[aprssig] HF APRS Transceiver

Steve Noskowicz noskosteve at yahoo.com
Mon Mar 10 16:02:24 EDT 2008

I've designed crystal oscillators and like the typical Engineer, have some
minor clarifications...

--- Robert Bruninga <bruninga at usna.edu> wrote:

> But the issue was always stability.  It was just not achievable
> with simple XTAL oscillators.  And no sense sending a boat to[...snip...]

   I can't address what has previously been tried/tested/done, but can echo an
earlier comment. Temperature variation is the issue here.   If this is sitting
at a fixed location at a relatively constant temperature My opinion is that it
should work quite well once the crystal (say an inexpensive 20-50  ppm type) is
'warped' on frequency.   
  Actually, if you tell the mfgr that it is for a room temp application, they
_might_ be able to use a blank which varries very little over +/- 10-15 degrees
F - but that's without knowing if they can cut a blank like that cheaply since
it may be unusual to do that.  I know they _can_ do it, but there's a cost if
it's out of the ordinary.

> sea if it could not stay on channel without constant attention.
> (Better than +/- 10 Hz).

   If this is what's needed, it's pretty darn tight.  I know-not what the APRS
decoders can handle, but am surprised to hear that 300 baud is that tight.

> True, but it is a very special calibrated, high-stabilty
> frequency reference grade xtal produced on a standard frequency.
> Such a xtal custom made on a single APRS frequency would cost
> more than the cost of the rest of the transmitter.

   I wouldn't characterize it quite like that.    They are what I would call
medium stability.  I'm used to relatively inexpensive crystals being 15-20 ppm
(over temp).  Metter than 5 ppm is considered more expensive.  The TH-F6
handheld spec it 8ppm  -20 to +60.  
  Perhaps I've been off the bench a while, but I would expect a 15 ppm crystal
to cost under $20 US. (some inflation correction applied)

> > ... most of the old style land line modems were only 
> > crystal clocked at best, and not a high spec xtal either! 
> Yes, because 20 parts-per-million at a moden tone of 1200 hz is
> only 0.02 Hz 

   Ditto.  Ya gotta watch those ppm (:-)

73, Steve, K9DCI

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