From: Ken Scharf on Jul 7, 10:19 pm

wrote:
From: Ken Scharf on Sun 26 Jun 2005 22:41
wrote:
From: Ken Scharf on Jun 25, 6:26 pm
wrote:


The 1206's dial only goes up to 400khz, but it tunes past that. Maybe
it goes to 420 or 450khz, I don't think it goes as high as 500khz.
(so maybe I don't have to expand the range for my needs).

The variable tuning capacitor max:min ratio is roughly 11:1 (no
external parallel capacity). Since the resonant tuning range is
the square root of that, you could have it tune 190 to 570 KHz
with a 9:1 max:min change in resonance capacity. The only real
problem is getting the variable LO tuning to track the front end
since it would tune 325 to 705 KHz (for the high-side) and that
would be a 2.169:1 frequency ratio or 4.706 capacity change ratio.

Well that doesn't take into account the distributed capacitance of the
coil windings, which would be considerable at this frequency what with
the required inductance. (ever wonder why a grid dipper covers a much
higher min-max frequency ratio as the frequency band goes up?) Also
IIRC there are trimmer caps in parallel with the tuning caps for
alignment as the coils are fixed. To make the oscillator track, a
series padder cap is used to reduce the max. capacitance value of the
oscillator section. -OR- a parallel padder could be used to swamp out
the range of the section.

As an aid to setting a tuning range, check out the February 1977
issue of Ham Radio magazine (under my byline). It covers adding
parallel caps, series caps, series-parallel and parallel-series
combinations. Easy formulas, all algebraic.

Worst case distributed capacity with typical inductances used at
this frequency might be 12 pFd.

Given a 32 to 352 pFd variable min:max, the C ratio = 11:1

Add 12 pFd to each section and C ratio = 8.0889:1 and F ratio =
2.8441:1. Add a total of 18 pFd in parallel and C ratio - 7.3800:1
with F ratio = 2.7166:1 and that is good from 190 to 516 KHz.

There's a near-infinite number of solutions for series-parallel
combinations for the LO but only a few will track adequately
with the antenna and RF amp stage. I suggest doing a simple
computer program for that kind of thing. It can check resonance
of the LO tank against the Antenna-RF tanks and find the
difference in resonance at as many frequencies within range as
you want.