IL43889A - System for evaluating ac potential - Google Patents

Description

The present invention relates to a.c. control signal analysis circuits for signals whose frequency, in particular signals of a relatively low frequency, can be altered in stages, for example.

The a.c. signal may be a pilot voltage utilised for level-monitoring, and/or for level control applications, for example.

For telecommunications, remote control or similar applications, control frequencies can be transmitted by arranging that the information which is to be conveyed is conveyed by .transmitting one or more of several possible frequencies.

For example, it may be of interest, in a particular station to know which of the possible frequencies is being received. On the other hand, the case can arise that each of several stations located in a transmission link is required to respond to a different frequency, so that it is sufficient to determine whether this single frequency is being received or not.

Communication channels may be monitored by a narrow-band filter with a steep characteristic, whose output is amplified and rectified. Because of the need to use relatively high pilot frequencies, to produce these pilot signals crystal-controlled generators which have high frequency stability need to be employed. At the receiving end, crystal-controlled filters can be used. Such arrangements are expensive.

The German Patent Specification No. 2,044,001 describes a device for the pilot-controlled stand-by switching of low ^ frequency transmission channels, employing a pilot signal to transmit four pieces of information about the state of two transmission channels. Here, the pilot can have one of four possible frequencies, namely 300 Hz, 315 Hz, 330 Hz and 345 Hz.

Using a frequency switchable pilot voltage, it is possible at the same time, by. analysing the pilot signal level, to carry out a measurement of level or to regulate the level, and by analysing the frequency to obtain control criteria, in particular for standby switching purposes. If band-pass filter's are used in the pilot receivers, then a relatively complex circuit arrangement is involved if several neighbouring pilot frequencies must be allowed appreciable tolerances.

The use of simple oscillatory circuits as frequency discriminators presents difficulties, insofar as in cases where the circuit quality factor is high, even tiny frequency shifts are sufficient to . jeopardise the reliability of response of the frequency discriminator, whilst when circuits having low quality factors are used, there may be a faulty response on the part of the analyser or processing devices which follow the oscillatory circuits, which can only be reliably avoided if the interval between the control frequencies is made correspondingly large. However, this leads to bandwidth requirement which in many cases is not available, in particular in a low frequency transmission channel, where several control frequencies need to be transmitted below the speech band. Other difficulties can arise if the pilot signal level is subject to fluctuations which may affect the frequency analysis. φ One object of the present invention is to provide a simple analysis circuit which ensures reliable control signal produc-tion whilst permitting reasonable tolerances for the frequencies being analysed, and fluctuations in the level of the a.c. voltage.

The invention consists in a system 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, wherein the received signal is applied to a plurality of tuned circuits each resonant at a respective one of said predetermined frequencies, wherein the outputs of said tuned circuits are rectified to provide respective d.c. voltages which are applied to a comparator circuit arranged to provide an output signal indicating when the voltage provided ty the tuned circuit resonant at said particular one of said plurality of predetermined frequencies has a higher magnitude than all the other ones of said voltages, and wherein the level of the received signal is determined by an independent level analyser circuit which is frequency-independentwithin the band occupied by said predetermined frequenciesi By these measures, particularly high reliability in signal production is ensured, even where a frequency exhibits relatively large deviations from its set point value. A further advantage resides in the fact that both for the production and the analysis of the frequencies, a particularly low cost of circuitry . suffices. This advantage is particularly effective where the frequency is in the low frequency range because in this range the design of filters having steep characteristics and of generators of accurate frequency, entail particularly heavy expenditure in terms of circuitry and space. Furthermore, the circuit is particularly insensitive to frequency shifts of the kind which can be expected as soon as the pilot signal has to be transmitted through a carrier frequency link.

The comparison of the reference voltages can be carried out in such a fashion that in a first stage the or each d.c. output voltage is compared with at least one other, and the results of this or these comparisons Subjected to logic combination with one another in a - second ■ stage. For example by using an AND-gate it is possible to determine whether all the comparisons of a reference voltage with the other reference voltages have yielded the result that a particular d.c. output voltage is the higher one. · . ■ . ponding oscillatory circuit, which is compared with a maximum d.c. value that is obtained without reference to any particular relationship with any other d.c. output voltage. The d.c. output voltage said to be obtained without reference to any particular relationship is intended to indicate that the resultant d.c. voltage represents the maximum value on its own, i.e. without any information about the relationship of this value to any specific oscillatory circuit frequency.

Preferably, the circuit may be such that a signal is only to be produced in the presence of a specific one of several possible frequencies, this in particular being the case with individual stations in a signal transmission path, each of which stations is only to respond to a specific frequency.

The analysis circuit arrangement may be so designed that for comparison, a voltage discriminator with a response threshold controllable by a control voltage is provided, to one input of which a d.c. output voltage for comparison with another is applied, whilst the other voltage is applied to an additional input to act as a control voltage.

The voltage discriminator may be "constituted by a Schmitt trigger built using two transistors, the emitter connection of each of the transistors being taken to the additional input, in particular via a resistor. A voltage discriminator comprising an operational amplifier circuit may be used, in which case one input of the operational amplifier has one of the d.c. output voltages applied thereto for comparison with another applied to its other input.

In order to obtain the maximum value of the other direct voltages in a manner which involves no special relationship each of the d.c. output voltages can be applied through a forward-operated diode to a common connection, so that the maximum value appears at this connection.

The invention will now be described with reference to the drawings, in which:- Figure 1 is a block schematic circuit arrangement for producing output control signals, as a function of an input a.c. frequency, variable in four stages, in one exemplary embodiment of the invention; Figure 2 is a circuit diagram of a Schmitt trigger with a controllable response threshold suitable for use in the embodiment shown in Figure 1; and Figure 3 is a circuit diagram of a voltage discriminator using an operational amplifier and having a controllable threshold, suitable for use in the embodiment shown in Figure 1.

Figure 1 shows part of a device for switching a faulty transmission channel to a standby channel, the analysis circuit shown serving to indicate not only the pilot signal level, but also the particular frequency, independently of one another. In this device, a pilot signal is used not only for level monitoring but also to transmit four pieces of information about the state of two communications transmissions channels. To this end, the pilot signal can have one of four possible frequencies, namely fl = 300 Hz, £2 = 315 Hz, f3 = 330 Hz, or f4 = 345 Hz.

In the analysis circuit, for this purpose, level measurement is separated from frequency detection, which makes it possible to use in that section which analyses the transmitted frequency, oscillatory circuits of low quality factor. By mutual comparison of rectified d.c. output voltages from the different oscillatory circuits, the particular frequency present can be unambiguously indicated within wide level and frequency tolerances. For level measurement, the pilot, voltage is directly rectified in a frequency-independent fashion At the transmitting end of the device, (not shown) a low frequency band with a range extending from 300 to 350 Hz is provided by using a band-stop filter. After the band-stop filter the pilot signal is injected. As Figure 1 shows, at the receiving end, this channel signal with its pilot signal is filtered out again by a band-pass filter 2.

In .the indicated example, we are dealing with a case where in addition to frequency analysis or frequency detection, level analysis is also to be carried out. For applications where it is only frequency detection which is of any interest, the level measurement can be discarded, as the actual level measurement is separate from the frequency measurement. The pilot voltage is rectified in a circuit 3 used for mean-value rectification, to provide at its output a frequency independent d.c. voltage U=.

For each of the four frequencies there is provided a respective oscillatory circuit, 41 to 44, tuned to one particular frequency ·:? .· ·..·*··.··"-'■-.·* .

The circuits 41 to 44 each have a quality factor of about 30, and ser\e to establish which frequency is being transmitted. The a.c. voltages which appear across the mutually decoupled parallel oscillatory circuits, are rectified using respective rectifiers 51 to 54. If one of the frequencies f2 to f4, is available, then signal production is required to be initiated in this application of the invention. To this end, for each -^' frequency responsible for signal production, a respective Schmitt trigger, 61 to 64 is provided, input el of the Schmitt trigger 61 being taken directly to the rectifier 52, input e2 of the Schmitt trigger 62 being taken directly to the rectifier 53, input e3 of the Schmitt trigger 63 being taken directly to the rectifier 54. At. outputs al to a3 of the Schmitt triggers 61 to 63 a digital indication is provided, indicating which of the frequencies f2 to f4 is being transmitted at any. instant. , The Schmitt triggers 61 to 63 each have a second input s , by means of which the respective response thresholds can be adjusted. Two possible examples of voltage discriminators with controllable response thresholds are shown in Figures 2 and 3.

In this application, the presence of the the frequency fl, is not required to initiate any signal production. For this reason, an individual Schmitt trigger for the frequency fl is not provided. The presence of the frequency fl can be detected . by virtue of the fact that none of the outputs al to- a3 are producing ; a signal at a given instant.

In this fashion, identification of which of the frequencies is producing the highest direct voltage can be effected using a number of voltage discriminators which is one less than the number of frequencies. To determine whether any one specific frequency is present, only one voltage discriminator is needed. Because the quality factors of the oscillatory circuits are low, fluctuations due to permitted frequency and tolerances . have only a minor effect upon the d.c. output voltages. The outputs of the rectifiers 51 to 54 are combined through separate diode combinations 611 to 613,*621 to 623, and 631 to 633, and taken respectively to inputs si to s3 of the respective Schmitt triggers 61 to 63, so that the response thresholds of each trigger circuit changes with the level produced by its associated combination. Each combination is so contrived that each rectifier is in each case connected through an associated separate diode, to the control inputs of all the Schmitt triggers, with the exception of that Schmitt trigger which has its signal input taken directly to this rectifier. Thus, for example, the rectifier 52 is connected via the diodes 622 and 632 to the inputs s2 and s3 of the Schmitt triggers 62 and 63.

The output voltage from a rectifier thus shifts the thresholds of the Schmitt triggers whose signal inputs are not connected to this particular rectifier. In this fashion, the result is achieved that in each case the highest of the four rectified voltages increases the threshold of the other Schmitt triggers to such an extent that they cannot respond, although, because of the low quality factor, they receive a voltage from the oscillatory circuits assigned to them. The quality factor of the oscillatory circuits is so dimensioned that taking into account all the possible frequency deviations and oscillatory circuit tolerances, within the working range around the resonance point of the oscillatory circuit characteristic a safety interval', is maintained with respect to the point of intersection of its ^ resonance curve with the neighbouring resonance curves.

The Schmitt trigger shwon in Figure 2 is formed by transistors 21 and 22, whose emitters are directly connected together and connected via a resistor 27 to a negative supply voltage terminal. The input voltage Ui is fed via a resistor 24 to the base of the transistor 21, which is connected via a resistor 25 to the negative supply voltage terminal. The collector of the transistor 21 is connected via a resistor 23 to the posi tive supply, terminal, as is that of the transistor 22 via a resistor 28.

The base of the transistor 22 is connected via a resistor 26 to the collector of. the transistor 21 and also via a resistor 22 to the negative supply voltage terminal.

In this Schmitt trigger, the emitter connection between the transistors 21 and 22 is connected via a resistor 30, to its additional control input _s, so that the voltage Ug applied to this control input determines the response threshold of the Schmitt trigger. In this fashion the result is achieved that the Schmitt trigger only responds when the input voltage Ug is higher than the control voltage Ug.

Because the output voltage from the Schmitt trigger is dependent upon the particular threshold voltage, conveniently a switching stage, (not illustrated) is provided after the trigger stage, in order to produce a voltage which is independent of the threshold.

The alternative voltage discriminator circuit shown in Figure 3 is also of Schmitt trigger design, but comprises an ~? operational amplifier 16, and can be controlled at the expense of particularly little power. The negative input of the operation amplifier 16 is connected via a resistor 13 to a negative terminal supply voltage Ug2 which is at a potential half that of the supply voltage Ug for the operational amplifier. Thus, as long as the diodes 611 to 613 are blocked, the negative input is at medium potential. In respect of the direct voltage supplied from the associated rectifier, e.g. 52, via a resistor 12 to the positive input of the operational amplifier, the arrangement acts as a non-inverting amplifier for the input signal.

The operatiotal amplifier 16 is provided with a feedback from output which is earthed via a voltage-divider formed by resistors 17 and, 18, whose junction is connected via a resistor 15 to the positive input of the operational amplifier. The positive input is also connected via a capacitor 19 to the negative input, and via a resistor 14 to the supply voltage UB1. The resistors 14 and 15 form. voltage-divider which determines the response threshold.

In respect of the d.c. voltage applied to the positive input, the circuit described acts like a Schmitt trigger.

To control the response threshold, of the voltage discriminator, the maximum value of the other direct voltages, said value being formed with the help of the diodes 611 and 613 in this particular example, is applied via a resistor 11 to the negative input. In so doing, the resistors 11 and 13 form a voltage divider with the help of which the control voltage at the negative input is adjusted so that the voltage discriminator does not respond if the said maximum value is higher than the d.e. voltage produced by- the rectifier 51.

Claims (8)

43889-2·

1. A system 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, wherein the received signal is applied to a plurality of tuned circuits each resonant at a respective one of said predetermined frequencies, wherein the outputs of said tuned circuits are rectified to provide respective d.c. voltages which are appl ied to a comparator circuit arranged to provide an output signal indicating when the voltage provided by the tuned circuit resonant at said particular one of said plurality of predetermined frequencies has a higher magnitude than all the other ones of said voltages, and wherein the level of the received signal is determined byan~independent-level--analyser-ic-ircuit-whieh-is-frequency- independei within the band occupied by said predetermined frequencies.

2. A system as claimed in Claim 1, wherein said d.c. voltages are applied to a plurality of comparator circuits each arranged to provide an output signal when the voltage provided by a respective particular one-of said tuned circuits has a higher magnitude than all the other ones of said voltages, the presence of an output signal from any particular one of said comparator circuits indicating that the received signal has a respective particular one of said predetermined frequencies.

3. A system as claimed in Claim 1 or Claim 2, in which said comparator circuit, or each of said comparator circuits is constituted by a vol tage discr imin- ator having a response threshold that is controllable by a control voltage derived · from said other ones of said voltages. VPA 72/6762 IjSR

4. A system as claimed in Claim 3, in which said voltage discriminator, or each of said voltage discriminators is constituted by a Schmitt trigger using two transistors having their emitters connected to said control voltage.

5. A system as claimed in Claim 3. in which said voltage discriminator, or \ each of said voltage discriminators comprises an operational ampl if ier , one input of which is connected to the voltage provided by the respective particular tuned circui and the other input of which is connected to said control voltage.

6. A system as claimed in any of Claims 3 to 5, in which said other ones of said voltages are applied to said discriminator or each of said discriminators through respective forward-operated diodes.

7. A system as claimed in any preceding Claim, , in which there are four of said tuned circuits resonant respectively at 300 Hz, 315 Hz, 330 Hz and 345 Hz, each of said tuned circuits having a quality factor of approximately 30.

8. A frequency and level determining system substantially as described with reference to Figure 1, or Figure 1 and Figure 2, or Figure 1 and Figure 3. VPA 72/6762 ISR