Mark S. asks:
> For you more experienced homebrewers - can anyone tell me why varactor
> diodes are not more widely used in QRP VFOs? It seems to me that their
> much smaller size and mechanical stability, compared to air variable
> caps, would make then ideal for QRP gear, especially backpackable gear.
Yes and no. In addition to the undesirable temperature characteristics
others have mentioned (though compensation techniques are well known), the
DC you apply to the varactor must be absolutely clean or it will frequency
modulate the oscillator's output. Which is how many FM transmitters do it,
but is not what you want for a CW or SSB transmitter.
This includes control voltages. The pot you use to adjust the oscillator
frequency must be a good one to avoid transients making a mess of your
frequency, and also so that it won't wear out from excessive use, which
cheap pots aren't designed for. You want the highest quality 10 turn pot you
can lay your hands on for such applications.
Since any reverse-biased PN junction will exhibit a space charge and thus
variable capacitance, many projects that use varactors can work fine with
many diodes. Zener diodes can work well as varactors, and even a lowly 1N914
will do well if you choose the right one (the Ramsey DC receivers use them,
with a Zener diode for voltage regulation). The junctions in many bipolar
transistors will work well too.
73 from Burnaby
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