Post by thread:Trying to build a VCXO

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

> 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

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_OUTjfTakeThisOuTdetektor.de

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