A New System of Inductive Tuning

A sliding-contact variable inductance is described wherein contact reliability over long life is secured by a unique carriage arrangement and parallel nibs at the point of contact. The coil is rotatable, the high-potential contact being taken off a ring at one end. The contactor and unused end of the coil are at ground potential. The entire variable coil is used and the high-frequency limit is determined by a separate adjustable "end" inductance which improves the performance throughout the high-frequency range. The high-frequency limit of a continuous operating range for a given coil is near the natural frequency of the unused part of the coil when the contact is near the high-potential end of its travel. Wide frequency tuning ratios of the order of six to eight are feasible and all-wave continuous coverage may be effected with two switch positions instead of three as required with variable-condenser tuning. Oscillator circuits are described which yield substantially uniform voltage over a ten-to-one frequency range. This widerange characteristic may be obtained even in the ultra-high-frequency region. Wider ranges necessitate improvement in the tracking of the usual superheterodyne input circuit. A method for producing a fourth tracking crossover when the inductive tuner is designed for superheterodyne circuit tuning is described.