[K6BW] VFO Drift

[Bill Smith]

Here is an excerpt from an e-mail concerning the origins of drift in an
analog circuit.  The email originated from the qrp-l newsgroup.
Not quoted, he goes on to recommend DDS circuits.  :-)

<snips>

Drift can come from many sources, which makes it hard to pin down
sometimes. Most of the drift comes from the caps and inductor.

Drift in the inductor is mostly from external temperature changes, which
expands or compresses the wire and of the coil form or permeability of the
core.

Ceramic disk caps are prone to self heating. They have an AC impedance, so
just like a resistor, AC currents flowing through them can make them heat
up and their value to drift. That's why some VFO circuits use several caps
in parallel, to distribute the currents and reduce the internal heating.

Polystyrene caps are zero drift, more or less, and are much less prone to
self heating. Therefore, they reduce warm up drift. So long as the ambient
temperature stays about the same, drift will be minimal.

Once you get the initial warm up drift under control, drift over a range of
ambient temperature changes is a whole 'nother can of worms. If your lucky,
all the parts will drift in equal and opposite amounts, canceling each
other out. This is rarely the case, which makes finding the right
combination of parts often time consuming and frustrating.

Steve, KD1JV
http://www.qsl.net/kd1jv/
http://kd1jv.qrpradio.com