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Just curious if we have any gurus here that understand the intricacies of different modulation techniques and their respective impacts on modulation bandwidth and things like second and third order distortions.

If so, I have a few questions.

Well shoot out the questions. Let's see what you got.

If I were to digitally modulate a carrier, using a data rate exactly matching the carrier frequency and synchronized such that each single wave (starting at the upward zero-crossing point)is either at the full level (0% modulation) or reduced in amplitude by a set amount (say, 50%), what would the sidebands look like, or how much bandwidth would be used?

This would represent 0101010101:

I would predict that the Earth would lose gravity and that we would all go spinning into space.

FM bandwidth is deviation plus audio frequency.

AM bandwidth is just audio frequency.

If the transition occurs at the zero crossing points, I was thinking this would be like morse code. Narrow bandwidth, but not zero bandwidth.

EDIT: I have spectrum plots I created in Cool Edit. 50% is narrower than 0%.

audiophile wrote: ↑

Thu Nov 02, 2023 4:39 am

FM bandwidth is deviation plus audio frequency.

AM bandwidth is just audio frequency.

If the transition occurs at the zero crossing points, I was thinking this would be like morse code. Narrow bandwidth, but not zero bandwidth.

EDIT: I have spectrum plots I created in Cool Edit. 50% is narrower than 0%.

Why not zero? If I have a clean unmodulated CW carrier at 400Mhz and 10dBm, it's bandwidth is zero. If I have a clean unmodulated CW carrier at 400Mhz and 5dBm, it's bandwidth is also zero. If I toggle between them at precisely a zero-crossing point, then why would I have any bandwidth other than zero? Practically, if there is some 'slop' in the zero-crossing then I would have some bandwidth incurred, but would that also be at very, very low power levels since it is when the carrier is at near-zero dBm? And that bandwidth transmitted could be impacted by a filter or other anomaly and it wouldn't have any impact on the received carrier, because it doesn't actually carry any of the useful information.

To extrapolate the idea, what if I have 8 or 16 carriers all at the same exact frequency but just differentiated in power and switch between them at the zero-crossing point? This would be like one of the QAM carriers in a 256QAM application, except that the bandwidth of the modulated signal would be (essentially) zero vs taking up 6 or 8 MHz. You could reduce the spacing on the adjacent channels significantly, increasing the overall throughput of the system.