I'm playing with Bob Weaver's excellent superhet tracking calculator
and am surprised at the large effect on tracking of tiny changes in L or C, and am wondering how one actually aligns such a tracked set in practice since small errors will have large effects on the tracking.
For example, my SW1 range will be 3000-6000 kHz, with an IF of 2000 kHz. My varactors go up to 530 pF. Plugging this into the calculator gives a nice tracked response:
OK, so I try to substitute the capacitance values with some caps I have on hand, and the tracking gets way off:
Even a change of 0.01 uH in the inductance makes a huge change in the tracking. Here the LO coil is increased by 0.01 uH.
Here the LO coil is decreased by 0.01 uH.
So I'm wondering, how do you do the alignment in practice if tiny changes in L or C can throw the tracking so far off?
One idea I've considered is to make the front-end aerial tank oscillate at its resonant frequency by attaching a JFET and a tickler coil to the aerial coil. Then as the LO tank capacitance is tuned (ganged with the aerial tank capacitance), the front-end aerial tank's oscillation frequency will also change, above and below the IF of 2000 kHz. Assuming I have a working 2000 kHz product detector attached to the mixer output, I should be able to audibly "hear" the tracking as an audible heterodyne and hear as it goes above and below the IF of 2000 kHz. The LO tank L can be adjusted (in 0.01 uH increments!) until the tracking is reasonably correct.
I feel there is probably a better way to do the alignment. Any suggestions?
Also, I was considering hand-winding toroidal coils for the LO and aerial tanks, but seeing as how changes of 0.01 uH make such a large change in the tracking, I don't see how to adjust the L in such small increments.
Maybe it's just not feasible to homebrew accurately-tracking LO/aerial tanks over such large frequency ranges due to the precision required. Thoughts?