Varactor VFO Drift (i.e. Centenial 75m)


From: Daniel C Winkler (
Date: Mon Jun 23 1997 - 20:46:10 EDT

Hi gang,

On Fri, 20 Jun 1997Don C. Faith wrote:

>... mods... to reduce the amount of drift from the Varactor VFO ...?

      Doug DeMaw addressed this problem a long time ago. He
wrote an article in QST (Sept 87, p25), which is reprinted in
QRP Classics (p111). He used a temperature-compensating
diode to reduce the varactor drift.

      The 7 MHz VFO is tuned with an MV2109 varactor. The
varactor is RF isolated from the tuning circuit by a 500 uH choke.
Tuning voltage (1.6 to 7.5 volts - from a 10k pot with appropriate
resistors at each end) goes through the base---> emitter junction
(diode) of a 2N3904 NPN transistor (collector left floating). The
emitter is tied to the "cold" end of the 500 uH choke, and to a
270k resistor going to ground, allowing some current to flow
through the BE junction. This diode, the2N3904 BE junction,
acts as the temperature compensation diode.

      Doug wrote:

      "As the ambient temperature changes, so does the resistance
 of the... diode. The small resultant resistance change causes
the reverse voltage at D1 to change slightly, thus compensating
for changes in the (varactor) diode capacitance that are caused
by heat."

      The oscillator is a Colpitts type, using a 2N2222A. He used
680pf polystyrene caps from base to emitter and emitter to ground,
and NP0 caps (100pf) for the tank and coupling caps. The inductor
was a 2.6 uH slug tuned inductor (#6 material for its temperature
characteristics, Amidon L-57-6 transformer assembly).

      "I measured the drift at room temperature (72 F) with the cover
in place. The initial drift took place in a 30 second spurt.
the drift was gradual, and stabilization (+/- 2 Hz) was noted after 10
minutes. The long term drift was measured as 80 Hz. Do not
measure your VFO drift for at least an hour after all soldering on the
PC board is completed. The VFO module should be mounted
in place and enclosed in a cabinet before measurements commence.
Even slight stress on the VFO board will cause frequency changes."

      I guess the circuit never caught on, for I never saw anyone else
use it in a published article.

      Please do not ask for a copy of the article. I just don't have
time to do that.
73, Dan Winkler N7IVR Seattle, WA


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