[Lowfer] trimmer for pierce oscillator

[Dick Cappels]

Add capacitance from the base to ground and from the collector to ground.
Make the capacitors as large as you need to in order to obtain the desired
frequency shift. BUT -you may well find that you cannot pull the crystal
this far before it stops oscillating.

To drop the frequency a little lower, remove the crystal from the package
and rub some solder or pencil lead on it to add mass. If you are lucky, you
might get to pull it all the way down to 290 Hz lower than it is now.

Another approach, which I can only vaguely remember something about, is to
run the crystal in series mode (the connection in this oscillator is in
parallel mode). This will shift the frequency significantly, though I cannot
remember whether it would shift it up or down. I hope somebody else can
remember.



On 5/25/07 10:56 PM, Douglas D. Williams at iguana at chartertn.net wrote:

> Years ago I built an LF converter from a design by Doug DeMaw and Jay
> Rusgrove that appeared in the June, 1977 issue of QST. It has proven to be
> an excellent design, outperforming two different commercial converters that
> I have tried out.
> 
> The local oscillator circuit in the converter is a simple Pierce oscillator
> designed around a 2N2222A transistor and a 1.7 MHz crystal. The crystal I
> have is 290 Hz high and I would like to add a trimmer capacitor to be able
> to adjust the frequency dead on so I don't have to do any mental gymnastics
> when trying to tune a certain frequency.
> 
> Here is a link to the schematic of the oscillator circuit:
> 
> http://i81.photobucket.com/albums/j231/Goranothos/oscillator.jpg
> 
> 
> My question is where should I add the trimmer capacitor and what value
> should I use? 
> 
> Thanks in advance for any help.
> 
> -Doug KB4OER
> 
> 
> 
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