Equal Value Superhet Tuning Capacitor Tracking Calculator
In superheterodyne radio receivers, a local oscillator (LO) must produce a frequency greater (or less) than the received frequency by a fixed value, the intermediate frequency (IF). This is then mixed with the received signal to produce the desired IF signal for the succeeding IF amplifiers.
Early radio sets used two basic approaches to the design of the tuning capacitors. One approach had two dissimilar variable capacitors ganged together, one was for tuning the station frequency, and the other was for tuning the local oscillator. Obviously, with the correct design and adjustment of these two capacitors one could (theoretically) achieve perfect tracking of the IF.
Another approach was to use a dual-ganged tuning capacitor where both sections were identical. A “padder’ capacitor in series with the oscillator section would cause the oscillator frequency to track at a fixed frequency above the received frequency. However this arrangement produced only a close approximation of the intermediate frequency required. This simple spreadsheet produces a graph of the deviation from the ideal IF across the AM broadcast band. It allows the selection of all capacitor and inductor values, and permits “tweaking” of the trimmer and padder capacitors for optimization of the tracking error.
(This spreadsheet was originally published on the web in response to discussions on the Antique Radios Forum regarding a one-tube reflex superhet design.)
Tracking Calculator Spreadsheet Instructions
Select the maximum variable capacitor value, Cmax(LO,RF) in Farads (i.e. 365E-12).
Select the desired IF frequency, IF (kHz)(i.e. 455).
Enter a value for C Step that causes dial scale 30 to reflect the usable upper range of the tuning capacitor (C Step is usually around .90 to .99).
Or first select an RF inductance (or antenna coil) value, L(RF), to give you the lowest desired frequency (dial scale 1) in the RF(kHz) column, and then select a C Step value that gives you the upper frequency in the RF(kHz) column.
Now work with different values of L(LO), C LO Trim, and C LO Pad to obtain acceptable tracking as viewed by the graphs. Observe the relationship of these components to the amount of tracking or mistracking.
The circuit modeled is a variable tuning capacitor and a trimmer capacitance in parallel. The padder capacitor is in series with this parallel combination.
This spreadsheet has been checked for accuracy, but is not guaranteed to be error free.