On Nov 26, 2013, at 11:30 AM, Bill Turner wrote:
> So it sounds like since it is phase continuous it doesn't have the multipath
> problems of PSK? Other than selective fading of course.
No, phase continuous just means that there is no sudden jumps in the RF from
the transmitter as you change between Mark and Space tones.
It is used not for any other reason than to keep the keying sidebands of an FSK
signal narrow.
You can see the difference between phase coherent FSK (Mark and Space tones are
picked from two different generators, and there is no additional filtering) and
phase continuous FSK in this old article:
http://www.w7ay.net/site/Technical/RTTY%20Sidebands/sidebands.html
The first picture is phase coherent (random phase would also produce a similar
picture). The next picture is the spectrum for phase continuous. Notice how
much less QRM it produces.
The third picture shows that you can do even better by filtering and/or
waveshaping the AFSK signal.
The fourth picture shows the case of applying too much filtering.
73
Chen, W7AY
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