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4:58 AM
If I overlay two sine waves at the same frequency 90 degrees phase-shifted apart, does that still count as transmitting at the one frequency? (...how much do I clearly need to learn about electromagnetics, ugh)
e.g. if I'm doing QAM at 2.4GHz, do the two added waves still count as broadcasting on 2.4GHz, even though they might have some odd beat frequency stuff going on
 
 
9 hours later…
2:21 PM
the electromagnetic field is linear — that means that if you add two signals of any kind (whether this be digital, analog, or by means of having two separate transmitters) this does not result in there being signals at other frequencies than you started with
do note that just like any approach to modulation, there's a nonzero bandwidth, a range of frequencies that the signal occupies, once you start doing anything more than the carrier
 
 
2 hours later…
4:16 PM
@KevinReidAG6YO Okay, cool, that's good to know. I'm thinking of trying to implement some phase-shift keying and it'd be mildly inconvenient if I had to worry about what other frequencies were being output. Thanks!
 
4:30 PM
@NicHartley If you're doing QAM, then you are increasing the bandwidth. Your center frequency will still remain 2.4GHz*, but it will now occupy 2.4GHz +/- 100kHz. The band width depends on how you modulate the signal.
 
5:05 PM
from some quick Google seraching, it appears that "bandwidth" means any of dozens of things, so I'm going to put off actually figuring that out until I get home
 
 
1 hour later…
6:29 PM
there's lots of different ways in which it appears but in general it is always — there is some range of occupied/permitted/relevant frequencies [f_1 … f_2] and f_2 - f_1 is the bandwidth
and the way in which it is relevant to modulation is that:
A theoretically perfect unmodulated carrier has zero bandwidth, but also can transmit zero information. Any way you modulate it increases the bandwidth.
 
 
2 hours later…
8:31 PM
@KevinReidAG6YO Right, I get that, but how do you calculate the bandwidth of a signal?
Like, if I'm doing PSK let's say -- how do I know how much bandwidth I'm taking up when making any given transmission?
 
The math is nontrivial and the practical engineering answer is "look it up for the modulation you're using".
And it depends on a bunch of parameters
I think you might benefit from reading through some introductory text on digital communications — there's a lot of things about modulation and fundamental communications concepts that are all tied together
 
Probably
 

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