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Unknow demodulator - RF Cafe Forums
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magnetra |
Post subject: Unknow demodulator
Posted: Mon Dec 26, 2005 10:27 am
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Joined: Fri Aug 05, 2005 2:56 am Posts: 2
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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
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gnomus
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Post subject:
Posted: Thu Dec 29, 2005 12:48 pm
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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.
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potis21
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Post subject:
Posted: Tue Nov 28, 2006 3:43 am
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Joined: Sun May 29, 2005 5:26 am Posts: 2
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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!
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Posted 11/12/2012
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