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Beat signals

by Niles
Tags: beat, signals
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f95toli
#19
Mar15-12, 05:11 AM
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Quote Quote by NascentOxygen View Post
I'm a little uneasy with the concept of two sinusoids of different frequency being of opposite phase--wouldn't they be in-phase for exactly as much time as they'd be opposite-in-phase? But the maths looks good, and that's the main thing.
It is because the sidebands are the results of a carrier being passed through a phase modulator where is is mixed with the IF, i.e. they are the products of the same process which means that they maintain a phase relationship when the signal is downmixed again in the lock-in.

I use a variation of this technique in my experiment (although not for spectroscopy), and I can assure you that the math not only looks good but also works in practice
NascentOxygen
#20
Mar15-12, 05:39 AM
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Quote Quote by NascentOxygen View Post
In this phase modulation, one of those low-frequency components is in anti-phase to the other.
I'm coming to the conclusion that there is a bit of poetic licence here. When the author says that the upper sideband is out of phase with the lower sideband, I think he probably being conservative with words, and what he really means is that the modulation carried on that upper sideband, when demodulated returns a demodulated signal which is in antiphase with the demodulated signal recovered from the lower sideband.

In which case, any mathematics is moot.

If my hunch is right, this probably arises so often in communications engineering that it is a well-understood shorthand. Let's see if anyone can support me on this.


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