kd2bd@ka2qhd.UUCP (John Magliacane) (02/04/90)
This is in response to the discussions concerning SCA receivers.
SCA transmissions are made on 41 KHz, 67 KHz and 92 KHz NBFM subcarriers.
41 KHz is used only when stereo transmissions are not employed. 92 KHz is
a new allocation that has been available for a few years now. 67 KHz SCA
subcarriers are transmitted at only 10% of the total main carrier deviation.
The amplitude of the 92 KHz subcarrier (if used) is even less than 10%.
Most FM broadcast stations use SCA subcarriers to transmit limited audience
programming. Programs include PRN (the Physican's Radio Network), background
music (Muzak), readings for the blind, digital signals, FAX, and foreign
language programming.
53 KHz carriers (19 KHz x 3) are used for the ARI traffic information system.
I designed several SCA receivers in the past ten years. My first receiver was
built around an LM565 PLL working as a narrow-band FM detector. Reception was
improved with the inclusion of an input amplifier and high-pass filter.
Later, I obtained even better performance by replacing the input amplifier
with a two-stage, discrete component, active filter. Further improvements
were made with the inclusion of a carrier-operated mute (squelch) and output
low-pass filtering. The mute was helpful in quieting noise when no SCA signal
was present, as some stations drop their subcarriers between music selections
(their subcarrier generators use "VOX-type" circuitry).
Wide-band output noise was always a problem, especially on weak signals.
While this circuit worked fine living 20 miles from an broadcast transmitter,
it gave poor performance when I moved 50 miles from stations carrying SCA,
so I decided to tackle SCA using a different and more sophisticated approach.
My latest SCA adapter is a single conversion super heterodyne design (!!).
It is a three channel receiver with AFC and a unique S/N squelch circuit.
SCA signals are first amplified by a two-section active filter, using an
LF353N op-amp. The filtered signals are then routed to a carrier detector
and a doubly-balanced mixer (LM1496N). An LM566 VCO supplies the local
oscillator injection voltage for the mixer. The oscillator operates at
455 KHz - f and the mixer output is a standard 455 KHz narrow-band FM signal.
Output from the mixer is filtered through a single section 455 KHz ceramic
filter (10 KHz bandwidth) and then fed into an MC1358/LM3065 amplifier and
FM quadrature detector. The detected audio from this chip is diverted in
several different directions.
First, the DC component of the detected signal is low-pass filtered and fed
back to the LM566 for AFC (automatic frequency control). The output is also
fed to an active high-pass filter and detected. This DC is then differentially
combined with the output of the carrier detector to yield a DC output that is
proportional to the signal-to-noise ratio of the SCA subcarrier. This output
then feeds a Schmitt trigger, which controls a 1N914 switching diode acting as
as a series audio switch for audio muting purposes. It also drives an LED
that acts as an SCA indicator light.
The purpose of the "fancy squelch" is to avoid triggering on false signals.
A noise operated squelch does not work well because strong FM stations void
of an SCA subcarrier might quiet the SCA detector's output enough to open the
squelch. A carrier operated squelch does not work well either because weak
signals might contain enough high-frequency noise that could activate the
67 KHz carrier amplifier/detector and open the squelch on pure noise.
The squelch circuit I developed works extremely well. It is very sensitive to
weak signals and does not create an annoying "squelch-tail" during operation.
A three position rotary switch selects the frequency to be received.
The switch controls the center frequency of the input amplifiers as well as
the operating frequency of the local oscillator. The receiver tunes 67, 78
and 92 KHz. Since almost all broadcast stations transmit in stereo, I didn't
include a 41 KHz position on the channel selector switch. Instead, I included
78 KHz, the subcarrier frequency used by TV SAP (second audio program)
channels.
The detected SCA audio is then fed through a 7 KHz low-pass filter and made
available for external audio power amplification. The entire receiver
operates on 12 volts and is small enough to fit in the palm of your hand.
I recently considered trying an XR2211 PLL as an SCA detector. The XR2211 is
a popular Bell 202 FSK demodulator, but it can be used for linear demodulation
as well as FSK detection. Some of the advantages of this chip over an LM565
include a single ended audio output, and a PLL lock detect output suitable for
driving an indicator LED and squelch circuitry.
I admit, my "super" design is a bit "robust" for SCA, but it is a design that
works well. I never do anything the "easy way" anyway. :-)
It is the result of many years of "tinkering", and I have been able to use
some of the concepts developed here in other projects as well.
I hope the information contained here might help others interested in SCA
reception and subcarrier FM receiver design.
73, de John KD2BD
--
AMPR : KD2BD @ NN2Z (Neptune, NJ)
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