What should be the min. frequency of carrier being modulated to get good demodulation
What should be the minimum frequency of the carrier being modulated in order to get good demodulation?
In other words, will I get good results ie. with 1MHz square wave modulating 10MHz carrier? how to calculate that?
The carrier and modulation frequencies dont depend on each other. You may choose any carrier.
But actually there are some legacy limitations: for 1 MHz modulation frequency you need at least 2 MHz RF bandwidth, so you should choose the frequency band where such a channel bandwidth is permitted.
I understand your point. But assume I don't send anything in air. 1MHz modulating 2MHz means there is only 2 cycles of carrier per bit. So is that 2 cycles enough for the demodulator to recognize the phase reliably ?
I guess this depends on your demodulator. In general only one period of the carrier is enough.
In theory, with the right demodulator, you can recover the modulating waveform, even
if it has a frequency very close to the carrier, and even if only one side-band is available
Unless the waveforms are beforehand known to be synchronous in some way,
you normally need at least 3 "samples" to recover the phase of a waveform.
I suspect there are probably special ways to do better. In practice, you need x4 or more.
You need enough to get a good signal-to-noise ratio out of the demodulator,
and any poor information about the true phase counts as noise.