Hi STeve
With all modulation schemes the components are never single spectral
lines, as of course the tones are modulated and thus the modulation will
show depending how the spectrum analysis is captured.
You are right it is a dirty process switching from one to another.
However it is a controlled process- the data is applied to a low pass
filter before heading to the modulator (or should be) . So the dirt is
controlled. Various filter schemes are available depending on whether
lowest SNR, lowest bandwidth , minimum ISI etc is desired.
If the tones were directly switched at the datarate, ie no low pass
filter, this is the same as the individual tones being modulated with a
square wave.
How it will look on spectrum is highly dependent with how the spectral
analysis is performed . IE time averaged periodic, sync (IE is the
sample and sweep rate synchronous with the data clock and sample clocks
of the modulator etc ?)
But the dirt can also indicate a modulator bug, for sure, certainly a
chance of that
cheers
On 14/06/2018 6:27 AM, Steve wrote:
In modulating the 4FSK using a random number (rand() % 4) I get this
spectrum:
4fsk-rand.png
Looks good. Now if I modulate it with the vocoder bits instead:
4fsk-voice.png
Well, isn't that special. I have no idea where those harmonics comes
from, but there is always a pip at 1400 (center frequency). Plus the
garbage on the right. Heck, there are only 4 frequencies possible in
the modulator.
I guess swinging from say F1 in one instant to F4 in the next, must be
a dirty process. More than likely, it's just a well hidden bug.
It's a puzzle so far :-)
Steve
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