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I am reading about how radio communication works and I have an unanswered question.
We have wideband channels with 25kHz of radio spectrum. The transmitter modulates a signal and sends it.
The receiver that captures that particular channel and receives the signal. But how does it know to which frequency it has to tune in?
There is a 25 kHz channel size, so there are so many different frequencies that it could be as the signal is modulated. How does the receiver decide?
The signal is not at exactly one frequency. The only signal that exists exactly at one frequency is an unmodulated carrier, and such a signal contains no information.
As soon as the carrier is modulated, the signal's energy is spread over a wider bandwidth. So to receive any signal containing information, the receiver must listen to some range of frequencies, in your case a 25 kHz channel.
It may be easier to think of the receiver as receiving all frequencies. The job of the receiver is not to tune to a single frequency. Its job is to filter out everything that's not at that frequency, or in a band around it.
So when you tune your FM receiver to 144.2500 with a 25 kHz bandwidth, you're telling it to reject all frequencies that are more than 12.5 kHz below 144.2500, and all frequencies that are more than 12.5 kHz above 144.2500.
Other frequencies are still there, they still hit your antenna, and still show up (to some degree or another) at the antenna connector of your receiver. A big part of the receiver's job is to filter out all the frequencies you don't want to listen to - leaving only the signal that you do.
The receiver "listens" to any and all signals in its receive bandwidth (talking about AM, CW, or sideband here, FM handles everything differently). Generally, there will be only one strong signal that gets through the receiver's filter, and that (plus atmospheric or man-made noise) is all you'll hear.
If there are two stations within the filter width, however, they'll get mixed together (you can sometimes hear this in AM broadcast, if stations in adjacent cities are on adjacent frequencies, or at night when the signals can propagate hundreds of miles). In this case, your radio doesn't "know" what signal to listen to -- it just detects and amplifies whatever signal it receives.
Some radios have adjustable filters that let you narrow the receive band -- this is especially common with SSB and CW, where the signal you're after can be quite narrow (around 3 KHz for SSB and as little as 150 Hz for CW). In this case, you can adjust filter width wider to make it easier to find a signal, then narrow it to ensure you can hear the one you're after. Still and always, however, the radio just reproduces whatever signal comes in through the filter width.
So everyone is talking about frequencies and almost nobody mentioned modulation mode. This is important because how close you have to get in frequency depends on the mode.
A signal with information is not one frequency; it has a bandwidth, or a range of frequencies that contain the information being transmitted.
The radio has filters that allow it to receive a slice of spectrum in the "pass band" which is usually slightly wider than the signal you want.
If the mode is AM, the pass band is usually the same as the desired signal, the carrier is suppressed (hopefully), and the amplitude of the RF is converted to audio amplitude to demodulate the signal. If multiple signals are present, they will all be present in the demodulated audio, like people in a room all talking at once. If the pass band is not aligned with the transmitted signal, the audio will be clipped and distorted, although with commercial AM, the signal is so wide that you'd only notice that with loud music if it's just a little off. Also, if the frequency is not aligned, carrier suppression may fail which will add a whistle to the demodulated audio. Some advanced AM receivers are able to lock onto the carrier or look for the symmetry in the signal to lock on, but most AM receivers (especially older ones) are built too cheaply to do this and just try to suppress the whistle in the demodulated audio.
If the mode is SSB, the upper or lower range of frequencies is isolated, and the opposite side is reconstructed, and the result is then handled similarly to AM. If the frequency is not aligned with the transmitted signal, the audio will be distorted badly and garbled, as the reconstructed portion will not be correct. If it is only slightly off, the frequency of the audio will be higher or lower than the original along with some distortion, and it is sometimes possible to align the frequency by ear.
If the mode is FM, the FM signal is mostly symmetrical and the demodulation method has a capture effect that uses that symmetry to lock on to the signal. (This allows FM receivers to have sloppy frequency control while still having good quality demodulation.) If the signal is within the (usually larger) pass band, the demodulated signal won't be distorted; if it is not entirely in the pass band, it will be clipped. If there are multiple signals present, only the strongest will be demodulated, although there may be buzzing as a result of the secondary signals. If multiple signals are close to the same strength, the receiver may alternately lock on each of them in turn, which may make all of them unintelligible depending on how fast it switches between them.
