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So we may have a radio station on AM 1400, or AM 1450, or it can be FM 96.5 or FM 98.1.

My question is, it seems the "1400" means, we pick up a signal (either 0 or 1, or an amplitude) every 1 / 1,400,000 seconds, because it is 1400 kHz, I believe.

So if we use the traditional radio, where we probably cannot tune to exactly 1400, but it is at 1400.1, does that mean after 1 / 14,000,000 seconds, we get the wrong signal (that belongs to another radio station), and so on and so forth: we will get the wrong signal, until about 14,000,000 times later, then we get the right signal intended for this radio station again. So that will mean the signal is close to all wrong.

But in fact, if it is slightly out of tune (at 1400.1 or 1400.005), we still hear it alright.

I think it is similar to, if there are 50 people out there (as in 50 radio stations), sending a 0 or 1 (for simplicity let's just use 0 or 1), and they change the signal at different frequencies, and the final outcome is the superposition of everybody. (if all 50 people send a 1, then it is sending out a 50. If only 10 people send out a 1, then the value being sent is a 10. So from this, we are able to get the signal of, let's say, person 12, who changes his signal at a rate of 1400kHz? And we can also get the signal of person 15, who changes his signal at a rate of 1450kHz?

How exactly does it work?

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    \$\begingroup\$ Hz or Hertz is a measure of Frequency ie cycles per second. You may benefit from checking that and then re-considering what you ask in your question. \$\endgroup\$
    – Solar Mike
    Commented Nov 11, 2023 at 20:06

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Both AM broadcast signals and FM signal reception are somewhat tolerant to frequency offsets, because the receiver by design doesn't notice small frequency offsets:

  • AM, as you know it from broadcast, has an unsuppresed carrier, which is used to mix down the message signal (directly in a homodyne receiver, or to correct the frequency of a different receiver architecture)
  • FM only cares about frequency deviation over time, anyways.

we will get the wrong signal, until about 14,000,000 times later, then we get the right signal intended for this radio station again.

err, no. That's not how that works. The signal is there all the time. If you tune to a different station than you mean to, you get that different station all the time.

You will have to look at what the spectrum is, and what frequency-selectivity (as in band-pass filter) means.

I think it is similar to, if there are 50 people out there (as in 50 radio stations), sending a 0 or 1 (for simplicity let's just use 0 or 1), and they change the signal at different frequencies, and the final outcome is the superposition of everybody. (if all 50 people send a 1, then it is sending out a 50. If only 10 people send out a 1, then the value being sent is a 10. So from this, we are able to get the signal of, let's say, person 12, who changes his signal at a rate of 1400kHz? And we can also get the signal of person 15, who changes his signal at a rate of 1450kHz?

You're on a good track, but your example makes it harder to understand what you really mean. Let me quickly elaborate:

Different radio stations have different carrier frequencies. That means, as long as there a station transmits "silence", it's just transmitting a single, static tone, for example, at a frequency 1.4 MHz. That frequency is not "how many times it's on or off", it describes how often it repeats per second. The shape of a pure tone is a cosine. The frequency is the inverse of that cosine's period.

By the nature of how physics works out, we can separate tones inherently by their frequency. The electromagnetic wave that transmitter A transmits at 1450 kHz has no effect on what is happening at 1400 kHz.

That's basically the whole magic: if you build a radio receiver circuit that selects a range of frequencies that is narrow enough to only let through one of the two tones, then you only hear one.

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    \$\begingroup\$ It might be worth mentioning that in the simplest AM receivers (envelope detectors), the audio signal is simply a measurement of the total energy at the output of the frequency-selective filter. Which is why, if your filter is not selective enough, you'll hear multiple signals superimposed. \$\endgroup\$
    – Dave Tweed
    Commented Nov 11, 2023 at 20:54
  • \$\begingroup\$ The point about using the unsuppresed carrier to recover the audio is important. This is why the recovered audio does not change in pitch as the receiver is tuned away from the carrier frequency. It also explains why, in a single sideband (SSB) system, the pitch does change as the receiver is tuned. \$\endgroup\$
    – SteveSh
    Commented Nov 14, 2023 at 12:48
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You seem to be confusing elements of analogue and digital electronics here. In general everything we discuss here is analogue domain. O's and 1's do not come into it. (Some receivers may use digital techniques to do what was once done in a purely analogue way, but that is not really the point.)

The 1400kHz means that the carrier of the AM signal is a sine wave of that frequency. AM means that the information (in this case, low bandwidth analogue audio) is encoded by varying the amplitude of the carrier signal.

At the receiver, a tuned circuit is used to pick that 1400kHz carrier out of the background noise. The tuned circuit will have a defined bandwidth which will be fairly narrow. This means that the output of the circuit falls off (in amplitude) either side of 1400kHz. There will generally NOT be another signal at 1401kHz - the way the band is allocated (that is, who can transmit on what frequency, and with what power, is VERY tightly controlled) ensures that. So when tuning to 1401kHz you will still get enough of the 1400kHz carrier to be able to de-modulate and retrieve the audio.

In fact there is some modulation theory that says that using AM on a carrier with a much lower frequency signal creates a band of frequencies which is a function of both (and modulation depth). That band is centred on the 1400kHz carrier and about 5kHz either side of it (if the audio bandwidth is 5kHz). That is why the carrier must be much higher in frequency than the highest frequency you are modulating with. For AM radio the lowest frequency carrier is generally about 150kHz, and stations are 9kHz apart. This is only generally used for low quality audio.

If you wish to go into more depth on AM, here is one of many sources on the web.

FM is a similar situation, it is just that the method of modulation is different. But the same general statements remain true.

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I'm going to focus on AM radio in the US for the moment.

AM radio signals have a certain bandwidth, or range of frequencies associated with them. It is generally up to 5 kHz below and 5 kHz above the center frequency. So for example, an AM radio station transmitting at 640 kHz can have frequencies from 635 kHz to 645 kHz.

enter image description here

This 5 kHz deviation is related to the highest audio frequency, 5 kHz, that can be transmitted. This is why AM radio is generally considered low fidelity.

Because of this, radio stations are placed at least 10 kHz apart in a given listening area, and usually more. The frequency selection devices in a radio receiver - the tuners - are not perfect. That is, they are not what we might call brick-wall filters.

enter image description here

So two radio stations 10 kHz apart and transmitting at the same power and the same distance away will probably both be received at certain tuner settings, and a listener will have to slightly de-tune the receiver in order to reduce the strength of the undesired station.

enter image description here

This also explains why you can receive and listen to a particular station over a small range of frequencies around the stations transmitting frequency.

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    \$\begingroup\$ All of this is true, but it doesn't address the actual question about imperfect tuning of the receiver. \$\endgroup\$
    – Dave Tweed
    Commented Nov 12, 2023 at 3:01
  • \$\begingroup\$ The point I was trying to make is that you can sometimes improve reception by deliberately tuning away from the carrier frequency. Yeah, I probably could have been clearer in that regard. \$\endgroup\$
    – SteveSh
    Commented Nov 12, 2023 at 11:43
  • \$\begingroup\$ KHz = kelvinhertz. kHz = kilohertz. \$\endgroup\$
    – winny
    Commented Nov 14, 2023 at 7:40
  • \$\begingroup\$ Yes, thanks winny. Always get confused over that. \$\endgroup\$
    – SteveSh
    Commented Nov 14, 2023 at 12:40

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