On 24 Apr 2007 at 22:44, Dave G3WCB wrote:

> I've done a few tests with trying to "pull" crystals and resonators up
> into the 501-504 kHz band. I used a single NPN transistor in a Colpits
> configuration with a 3 - 30pf trimmer capacitor in series with the
> crystal.

Dave's tests make interesting reading, and it is because of this 
difficulty of pulling 500kHz crystals in VXOs that I did not even 
consider doing it this way.

My twin VXO, as previously mentioned, uses 5.9 and 6.4MHz crystals. I 
had already used a VXO in my 5MHz rig and the circuit I used is 
virtually identical to that one - see 
http://www.davesergeant.com/fivemegs/60mtx.htm

I made the 6.4MHz crystal the 'tunable' one and found with a single 
50pF variable cap in series it would pull around 4kHz high. I needed 
to pull the 5.9MHz one low to give a 501kHz difference and achieved 
this by replacing the series cap with an inductor. I tried quite a 
few inductors (miniature fixed parts) and was amazed how far I could 
pull it - the first attempt with 100uH dragged it down about 20kHz! I 
found 10uH achieved the required 1kHz downward shift - this vxo is 
fixed, all tuning is done by pulling the 6.4MHz crystal.

In place of the fet buffer in the 5MHz design I replaced it with a 
3N201 dual gate mosfet and used this as a mixer. A 470kHz mw if 
transformer conveniently tuned the output to 501kHz.

This method seems to be a very staightforward way of getting on the 
band. I have quite a few crystals in the 5.5-6.4MHz range which I 
could offer to others in 500kHz spaced pairs.

I have been diverted the past few days on other matters but hope to 
get the pa side wired up pretty soon.

73 Dave G3YMC


http://www.davesergeant.com