[aprssig] HF APRS Transceiver

[Ray Wells]

Ben Jackson wrote:

>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.
>
>  
>

The use of ex-commercial SSB rigs for HF packet is not unusual over 
here. I have two such rigs here for packet, one on 40m and the other on 
20m, and their performance is more than acceptable. I only ever buy 
commercial spec crystals that are manufactured to the radio 
manufacturer's specifications for the radio. There's no such thing as a 
one size fits all when it comes to crystals, and the extra cost for the 
right crystal pays dividend.

Most reasonable crystals perform better in a stabilised temperature 
environment, such inside a rig that is never turned off. There are 
reasonably simple ways to heat a crystal to quickly bring it to a 
stabilised temperature. A purpose design/manufactured rig could 
incorporate a simple oven, if temperature shift was thought to be a problem.

The design of both of my radios (one is a Codan and the other a Tracker 
Lynx)  is such that a single crystal is used for both tx and rx ( a 
common IF at 1.65MHz is used) so the crystal is always running and 
always "up to temperature". The NZ version of the Codan radio has an 
operating temperature range of -10 to +50 Celsius, the Australian 
version is rated 0 to 60 Celsius. The design allows for either positive 
or negative temperature coefficient capacitors to "trim" crystal 
performance, if necessary.

Whilst temperature stability is obviously a requirement, I believe that 
single factor shouldn't be an impediment to the design of a single 
frequency, crystal locked SSB transceiver for APRS/HF packet.

Ray vk2tv