[Lowfer] Hifer FSK?

[Ed Phillips]

Bill Ashlock wrote:
> 
> >At best that's a lousy way to FM a crystal oscillator.  For the small
> >shifts which would probably be employed in a beacon a varicap in series
> >with the crystal would be preferred.  You might have to build the
> >oscillator but that's no big deal.
> 
> Ed, the Xtal route is that's exactly what I've been using over the last four
> years with my patented slant ID - and the FM modulation possibilities are
> much wider than the .2 to .5 Hz I typically use on QRSS30 and 60.....
> possibly 500ppm or 100Hz!
> 
> I still wouldn't rule out FM modulation of the Epson oscillators as long as
> most of the ID occurs when the Vcc is at 6v. This way the base line of the
> signal should be flat and of course that's all that counts, right? Could be
> an easy way to achieve some unique IDing - and that's worth a million when
> it comes to finding a beacon in a screen full of noise and misleading
> signals.
> 
> Bill A

	If it works I guess it's OK, but the sensitivity of the frequency of an
XO to the supply voltage is due to such uncontrolled factors as the
change in junction capacitance with voltage (have no idea what the
internal oscillator circuit looks like) or even change in amplitude of
oscillation.  Since the guys who design these things presumably have
tried to minimize these effects, the residual is pretty unpredictable.  

	The varicap method is much more controllable, even if more complicated,
and is certainly capable of much wider frequency deviation.  You can go
beyond that by using a series LC circuit.  At resonance it does nothing
but add a little resistance in series with the crystal, with low values
of C the frequency is higher, large values the reactance is inductive
and the frequency is lower (than series resonant value).  Not worth the
trouble unless you want a lot of swing and, of course, the added tuning
elements in general are less temperature stable than the crystal.  For
the FM ranges being considered here such complications certainly aren't
justified.

Ed