Comment Re:Why you can't use an FM radio RECEIVER on a PLA (Score 1) 237
the "beat frequency" he's talking about isn't the frequency used as a carrier to decode the FM, it's the intermediate frequency used in a super-het FM radio.
Here's the deal: Say I want to be able to demodulate FM from 80 MHz to 110 MHz. Do I build a bunch of demodulators that can each demod a single FM station? That would be expensive, so I build a single FM receiver, and I multiply the incoming transmission by a variable frequency, and tune the variable frequency such that the signal I want to receive is mixed down (by the multiplication) to the frequency my FM receiver is built for.
Wait, I said "mixed down to." Why wouldn't some of them be mixed up to the frequency of the receiver, and some of them mixed down? In general, for a fixed bandwidth (all FM stations have an identical bandwidth, just different center frequency), the higher the frequency, the more expensive the equipment. So to cut costs, our FM receiver doesn't operate anywhere near the FM band, it'll be way lower.
I don't know what center frequency the radio manufacturer will build the receiever to, but let's just guess that it would be 10 MHz. So, if I want to listen to 101.5 FM, which operates around 101.5 MHz, my variable frequency would have to be set to the difference, which is 81.5 MHz.
The grandparent is sugesting that the variable frequency used by most FM receivers varies over a range that interferes with air-ground transmissions. Now, your receiver isn't trying to broadcast anything, but when you build devices with radio-frequency currents running through them, the energies have a habit of wandering of the circuit, and you have to work really hard to prevent all leaks.
Here's the deal: Say I want to be able to demodulate FM from 80 MHz to 110 MHz. Do I build a bunch of demodulators that can each demod a single FM station? That would be expensive, so I build a single FM receiver, and I multiply the incoming transmission by a variable frequency, and tune the variable frequency such that the signal I want to receive is mixed down (by the multiplication) to the frequency my FM receiver is built for.
Wait, I said "mixed down to." Why wouldn't some of them be mixed up to the frequency of the receiver, and some of them mixed down? In general, for a fixed bandwidth (all FM stations have an identical bandwidth, just different center frequency), the higher the frequency, the more expensive the equipment. So to cut costs, our FM receiver doesn't operate anywhere near the FM band, it'll be way lower.
I don't know what center frequency the radio manufacturer will build the receiever to, but let's just guess that it would be 10 MHz. So, if I want to listen to 101.5 FM, which operates around 101.5 MHz, my variable frequency would have to be set to the difference, which is 81.5 MHz.
The grandparent is sugesting that the variable frequency used by most FM receivers varies over a range that interferes with air-ground transmissions. Now, your receiver isn't trying to broadcast anything, but when you build devices with radio-frequency currents running through them, the energies have a habit of wandering of the circuit, and you have to work really hard to prevent all leaks.
