why capture effect does not happen in AM

I have some doubts regarding capture effect that is usually more prominent in FM. If there are two nearby frequencies and I use a PLL for demodulation of FM, the following statement is true- "The PLL takes the one which has more amplitude than the other". Why does this not happen in AM? Is that because in AM we are tuned to a single frequency?

Thanks in advance.

I did some empirical experiments once using cmos dividers, and they always "locked on" to the bigger of the two signals, even if one was only 3 dB higher than the other. An interesting effect. Not sure what physics causes that.

Of course, the other signal is still there as a phase modulation sideband.

Okay I have a basic question related to PLL. I have two or more frequencies. I want to switch between these frequencies. I am using PLL. How would you do that?

That is a very different, and simpler, problem. It seems you want to have a vco switch between two output frequencies. If that was the case, you use one reference frequency, and change the divisor ratio between N1 and N2 so that it cause either frequency F1 of F2.

So this is how you actually do in real time systems? Having a divisor and switch between them if you want to switch between the frequencies.

AM receivers operate at a low frequency (<30 MHz) where frequency stability is not an issue. Therefore there are no AFC circuits used to pull the receiver to a selected AM signal.
In FM receivers that operate at >80 MHz, oscillators are unstable. Therefore AFC circuit was invented to pull receiver oscillator to the FM spectrum center, to optimize the tuning and prevent distortion.
AFC circuits cannot recognize which signal is desired by you, therefore they may pull the receiver to one of two adjacent spectra centers, mostly by signal amplitude.
Due to the use of hard limiters in FM receivers, you are not aware that in FM frequency band signal amplitude varies very wildly and over a wide range, from uV to V, for example when the FM receiver is in your car.

FM transmitters are usually organized over the territory and using frequencies selected so that overlaps do not happen. Driving a car with a FM receiver, however, often breaks this organization; mostly in a hilly terrain, two signals on one frequency (or adjacent ones) can reach your antenna and you hear the receiver confused by which signal to process.

---------- Post added at 17:55 ---------- Previous post was at 17:51 ----------

To switch between two frequencies generated in one PLO, you must establish the reference frequency so that its harmonics fall to the desired frequencies. By tuning the PLO "high-frequency" oscillator, it alternatively locks to one or another frequency.
Frequency synthesizers utilize the same principle but the resulting frequency is selected by a combination of reference-frequency multiplication and division, so you can set the output frequencies in a predesigned series.

Yes, scope this out:
http://www.eetimes.com/design/analog...ency-synthesis