Patent GB-1191423-A (GB)
Title: Signal Translating Arrangement.
Inventor: HARWOOD LEOPOLD ALBERT
IPC: H04Q9/10
Filed: 1968-05-17
Abstract: 1,191,423. Transistor circuits; controlling receivers by supersonic signals; voice frequency signalling. R.C.A. CORPORATION. 17 May, 1968 [18 May, 1967], No. 23708/68. Headings H3Q, H3T and H4R. [Also in Division G4] A desired frequency is selected by applying it to the base of a transistor having a circuit connected to its collector which rings at a higher frequency and is such that alternate half-cycles cause the base-collector path to conduct, the resulting output being applied to a filter selective to the desired frequency. The selector is described as used in a remote control system (Fig. 1, not shown), for a radio receiver in which radiated supersonic control tones are separated at the receiver from spurious noise by circuits in accordance with the invention, the outputs from the circuits operating corresponding relays. Briefly, the incoming signals are picked up by a capacitor microphone polarized by an input coupling unit 200-182. From there the signal is passed through a high-pass amplifier including asymmetrical limiters 140, 150 and the output, in the form of pulses, is further limited and narrowed by the input circuit of a stage 170 in accordance with the invention. The output of this stage causes an output transformer 31 to ring at a constant amplitude and the resultant signal is applied to a filter (not shown) to operate the respective relays. The amplifiers and limiters have D.C. stabilization. It is explained that the circuit distinguishes between desired and spurious signals even when the limiting is not effective. Amplifier limiter 130 is biased so that the collector of transistor 131 is normally at a low voltage, accordingly the input signal (3a) will swing the collector further to cut off than to saturation and will therefore be symmetrically limited (Fig. 36). Negative feedback through resistor 23 effects D.C. stabilization and also causes the circuit to operate as a high-pass filter. Amplifier limiter 150 likewise effects asymmetrically limiting and together with the capacitive input coupling further modifies the wave shape (Fig. 3c). Stabilization is effected by circuit 180 described below. Limiter and ringing circuit 170 includes an input coupling capacitor whereby the received pulses (Fig. 3c) are converted at the input to narrow positive pulses (Fig. 3d). The transistor is biased by a stabilizing circuit 190 (described below) to be heavily conducting and its output circuit includes a transformer resonant at such a frequency that one half cycle has a period equal to the pulse duration or to a sub-multiple thereof. At the end of the half-cycle the collector base path becomes conducting. Ringing ceases after one or more cycles depending on the duration of the pulse, so that one or more output pulses are produced. Stabilizing circuit 180 comprises a transistor 182 similar to 151 (because it is on the same chip) and having the same load. Current through the transistor is stabilized by 181 and the voltage at the emitter of 181 is fed to 151 via a resistor 186 similar to 185 so that the current through 181 is also stabilized. The current through 171 is similarly stabilized by a circuit 170.