A signal detecting circuit for detecting a signal specified in a designated frequency

Abstract: A designated burst signal specified in a designated frequency is detected from an all-inclusive signal (S i ) specified in a plurality of frequencies, by multiplying a frequency of the all-inclusive signal (S i ) by a multiplying factor so as to produce a multiplied output signal (S33), performing a phase-lock loop operation between the multiplied output signal (S33) and an oscillated signal produced for performing the phase-lock loop operation and specified in a multiplied frequency of the designated frequency by the multiplying factor, so as to produce a phase-lock output signal (S41) and comparing the multiplied output signal (S33) and the phase-lock output signal (S41) for producing a detected output signal (S d ) representing whether the designated burst signal is in the all-inclusive signal.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a signal detecting circuit for detecting a designated signal specified in a designated frequency, from an input signal to the signal detecting circuit, having a plurality of frequency components. With respect to the above detection, there is an example of a public telephone set in a telephone system. When a calling party speaks with his called party by using a public telephone set through a central office, a metering signal is sent for accounting from the central office to the public telephone set in an accounting cycle. The metering signal is a pulse-shaped burst signal of a sine wave specified in a designated frequency such as 50 Hz, 12 kHz or 16 kHz, and the metering signal is sent to the public telephone set together with a voice signal transmitted between the calling party and the called party. Therefore, the metering signal must be detected from a signal consisting of the voice signal and the metering signal, for accounting at the public telephone set. The same kind of example can be cited from PBX in a telephone system when a subscriber of PBX speaks with his called party accommodated to a central office of the telephone system. That is, a metering signal same as mentioned above is sent to the PBX from the central office for accounting together with a voice signal transmitted between the subscriber and his called party. Therefore, in order to perform the accounting, the metering signal must be detected from a signal consisting of the voice signal and the metering signal at the PBX. Same as the above examples, in other communication systems or signal processing system, there are many cases that a designated signal specified in a designated frequency component is required to be detected from an input signal having proper frequency components. In order to detect this kind of designated signal, a phase-lock loop circuit has been widely used in the telephone system, called PLL. Description of the Related Art Fig. 1 shows an example of a signal detecting circuit (500) of a related art used in a telephone system, and signals appearing in signal detecting circuit 500 in Fig. 1 are illustrated in Figs. 2a, 2b, 2c, 2d and 2e respectively. A similar circuit is also shown on pages 52-53 in the book "Phase lock Techniques", author F. M. Gardner, 1966, John Wiley & Sons, New York, London, Sydney. As well known, signal detecting circuit 500 consists of a Phase-Lock Loop circuit (PLL) 1 and a phase-lock detector 2 and outputs a detection signal S d (see Fig. 2e) when signal detecting circuit 500 receives an input signal S i (see Fig. 2a). In Fig. 2a, input signal S i is depicted in a style of a rectangle-shaped burst wave and the voice signal mentioned before is omitted to be illustrated, for simplicity. This rectangle-shaped burst wave is produced by limiting and waveform shaping the burst sin wave metering signal mentioned before. Means for limiting and waveform shaping the metering signal is not depicted in Fig. 1. The PLL 1 consists of a Voltage Controlled Oscillator (VCO) 11, a Phase Comparator (PC) 12 and a Low Pass Filter (LPF) 13. The VCO 11 produces a rectangle-shaped oscillation signal S₁₁ specified in an oscillation frequency f₁₁ proportional to a DC voltage V₁₃ output from LPF 13. The PC 12 compares phases of input signal S i and oscillation signal S₁₁ every cycle of S₁₁ and produces a rectangle-shaped signal S₁₂ in a duty ratio corresponding to a phase difference (lead or lag) between phases of input signal S i and oscillation signal S₁₁ or to a frequency difference between frequencies of S i and S₁₁. And LPF 13 outputs DC voltage V₁₃ to VCO 11 by performing low pass filtering to signal S₁₂. When input signal S i (metering signal) is not given to the signal detecting circuit, the duty ratio of signal S₁₂ is kept to 50%, DC voltage V₁₃ is kept to a proper value and oscillation frequency f₁₁ or signal S₁₁ is also kept to a proper frequency so-called free-running frequency (f₀). When input signal S i is given to the signal detecting circuit and a frequency f i of input signal S i is higher than oscillation frequency f₁₁ or when a phase  i of input signal S i leads a phase ₁₁ of signal S₁₁, the duty ratio of signal S₁₂ becomes larger than 50%, resulting in increasing DC voltage V₁₃ and oscillation frequency f₁₁. On the contrary, when frequency f i is lower than frequency f₁₁ or when phase  i lags behind phase ₁₁, the duty ratio of signal S₁₂ becomes less than 50%, which results in decreasing DC voltage V₁₃ therefore lowering oscillation frequency f₁₁. When frequency f i is equal to free-running frequency f₀, the comparison between the phases (or frequencies) of input signal S i and signal S₁₁ is repeated in PLL 1 until the phase-lock is established between input signal S i and signal S₁₁. When the phase-lock is established, oscillation frequency f₁₁ becomes equal to frequency f i and the phase difference ( i - ₁₁) between the phases of signals S i and S₁₁ is kept to a fixed value of π. If free-running frequency f₀ or a frequency nearby free-running frequency f₀ is not included in input signal S i, frequency f₁₁ and phase ₁₁ of signal S₁₁ cannot be locked, establishing no phase-lock. The phase-lock detector 2 consists of an exclusive OR gate 21, an LPF 22, a voltage comparator 23 and a reference voltage source 24. The exclusive OR gate 21 performs exclusive OR operation to input signals S i and S₁₁, and produces output signal S₂₁. The LPF 22 performs low pass filtering to signal S₂₁ and produces a DC voltage V₂₂. The voltage comparator 23 compares DC voltage V₂₂ with a reference voltage V₂₄ from reference voltage source 24 and produces detection signal S d which is equal to the output from the signal detecting circuit 500. The detection signal S d becomes a signal representing an undetected state such as logic "0" when DC voltage V₂₂ is lower than reference voltage V₂₄ and becomes a signal representing a detected state such as logic "1" when V₂₂ is higher than V₂₄. In Fig. 1 and Figs. 2a to 2e, since input signal S i is not given to signal detecting circuit 500 till t₁ (see Fig. 2a), VCO 11 oscillates in free-running frequency f₀ till t₁ as shown in Fig. 2b, so that signal S₂₁ becomes a rectangle-shaped signal having 50% duty cycle till t₁ as shown in Fig. 2c. In such state, DC voltage V₂₂ from LPF 22 is maintained lower than reference voltage V₂₄, so that voltage comparator 23 outputs detection signal S d representing the undetected state (logic "0"). When input signal S i specified in frequency f i equal or nearly equal to free-running frequency f₀ is given to signal detecting circuit 500 in a time interval from t₁ to t₃ (see Fig. 2a), VCO 11 operates so as to bring phase ₁₁ close to phase  i by varying oscillation frequency f₁₁ of signal S₁₁ around free-running frequency f₀, so that the phase difference ( i - ₁₁) approaches π (compare Figs. 2a and 2b near t₃). As the phase difference approaches π, the duty ratio of output signal S₂₁ from exclusive OR gate 21 gradually increases (see a waveform at the time interval from t₁ to t₃ in Fig. 2c) and DC voltage V₂₂ from LPF 22 also rises as shown in Fig. 2d. Then, the phase-lock is established in PLL 1 and the phase difference ( i - ₁₁) becomes π. When the duty ratio in signal S₂₁ reaches 100%, DC voltage V₂₂ becomes reference voltage V₂₄ at t₂ as shown in Fig. 2d and voltage comparator 23 outputs detection signal S d representing the detected state (logic "1") as shown in Fig. 2e. Thus, the free-running state of PLL 1 till t₁ is changed to a phase-lock state at t₂. In other words, the time interval from t₁ to t₂ is a transition time for changing the state of PLL 1 from free-running to phase lock. When input signal S i is ended at t₃, PLL 1 starts to bring back the state from phase-lock to free-running by a process opposite to the above, passing through the similar transition time to the above, which is not depicted in Figs. 2a to 2d. Problems in the Related Art However, in the signal detection circuit of the related art, a long transition time is required in PLL1 to change the free-running state to the phase-lock state and vice versa as shown in Figs. 2b and 2c, which has been a problem in the related art. In particular, this problem becomes more remarkable in the telephone system when the metering frequency, which is equal to frequency f i of input signal S i is as low as 50 Hz. Because, in some telephone system, such low metering frequency as 50 Hz must be employed in accordance with rules of the telephone system and a time to get the phase-lock is required to be as short as 120 millisecond. From a viewpoint of the reliability of operation, it has been hard to employ such low frequency for the metering frequency as far as the related art is applied to the telephone system. SUMMARY OF THE INVENTION Therefore, an object of the present invention as it is defined in the appended claims is to shorten a time for detecting a designated signal specified in a designated frequency from a signal having a plurality frequency components, in a signal detecting circuit including a phase-lock loop circuit (PLL). Another object of the present invention is to increase detecting efficiency of the signal detecting circuit. Still another object of the present invention is to increase detecting reliability of the signal detection circuit. The above objects are achieved by providing a frequency multiplier for multiplying the designated frequency, on the passing way of the designated signal given to PLL. The frequency multiplier multiplies the designated frequency by a frequency multiplication factor. Therefore, when the designated signal is given to PLL through the frequency multiplier, the designated frequency is multiplied by the frequency multiplying factor. As well known, PLL includes an oscillator for producing an oscillation signal specified in a free-running frequency previously determined to be equal to a frequency of a signal to be detected at PLL. When no signal to be detected is given to PLL, the oscillator performs a free-running oscillation, producing the oscillation signal specified in the free-running frequency, so that the signal detecting circuit produces an output representing that a signal to be detected is not detected by the signal detecting circuit. When a signal to be detected is given to PLL with the same frequency as the free-running frequency, phases of the signal to be detected and the oscillation signal are compared and the phase of the oscillation signal is varied untill the both phases are coincided to each other. When the phases are coincided to each other, which is called "phase-lock", the signal detecting circuit outputs a signal representing that the signal to be detected is detected by the signal detecting circuit. In the process of becoming the phase-lock, it takes a time (transit time) to change the state of PLL from free-running to phase-lock after the signal to be detected is given to PLL, and similarly to the above, it takes another transit time to change the state of PLL from phase-lock to free-running after the signal to be detected disappears. However, precisely considering the transit time, there is a fact that the transit time is inversely proportional to the frequency of the signal to be detected or the free-running frequency. This is a point of the present invention. That is, in the present invention, because of increasing the frequency of the input designated signal by multiplying the frequency at the frequency multiplier, the transit time can be shortened when the free-running frequency in PLL is also increased same as the multiplied frequency of the input designated signal. As a result of shortening the transit time thus, the time for detecting the designated signal can be shortened, so that the detecting efficiency and reliability of the signal detecting circuit can be increased. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a signal detecting circuit of the related art; Fig. 2a is a waveform illustrated for an input signal to a signal detecting circuit; Fig. 2b is a waveform illustrated for an output signal from a voltage controlled oscillator (VCO) in a phase-lock loop circuit (PLL) of the related art signal detecting circuit; Fig. 2c is a waveform illustrated for an output signal from an exclusive OR gate in a phase-lock detector of the related art signal detecting circuit; Fig. 2d is a DC voltage illustrated for an output signal from a low pass filter in the phase-lock detector; Fig. 2e is an illustration of an output signal from a voltage comparator in the phase-lock detector; Fig. 3 is a block diagram for illustrating a principle of a signal detecting circuit embodying the present invention; Fig. 4 is a block diagram of a signal detecting circuit embodying the present invention; Fig. 5a is a waveform illustrated for an input signal to a frequency multiplier in the signal detecting circuit; Fig. 5b is a waveform illustrated for an output signal from a diode bridge in the frequency multiplier; Fig. 5c is a waveform illustrated for an output signal from a low pass filter in the frequency multiplier; Fig. 5d is a waveform illustrated for an output signal from a waveform shaper in the frequency multiplier; Fig. 5e is a waveform illustrated for an output signal from a voltage controlled oscillator in a phase-lock loop circuit of the signal detecting circuit; Fig. 5f is a waveform illustrated for an output signal from an exclusive OR gate in a phase-lock detector of the signal detecting circuit; Fig. 5g is an illustration of an output signal from an low pass filter in the phase-lock detector; and Fig. 5h is an illustration of an output signal from a voltage comparator in the phase-lock detector. DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 3 illustrates a principle of a signal detecting circuit 600 embodying the present invention. In Fig. 1 the same numeral or symbol as in Fig. 1 designates the same unit or signal as in Fig. 1. In Fig. 1, signal detecting circuit 600 consists of a frequency multiplier 100 of the type similar to that disclosed in GB-A-1469 374, a PLL 200 and a phase-lock detector 300. The frequency multiplier 100 is a circuit newly provided in signal detecting circuit 600 for the present invention and PLL 200 and phase-lock detector 300 are functionally same as PLL 1 and phase-lock detector 2 in signal detecting circuit 500 of the related art respectively. The frequency multiplier 100 multiplies a frequency f i of input signal S i to signal detecting circuit 600 (frequency multiplier 100) by a multiplying factor N, producing output signal S₁₀₀. The PLL 200 performs phase-lock so that a free-running oscillation signal in PLL 200 is locked in a designated signal specified in a designated frequency in signal S₁₀₀ received from frequency multiplier 100, producing an output signal S₂₀₀. The phase-lock detector 300 compares signal S₁₀₀ with signal S₂₀₀ for detecting whether there are signals being in a phase-lock state each other and produces detection signal S d . The detection signal S d becomes the signal representing the detected state if there is a signal specified in a frequency being one Nth of multiplied frequency in input signal S i . Since the multiplied frequency is used, the phase-lock becomes possible to be performed in PLL 200 even though the time interval of the pulse-shaped input signal S i is short, which extremely increases the detecting ability of the metering signal and therefore increases a processing speed of the telephone system. A preferred embodiment of the present invention will be explained in reference to Fig. 4 and Figs. 5a to 5h below. Fig. 4 shows a schematic diagram of signal detecting circuit 600 embodying the present invention and Figs. 5a to 5h illustrate the waveforms of signals appearing in the schematic diagram shown in Fig. 4. In Fig. 4 and 5a to 5h, the same reference numeral or symbol as in Figs. 1 and 2a to 2e designates the same part, circuit or signal as in Figs. 1 and 2a to 2e. The signal detecting circuit 600 consists of a frequency multiplier 3, a PLL 4 and a phase-lock detector 5 as shown in Fig. 4. In Fig. 4, frequency multiplier 3, PLL 4 and phase-lock detector 5 are provided as frequency multiplier 100, PLL 200 and phase-lock detector 300 in Fig. 3 respectively. The frequency multiplier 3 is provided so as to operate under a condition that N = 2 in this embodiment, so that frequency multiplier 3 consists of a diode bridge 31 including four diodes D₁, D₂, D₃ and D₄ and additional circuits such as: an LPF 32 consisting of an operational amplifier A₁, resistors R₂, R₃, R₄ and R₅ and capacitors C₁ and C₂; and a waveform shaper 33. The diode bridge 31 produces an output signal S₃₁ specified in a doubled frequency of frequency f i of input signal S i by performing fullwave rectification to input signal S i . The LPF 32 produces an output signal S₃₂ by extracting a base frequency component being a doubled frequency component of input signal frequency f i . The waveform shaper 33 produces output signal S₃₃ formed to a pulse-shaped wave by shaping signal S₃₂. Therefore, frequency multiplier 33 produces output signal S₃₃ formed to a rectangle-shaped wave specified in a doubled frequency of frequency f i of input signal S i . The PLL 4 consists of VCO 41, PC 42 and LPF 43 each having the same function as VCO 11, PC 12 and LPF 13 in PLL 1 in Fig. 1 respectively. That is, the VCO 11 has a free-running frequency f₄₀ equal or nearly equal to a doubled frequency f₃₃ of a designated signal (metering signal) included in input signal S₁. As a result, when signal S₃₃ from frequency multiplier 3 includes a signal specified in the doubled frequency f₃₃, PLL 4 performs the phase-lock between the doubled frequency f₃₃ and a oscillation frequency of VCO 41 and produces output signal S₄₁ having a phase different from a phase of the designated signal as much as π. A phase lock detector 5 in Fig. 4 consists of an executive OR gate 51, a LPF 52, voltage comparator 53 and a reference voltage source 54 and detects that PLL 4 is in the phase-lock state with the designated signal in signal S₃₃, same as phase lock detector 2 in Fig. 1. The above operation of signal detecting circuit 600 will be further explained in reference to Figs. 5a to 5h. Fig. 5a shows a waveform of input signal S i . However, in Fig. 5a, only a designated signal such as a metering signal is depicted in a style of burst signal appearing in a time interval between t₁₁ and t₁₃, omitting other signals such as a voice signal for simplicity. when the designated signal is given to diode bridge circuit 31 as shown in Fig. 5a, diode bridge circuit 31 performs full-wave rectification, producing signal S₃₁ as shown in Fig. 5b. The LPF 32 extracts a doubled frequency component of she designated signal so as to produce signal S₃₂ as shown in Fig. 5 c. Then waveform shaper 33 shapes signal S₃₂ so as to produce signal S₃₃ having squared-shaped wave as shown in Fig. 5d. Since no signal is given to PLL 4 till t₁₁ as shown in Fig. 5d, VCO 41 oscillates signal S₄₁ specified in a free-running frequency f₀ in 50% duty ratio till t₁₁ as shown in Fig. 5e. In such state, exclusive OR gate 51 outputs signal S₅₁ in 50% duty ratio till t₁₁ as shown in Fig. 5f, and output voltage V₅₂ from LPF 52 is maintained a voltage lower than reference voltage V₅₄ as shown in Fig. 5g, so that voltage comparator 53 produces detection signal S d representing an undetected state (logic "0") as shown in Fig. 5d. When t₁₁ passes, the phase of the signal S₄₁ specified in the free-running frequency f₀ varies so as to lock to the phase of signal S₃₃ in PLL 4, taking a transit time (from t₁₁ to t₁₂) as seen from comparison between Figs. 5e and 5f in an interval from t₁₁ to t₁₂. That is, during the transit time, PLL 4 compares phase ₃₃ of signal S₃₃ with phase ₄₁ of signal S₄₁ and varies the phase ₃₃ so that the phase difference (₃₃ - ₄₁) becomes π by making phase ₃₃ follow phase ₄₁. In phase lock detector 5, as the phase difference (₃₃ - ₄₁) closes to π, the duty ratio of signal S₅₁ from exclusive OR gate 51 increases gradually and therefore output voltage V₅₂ from LPF 52 increases. when the phase difference becomes π, the duty ratio of signal S₅₁ reaches 100% as shown in Fig. 5f and output voltage V₅₂ exceeds reference voltage V₅₄ at t₁₂ as shown in Fig. 5g, so that voltage comparator 53 produces detection signal S d presenting the detected state (logic "1") and the detected state continues as far as V₅₂ exceeds reference voltage V₅₂ as shown in Fig. 5h. When the burst input signal S i is over at t₁₃, TLL 4 is brought back to the free-running state at t₁₄ in a opposite process to the above phase-lock process through a transition time from t₁₃ to t₁₄ as shown after t₁₃ in Figs. 5f, 5g and 5h. As seen from the above explanation of the embodiment, since frequency multiplier 3 doubles the frequency f i of input signal S i and the phase-lock is performed by using the oscillation signal specified in the doubled frequency in PLL 4, the transit time (from t₁₁ to t₁₂) for establishing the phase-lock state on PLL 4 and the transit time (from t₁₃ to t₁₄) for bringing PLL 4 back to the free-running state can be shortened as less as one half of those in the related art. As a result, the signal detection can be performed, leaving a sufficient margin though the signal to be detected is given to the signal detecting circuit 600 in a short time. In the above explanation of the embodiment, the detection of the metering signal in the telephone system has been discussed. However, the present invention is not limited to apply the telephone system. The present invention can be applied to other communication system or signal processing system. The block diagram shown in Fig. 4 is nothing but an embodiment of the present invention. Other circuits can be considered to achieve the object of the present invention. In the explanation of the embodiment in reference to Fig. 4 and Figs. 5a to 5h, the multiplying factor N is set to two in frequency multiplier 3, however, other numeral larger than two is applicable to N. The constitution of frequency multiplier 100, PLL 200 and phase lock detector 300 in Fig. 3 is concretely shown in frequency multiplier 3, PLL 4 and phase lock detector 5 in Fig. 4 respectively, however, the constitution in Fig. 3 is not limited to that in Fig. 4.

Claims: A signal detecting circuit for detecting a designated signal specified in a designated frequency, from an input signal (Si), specified in a plurality of frequencies, to the signal detecting circuit, said signal detecting circuit comprising: a frequency multiplier (3, 100) for multiplying an input frequency, which is one of the plurality of frequencies, of the input signal (Si), said frequency multiplier having a multiplying factor and producing a multiplier output signal (S₃₃) specified in a frequency obtained by multiplying the input frequency by the multiplying factor; a phase-lock loop (4, 200) circuit for performing phase-lock between a phase of the multiplier output signal (S₃₃) and a phase of an oscillation signal (VCO) produced in said phase-lock loop circuit and producing a phase-lock output signal (S₄₁, S), said oscillation signal being specified in a multiplied frequency of the designated frequency by the multiplying factor and said phase-lock being performed by comparing (42) phases of the multiplier output signal (S₃₃) and the oscillation signal (VCO), producing a comparison result and shifting the phase of the oscillation signal (VCO) in response to the comparison result so that the phases of the multiplier output signal (S₃₃) and the oscillation signal (VCO) are synchronized with each other; and a phase-lock detector (5, 300) for producing a detected output signal (Sd) of the signal detecting circuit for representing whether the designated signal is in the input signal, by comparing the multipier output signal (S₃₃) with the phase-lock output signal (S₄₁, S). A signal detecting circuit according to claim 1, wherein said frequency multiplier comprises a diode bridge circuit (31) performing fullwave rectification of the input signal for producing a multiplier output signal specified in a doubled frequency of the input frequency. A signal detecting circuit according to claim 2, wherein said frequency multiplier further comprises: a low pass filter (32) for extracting a filter output signal specified in the doubled frequency, from the multipier output signal specified in the doubled frequency; and a waveform shaper (33) for shaping a waveform of the filter output signal into a rectangle-shaped wave. A signal detecting method for detecting a designated signal specified in a designated frequency from an all-inclusive signal (Si) to be examined, specified in a plurality of frequencies; said method comprising the steps of: multiplying (3, 100) a frequency of the all-inclusive signal by a multiplying factor, producing a multiplied signal (S₃₃) specified in a multiplied frequency of the frequency of the all-inclusive signal by the multiplying factor; comparing (4, 200) a phase (first phase) of the multiplied signal (S₃₃) with a phase (second phase) of an oscillated signal (VCO) specified in a multiplied frequency of the designated frequency by the multiplying factor, producing a comparison output signal; shifting the second phase in response to the comparison output signal so that the first phase is locked with the second phase; repeating said steps of phase comparison and phase shift until phase-lock between the first and second phases is established and producing a phase-lock output signal (S₄₁, S); and comparing (5, 300) the phase-lock output signal with the multiplying signal for producing a detected output signal (S d ) representing whether the designated signal is in the all-inclusive signal.

Inventors: SATO TOSHIYUKI, SATO, TOSHIYUKI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1991
Main classification: H03D3
IPC codes: H04M15, H04Q1, H03K5, H03B19, H03L7, H03D3, H04L27