Frequency-shift signaling circuit for 45-type carrier systems

Signaling equipment used with short-haul carrier systems is now supplied largely on an out-of-band basis. On-hook, off-hook signals are obtained either through turning a single tone on and off, or through shifting the tone alternately between two frequency positions. Signaling with Lenkurt 45-type carrier systems makes use of frequency-shift (FS) methods. Laboratory tests with artificially induced noise and field tests during lightning periods show at least a 6-decibel (db) advantage of FS over on-off signaling arrangements in freedom from pulse distortion effects. FS arrangements also show little impairment in performance over all expected operating conditions in a telephone office. The FS signaling arrangement described in this paper also provides automatic channel level regulation of the speech channel through availability of the signaling tone as an ever-present regulating tone. Varistor keying of a transistor oscillator is used in the transmitting terminal and a polar relay in the receiving terminal to supply all normal varieties of M and E lead signaling. Mechanical arrangements provide very compact assemblies through plug-in subunits and utilization of printed wiring.