Re: More on IQ modulation

[Christophe Seguinot]

Hi

Some rapid answers inside the original email

Christophe

On 22/02/2021 19:48, John Byrne wrote:

 As someone with not much radio knowledge, I'm trying to understand IQ modulation. I have a few questions if anyone can help. I recently read the interesting thread on IQ signals, and I also came across this site which has a few articles that helped (https://www.markimicrowave.com/blog/the-why-and-when-of-iq-mixers-for-beginners/).

    this article is only related to SSB

But there's still a few things that confuse me. The way I understand it now is in terms of how the sidebands of the modulated I and Q signals add to produce the final RF signal: on the upper side of the carrier, Q is added to I, but on the lower side, Q is subtracted from I (due to the -sin term in the trig identify for cosA*sinB).

You conclusion is quite right, however, this is not due to -sin term. This can be obtained only if I and Q signal are related to each other by a constant phase shift  of -90° at ALL frequencies including positive and negative frequencies .This correspond to Hilbert transform as explained here figure 1  which is not physically existing.

So if Q(t) is the Hilbert transform of I(t), the output of the IQ modulator is and Upper or lower side band SSB.

So we end up with a signal that has two sidebands, but they contain different info. At the receiver, the process happens again: the signal coming out of the I mixer has 2 positive copies of I (one in each sideband), and one positive copy of Q and one negative copy of Q. Combining all these gives 2I, and the opposite thing happens with the Q mixer, so we end up with 2I and 2Q (or maybe they're not x2 because the original signal's power was divided between 2 sidebands?)

Is that the gist of it? I hope so because it makes sense to me. But what's giving me trouble now is this: what happens if either the Tx or the Rx don't use IQ modulation?

 Regarding your reference https://www.markimicrowave.com/blog/the-why-and-when-of-iq-mixers-for-beginners/  we found 3 methods of obtaining an SSB signal. You can combine any modulator (1 out of 3) with any demodulator (1 out of 3) to correctly demodulate the incoming signal.

It can be shown that an AM demodulator can be used to demodulate an SSB modulated signal

The best answer I can come up with is as follows: if the Tx uses IQ but the Rx doesn't, then the Rx would have to use a local oscillator that's offset from the carrier so that the whole signal appears as a single sideband - otherwise the 2 different sidebands will interfere with each other. Is that right? And conversely, if the Tx just produces 2 identical sidebands, but the Rx uses IQ, then the I and Q signals in the Rx will be identical to each other, except for a phase difference right? And we need to combine them to get the full power?

    In any case I mentioned above, the carrier frequency should be the
    same in modulator and demodulator without any shift.

Thanks

John