Hidden signals

Inspired by an idea of Ruud, I connected Winradio's input via a 4,7nF capacitor to the demodulator output of a FM radio. Winradio can be tuned down to 9 KHz and its IF ouput is connected to Spectrum Lab. Now we can see what the multiplex signal carries. Instead of Winradio you could connect a 192 kS/s soundcard to the multiplex signal and use Spectrum Lab.

This shows the audio spectrum ending at 15 kHz, the 19 kHz pilot tone and the lower sideband of the 38 kHz difference signal ending at 23 kHz. The attenuation above 27 kHz is due to Winradios limited bandwith. Ignore the spectral line at 14kHz, it is a spur of Winradio at these low frequencies.


AFN 98,7 audio spectrum is limited to 10,5kHz. No idea why they do it.


This shows both sidebands of the difference signal centered at 38kHz. The audio was a phone interview, you can see the phone part, it was mono and the difference spectrum is a lot smaller. This signal is audible in USB or LSB mode.


At 57kHz the RDS signal, the upper 38Khz sideband ends at 52,5kHz.
In Holland some Tx now carry additional signals above 59kHz, called FMeXtra




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