I've been brainstorming ways to make a cheap HF APRS beacon. HF APRS
is on 10.151, or more precisely, mark = 10.149.200 and space = 10.149.400.
It occured to me that if I had an appropriate crystal, FSK could be
a simple matter of driving two different voltages into a VXO. Of
course it's not easy to find a suitable crystal. I grabbed the list of
all crystals on Digikey and paired them up and sorted for the best
pairs producing mixer products near 10.149. The most promising combo
is 25MHz - 14.85MHz = 10.150MHz requiring 800Hz worst case pull to MARK
which is .006% of 14.85MHz.

So the idea is to use something like SA602A with the 25MHz LO and a
VXO driven by a microcontroller providing a ~14.85MHz "IF" with the
FSK signal (300 baud packet).

Has anyone tried something similar? I'm not sure if the VXO will
slew fast enough or accurately enough. I'm also wondering if a one-
time calibration of the VXO voltages would keep the output accurate
enough for a normal radio+TNC setup to consistently receive it.

Any ideas appreciated!

--
Ben Jackson

http://www.ben.com/