[aprssig] RFM22B UHF Transceiver Module

Jason KG4WSV kg4wsv at gmail.com
Wed Dec 12 09:27:59 EST 2012

On Wed, Dec 12, 2012 at 3:21 AM, John Wiseman <john.wiseman at cantab.net> wrote:
> The PIC has enough power
> to do this for a 1200 link, but I'm not sure it it could cope with 9600.

The old PICs (16F?), IIRC, ran at max 20MHz and used 4 cycles per
instruction, which would leave you with 260 instructions per bit time
at 9600 bps.

An ATmega on an Arduino board runs at 16MHz and does better than 2
CPI, which would give you around 1000 machine instructions per bit
time at 9600 bps.

I'm pretty sure either one could keep up, although I could see that
sub-optimal code could have problems with it.

IMO, these modules will only be of academic interest unless they can
be made to interoperate with existing equipment; I could live with
9600 baud only, not sure how other APRS users feel about it. If, on
the other hand, we could get cheap 9600 baud transceivers out of it,
it could be a game-changer.

Our cubesat students are working with some modules that I think are
based on the same chipset (TI CC1100 ?) and are having some challenges
getting 9600 baud AX.25 working.  I think the ham equipment receives
their data fine, but the module doesn't receive the AX.25 data.  This
made me wonder if FSK -> GFSK is compatible  I can see that an FSK
receiver wouldn't have a problem with GFSK, because as I understand it
the GFSK is FSK where the frequencies all shift at a zero crossing of
the carrier.  If, on the other hand, the FSK gear switches somewhere
other than zero, I could see where the GFSK receiver may not detect
it.  Of course, the problem could also be getting data in and out of
the radio in a "raw" mode.


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