Abstract: A failsafe bandpass filter/decoder system comprising: an active bandpass filter which comprises a high-pass filter section having selected resistors which can only decrease in value during a failure mode and, in series with the high-pass filter section, a low-pass filter section having selected resistors which can only increase in value during a failure mode, whereby under any failure mode, the pass band of the filter will be reduced; and a level detector having an upper threshold voltage trip point and a lower threshold voltage trip point, both trip points having to be exceeded by the input wave form to produce a proper (square wave) output.

Abstract: An audio frequency (AF) track circuit whereby a railway track tuned loop transmitter operating in a dual resonant mode, for handling, two signal frequencies, can be coupled to the rails. Each frequency is conducted through a tuning unit having series and parallel branches tuned to each frequency. The track circuit using the tuned loop transmitter eliminates DC return current imbalance effects while maintaining suitably high track circuit shunting sensitivity.

Abstract: A controllable frequency ultrasonic signal generator for driving an ultrasonic transducer via a switching type output circuit for the efficient transfer of energy thereto. The signal generator includes a phase lock loop feedback control for locking the operation of the oscillator to the resonant frequency of the transducer for following variations in transducer resonant frequency due to loading changes, temperature changes, tool tip variations, and the like.