GB1451816A - Systems for monitoring the level of a received alternating- current signal and for indicating whether said received signal has a particular one of a plurality of predetermined frequencies - Google Patents

Abstract

1451816 Measuring voltage amplitude SIEMENS AG 4 Dec 1973 [29 Dec 1972] 56110/73 Heading G1U [Also in Division H3] In a system for monitoring the level of a received A.C. signal and for indicating whether the received signal has a particular one of predetermined frequencies, a received signal is applied to tuned circuits 41-44, each resonant at one of the frequencies, the outputs of which are rectified at 51-54 to provide D.C. voltages which are applied to a comparator circuit 61-63 arranged to produce an output signal indicating when the voltage provided by the tuned circuit resonant at the particular one of the frequencies has a higher magnitude than all the other ones of the voltages and the level of the received signal is determined by an independent level analyzer circuit 3 which is frequency independent within the band occupied by the predetermined frequencies. Fig. 1 shows part of a device for switching a faulty transmission channel to a standby channel. The received A.C. signal can be a pilot signal and the voltage comparator can be formed by Schmitt trigger circuits 61-63 which can be shown in Fig. 2 and have control inputs S which determine the response threshold. The Schmitt trigger circuit only responds when the input voltage Ue is higher than the control voltage Us so that the D.C. output voltage from 52-54 having the higher voltage than that from 51 -54 provides an output from the respective Schmitt trigger 61-63. The presence of the frequency fl is not required to initiate any signal production so that a Schmitt trigger circuit for the frequency f1 is not provided. The presence of the frequency fl can be detected by the fact that none of the outputs a1 to a3 are producing a signal. The pilot voltage is rectified at 3 which is used for mean value rectification to provide a frequency independent D.C. voltage U=. An alternative Schmitt trigger circuit is disclosed in the form of a voltage discriminator Fig. 3 (not shown) which includes an operational amplifier (16) having feedback to its positive input from its output provided by resistors (14, 15, 17, 18). The resistors (14, 15) determine the response threshold. As long as the diodes such as 611, 613, 614 connected to the negative input of the operational amplifier (16) are blocked the negative input is at a medium potential. The voltage discriminator does not respond if the control voltage applied via the diodes is higher than the D.C. voltage produced by the rectifier 52.