Q&A
with TJ Byers
Electronics Q&A
In this column, I answer
questions about all aspects
of electronics, including
computer hardware,
software, circuits, electronic
theory, troubleshooting, and
anything else of interest to
the hobbyist.
Feel free to participate
with your questions,
SCA ... On the Air
Q. Lately, I’ve been experimenting
with SCA adapters: a kit from a
1960 Popular Electronics article, one
from another magazine article, and a
kit from Ramsey. I have found that they
all work with some receivers and none
will work with other receivers —
receivers that, to me, appear to be quite
similar. I don’t have a lot of FM test
gear and can’t seem to put my finger
on the difference. Any pointers? How
about an SCA adapter schematic?
David Herman
Laurel, MT
comments and suggestions.
You can reach me at:
TJBYERS@aol.com
What's Up:
E
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NUTS & VOLTS
F
o
r
A bunch of magic. I
can make FM carriers
disappear, colored lights
appear, and create a
magic wand under the
hood of your car. Well ...
almost. What I really do
is describe FM transmis-
sions, build a magic box
and show you how to
gauge the power in your
RC battery package.
A reader has a unique
E
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r
y
t
h
i
n
g
test instrument, too.
A. Let’s first define SCA —
Subsidiary Communications
Authorization. The FM channel is 150
kHz wide (± 75 kHz). Within this bandwidth, the broadcast station inserts a
mono and a stereo signal, with appropriate decoding tones at 19 kHz and
38 kHz (which is actually a suppressed
carrier). This leaves about half the
bandwidth free for something else.
FM stations are allowed to place
other subcarriers in the unused portion
of the spectrum to transmit other information, like subscriber background
music, foreign language services, or
stock market quotes. Usually there are
two subcarriers, one at 67 kHz and one
at 92 kHz (Figure 1). Data transmissions sometimes use 71 kHz, although
other frequencies may be used, too.
The subcarriers are limited only by the
bandwidth of the allotted frequency
slot. To receive SCA transmissions,
you need an FM receiver and an SCA
decoder. Any FM receiver will do. It
doesn’t have to be stereo. However —
and this is a big however — the SCA
signal won’t be available if the radio’s
audio output is filtered to cut out frequencies above 15 kHz (common in
cheap receivers). If this is the case, you
have to open up the radio and tap off
the audio before filtering — preferably
just after the detector.
Most SCA decoders today use a
phase-locked loop (PLL), like the
LM565. A very simple SCA decoder
built around a LM565 (then the
SE565) appeared in a Signetics
Figure 1
12
SEPTEMBER 2005
