Doug has thrown out the 38s DX challenge.
That means many will be lurking down at the bottom
end of the band, where the 38s's tuning range is
pretty quick.
The tuning pot controls the D.C. VXO voltage
going to the 1N4004 tuning diode. One suggested
solution to help linearize the tuning range adds
a resistor from wiper to ground. Good idea.
This resistor will most affect the top end of the
band, where tuning is very slow. But the bottom
end could have still slower tuning.
So we need a non-linear element to give
us something like an exponential rise in VXO
voltage, while the pot wiper voltage rises
linearly. Sounds like some kind of diode fits
the bill. Here's one possible circuit:
+8v_____
|
| ____330K____
P1 | |
100k | diode | 17
tuning /___|___|\|_____|_____....to VXO
pot \wiper |/| |
| |
| 100k
| |
gnd gnd
The diode will most affect the tuning range down
at the lower end of the band, stretching it out. You'll
lose a little frequency range, because the VXO
voltage can't get all the way up to +8v. So the rig
won't go quite as high in frequency as it did before.
You could use a 1N914 type diode here, but
a RED light-emitting-diode will extend the slower tuning
a little higher in frequency, a GREEN L.E.D. will
extend a bit more.
You can play with the two resistor values
to change the tuning effect. Decreasing the 100k
resistor will slow the tuning more, as will increasing
the 330k.
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