Receiving a radio signal...

[Externet]

Hi.
When a radio receiver is tuned to say, 100 MHz; its local oscillator is at say 110.7MHz, the mixer stage produces a 10.7MHz intermediate frequency for later demodulation.

If the broadcast ceases to emit, goes off the air;
Or, shifting tuning to a blank unoccupied frequency;
Or, shorting to ground the antenna input;
is 10.7MHz still present at the mixer output ?

And, do you know of an IC that can be used to detect the presence of 10.7 MHz ?

 

[davenn]

If the broadcast ceases to emit, goes off the air;
Or, shifting tuning to a blank unoccupied frequency;
Or, shorting to ground the antenna input;
is 10.7MHz still present at the mixer output ?

the 10.7 IF is only there when the RF and the LO are mixed
if either is missing then there is no IF out

And, do you know of an IC that can be used to detect the presence of 10.7 MHz ?

a google search would have answered that for you.
there are many IF IC's that have a 10.MHz input eg


MC3357 --- NBFM dual conversion IF, 10.7MHz In, Audio out

MC3359 --- NBFM dual conversion IF, 10.7MHz In, Audio out, less external components than 3357 and addition IF limiter stage

MC3361 --- NBFM dual conversion IF, 2-8V supply, up to 60MHz I/P

MC3362 --- NBFM dual conversion IF, 2-6V supply, usable with PLL, carrier detect indicator, up to 200 MHz I/P


just some of the many in my own collection


Dave

 

[duke37]

Most FM radios have a muting control to turn off the sound if no signal is being received. The gain of FM recievers is very high and if there is no signal then detected noise produces a rushing noise.

 

[Externet]

Thanks, gentlemen.
Dave : I meant knowing if 10.7MHz is present. No audio detection desired. To work on silent unmodulated signals. My searches found complete receivers as your suggested chips.
Intention is to know if a carrier is present, on a receiver chip that does not have tuning indicator feature. Perhaps using a section of some other chip that could be added to an existing receiver.
Will explore portions of the MC3362, for presence of 10.7MHz telling there is carrier.

 

[Ylli]

If you are looking for a dead carrier, the best way may be to look at the noise. With no signal, the output of an FM detector will have large amount of white/pink noise. When there is a carrier present, that noise goes away. The spec on this is "Quieting". This looks like an informative article: https://radio-electronics.com/info/rf-technology-design/fm-reception/capabilities-specifications.php

 

[Audioguru]

I agree that the 10.7MHz from the mixer will never be zero. It will have either signal or noise. Maybe its muting circuit detects the amplitude fluctuations of the noise?

 

[davenn]

Thanks, gentlemen.
Dave : I meant knowing if 10.7MHz is present. No audio detection desired. To work on silent unmodulated signals. My searches found complete receivers as your suggested chips.
Intention is to know if a carrier is present, on a receiver chip that does not have tuning indicator feature. Perhaps using a section of some other chip that could be added to an existing receiver.
Will explore portions of the MC3362, for presence of 10.7MHz telling there is carrier.

As I stated before

If either the LO signal or the RF signal is absent, there will be NO 10.7MHz IF signal generated (in the mixer) that then goes to the IF input pin of the IF IC