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Tech tip for part 15 FM transmitters

R

radio8z

Hello all,

If you are using a $100 or so FM part 15 transmitter here is something to consider:

The stereo generator uses a pilot frequency of 19 kHz. which the receiver uses to lock on to the signal in order to recover the stereo signal centered around 38 kHz. If the audio has frequencies in the 19 kHz. range, it will interfere with the pilot and cause distortion. The remedy is to filter your audio to cutoff lower than 19 kHz. I use 15 kHz. as the cutoff and it does improve the signal.

If you have noticed some wierd artifacts in your signal, try this.

MY COMMENTS APPLY ONLY TO PART 15 FM, NOT COMMERCIAL STATIONS! The last time I posted about audio bandwidth there was much confusion. I hope there will be no misunderstandings this time.


Neil
 
A good observation. And if commercial stereo FM stations had program content around 19 kHz, the same thing would happen to them. However the FCC prohibits commercial stereo FMs from doing that.

If Part 15 FMs want to maximize their received signal-to-noise ratio, they should turn off their stereo generator and transmit in mono. There is large noise penalty in stereo FM transmission due to the greater r-f bandwidth it occupies -- in theory, a 23 dB reduction.
//
 
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