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Author Topic: Use your ham rig to listen to FM broadcast station subcarriers  (Read 475 times)

WA2ISE

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Use your ham rig to listen to FM broadcast station subcarriers
« on: January 02, 2020, 03:07:57 PM »

Many FM broadcast stations have subcarriers above their baseband audio and the stereo difference channels, usually at 67KHz and 92KHz.  These subcarriers are FM modulated, and you can, with some mods to a stereo tuner, listen to these subcarriers with a rig like an Icom 756 Pro. 

From my web page http://www.wa2ise.com/radios/ham.htm#12ba7 (more details there if you want)
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).

Above, I did this in a Technics STZ780 tuner, and added a buffer emitter follower output circuit. I coupled a 6uF cap to the FM demodulator chip's audio output (between that and the stereo demodulator chip) and fed that to the emitter follower. I selected the output coupling cap 0.15uF to act like a high pass filter when the 50 ohm input rig is connected, to minimize baseband audio. You can see on the Icom 756 Pro's panadaptor display the SCA's spectrum space. You can see in the lower middle pictures below the emitter follower.
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KC6RWI

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Re: Use your ham rig to listen to FM broadcast station subcarriers
« Reply #1 on: January 02, 2020, 04:56:54 PM »

Thats great stuff, I remember assembling a kit that used a led when It sensed the subcarrier.
On a similar note, I also enjoyed picking up news text, you'll find up to 4 text channels that you can select from your tv menu.
I had one channel that gave me text news all day long. But later I could not find any tv channels that used the text feature.
I even emailed a carefully worded note to the fcc asking for a guide that would tell me what channels had text info. They answered with the wrong info.
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WA2ISE

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Re: Use your ham rig to listen to FM broadcast station subcarriers
« Reply #2 on: January 02, 2020, 05:07:26 PM »

There was stuff like this on TV sound carriers, but since digital TV replaced analog TV, no more. 

Here's a more detailed mod schematic of a AM/FM stereo tuner.
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N8YX

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Re: Use your ham rig to listen to FM broadcast station subcarriers
« Reply #3 on: January 08, 2020, 08:29:17 AM »

You may also be able to do this with a general coverage VHF receiver which has a wideband IF output.

I have a few R-7000s kicking around here and might try coupling one to an HF receiver then set the latter to the R-7000's IF frequency, go find an FMBC signal with the 7000 then tune the HF receiver to see what pops up.
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