Tuneable LF Converter Schematic

 

Below is tuned circuit data for the 4 band LF tuneable converter. A 10-365 pF 3 gang air variable tuning capacitor is used in this design (Delta C = 355 pf) with a minimum capacitance of 10 pf. All frequencies are in kHz.

f.l. = low freq tracking point ......f.i. = midfrequency tracking point ........f.h. = high frequency end tracking point

The constants allow exact tracking at these three frequencies in each band. In a superheterodyne receiver there is always a small tracking error at most frequencies in the band. This can be minimized by careful choice of oscillator coil inductance, and padding and trimmer capacitance.

The local oscillator operates on the high side of the signal and will be found between 630 and 1155 kHz depending on signal frequency.

The oscillator coils are assumed to have 30 pf distributed capacity. RF circuit constants tuning to the signal frequency are shown on left side, and the padder and trimmer values for the oscillator circuit are on right. The values shown are for a converteroutput IF frequency of 620 kHz

Coil Q of 60-80 is assumed (typical for smaller coils in this frequency range). Below is also shown the calibrated dial scale for this converter. It will be correct only if the exact components specified are used.

This information is presented for illustrative purposes only. The original converter was constructed in the early 1970s and the parts specified were easily obtainable at that time. This converter is still in use (Spring 2010) and continues to work very well. Some parts (coils, tuning capacitor, switch) are no longer generally available from distributors, and may now be difficult to obtain except from sources such as flea markets and from individual hobbyists. Coils will probably have to be made from scratch.

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