Re: Pulling crystal

On 5 Nov 2005 05:16:02 -0800, "Apparatus" <apparatus.home@xxxxxxxxx>
wrote:

>Hi,
>
>I am working on building a VCO for FSK with 850 Hz separation between
>mark and space. I am building it using a 16.000 MHz 32pf parallel load
>crystal. I am pulling the crystal via a varactor to implement the FSK.
>However, the amplitude between mark and space is an unacceptable 10 dB
>with about 700 Hz of shift. Any ideas on how to pull the crystal
>without changing the amplitude of the mark and space frequencies
>considerably?
>
>A bit about my circuit: have an 8 - 50 pF tuning cap across pins 6 and
>7; a 22pF cap shunts pin 7. The crystal shunts pin 6. A 100nF DC block
>cap separates pin 6 from the 8 - 29 pF (3 to 10 V reverse bias)
>varactor. A 1mH inductor isolates AC from the varactor biasing.
>
>I tried adding 1uH of inductance in series with the crystal. This moved
>the oscillation frequency up, but the amplitude problem remained. I
>tried putting the varactor across the tuning cap. This configuration
>exhibited the same amplitude shift problem, but shifted the frequency
>up rather than down. I tried combining the two shifts via two varactors
>and a JFET to invert the biases, one across the crystal (as originally)
>and one across the tuning cap. This didn't work well (very noisy
>spectrum).
>
>Any ideas as to what could be the problem and how to fix it?
>
>Cheers,
>Chris

I'd look at your circuit. Thats a very large change for a small shift
at 16mhz. Nominally I'd expect only a tiny change at 16mhz and no
where near 10db. From what you say it sould like your using
a NE/SE602 or similar as the osc and it may be the constants
you are using are off. it also may be that the varactor you using
is getting foward biased and that will alter the oscillartor output.

For a simple shift a series cap and a diode switch is adaquate for
a shift like that. To dial it in the cap can be variable. There are
plenty of published circuits that already do this in various CW
tranceiver designs for RX to TX shift (sidetone too).

An alternate is live with it and add a limiting stage after it
(74HC04s) to take out the amplitude change.

Allison
.

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