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/

Ben Jackson wrote:

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!

For 300 baud you should be all right -- I'm dinking with frequency-shift
pulse position modulation with 1ms pulses, which work fine on a ~16MHz
crystal.

I'd build it to adjust the 25MHz LO down a bit with some excessive
capacitance in parallel, or with a coil in series. If you arrange for a
trimmer in there someplace you won't have to play with adjusting
voltages to the XCO -- in fact you could implement the XCO with a
switching diode to a cap.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html