Dear OM Peter, G3LDO,
I also had bad results with VXO drift both in single X-tal and in mixed
settings. I found out that the old big type X-tals in the range between 5 and 9
MHZ are most suitable for use in VXO's. These x-tals give a tuning range well
over a 100kC's where the small type X-tals will not tune over a range of more
than 30-40kC's The coil in series with te X-tal can play tricks as far as
stability is concerned. This can be observed by putting your finger near the
inductor. I found out that when the VXO does not return rapidly to its
original frequency i had constructed a VFO instead. I had better results when
two or more inductors (range at least 1:10 in uH) are put in series, this also
limits the influence of paracitic capacitance in the inductors. An other test i
found out is to warm one of the x-tals with a finger and observe the drift back
to the original frequency when 'cooling off'. When the original frequency is
reached i am sure the X-tals have the main influence on the osc
illating frequency.
An other point a observed is the value to be used for the capactors dividing
the voltalge in the collpits should be carefully designed. To mutch
amplification makes the oscilator easy to start with a rock-solid frequency. To
little amplification will not allow the circuit to oscilate at all. Sometimes a
extra buffer/amplifier stage is needed to make sure the mixer works better but
this does not influence stability.
Good luck on LF and best 73's
Mike, PA3BSH
Hoogeveen, The Netherlands
e-mail also to <[email protected]>
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