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HR40 • 21
In planning any ‘bandspread’ improvement, it is useful to understand exactly
how varactor tuning works. The control level varies the amount of DC voltage
applied to the diode. The highest produces the lowest capacitance, while the
lowest voltage yields the highest capacitance. The spread is typically about 30
to 40pF. from minimum to maximum.The goal in achieving bandspread or
‘finetuning’ is to achieve a smaller variation in capacitance with the
comfortable tuning of a knob.
Any resistor in a range from 500 ohms to 3000 ohms, connected in series with
the ground lead of R2, will set a limiting effect on varactor capacitance range
and therefore create a “bandspread “ effect. The higher the resistance value,
the shorter the available tuning range.
If you wish to reduce the tuning range of R2 down to 40 or 30KHz or even less
for a favorite band segment or calling frequency, reducing the value of C4 will
decrease the capacitance effect of the varactor diode, D1. Values from 2 to
5pF may give you exactly the tuning range you are looking for.
If you are using your own enclosure and have room for a separate fine tuning
control, one useful choice is a 250 ohm speaker attenuation rheostat (Radio
Shack model no. 271-265) wired in series with either outside ‘leg’ of the main
tuning control. Such a control can make fine adjustments to a CW signal so
the tone is pleasant and smooth sounding.
If you want the opposite of bandspread (that is as MUCH frequency range as
possible over full rotation of the tuning knob ) increase the value of C4. This
will cause the varactor diode to have greater effect on the tuning. Try values of
33pF up to 100pF.
FREQUENCY STABILITY:
A kilohertz of drift is not a big deal in FM circuits or a shortwave broadcast
configuration, but ANY frequency instability is annoying when trying to receive
CW or SSB. The varactor - controlled Local Oscillator is indeed a VFO, and
therefore needs all the consideration given to any VFO circuit. Even though
this tunable oscillator design takes good advantage of the SA602’s capabilities
and offers nice tuning range from a simple varactor circuit, we do not
represent it as suitable for transmitter frequency control or for demanding
receiver applications. In any ham VFO design, ANY unshielded or
uncompensated oscillator components are susceptible to the influences of
temperature change and of nearby moving objects. The use of a suitable
enclosure and secure mounting of the PC board within that enclosure will
maximize the stability of the oscillator. One to two KHz of slow drift may be
expected as components warm to operating temperature; after that the
oscillator is reasonably stable