Quote:
Originally Posted by Neil Purling
I do remember that when I tried a Superhet before I had a variable capacitor with 2 equal gangs & it took a 1000pf trimmer across the osc coil to get the necessary off-set. Not sure whether there's the room to mount a coilpack under the chassis.
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This is one of the most important parts of a superhet design but often gets glossed over and some just pick a padder capacitor value from another set when making a homebrew superhet.
Normally, if the two gangs on the V/C are equal capacity, the gang for the oscillator has a capacitor called a padder, placed in series with that gang, to lower the overall capacitance, but it has to be the correct value.
Normally the local oscillator runs above the received frequency by an amount equal to the IF frequency.
For correct tracking the oscillator minus the IF frequency, for any mechanical position of the V/C's shaft, equals the received frequency peaked by the antenna tuned circuit on the other gang on the V/C, but this can only happen in three exact places, near the beginning of the band, in the middle and near the end. Between those places are tracking errors.
Therefore, in other places on the dial the tracking is always off by a little (tracking errors) with an error below and above the mid-band received frequency. In these places the gang tuning the antenna coil is peaking the resonance of that at a frequency a little above or below the frequency of the oscillator minus the IF (which is the received frequency), so its "mis-tracking" in these zones . When the padder capacitor value is right, the tracking errors on each side of center are equal and minimal.
The best way I have found over the years to get the padder value right was initially with the equations in Terman, but it uses a side of A4 and wears down a pencil, then a number of software utilities appeared on the net to do it, making the whole process so much easier.
In transistor radios you will see often the gang on the V/C for the oscillator was a smaller capacity anyway and this dispensed with the need for the padder capacitor and the V/C itself is manufactured for the correct tracking.
In a lot of valve radios, the padder capacitor itself was also used as a tool for setting/trimming the low end of the band oscillator frequency when there was no slug or no adjustable slug in the oscillator coil.
I'm sure there are a lot of folks here very familiar with the tracking and padder issue and can make remarks or corrections to the above.