In varactor tuning, your goal is to maximize the capacitance
variance from one end to the other.
Therefore, the real questions are two: what varactor is
being used, and what is the voltage range it is operating
under.
Note that most datasheets show varactor variance at
best linearity and maximum variance at a particular
voltage range, usually the lower end. So the first thing
is to determine the voltage range. Next, figure out a resistance
range that will exploit that. Usually, the voltage range is about
8v, and the full resistance range is about 10-15k.
It is a relatively simple matter to use 2 pots in series, one of
a higher value to set the frequency position, and one of a lower
range to fine tune. The only real advantage of a 10-turn pot is
to get better gross resolution, but a 10k and 1k regular linear
pot in series will accomplish about the same thing.
As to adding capacitance, that is what the varactor is doing.
It may be that more capacitance is needed to change the
base frequency of the VFO. Usually experimentation is
required, though a rough estimate can be made by figuring
out the total LC ratios. Where to add that capacitance
is another question: probably it will require some in the
main tank and some in the DC isolation of the varactor circuit.
For instance, I modified the VFO for the BITX20 v.3 as follows,
following the example of the BITX20A:
Q5 is a 2N3904; the two main VFO caps are 470 pF polystyrene.
The L4 coil is a T68-6 with 40t, measuring about 9 uH free-standing.
One of the secrets to a stable VFO was paralleling that coil with a
180 pF s.m capacitor. Main tuning is with a 365 pF variable cap,
in series with 2 parallel 220 pF caps, and then into the VFO circuit
of 5-45 pF range-finding cap in parallel with a 220 pF s.m. cap.
The basic tuning range is about 2.895 - 3.295 MHz. The original
test tuning range without the variable cap and a 220 pF in its place
was 2.95640 MHz to 2.965445 MHz (varying the 5-45 pF cap).
For fine tuning, I added a 78L08 regulator running to a 10-turn pot
with a 1K resistor to ground. That ran through a 200K resistor to
a MV2109 (to ground) and then to a 100 pF npo cap leading back
to the main oscillator. This gave a full range of about +/- 25 KHz for the fine
tuning.
So, in essence, the varicap mod added fine tuning. The main VFO
remained the principal tuning for the rig. It works well and is quite
stable.
The IF is 11.092 MHz, not the 10 MHz of the original BITX.
All of this information comes from my original pages of builder's
notes form Leonard's manual. I can publish the pages if you want,
but they are a bit hard to read if you didn't make them up originally.
The rig tunes from about 6.95 to 7.34 MHz.
Hope this helps.
john
AD5YE
