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Master1022

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- TL;DR Summary
- How are the two carrier frequencies for FSK modulation chosen?

**Question:**

For frequency shift key modulation in the binary case, how are the two carrier frequencies chosen?

From online reading, I have seen that the carriers are chosen to be different but also with the goal of minimising bandwidth and without any overlaps of the spectra, etc.

However, I have been told by a tutor that we ought to choose one carrier frequency and then multiply that value by one of the amplitudes of the wave to get the other carrier. However, I have looked on the internet (where I cannot find much clear information about this) and cannot find any source to support this statement. This suggestion runs into issues when there is a 0 amplitude (perhaps an offset would fix that) and a -1 amplitude, the latter of which leads to a case where the two carriers aren't distinct and the frequency spectra would overlap?

For example, if we had a pulse signal that was +1, -1, +1, -1, +1, -1, ... and we wanted to modulate it onto a cosine wave then, according to this suggestion, the points where the pulse changed levels would not lead to any difference in the cosine wave as ## cos(\omega_c t) = cos(-\omega_c t)##. Choosing two distinct carrier frequencies seems to make more sense to me in such a scenario.

I would appreciate any guidance and/or clarification regarding this topic.

For context, I have only been exposed to these concepts at an introductory level (i.e. undergraduate)

Thanks.