CRYSTAL MIXER EXCITER FOR 137KHZ
All my attempts to construct any sort of VFO/synthesiser/divider have
ended in failure. I am not sure why I have such a reliable failure
rate with digital circuits - I strongly suspect incompetence.
It is not the case that my amplifiers oscillate and my oscillators
don't. I seem to be able to make ugly construction RF and AF
circuits that work first time so I was looking a solution that did
not involve digital circuits.
From crystals box I found two crystals on 10489.37 and 10355.0 kHz
respectively. I built two FET Colpits oscillators, firstly with a
variable capacitor across the lower frequency oscillator crystal but
then found that it worked best (even output across the frequency
range) with the variable capacitor in series higher oscillator crystal.
While trying to decided on a suitable mixer/filter I saw this e-mail
from Peter DF3LP who described a very similar exciter. Peter kindly
sent me a copy of his circuit (which uses 5MHz crystals) , which gave
me enough ideas to complete the exciter.
Apparently the first circuit of this type (that I am aware of) was
built by DJ1ZB.
The main attraction of this circuit is that if both oscillators are
built in the same enclosure with identical components the output
frequency is held constant because any drift in one oscillator is
compensated for by an equivalent drift in the other.
My circuit is made from junk box components and the measured drift
was 137.6530kHz to start and 137.6524kHz after 3 hours.
In my last e-mail I described a SCW QSO with DJ8WL (using the above
exciter I might add) whos name I gave as Geri! Sorry Peter.
Regards, Peter, G3LDO