In a message dated 8/13/02 2:54:19 PM Eastern Daylight Time,
[email protected] writes:
<< I am worried about drift but also thinking that they may be stable enough
if kept in the 1 - 2MHz region. As well, they should (if identical) drift by
the same amount and thus be self-correcting? Is this a reasonable avenue to
pursue or should I just break down and buy a couple of xtals?? >>
I'd go for the crystals, Steve. The stability of even relatively inexpensive
crystals in the HF range is much better than you can expect from VFOs.
Your oscillators will drift by the same percentage amount, not the same
absolute frequency amount. Remember those halcyon days of algebra class, and
working with the good old associative property? Lets imagine two 2MHz
oscillators each maintaining a rather good 50ppm over the temperature range
of the room in which they are kept, and if they do track comparably, f1 will
have a potential drift of 50ppm of 2MHz and f2 will have a potential drift of
50ppm of 2.136MHz (that is, a few more Hertz of drift than the f1
oscillator)--which, when one takes the difference of f1 and f2, is the same
error as 50ppm of (f2-f1). Thus, the output has the same percentage
tolerance as the original oscillators.
Unless they were unusually well stabilized, VFOs would not be good for
more than the shortest QRSS dot times, and probably not at all suited for a
JASON transmitter.
73,
John
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