Patent GB-1340739-A (GB)
Title: Voice frequency signalling receivers
IPC: H04Q1/45
Filed: 1972-03-23
Abstract: 1340739 Voice frequency signalling PLESSEY CO Ltd 23 March 1972 [3 June 1971] 18770/71 Headings H4R and H4K A voice frequency receiver comprises two receiver units which are connected in shunt to a single input path and are substantially identical except as regards their frequency-tuned characteristics, each receiver unit including a transistor pre-amplifier stage incorporating a first tuned circuit arranged to appreciably attenuate only the frequency component of an input signal which corresponds to the operative frequency of the partner receiver unit, the output of said pre-amplifier being applied through amplitude limiting means to a second tuned circuit resonant at the operative frequency of the particular receiver unit, said second tuned circuit providing two separate output paths one of which is activated in the presence of the operative frequency of the receiver unit and utilized to effect operative response of a transistorized comparator controlling a switching device whereas the other path is activated in the presence of other frequency conditions to inhibit the transistorized comparator. General.-Such a receiver is shown coupled to the "receiver" path R of a 4-wire telephone transmission circuit, the left hand conductors of the path being incoming from a remote exchange and extending through a known form of buffer amplifier interposed between balanced input and output transformers T1 and T2, transformer T2 being connected to the forward-going end of the receiver path by a matching network MN. The receiver is arranged to operate two relays RX and RY, one coupled to each receiver unit RECA, RECB of which only receiver RECA is shown in detail, the circuitry of the two receiver units being identical except for certain component values associated with tuned circuits T4 and T6. The arrangement is designed so that one or both of relays RX and RY are operated according to whether one or both of two frequency signals are received. For example a compound signal i.e. a signal comprising both frequencies, may be used for functions such as "seize" and "clear" and single frequency signals for digital impulsing. Contacts of the relays may also be used for line splitting in that when a relay is in the operated state, its contacts are effective in applying earth to lead LS associated with transformer T2 of the speech path to disable the buffer amplifier BA. Signal frequencies of 2040 Hz and 2400 Hz are given as examples.  Details of the receiver units.-Only the 2040 Hz receiver unit RECA will be described. Transistor TR1 is arranged as a class A pre-amplifier and has the incoming signals applied to its base. All incoming signals are amplified with the exception of signals having the operative frequency, e.g. 2400 Hz, of the other receiver unit which signals are attenuated by about 18 db  due to the action of tuned circuit T6 having a resonance frequency of 2400 Hz and a band width of the order of 100 Hz. This is to prevent the operation of the guard arrangement of the receiver unit when a two frequency signal is present at the input. To prevent transistor TR1 from saturating for high levels of audio frequency input, clamping diode D7 is provided which together with resistor R17 suitably limits the peak collector current. The output of the pre-amplifier is amplitude limited by a clipping element involving diodes D5 and D6 and resistors R18 and R19 and the clipped waveform is supplied to transformer TR3 by current amplifying transistor TR2. The primary winding of transformer TR3, which is a step down transformer, is coupled to a terminating resistor 24 and a series tuned circuit T4 tuned to the operative frequency 2040 Hz, of the receiver unit. Capacitor C7 is connected across the winding for reducing harmonics so as to reduce the possibility of the receiver unit being disabled by harmonics of its own operative frequency. The voltage developed across the tuned circuit T4 is used to control the collector current of transistor TR3 which is coupled to a transistor TR5 associated with the relay RX via transformer T5. Transistor TR3 is turned on by negative excursions of a signal generated in the tuned circuit and causes transistor TR5 to be turned on and off in alternate half cycles. Capacitors C10 and C11 perform a smoothing function in respect of the pulsating waveform. Relay RX is therefore held on for as long as transistor TR3 is switched on and off. If frequencies other than that of the receiver unit are present a guard voltage is developed at the base of guard transistor TR4 to inhibit transistor TR4, which acts as a rectifier to enable capacitor C9 to charge to the peak of the guard voltage by way of resistor R25. This causes transistor TR3 to be biased off, thereby providing immunity against speech signals. Resistor R32 and diode D8 act as a surge protector for transistor TR5. They also control the release lag of the relay to the order of 7 ms. This release lag can be varied by making resistor R32 variable. The receiver may be fabricated in the form of printed conductor board assemblies with one receiver unit to each board.