markz@ssc.UUCP (Mark Zenier) (08/21/90)
Here is a description of an SCA decoder using the Motorola Mc3359
Low Power Narrowband FM IF integrated circuit.
The output is from a set of awk scripts and shell procs I use
for creating wirewrap lists.
Mark Zenier
markz@ssc.uucp
------------------------
What is SCA
SCA or Subsidiary Communications Authorization is a method
of piggybacking signals on the normal audio programming from FM
Broadcast stations. Normally, the audio program occupies
the frequencies (after FM detection) from 0 to 15 Khz, there
is a 19 Khz pilot tone for stereo, then the Double Sideband
Supressed Carrier Stereo Difference signal takes 38 Khz +/- 15 Khz.
There is room above all that for other signals, using the range
60 to around 100 Khz. About the only limitation the FCC places
on the signal is how much the SCA signals modulate the main RF
carrier. Monophonic FM stations can use subcarriers in the range
of 20 to 100 Khz.
The usual audio signals are narrowband FM subcarriers with a
deviation of +/- 5 Khz. Common subcarrier frequencies are 67, 75, and
92 Khz.
There are also FSK and PSK data subcarriers for such uses as
stock market tickertapes and paging.
Content, Property Rights and ECPA
The signals can be used for whatever the station can
get money for. Talking newspapers for the blind, "Muzak", and
ethnic programming are signals I've monitored in my area.
These are not considered public broadcasts, and are covered
under some form of copyright or performance right. There is some
question that the Electronic Communications Privacy Act may disallow
listening to SCA signals. So don't try to go into competition with
the program providers, because they are the ones collecting the
subscription payments and the law is on their side.
Theory of Operation
The Mc3359 is one of several similar chips designed to
be the second IF for a narrowband FM receiver. It has a local
oscillator, a balanced mixer, a limiting IF amplifier, a FM
quadrature detector. Also included is an OP amp, and a squelch/
mute circuit which is not used. It is documented in Motorola's
Linear and Interface Integrated Circuits databook (DL128).
The specifications state that it produces a useful audio signal
with and input of around 10 microvolts. (Quite a change from
logic thresholds for this digital hacker.)
In this case, the signal is obtained from the output of the FM
detector inside a FM tuner. The input resistor (R1) is large to prevent
loading down the tuner's circuitry. Then a parallel LC tuned circuit
(C1,C2,L1) selects the desired subcarrier. The Local Osc. frequency
is set by L2, C4 and C5. This should be in the range of 388 (455 - 67)
Khz to 363 (455 - 92) Khz. The output of the mixer is filtered by a
three terminal ceramic bandpass filter, X1, (Not a ceramic resonater
used for oscillators). The limiter input (the output of the filter) is the
pin most senstive to spurious signal pickup. Keep that wire short.
The limiter output is processed through a quadrature detector, a balanced
mixer combined with a tuned circuit (L3 with an internal capacitor),
to produce the audio frequency signal, deemphasized and filtered by
R3, c11, c12, r4, etc.
Parts
First try the nearest Motorola distributor. I got the IC
at a local (Seattle area) retailer called Supertronix. (Sort of
a local "Radio Shack on Steroids" with a large selection
(206) 251-8484 ).
It's also in the NTE (and ECG) inventory as a NTE-860 ($$$ warning).
A Mc3357 is similar, but requires more external parts.
The inductors and bandpass filter are available from DIGI-KEY.
I haven't yet tried the Toko 154ANS-T1020Z for L2, should work. ;-)
The J. W. Miller 9001 is a big ugly wide range thing.
Adjustment
First, power the circuit up with no input signal, 4 to
9 volts, 3 mA. Hook the audio output up to your amplifer (about
.5 V signal). With a DC voltmeter or 'scope across C11, adjust L3
across it's range. At one end, the voltage should be about .5 V DC,
at the other end the voltage should be about 1 volt below the power
voltage. The transition should be abrupt. In the transition area,
adjust L3 so the voltage is halfway between the two extremes. This
should be the point for the loudest output of random noise.
If you have a frequency counter, scope, or LF receiver, look
at pin 2 and adjust L2 for the subcarrier you want to try first. 388
Khz is a good bet. With L3 adjusted, it really isn't that critical.
Hook the input to the FM detector output inside your FM receiver/tuner.
(You'll have to figure this out yourself), and tune to the station
with a SCA subcarrier. Quality of reception is important. If the
normal broadcast audio isn't clean, the subcarriers will be drown out
by noise and distortion. Tune L2, when a signal is there, you'll get
a quiet output, or hear the music.
Even with L1 misadjusted, the the chip is sensitive enough to pick the
subcarrier up. Adjust L1 for the lowest extraneous noise.
Alternatively, you can hook an audio oscillator to the input, instead of
the FM receiver, set to 67 (or whatever) Khz, .1 to .5 Volt, look at pin 18,
and tune L1 for a peak AC voltage.
Performance
There is still a lot of crud in the audio, but it's good enough to tell
you what's out there.
scad.net
location device function
c01 capacitor 3300 pf c1+c2 = 6000 pf
c02 capacitor 2700 pf
c03 capacitor 4700 pf input couple
c04 capacitor 3300 pf
c05 capacitor 4700 pf
c06 capacitor limiter bypass .1 uF
c07 capacitor limiter bypass .1 uF
c08 capacitor decoder bypass 100 pf
c09 capacitor power bypass .1 uF
c10 capacitor power bypass 100 uF 25v
c11 capacitor deemphasis .02 uF
c12 capacitor output couple .01 uF
c13 capacitor output couple .01 uF
ic1 mc3359 mixer if fm detector
j1 connector signal input
j2 connector audio output
j3 connector power
l1 inductor5 .68 mH toko RMC-2A6597HM
l2 inductor 80 - 90 uH miller 9001 or toko 154ANS-T1020Z
l3 inductor5 quadrature coil toko RMC-2A6597HM
r1 resistor input resistor 100k
r2 resistor quadrature deQ 68k
r3 resistor deemphasis 7.5 kohm
r4 trimpot output volume 10 kohm
x1 bandpass 455 kHz 10 khz toko HCFM2-455D
-----------------------------------------------------------------------
location c01
device capacitor capacitor
function 3300 pf c1+c2 = 6000 pf
pin function net
01 plus infilter
02 minus gnd
location c02
device capacitor capacitor
function 2700 pf
pin function net
01 plus infilter
02 minus gnd
location c03
device capacitor capacitor
function 4700 pf input couple
pin function net
01 plus infilter
02 minus rfin
location c04
device capacitor capacitor
function 3300 pf
pin function net
01 plus oscbase
02 minus oscemitter
location c05
device capacitor capacitor
function 4700 pf
pin function net
01 plus vcc
02 minus oscemitter
location c06
device capacitor capacitor
function limiter bypass .1 uF
pin function net
01 plus pin6
02 minus pin7
location c07
device capacitor capacitor
function limiter bypass .1 uF
pin function net
01 plus vcc
02 minus pin7
location c08
device capacitor capacitor
function decoder bypass 100 pf
pin function net
01 plus demodbyp
02 minus gnd
location c09
device capacitor capacitor
function power bypass .1 uF
pin function net
01 plus vcc
02 minus gnd
location c10
device capacitor capacitor
function power bypass 100 uF 25v
pin function net
01 plus vcc
02 minus gnd
location c11
device capacitor capacitor
function deemphasis .02 uF
pin function net
01 plus deemph
02 minus gnd
location c12
device capacitor capacitor
function output couple .01 uF
pin function net
01 plus deemph
02 minus trimin
location c13
device capacitor capacitor
function output couple .01 uF
pin function net
01 plus trimout
02 minus audioout
location ic1
device mc3359 low power narrowband FM if
function mixer if fm detector
pin function net
01 i osc base oscbase
02 x osc emitt oscemitter
03 o mixer mixerout
04 vcc vcc
05 i limiter limiterin
06 x decouple pin6
07 x decouple pin7
08 x quad coil quadcoil
09 x demod filt demodbyp
10 o audio detectout
11 o afc
12 i opamp - gnd
13 o opamp
14 i squelch gnd
15 o scan
16 o mute
17 gnd gnd
18 i rf rfin
location j1
device connector connector
function signal input
pin function net
01 multiplex
02 gnd
location j2
device connector connector
function audio output
pin function net
01 audioout
02 gnd
location j3
device connector connector
function power
pin function net
01 vcc
02 gnd
location l1
device inductor5 5 pin min. inductor
function .68 mH toko RMC-2A6597HM
pin function net
01 coil 1 infilter
02 center
03 coil 1 gnd
04 coil 2
05 coil 2
location l2
device inductor inductor
function 80 - 90 uH miller 9001 or toko 154ANS-T1020Z
pin function net
01 1 vcc
02 2 oscbase
location l3
device inductor5 5 pin min. inductor
function quadrature coil toko RMC-2A6597HM
pin function net
01 coil 1 vcc
02 center
03 coil 1 quadcoil
04 coil 2
05 coil 2
location r1
device resistor resistor
function input resistor 100k
pin function net
01 1 multiplex
02 2 infilter
location r2
device resistor resistor
function quadrature deQ 68k
pin function net
01 1 vcc
02 2 quadcoil
location r3
device resistor resistor
function deemphasis 7.5 kohm
pin function net
01 1 detectout
02 2 deemph
location r4
device trimpot trimmer pot
function output volume 10 kohm
pin function net
01 top trimin
02 wiper trimout
03 bottom gnd
location x1
device bandpass 3 terminal bandpass filter
function 455 kHz 10 khz toko HCFM2-455D
pin function net
01 input mixerout
02 gnd vcc
03 output limiterin
-----------------------------------------------------------------------
net = audioout
c13 02 minus output couple .01 uF
j2 01 audio output
count = 2
net = deemph
c11 01 plus deemphasis .02 uF
c12 01 plus output couple .01 uF
r3 02 2 deemphasis 7.5 kohm
count = 3
net = demodbyp
c08 01 plus decoder bypass 100 pf
ic1 09 x demod filt mixer if fm detector
count = 2
net = detectout
ic1 10 o audio mixer if fm detector
r3 01 1 deemphasis 7.5 kohm
count = 2
net = gnd
c01 02 minus 3300 pf c1+c2 = 6000 pf
c02 02 minus 2700 pf
c08 02 minus decoder bypass 100 pf
c09 02 minus power bypass .1 uF
c10 02 minus power bypass 100 uF 25v
c11 02 minus deemphasis .02 uF
ic1 12 i opamp - mixer if fm detector
ic1 14 i squelch mixer if fm detector
ic1 17 gnd mixer if fm detector
j1 02 signal input
j2 02 audio output
j3 02 power
l1 03 coil 1 .68 mH toko RMC-2A6597HM
r4 03 bottom output volume 10 kohm
count = 14
net = infilter
c01 01 plus 3300 pf c1+c2 = 6000 pf
c02 01 plus 2700 pf
c03 01 plus 4700 pf input couple
l1 01 coil 1 .68 mH toko RMC-2A6597HM
r1 02 2 input resistor 100k
count = 5
net = limiterin
ic1 05 i limiter mixer if fm detector
x1 03 output 455 kHz 10 khz toko HCFM2-455D
count = 2
net = mixerout
ic1 03 o mixer mixer if fm detector
x1 01 input 455 kHz 10 khz toko HCFM2-455D
count = 2
net = multiplex
j1 01 signal input
r1 01 1 input resistor 100k
count = 2
net = oscbase
c04 01 plus 3300 pf
ic1 01 i osc base mixer if fm detector
l2 02 2 80 - 90 uH miller 9001 or toko 154ANS-T1020Z
count = 3
net = oscemitter
c04 02 minus 3300 pf
c05 02 minus 4700 pf
ic1 02 x osc emitt mixer if fm detector
count = 3
net = pin6
c06 01 plus limiter bypass .1 uF
ic1 06 x decouple mixer if fm detector
count = 2
net = pin7
c06 02 minus limiter bypass .1 uF
c07 02 minus limiter bypass .1 uF
ic1 07 x decouple mixer if fm detector
count = 3
net = quadcoil
ic1 08 x quad coil mixer if fm detector
l3 03 coil 1 quadrature coil toko RMC-2A6597HM
r2 02 2 quadrature deQ 68k
count = 3
net = rfin
c03 02 minus 4700 pf input couple
ic1 18 i rf mixer if fm detector
count = 2
net = trimin
c12 02 minus output couple .01 uF
r4 01 top output volume 10 kohm
count = 2
net = trimout
c13 01 plus output couple .01 uF
r4 02 wiper output volume 10 kohm
count = 2
net = vcc
c05 01 plus 4700 pf
c07 01 plus limiter bypass .1 uF
c09 01 plus power bypass .1 uF
c10 01 plus power bypass 100 uF 25v
ic1 04 vcc mixer if fm detector
j3 01 power
l2 01 1 80 - 90 uH miller 9001 or toko 154ANS-T1020Z
l3 01 coil 1 quadrature coil toko RMC-2A6597HM
r2 01 1 quadrature deQ 68k
x1 02 gnd 455 kHz 10 khz toko HCFM2-455D
count = 10
total count = 64