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'Trying to build a VCXO'
1999\01\04@072418 by Jens Peter Jacobsen

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I am trying to build a VCXO to make a 16C54 control its own oscillator
frequency. I need it to oscillate a little below and a little over 20 MHz. I
tried different cirquits with a 20 MHz parallel resonance crystal and a
BB405B varactor diode, but the oscillator always oscillated ABOVE 20 MHz
unless I have large caps to GND. (around 100 pF on each crystal pin) But the
oscillator became unstable and stopped running for a while whenever the
capacity of the BB405 was changed.

Has anyone ever made their own VCXO ?
Any Example cirquits ?

Jens

1999\01\04@082712 by Jens Peter Jacobsen

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> I am trying to build a VCXO to make a 16C54 control its own oscillator
> frequency. I need it to oscillate a little below and a little over 20
> MHz. I
> tried different cirquits with a 20 MHz parallel resonance crystal and a
> BB405B varactor diode, but the oscillator always oscillated ABOVE 20 MHz
> unless I have large caps to GND. (around 100 pF on each crystal pin) But
> the
> oscillator became unstable and stopped running for a while whenever the
> capacity of the BB405 was changed.
>
> Has anyone ever made their own VCXO ?
> Any Example cirquits ?
>
> Jens

1999\01\04@095401 by Jochen Feldhaar

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Hello Jens Peter,

I did build such VCXO several times, for 4.4 and 8.8 MHz, and I always
found that including an inductor between the crystal and the tunable
capacitor to GND (varicap...) will give VERY extended tuning ranges.
You will have to experiment a bit with the inductance.

If you will use this on a PIC as standalone oscillator, you have a problem,
because I reckon that the gain from the PIC is not high enough to feed the
crystal hindered by the inductor. So an external transistor will be needed,
connected as a crystal oscillator.

I have found that a 16 MHz signal can be moved as much as 500 KHz below
nominal frequency using this method. BUT: The precision that comes along
with a crystal is now dependent almost solely on the tolerances in external
circuitry. There is no such thing as a very stable and very variable VXO.

Hope this helped...

Greetings,
Jochen Feldhaar
spam_OUTjfTakeThisOuTspamdetektor.de

1999\01\04@165353 by Dwayne Reid

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>I am trying to build a VCXO to make a 16C54 control its own oscillator
>frequency. I need it to oscillate a little below and a little over 20 MHz. I
>tried different cirquits with a 20 MHz parallel resonance crystal and a
>BB405B varactor diode, but the oscillator always oscillated ABOVE 20 MHz
>unless I have large caps to GND. (around 100 pF on each crystal pin) But the
>oscillator became unstable and stopped running for a while whenever the
>capacity of the BB405 was changed.

Just a quick thought - did you use a capacitor between the varacter and the
crystal?  If not, the DC bias on the varacter is probably upsetting the
oscillator.

dwayne


Dwayne Reid   <.....dwaynerKILLspamspam@spam@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(403) 489-3199 voice     (403) 487-6397 fax

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