[aprssig] HF APRS Transceiver

Dave Baxter dave at emv.co.uk
Mon Mar 10 05:23:49 EDT 2008



Most Ham rigs are crystal stabilised anyway, even if they use a
synthesised VFO, the base clock is always a free running crystal.
Though some "base station" radios offer a high stability option of an
ovened oscillator, many do not.   They all seem to work OK on AFSK RTTY,
PSK, APRS etc etc...  So overall accuracy is not what's required, just

In any case, most 300bd modem chips, or software implementations I've
come across, are surprisingly tolerant of any slight frequency offset,
after all, most of the old style land line modems were only crystal
clocked at best, and not a high spec xtal either!  And some even used a
ceramic resonator to keep costs down.  They still worked OK...

There are direct conversion phone radio kits out there, but I'd expect
the image issue to be a limiting factor, plus you'd transmit two sets of
tones too.  Unless you implemented something like the "Third Method" (or
I/Q modulation/mixing) of SSB generation.  That'd probably work quite
well at a guess, for a limited audio bandwidth, but is the effort really
worth the payback?

As someone else said, otherwise sick early 706's (no working 2m)
FT100's, Alinco's etc, would all do the job.   Even (at risk of getting
flamed!) a converted multimode CB set?

Even better... Military surplus kit?  Extremely high spec
accuracy/stability wise, often chanelized & with internal ATU's etc, and
ideal for remote/mobile/rugged use.  Also often available at good prices
too, look on eBlag (and other sites) for starters...    Expect much of
it to work on 24/28V though, rarely 12V....   You could possibly fit a
tracker inside some of them, just needing a GPS "puck" on the outside to
make a self contained HF Tracker.

This is "Amateur" radio of course, so get the tools out and get in


Dave G0WBX.

> -----Original Message-----
> From: Ben Jackson [mailto:ben at ben.com] 
> Sent: Friday, March 07, 2008 9:29 PM
> To: TAPR APRS Mailing List
> Subject: Re: [aprssig] HF APRS Transceiver
> On Fri, Mar 07, 2008 at 01:04:03PM -0800, Ev Tupis wrote:
> > Not for nuthin, but I phoned MFJ a couple years ago asking 
> if they'd 
> > consider a crystal controlled version of their SSB transceiver 
> > specifically for data-mode use.
> Crystal control is not sufficient.  20ppm at 10MHz is 200Hz 
> which is an order of magnitude or more too high (consider 
> that the shift of the FSK is only 200Hz).  You probably need 
> around 1ppm accuracy.  The operator-free solution is a 
> reference clock slaved to the GPS.  With DDS it shouldn't be 
> that hard.
> --
> Ben Jackson AD7GD
> <ben at ben.com>
> http://www.ben.com/
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