Patent GB-1377021-A (GB)
Title: Signal receiver for receiving signals of different frequency
IPC: H04Q1/453
Filed: 1972-02-02
Abstract: 1377021 Voice frequency receivers; automatic exchange systems; time delay circuits PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 2 Feb 1972 [5 Feb 1971] 4895/72 Headings H4R H4K and H3T In a multifrequency signal receiver of the type in which a threshold device is common to a group of channels each including a frequency selecting filter and a detector, the threshold device is connected between the group of channels and a signal validity testing device and is characterized in that it has a variable first threshold level which may have a minimum level of zero, the arrangement being such that the threshold device causes the signal validity testing device to be enabled for as long as a signal received from said channels is increasing and further causes the signal testing device to remain enabled for as long as said signal received from said channels exceeds a second threshold level, irrespective of whether the signal is increasing or not. Such an arrangement causes the signal testing device to be enabled immediately a signal, however small, is received from the channels and hence reduces the testing time of a signal by the delay time which arises due to the frequency selecting filters having to build up an output which exceeds at constant threshold level. Such a build up time may be up to 10 ms. Figure 1 shows a block diagram of a multifrequency signal receiver which is apparently known. The signalling system employs 2 x (1 out of 4) frequency signalling. An incoming tone is fed via a filter 2 and amplifier 3 to two band pass filters 4, 6 which separate out the two frequency bands. The output of each band pass filter is amplitude limited in respective limiters 5, 7 and is fed to four frequency channels corresponding to the four frequency signals of the pass band. Each such channel includes a selection filter 10 and a detector 11 having two outputs, the first connected to one of a number of output sources 17-1 to 17-8 and the second to a threshold device 12-1 or 12-2 in multiple with the second outputs of the other three detectors associated with the three filters in the same pass band. The outputs of the threshold devices are fed to a signal testing device 13 including a signal-recognition device 14 and a signal timing device 15. In known manner the testing device causes the output sources to be enabled if signals received from the threshold devices are concurrent for longer than a predetermined time. In the known arrangements the threshold devices 12-1, 12-2 provide a fixed threshold level which has to be exceeded by the output of one of the filters in their respective groups to cause the testing device to be enabled. However in the described arrangement, Fig. 2, a variable initial threshold level is set. Fig. 2 shows one of the selecting filters 10, a detector 11 and a threshold device 12. When no signal is being generated in the filter circuit transistor 31 is biased off, capacitor 42 is uncharged, and transistor 50 is switched on. When a signal is received containing a frequency component corresponding in frequency to that of the resonance frequency of the filter oscillations start to build up across the inductor 24. As capacitor 42 is uncharged the emitter of transistor 31 is at earth and hence transistor 31 is switched on on the positive half cycle of the signal generated in the filter circuit. This causes a negative output at terminal 41 which is sufficient to exceed a threshold level DN2 for switching off transistor 50, this threshold level being determined by Zener diode 49 and resistor 52 and 53. The change in potential at terminal 54 is used to enable the signal testing device for at least one period of the frequency signal generated in filter 10. The conduction of transistor 31 charges up capacitor 42 to the voltage of the positive voltage peak of the received signal. On the negative excursion of the signal appearing across the inductor capacitor 42 slowly discharges through resistor 43. On the next positive excursion of the filter signal, the emitter of transistor 31 will be above earth potential and hence the positive peak of the filter signal must be above a certain level, which depends on the voltage peak of the last positive signal half cycle received, in order to turn transistor 31 on. The decay constant of capacitor 42 through resistor 43 and the threshold DN2 of transistor 50 are such that, if the second positive peak of the filter signal is greater than the previous peak, transistor 31 turns on and causes transistor 50 to turn off, thereby enabling the signal testing device for a further period. If, however, the positive peak of the filter signal is equal to, or below, the previous peak transistor 50 is not turned off irrespective of whether transistor 31 turns on or not, subject to the positive peak being below a second threshold level DN1. This second threshold level is equal to the voltage level to which the signal of the filter should build up if the filter is energized by an undisturbed signalling tone i.e. one not subj ect to amplitude variations due to noise inference and is achieved by the diode 44 and resistors 47 and 48 in conjunction with Zener diode 49. If the variable threshold level produced by capacitor 42 and resistor 43 exceeds the voltage at the point G1, the capacitor 42 rapidly discharges via resistor 47 which has a smoothing capacitor 44 in parallel with it, to the voltage at point 61 after which it discharges through resistor 43. Hence the variable threshold is subject to an upper limit, the second threshold level DN1, and provided the positive peaks of the filter signals exceed this level, transistor 50 will be turned off to energize the signal testing device during a further period. The receiver may be used in tone-frequency push-button signalling systems for subscriber selection in automatic telephone systems.