METHOD FOR DETECTING A SIGNAL

Abstract: The present invention relates to a method for detecting an alternating current signal of a predetermined frequency in a received signal. In order to achieve a simple and easily applicable method, it comprises the steps of feeding said received signal to an input (0) of a switching means (MUX) which comprises several outputs (1 - 4) as well as means for alternately connecting the input to each of said outputs (1 - 4), synchronized by a clock pulse fed to the switching means, feeding a clock pulse to the switching means whereby the clock pulse frequency is chosen so that the input (0) of the switching means is once connected to each output (1 - 4) of the switching means essentially during one cycle of a signal to be detected, filtering the signals obtained from the outputs of the switching means, monitoring the filtered signals, and detecting the presence of a signal to be detected as the voltage level of any of the monitored signals exceeds a predetermined level, or as the voltage difference between two monitored signals exceeds a predetermined threshold level. The invention further relates to a detector circuit.

Description: Method for detecting a signal The present invention relates to a method for detecting an alternating current signal of a predetermined frequency in a received signal. The invention further relates to a detector circuit for detecting an alternating current signal of a predetermined frequency in received signals. The invention in particular relates to a detector circuit by means of which signalling forwarded through a double conductor of a telephone exchange having an analog interface can be detected and sent forward to a PCM (Pulse Code Modulation) multiplexer. Such a circuit must be able to detect at least home metering pulses, calling signals, double conductor polarity and breaks in the supply current from the telephone exchange. Prior art detector circuits are known, which utilize, among other things, expensive hybrids and PLL (Phase Locked Loop) circuits, or LC filters. In addition to said circuits being relatively expensive and complicated as to their structure, their tuning onto various detecting frequencies has proved to be difficult. With prior art circuits, achieving a narrow enough detecting frequency band has also proved most troublesome. It is an object of the present invention to solve the aforementioned problem and to provide a simple and easily applicable method for detecting an alternating current signal of a predetermined frequency. This object is achieved by the method of the invention, characterized by feeding said received signal to an input of a switching means which comprises several outputs as well as means for alternately connecting the input to each of said outputs, synchronized by a clock pulse fed to the switching means, feeding a clock pulse to the switching means whereby the clock pulse frequency is chosen so that the input of the switching means is once connected to each output of the switching means essentially during one cycle of a signal to be detected, filtering the signals obtained from the outputs of the switching means, monitoring the filtered signals, and detecting the presence of a signal to be detected as the voltage level of any of the monitored signals exceeds a predetermined level, or as the voltage difference between two monitored signals exceeds a predetermined threshold value. The invention is based on the idea that as the clock pulse frequency of a signal fed to the switching means is chosen so that the input of the switching means is once connected to every output of the switching means during one cycle of the signal to be detected, and as the signals obtained from the outputs of the switching means are filtered by means of, for example, RC filters, signals having different voltages can be obtained from the outputs of the various filters. The presence of a signal to be detected can thus be established by comparing to one another the voltage levels obtained from the filter outputs, or alternatively by comparing the signal obtained from each filter output to the ground level. Thus, the most significant advantages of the method according to the invention are simplicity and an easy adjustment of the detecting frequency bandwidth. By changing the dimensions of the filtering means it is possible the adjust the bandwidth of the detecting frequency to suit the purpose. Correspondingly, the change of the detecting frequency can be carried out by changing the frequency of the clock pulse which is fed to the switching means. Because the clock pulse is obtained from an accurate source, a further advantage is the accuracy of the detecting frequency. The concept "is once connected to each output of the switching means essentially during one cycle of a signal to be detected" here proposes that the frequency of the clock pulse fed to the switching means does not have to be an exact multifold of the frequency of the signal to be detected, but it is sufficient that the requirement above is met within certain precision. The magnitude of deviations allowed depends on the detecting frequency bandwidth, i.e. the dimensions of the filters. The present invention further relates to a detector circuit by means of which the method of the invention can be applied. The detector circuit of the invention is characterized in that it comprises a switching means having an input responsive to the received signals, and which comprises several outputs and means for alternately connecting the input to each output, synchronized by a clock pulse fed to the switching means, means for feeding the clock pulse to the switching means whereby during essentially one cycle of an alternating current signal to be detected the input of the switching means is once connected to each output of the switching means, filtering means responsive to the outputs of the switching means for filtering the signals obtained from the outputs of the switching means, and comparator means for receiving the signals obtained from the outputs of the filtering means, and for producing a predetermined signal and feeding it further as the voltage level of any received signal exceeds a predetermined level, or as the voltage difference between two received signals exceeds the predetermined level. The most significant advantage of the detector circuit of the invention is simple changing of the detecting frequency. The detecting frequency in the detector circuit of the invention is directly proportional to the frequency of the clock pulse fed to the switching means, whereby changing the frequency does not require any additional procedures in addition to changing the frequency of the clock pulse. The adjusting of the detecting frequency bandwidth can also be implemented very simply. Said bandwidth is namely directly proportional to the dimensions of the filtering means, whereby by changing the slope of the filter said bandwidth may be optimized as desired. The detection circuit detecting home metering pulses in a telephone exchange, for example, must be such as to allow an easy change of the detecting frequency, because the present telephone networks commonly employ two home metering signals of different frequencies, both of which are 200 mV - 2 V alternating current signals, but one of the frequency 12 kHz and the other 16 kHz. The detector circuit of the invention can thus be utilized in both cases with very simple modifications, for example, by changing the frequency of the clock pulse supplied to the circuit by means of software. The preferred embodiments of the method and detector circuit of the invention are disclosed in the attached dependent claims 2 - 3 and 5 - 7. In the following, the invention will be described in closer detail by means of a preferred embodiment of the invention with reference to the attached drawings, in which figure 1 illustrates a block diagram of a preferred embodiment of the detector circuit according to the invention, and figure 2 illustrates voltages obtained from the outputs of the switching means illustrated in figure 1. Figure 1 is a block diagram illustration of the first preferred embodiment of the detector circuit according to the invention. The detector circuit of figure 1 is connected by means of a double conductor a-b to a telephone exchange having an analog interface, whereby it can be utilized for detecting signalling sent by the exchange on the double conductor a-b. The circuit of figure 1 is able to detect the home metering pulses, calling signals, double conductor polarity and breaks in the supply current from the telephone exchange, after which the detected signals are coded into three bits, detO, detl and det2, prior to forwarding them to the PCM multiplexer. In the case of figure 1, the double conductor a-b from the telephone exchange is connected to the input of a high-pass type differential amplifier AMP. The differential amplifier AMP is so dimensioned that it is able to receive both strong, low-frequency calling signals (110 Vac, 25 Hz) and weak home metering pulses (200 mV - 2 V, 12 kHz) . The signals obtained from the output of the differential amplifier are fed through a DC breaking capacitor Cl to the input 0 of the switching means MUX. The switching means MUX illustrated in figure 1 is a multiplexer having an input 0, and four outputs 1 - 4 to which the input 0 of the switching means is alternately connected synchronized by a clock pulse fed to it. In the case of figure 1, the clock pulse fed to the switching means consists of two signals, one of which is of the frequency 12 kHz (the same as the detecting frequency) , and the second of the frequency 24 kHz (twice as high as the detecting frequency) . Said signals are fed to the switching means MUX through inputs CLK1 and CLK2. The signals obtained from the outputs 1 - 4 of the switching means are filtered by means of filters 6, after which the filtered signals are further fed to a comparator means 7. The filters 6 may, for example, be RC filters. The comparator means 7 produces and forwards the predetermined signal detO when the voltage level of the signals obtained from the output of filtering means 6 exceeds a predetermined reference level. This comparison is carried out by the comparator means 7 either by comparing signals from two filtering means 6 to one another or by comparing signals from each filtering means 6 to the ground potential. As the telephone exchange supplies on the double conductor a-b a 12 kHz home metering pulse to the detector circuit, voltages that are constant but different to one another are integrated over the filters 6 (cf. figure 2) . The comparator means 7 detects the change in voltages and feeds the predetermined signal detO further through its output as an indication of detecting the home metering pulse. This signal is detected by the PCM multiplexer as a change in the state of the output detO. The detector circuit of figure 1 is additionally capable of detecting the double conductor a-b polarity, the calling signal and a break in the supply current on the double conductor a-b. For this purpose, the detector circuit of figure 1 comprises a low-pass filter 5, detectors 8 and 9, a DC breaking capacitor C2 and a coder 10. The coder 10 codes said states into two bits, detl and det2, which the detector circuit forwards to the PCM multiplexer. The values of the bits detl and det2 are shown in the table below: State detl det2 pol+ 0 1 Break 1 0 pol- 1 1 Call 0 0 As a calling signal is sent by the telephone exchange, it means that the voltage between the double conductor a-b changes on a certain frequency. Due to said change, this signal passes the capacitor C2 whereby the detector 9 detects the calling signal. The coder 10 in such a case sets the value of the outputs detl and det2 to 0 for the duration of the negative phase (pol-) of the calling signal. Thus, the calling signal is not a steady state, but the PCM multiplexer must contain a processor or alike for controlling the change of the bits detl and det2 values in order to recognize the calling signal. For detecting the breaks and polarity changes in the double conductor a-b, the detector circuit utilizes the detector 8. In such a case, the coder sets the detl and det2 values in accordance with the table above. The capacitor 2 blocks the detector 9 from detecting the break or the double conductor polarity. Figure 2 illustrates the voltages which can be obtained at the outputs of the switching means of figure 1. Figure 2 illustrates a home metering pulse produced by the telephone exchange during two cycles. The horizontal axis of figure 2 denotes time t, and the vertical axis the voltage U. Due to the fact that the input 0 of the switching means MUX illustrated in figure 1 is once connected to each of its outputs essentially during one cycle of a signal to be detected, i.e. a home metering pulse in this case, it means that the same "part" of the home metering pulse is always outputted from the same switching means input. Figure 2 illustrates by means of vertical lines and numbers which "part" of the home metering pulse the switching means MUX outputs from the various outputs 1 - 4. Consequently, constant but different voltages are integrated over the filters of figure 1, which voltages are detected by the comparator means 7. If the frequency of the clock pulse fed to the switching means is not an exact multifold of the frequency of the signal to be detected, this results in that the signals which are integrated over the filters do not remain constant but slightly change with time. If the deviation is small, the voltage changes are, however, negligible whereby one of the comparators is always able to detect the voltage difference. If, however, the deviation is large, some other than the signal meant to be detected is in question (outside of the detecting frequency band) , indicating that the voltages do not remain constant whereby the filter (6) does not pass said signal, and that essentially the same voltage is produced over every filter, which results in the comparator means being unable to detect the voltage difference. It should be noted that the description above and the related figures are only intended to illustrate the present invention. Different kinds of variations and modifications of the present invention will become obvious for a person skilled in the art, without departing from the scope and spirit of the invention of the attached claims. For example, the number of switching means is not restricted to four, but switching means having six or eight outputs may advantageously be used.

Claims: Claims 1. A method for detecting an alternating current signal of predetermined frequency in a received signal, c h a r a c t e r i z e d by feeding said received signal to an input (0) of a switching means (MUX) which comprises several outputs (1 - 4) and means for alternately connecting the input to each of said outputs (1 - 4) , synchronized by a clock pulse fed to the switching means, feeding a clock pulse to the switching means (MUX) whereby the clock pulse frequency is chosen so that the input (0) of the switching means is once connected to each output (1 - 4) of the switching means essentially during one cycle of a signal to be detected, filtering the signals obtained from the outputs of the switching means, monitoring the filtered signals, and detecting the presence of a signal to be detected as the voltage level of any of the monitored signals exceeds a predetermined level, or as the voltage difference between two monitored signals exceeds a predetermined threshold value. 2. A method as claimed in claim 1, c h a r a c t e r i z e d in that said signal to be detected is a home metering pulse forwarded on a double conductor (a-b) from a telephone exchange. 3. A method as claimed in claim 1 or 2, c h a r a c t e r i z e d in that the signals obtained from the outputs (1 - 4) of the switching means (MUX) are filtered by RC filters. 4. A detector circuit for detecting an alternating current signal of a predetermined frequency in received signals, c h a r a c t e r i z e d in that the circuit comprises a switching means (MUX) having an input (0) responsive to the received signals, and which comprises several outputs (1 - 4) and means for alternately connecting the input to each output, synchronized by a clock pulse fed to the switching means (MUX) , means for feeding the clock pulse to the switching means (MUX) whereby during essentially one cycle of an alternating current signal to be detected the input (0) of the switching means (MUX) is once connected to each output (1- 4) of the switching means, filtering means (6) responsive to the outputs of the switching means (MUX) for filtering the signals obtained from the outputs (1 - 4) of the switching means, and comparator means (5) for receiving the signals obtained from the outputs of the filtering means (6) , and for producing a predetermined signal (detO) and feeding it further as the voltage level of any received signal exceeds a predetermined level, or as the voltage difference between two received signals exceeds the predetermined level. 5. A detector circuit as claimed in claim 5, c h a r a c t e r i z e d in that the circuit is connected by a double conductor (a-b) to a telephone exchange having an analog interface, whereby the circuit comprises a differential amplifier (AMP) to whose inputs said double conductor (a-b) is connected, and whose output is connected to the input (0) of said switching means (MUX) advantageously through a filtering means (5) and a capacitor (Cl) . 6. A detector circuit as claimed in claim 4 or 5, c h a r a c t e r i z e d in that the detecting frequency of the detector circuit is arranged to detect the home metering pulses from a telephone exchange. 7. A detector circuit as claimed in any one of claims 4 - 6, c h a r a c t e r i z e d in that the circuit further comprises detecting means (8, 9, C2) for detecting the polarity of the double conductor (a-b) , detecting means for detecting a calling signal, and detecting means for detecting a break in the current fed by the telephone exchange to the double conductor a-b, and a coding means (10) for producing and feeding further a two-bit signal (detl, det2) indicating the polarity, calling signal and break.

Inventors: HURME HARRI, NORDSTROEM CAJ, HURME, HARRI, NORDSTROEM, CAJ
Applicants: HURME HARRI, NOKIA TELECOMMUNICATIONS OY, NORDSTROEM CAJ, HURME, HARRI, NORDSTROEM, CAJ
Application year: 1995
Main classification: G01R19
IPC codes: G01R19, G01R23, H04M15, H04M19, H04Q1