[aprssig] HF APRS Transceiver

[Stephen H. Smith]

Keith VE7GDH wrote:
>
> That could actually be several crystals if the others are like the 80M
> Warbler. For that one, six crystals would be needed as they were used
> both in an oscillator and in a couple of bandpass filters. I actually
> looked into it last year. The cost of the crystals could change it from
> a budget project into an expensive one, but if you could get the
> crystals at a reasonable price (bulk purchase anyone?) it could be an
> interesting project. I just looked at some of the other models and at a
> glance, it looked do-able. In the 80M Warbler, all the crystals were the
> same frequency. In the others, several crystal frequencies would be
> needed for the change.
>


The Warbler is essentially a TRF (Tuned Radio Frequency - allusion to 
the EARLY days of radio!) receiver with a product dectector at the end 
instead of a diode detector for AM.   The receiver consists of a "lump 
of gain" at the operating frequency; i.e.no IF frequency.  All 
selectivity is actually on the operating frequency.   It just so 
happened that the customary operating frequencies for PSK on 80 meters 
coincided with the standard NTSC 3.579 MHz color burst frequency.  The 
designer took advantage of the cheap crystals to put all the selectivity 
at the operating frequency by constructing a lattice filter entirely 
made from color burst xtals.    [SIDE THOUGHT:   Are 3.579 color burst 
xtals going to become rare hard-to-find commodities with the switch to 
ATSC (digital) TV???]


The PSK-20, -30 and -40, on the other hand,  are true superheterodynes 
with an actual IF system at a fixed frequency regardless of operating 
freq.   They would only require changing a single L.O. crystal to tune 
onto a new frequency in the same band.



     http://smallwonderlabs.com   
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Stephen H. Smith    wa8lmf (at) aol.com
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