[aprssig] TNC-2 tuning and temperature stability

[scott at opentrac.org]

> Just curious, why not use the MX614?  It's a bit pricey, but I would  
> think it would pay off in reduced complexity.

1. It's a bit pricey, and more difficult to obtain
2. The complexity isn't that bad
3. The '2211 can be retuned for 300 baud
4. The MX614 can't transmit PSK31

Besides, I already had the TX side implemented on the MCU and didn't need
half of the MX614.

Scott
N1VG