Receiver tuning indicator

1,135,329. Tuning indicators. STANDARD TELEPHONES & CABLES Ltd. 8 July, 1966, No. 30764/66. Headings H3Q and H3T. The invention relates to tuning indicators for tone-keyed receivers where at any instant only one frequency of two possible frequencies is present, or a signal is present in only one of two possible signal paths in the receiver, e.g. in frequency-shift telegraphy. The former case arises when the incoming signals are converted to an I.F. of 100 KHz, displaced 100Hz on each side for the mark and space conditions respectively. Each 100 Hz sideband is accompanied by other sidebands of lower amplitude, and the invention avoids the possibility of tuning the receiver to one of these loweramplitude sidebands, or to receive the mark signal in the space channel or vice versa. In the other form of receiver the mark and space signals are both converted to the same frequency, e.g. 5 KHz, but appear in separate channels. According to the invention (Fig. 8) the signals in these two channels are combined and applied to a two-stage limiter 2, followed by a band-pass filter 3 which rejects the loweramplitude sidebands which are not 5 KHz. The constant-amplitude output is rectified by the base-emitter junction of transistor 40 and appears amplified at the collector. The signalfrequency components are removed by filter 51-53 and the D.C. component applied to a meter 61. When no signal appears, the meter shows a maximum reading. When either a space or a mark signal is present, the collector voltage of transistor 40 becomes less negative and the meter reading falls. When the receiver is correctly tuned, there will be at any instant either a space or a mark signal present, but off tune one or both of these will be absent or of lower amplitude. The minimum reading on the meter therefore shows the correct tuning point. If the mark and space signals are respresented by different I.F.'s the above circuit is duplicated (Fig. 9, not shown) using separate diodes as rectifiers instead of the base-emitter junction of transistor 40, and separate band-pass circuits tuned to the appropriate frequencies. Transistor 40 acts as common amplifier for both signals. This arrangement may be extended to more than two channels, and may also be used where the signals are of the same frequency to reduce errors due to beats. The channels may be separated after the limiter (Fig. 4, not shown). Variations of modulation rate due to successive signals being of the same kind (e.g. two successive