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Author Topic: Listening to FM radio SCA subcarriers with your modern ham radio HF radio  (Read 354 times)

Posts: 1074

« on: February 18, 2018, 02:34:22 PM »

Listening to SCA subcarriers on FM broadcast stations with the TS440S, IC-756Pro and similar radios: As the TS440S and IC-756Pro can receive signals all the way down to 30KHz all modes (AM, FM, USB, LSB, CW), you can "surf" the SCA subcarriers on the FM broadcast band. Usually found as FM modulation at 67kHz and 92kHz, and RDS/EAS at 57kHz (use the FSK demod on your rig at about 58 or 59kHz). Also those of any analog TV stations that might still be on the air, on their sound broadcasts. These are usually found as FM modulation at 78.67kHz (SAP channel), and a cue channel at 102.27kHz (usually found at some multiple or multiple and a half of the horizontal deflection frequency).

What you need to do is tap the output of the main FM carrier detector before the deemphasis circuit in an FM broadcast band radio or TV set. A cheapie AM/FM clock radio or TV set will do at first, one with a wide FM IF strip is better, so the sidebands carrying the subcarriers don't get clipped. The deemphasis circuit is usually an RC low pass filter. One way to "hunt" for this point is to hook up an audio amplifier to a test probe, and find a node on the circuit board that has main channel broadcast audio with lots of treble as compared to that found at the top of the volume control. If you can't find such, take a look for a small cap (on the IC pin with the audio demod out) that goes to ground which in combination with a resistor may be the deemphasis network. Try disconnecting the cap. One this spot has been identified, connect a 0.01uF cap in series with a 1K resistor, and that then in turn feeds the center conductor of a length of thin coax. The coax ground connects to the radio ground. The other end of the coax connects to the TS440S antenna input. Check that the FM broadcast radio still plays, as the coax cable is now loaded with 50Ω impedance.

If you have an older mono FM tuner like an Eico HFT90, connect to the multiplex output jack (the one intended to feed an external stereo demodulator) and use a resistor of around half a meg in the signal path from this jack to the TS440SAT or IC756Pro receiver. Or use a cathode follower or emitter follower to lower the impedance without much signal level loss. This will avoid excessive loading of the normal mono output signal (so you can listen for FM stations' main mono audio channels).

Disconnect the mic on the TS440S or IC-756Pro so you don't accidentally transmit into the FM radio! Tune in 38KHz in USB mode. Now tune around on the FM broadcast radio. You should be able to hear the "difference" (L-R) audio signal of FM stereo stations. You should be able to notice the usual lack of vocals in most songs, as compared to the FM broadcast radio's mono speaker output. Assuming success, try tuning in 67KHz FM mode on the TS440S or IC-756Pro. Now tune around on the FM broadcast radio. You may find various foreign language programs and data transmissions. Also try 92KHz FM on the TS440S or IC-756Pro. Expect about 10% of FM broadcast stations to have subcarriers. If you've tried to build the usual SCA decoders using PLL circuits or FM demod chips, you know that crosstalk from the main program material is a big problem, but the TS440 and the IC-756Pro receivers seem much better handling this problem, as their selectivity and dynamic range are much better. Similar ham transcievers should also work well with this (again, be sure to disable the transmitter). Don't overlook the college radio subband from 88 to 92 MHz.

SCA signals are about 12KHz wide. The freq divisions above are 5HKz. In addition to the 67KHz SCA subcarrier, you can see some of the 92KHz SCA subcarrier on this FM broadcast station, and some of the stereo difference DSB subcarrier.

You're essentially "surfing" the radio spectrum in two dimensions, one "axis" is the FM broadcast band, and the other is each FM station's subcarrier signal spectrum.

Posts: 54

« Reply #1 on: February 19, 2018, 04:10:23 PM »

Fantastic info, thank you!

73 de N0TLD

Posts: 1259

« Reply #2 on: February 19, 2018, 04:56:21 PM »

Fantastic info, thank you!

73 de N0TLD

Indeed. Another option is to use an $8 RTL-SDR dongle, and a couple of free software. Search the RTL-SDR Blog for the details.
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