Crystal VCO
 

Hi all,

I have a colpitts crystal oscillator with a 16.000 MHz parallel
resonance crystal that I'm turning into a FSK modulator with the
addition of a varactor / varicap diode. Two questions:

Originally, I had the varicap diode in parallel with the crystal. This
caused the oscillation to significantly attenuate (10 dB or so) as I
pulled the crystal 1 - 2 kHz. I found a reference design of a Hartley
oscillator that had two varicaps in series with a resonance capacitor,
so I decided to give this a try. This works a lot better, attenuating
my 1Vpp oscillation about 100mVpp. Why does the series configuration
work so much better?

I'm at home where I only have an analog tektronix 2213 60MHz scope (no
spectrum analyzer). How can I measure the frequency deviation / pull of
the crystal with the series configuration? Any ideas?

Cheers,
Chris

Crystal VCO
 

....usually, a crystal oscillator has a quite high voltage developing
across the crystal AND the varicap which is in parallel with it. this
AC voltage adds and substracts from the DC control voltage, causing the
instantaneous voltage to change and thus vary the capacitance. the net
result is a damping effect of the oscillations' amplitude.
when you have two diodes in series the AC is causing the voltage to
increase on one diode and decrease on the other. being in series, they
compnesate each other in some measure thus reducing the effect.the
additional cap you mention is more of a DC blocking capacitor, more
than anything else.
As for frequency measurement, the only real way is a counter. lacking
one, zer boat the oscillator on you radio, change the tuning voltage to
the other extreme and guesstimate the pitch of the beat note. nothing
much else you can do, except, may be, see if it works OK when
modulated.
Alex 4Z5KS