>
> i am using a Bell 202 compatible modem chip (mx614 - www.mxcom.com) in the design I
>am
> doing. Is this afsk? The datasheets I have only say fsk. I guess it is probably
>afsk
> since the chip outputs 1200hz/2200hz audio tones. Is fsk carrier frequency shifting?
The audio signal coming out of the chip is fsk, if you feed it into a FM
radio you will have afsk. If you choose to feed it into a SSB rig ($$ on
uhf) you will have fsk.
> >This signal happens to be a 1200hz tone that has an
> >arbitrary phase offset (it might be coherent fsk then no offset but that's
> >abnormal on ham bands as far as I know).
> Could you explain the above? I don't know what coherent fsk and arbitrary phase
>offset
> are.
Say you have two oscillators and you have a simple switch that switches
between the two and that feeds the transmitter. You send low tone-high
tone-low tone.
At the receiver you have two oscillators like you had at the transmitter.
Somehow you figure out which one was the same tone as the first symbol
(low). Now you measure the difference in phase between the sent low tone
and your low tone oscillator and write it down. You do the same thing for
the next tone (high). Then you come back to the third tone and measure the
frequency and discover that it is another low tone. This time when you
measure the phase you will find that it is the same as what you had
written down. You will rebuild your receiver to expect a certain phase
difference between the transmitter tones and the two local oscillators.
With the new receiver you will be able to decode weaker signals. This is
coherent fsk, and it's probably not much use to you. Your afsk signal
won't benefit from this type of modulation.
What you have is non-coherent fsk. The MX 614 sends 10101 the 1's
probably won't be in phase with each other for good reason. In many cases
switching between two oscillators of different frequencies will make a big
glitch that eats up bandwidth (take the fft of it sometime). Normally we
do something like this S[t] = Sin[wt] and we simply change w for the two
tones. All packet tncs are set up for non-coherent fsk on fm or just
afsk.
Your alinco rig is FM so you will be sending a packet-like signal. In
fact, you might think about using packet. There are some progams for PIC
microprocessors and MX614's that will do APRS. Then you would't have to
build a telemetry protocol and you could use normal TNC's to do the
demodulating. See TAPR and their PIC people:
http://www.tapr.org/tapr/html/Fmic-e.html
-------===Dustin Moore===---------
