565
Phase Lock Loop IC "tracks" the difference between the input signal and its
own internal reference oscillator. This tracking voltage is the recovered audio,
and is low passed by R8-10 and C12-14. A low level audio output is supplied
for an external audio amplifier, or a speaker may be used directly via the
amplified audio output at J3.
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