Unknow demodulator - RF Cafe Forums

 Post subject: Unknow demodulator

Posted: Mon Dec 26, 2005 10:27 am 

I had a bizzare experience today. I was testing a simple band pass filter in the lab. I had connected the output to a speaker. As I was setting up the ckt for testing, I hear the local FM station in the speaker. I was loud and clear. I couldn't believe it. How can that happen. The FM station's antenna is very close to my lab, but that doesn't explain how the FM signal got demodulated and got onto my speaker. Any explanations as to what might have happened? Or any one with a similar experience??

Magnetra

 Post subject:

Posted: Thu Dec 29, 2005 12:48 pm 

FM doesn't necessarily need to be demodulated in the normal limiter-detector fashon to be received. AM receivers will detect FM when the signal is tuned to the edge of the bandpass and the FM deviation results in a AM-like response. This is called slope-detection.

The output or input stage of semiconductor devices can sometimes rectify a signal sufficiently to pick up a radio station. In this case, the bandpass filter may have worked to provide a slope FM to AM response followed by a semiconductor junction providing the actual detection.

Years ago, there was a particular Japan made TV that had 19" leads to the speaker. This was just long enough to be 1/4 wave at the amateur radio 144 to 147 MHz 2-meter FM band. A stage in the audio amplifer provided the subsequent semiconductor junction and the audio stages amplifed it. Weak amateur radio conversations could be heard in the speaker. The speaker leads provided a low-Q response on which the FM was slope-detected. The fix in this case was to change the length of the speaker leads.

 Post subject:

Posted: Tue Nov 28, 2006 3:43 am 

There is another chance, though.

has happened to me several times.

a poorly designed FM tube amplifier can have a cavity of a very high Q.

sometimes a not so well trained engineer can mistune such an amplifier to have the output varied in amplitude as the frequency of the modulated carrier slips on the slope of the Q curve.

If the amplifier is powerful enough , the output power variations can be such that an AM modulation depth of several watts (even 100 watts @ kw) is observed (I have seen it with my own eyes (!) fwd meter dancing along with the beat.)

since it is already biased and self-slope discriminated, such a signal doesn't need detection, a capacitor is more than enough to filter rf and to provide crystal clear audio!