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
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