US 3710373 A
An improvement in a signal discriminating system used in a monitoring and control system for a low voltage commercial power distribution line, in which a high frequency signal is superimposed on the power current of 50 or 60 Hz, as a communication medium between a monitoring spot and consumers connected to the distribution line; the improvement resides in that in the centralized reading of the meters of respective consumers, the monitoring channel is time-divided or frequency-divided in order to provide a reference signal level peculiar to each consumer.
Claims available in
Description (OCR text may contain errors)
. United States Patent Watanabe et al. 51 Jan. 9, 1973  SIGNAL DISCRIMINATING SYSTEM 3,461,423 8/1969 Anderson ..340/3|0 X I 3,249,883 5/l966 Be'rneikc ..340/l83 X I lnvenwrszbelzl Wawnabe, Yokohama: K010 2,582,957 /1952 BOTSUm et 61.. ..340/l88.5 ux Ozaki, Tokyo; Hiroshi Oishi, Yoko- 3,266,018 8/1966 Higgins ..340/|83 X hama; Fumio Aohi, Yokohama; Shi- 3.268.665 8/1966 Miller et 111.. 340/177 CA ux geru Kawano, Tokyo all of japan 2,753,547 7/1956 Donath et al... ..340/l83 2,7l2,128 6/1955 Woodruff ..340/183 Assigneez Matsushita Communication Indus- 3,264,633 8/1966 Hellar ..340/183X trial Co" Ltd Yokohama, The 3,082,330 3/l963 Ward ..340/l83 UX Tokyo Electric Power Co., Tokyo, both of Japan Primary ExammerThomas B. Habecker Attorney-Stevens, Davis, Miller & Mosher  Filed: May 11, 1970 211 App]. No.: 36,368  ABSTRACT I An improvement in a signal discriminating system used in a monitoring and control system for a low volt- Foreign Apphcat'on Pnonty Data age commercial power distribution line, in which a May 14,1969 Japan ....44/38630 high frequency Signal is superimposed on the power May 14. 1969 Japan ..44/38633 amen 0f or as a CQmmmiCatiO" medium between a monitoring spot and consumers connected 52 us. (:1. ..340/310 307/3 340/171 distributim the improvement sides in [SI 1 Int Cl 6 15/10 that in the centralized reading of the meters of respecfive consumers,- the monitoring channel is [.58] Field of Search "340/177 vided or frequency-divided in order to provide a reference signal level peculiar to each consumer.
 References Cited 5 Claims 10 Drawin F UNITED STATES PATENTS 'gures 2,833,862 5 1958 T015011 ..340/1s3 x CONSUMER NO I i AUXF I I 1 ma I 1 I 1 CW7 1 l .J I L040 /02 M i 1 S/GNAL S/G/VAL INJECTOR LET/F670 T T 3 //5 //2(20/) SIGNZJL O/SCfP/Ml- #3 N47 /1765) SIGNAL DISCRIMINATING SYSTEM system, high frequency signals are used for the purpose of control and monitoring, the signals being injected to the distribution lines in a superimposed manner on the commercial current of 50 or 60 Hz. Usually, the voltage level of such a signal at consur'ners ends varies de- I pending on various causes including differences in the length of the distribution lines connected to consumers, variation in the injection level of the signals and differences or changes in the loads of the consumers.
The main object of this invention is to provide an improved signal discriminating method and system which v allows the above-mentioned reading of the consumers meters to be effected in a stable and reliable manner notwithstanding the above-mentioned variation of the signal level.
Another object of this invention is to provide such a method and a system also including functions of allowing the centralized control of the consumers loads and the confirmation of the results of the controlto be effected in a single monitoring cycle including the periods for the above-mentioned reading of the consumers meters.
In order to achieve the above objects, this invention provides an improvement in the signal discriminating method used in the monitoring and control system having functions of injecting high frequency signals to the distribution line at a monitoring spot, detecting a predetermined degree of variation in the injected signal due to a change in the channel of the signal and a function provided at each consumer for changing the state of the signal flow path depending on the condition which is the object of the monitoring, said change in the state of the flow path being made effective during predetermined periods allotted to the consumer; the improvement residing in that a further function is provided at each consumer to produce a reference state of the signal flow path during said allotted period regardless of said change in the flow path, and that said detection of the variation in the signal is performed by comparing the levels of two signals resulting from said two functions provided at the consumer.
The features and merits of this invention will be clarified by the following description given in connection with embodiments of the invention and in comparison with the prior artand further with reference to the accompanying drawings, in which;
FIG. 1 is a block diagram of the conventional monitoring or metering system for apower distribution line;
FIG. 2 is a diagram showing waveforms found in the system shown in FIG. I;
FIG. 3 is a block diagram of a monitoring system including an embodiment of this invention;
FIG. 4 is a diagram illustrating waveforms found in the system shown in FIG. 3;
FIG. 5 is a block diagram of an essential part of the system shown in FIG. 3;
FIG. 6 shows waveforms for explaining the operation of the circuit shown in FIG. 5;
FIGS. 7 and 9 are block diagrams relating to other embodiments of this invention; and
FIGS. 8 and 10 show waveforms observed at essential points in the systems shown in FIGS. 7 and 9.
First, the conventional system will be described with reference to FIGS. 1 and 2.
In FIG. 1, reference numeral 1 designates a signal injection terminal; 2 a signal injector (illustrated as a current injection); 3 a series resonant circuit tuned to the frequency of the' injected signal; 4 a synchronous motor; 5 an auxiliary circuit which integrates the signal received through the resonant circuit 3 in order to enhance the assuredness of the signal and then actuates a relay incorporated therein to start the synchronous motor 4; and numeral 6 designates another series resonant circuit. Though separate resonant circuits 3 and 6 are shown in the FIGURE for the convenience of explanation, it is a usual practice to use a single resonant circuit commonly for both circuits 3 and 6. Numeral 7 designates a switch operated with a timing peculiar to each consumer as the synchronous motor 4 rotates; 8 another switch to keep the resonant circuit 3 off the distribution line 14 after the start of the motor 4 until the end of the intended metering cycle; 9 a further switch arranged to close and open for once each time the consumer's watt-hour meter 10 reads another I kWI-I, for example; 11 a signal detector; 12 an output signal terminal; 13 the consumers load; and 15 another branch of the distribution line extending from a pole transformer 16 to other consumers.
In FIG. 2, index (a) designates a waveform of the signal at the signal injection terminal 1; (b) an example of waveform of the signal as detected at the signal detector 11, showing the changes in the signal level which reflect the states of watt-hour meters in the respective consumers; and (c) to (h) timing pulses which allow the distribution line to be used as the time-divided communication channel, each pulse corresponding to the operation of the switch 7 shown in FIG. 7.
The operation of the system shown in FIG. 1 will be described hereunder also with reference to FIG. 2. A radio-frequency or an audio-frequency signal (a) shown in FIG. 2 is injected into the distribution line 14 through the signal input terminal 1 and the signal injector 2 to be superimposed on the power current. The superimposed signal is led to the resonant circuit 3 through the distribution line 14. The resonant circuit 3 detects the signal and has the synchronous motor 4 started with the assistance of the auxiliary circuit 5. The same operations concurrently occur at other consumer devices connected to other branches l5 originating from the same transformer 16. Thus, the synchronous motors at respective consumers concurrently start to thereby close and open the respective switches 7 according to predetermined timing allotted to the consumers. When the switch 7 is in the closed state, the resonant circuit 6 is ready to be connected across the distribution line. The mutual relation in the timing of the operation of the switch 7 at the respective consumers is shown in FIG. 2 by waveforms (c) to (h) as mentioned previously. Whether the resonant circuit 6 is actually connected across the distribution line while the switch 7 is closed, or not, depends on the state of the switch 9 which represents the state of the watt-hour meter and which is connected in series with the switch 7. The switch 8 is opened after the I synchronous motor 4 is once started, so that the resonant circuit 3 is kept off the distribution line 14 during the allotted monitoring period. Therefore, the insertion of the resonant circuit 6 to the line is the decisive factor to greatly change the impedance of the signal channel or flow path. If the resonant circuit 6'is connected across the line reflecting a start of the watt-hour meter, the injected signal is led through the resonant circuit 6 and undergoes only a small loss. If, on the other hand, the resonant circuit 6 is removed from the line reflecting the other state of the meter 10, the signal is led by the route of the consumers load and undergoes a heavy loss. Thus, the state of the meter can be determined from the magnitude of the signal detected in the signal detector 11.
In the above-described conventional system, how- 'ever, the detected signals for different periods allotted to respective consumers have different signal levels even if a similar resonant circuit is in closed state at each consumer, as indicated by reference numerals 22, 23 in the waveform (b) shown in FIG. 2. Such difference in the detected signal levels isdue mainlyto the difference in the length of the branches. Further, signal level at the signal injector also is not necessarily constant. Therefore, the signal level detected while no resonant circuit is connected to the line, may also vary over a long period, as indicated by 21 and 24 in waveform (b)'of FIG. 2. Thus, it is possible to happen that the signal level detected in a readout cycle without a consumers resonant circuit 6 connected is very near to that detected in another readout cycle with another consumers resonant circuit 6 connected to the line. In such a case, it is impossible to set a proper threshold lever 25 for discriminating two kinds of signals.
Thus, according to the conventional system, an assured and reliable detection of the state of consumers meters cannot be achieved unless a limitation is set with respect to the variety in the length of respective branches, the magnitude of consumers loads and the injection level of the signal. I
Such a problem has been solved by this invention.
tected signal includes two pieces of information for each consumer. The signal in the first half period represents the information of the signal flow path with the resonant circuit 106 connected across the consumers load 113, the level of the signal depending on the property and state of the line leading to the con- Hereunder, this invention will be described in detail in connection with an embodiment of the invention.
The circuit shown in FIG. 3 is the same as that shown in FIG. 1, components 101 to 116 in FIG. 3 corresponding to 1 to 16 in FIG. 1 respectively, except'that the system shown in FIG. 3 is further provided with an auxiliary switch 117and a signal discriminator 118. In FIG. 4, index (i) designates the waveform of a signal detected at the signal detector 111; (k) and (m) are waveforms showing the operation of the auxiliary switches 117 which open and close according to timings allotted to the respective consumers; and (l) and (n) show the operation of switches 107 which correspond to switches 7in FIG. 1. g
The operation of the system shown in FIG. 3 will be described hereinafter also with reference to FIG. 4.
The signal injected through the signal input terminal 101 causes the synchronous motor 104.'to start by means of the resonant circuit 103 as explained in connection fwith the system shown in FIG. 1. The
sumer and of the consumers load as well as on the level of the injection signal. This detected signal is referred to as a reference signal hereinafter. The level of the signal in the second half period which is synchronized with the closure of the switch 107, varies depending on whether the switch 109 is closed or opened, that is, whether the resonant circuit 106 is connected across the load or not. This signal in the second half period is referred to as a readout signal hereinafter.
Thus, the entire sequence of the'signal appearing at the output terminal 112 of the signal-detector 111 during a monitoring cycle will be as shown in FIG. 4 (waveform (i)), for example. Such a signal is led to the,
signal discriminator 118, the operation of which will be explained with reference to the block diagram shown in FIG. 5.
In FIG. 5 which is a block diagram example of the signal discriminator 118, reference numeral 201' designates an input terminal; 202 an input signal to the clock pulse input terminal 207, the timing of the clock pulse corresponding to that of the allotment of a monitoring period to each consumer. On the basisof the clock pulse, the timing control circuit 206 provides the change-over circuit 202 with an appropriate timing pulse through the control line 208, by which the selection between the reference signal and the readout signal is performed in the change-over circuit. The
firstlyincoming signal, that is, the reference signal is led to the register 203, and the peak value of the signal is stored therein in an analogue mood. The reference voltage producing circuit 204 multiplies an analogue voltage imparted from the register 203 by a factor (not more than 1), for example, by dividing the given voltage through a resistance voltage divider to thereby,
produce the reference voltage. Meanwhile, the next signal received during the ensuing half period, that is, the readout signal, is changed over to the voltage comparator 205 by the change-over circuit 202, where the readout signal is compared with the reference voltage, an appropriate timing pulse being applied to the comparator 205 from the control circuit 206 through the control line 210. The result of the comparison appears AND gate;
at the output terminal 211. A meter-reading for a consumer being thus completed, a reset pulse is imparted to the register 203 through the control line 209, and the signal discriminator becomes ready for another meterreading. The same operation is repeated for each conmonitoring post and when the level of the injection signal is low. It will be seen that the threshold level 301 is very low. On the other hand, index (p) indicates a similar detected signal for another consumer who is situated near to the monitoring post and when the level of the injection signal is especially high. In such a case, the threshold level 302 becomes high.
As the threshold level is automatically adjusted as described above, the restrictions on the variety in the length of distribution lines and in the consumers loads as well as the limitation on the deviation of the signal injection level can be lifted, and a reliable readout operation is ensured.
Next, another embodiment of this invention will be described with reference to FIG. 7 which shows a block diagram thereof and FIG. 8 which shows waveforms observed at an essential point in the system.
In FIG. 7, circuit components designated by reference numerals 501 to 516 correspond to those designated by numerals 1 to 16 respectively. Therefore, explanation on these components will not be repeated. Reference numeral 517 designates a further resonant circuit tuned to a frequency near to but different from that of the resonant circuit 506; numeral 518 a switch which closes and opens in synchronization with the switch 507; numeral 519 a band-pass filter; 520 an output terminal of the filter 519; numeral 52] another band-pass filter whose center frequency is different from that of the filter 519;.numeral 522 an output terminal of the filter 521; numerals 523 and 524 integrators respectively; 525 a voltage comparator; 526 an 527 a control signal input terminal of the AND gate 526; and 528 the output terminal of the same'AND gate.
In FIG. 8, index (q) indicates an example of the waveform of a signal appearing at the output terminal 520, which has been selected from the output of the signal detector 511 through the filter 519; and the waveform indicated by index (r) is a signal appearing at the output terminal 522, the output of the signal detector 511 being filtered through the filter 521.
In the operation, two signals of near but different frequencies are concurrently injected to the distribution line through the input terminal 501 and the signal injector 502. In this respect, the said two resonant circuits 506 and 517 are designed so as to respectively tune with one and the other of the above two frequencies. In the same manner as previously described in connection with FIG. 1, one of the injected signals is picked up by the resonant circuit 503 and causes the synchronous motor 504 to start, which closes and opens the switch 507 in a timing such that the availability of the distribution line as the signal channel or flow path is allotted among all consumers by a time division process. Also in the same manner as described previously, the closure of the switch 507 makes it possible for the resonant circuit 506 tuned to the said one frequency to be connected across the consumer's load 513 when the switch 509 is closed. In this embodiment, however, the switch 518 is closed concurrently with the switch 507 to connect the resonant circuit 517 (which is tuned to the said other frequency) across the load 513. Therefore, the signal of the latter frequency necessarily passes the resonant circuit 517 during the period allotted to the particular consumer and it is detected by the signal detector 511. Such a detected signal is also referred to as a reference signal.
On the other hand, the other signal having the said other frequency is transmitted either through the resonant circuit 506 or by the route of the load 513 and undergoing a heavy loss, depending on the state of the watt-hour meter 510. Such a signal, as detected by the signal detector 511, is also referred to as a readout signal.
The reference signal and the readout signal appearing at the terminal 512 through the signal detector 511, are separated from each other through the band-pass filters 519 and 521 tuned to the respective frequencies I of two signals, and the readout signal as indicated by index (q) in FIG. 8 appears at the terminal 520, while the reference signal as indicated by (r) in the same FIGURE appears at the terminal 522. As is seen from FIG. 8, when the switch 509 is in closed state in response to the state of the watt-hour meter 510, level 601 of the readout signal is substantially equal to level 602 of the reference signal; whereas with the switch 509 in opened state, level 603 of the readout signal is lower than level 604 of the reference signal. The respective signals appearing at the respective terminals 520 and 522 are applied to the voltage comparator 525, after the assuredness of the signals has been enhanced through the respective integrators 523 and 524. Thus, the state of the switch 509 (therefore, the state of the watt-hour meter 510) in each consumer can be read out by extracting the output of the voltage comparator in a proper timing through the AND gate 526, namely, a clock pulse being applied to the clock pulse input terminal 527 of the AND gate 526 in synchronization with the previously described switch operating pulses ((c) to (h) in FIG. 2). With this embodiment, as described above, a distribution line as a signal flow path is utilized for both reference and readout signals through frequency-division, whereby the threshold level for discriminating the readout signal is automatically adjusted.
The application field of the method and device of this invention is extended by adapting the system so that the signal flow path, time or frequency-divided or in a frequency-divisional manner as described above, is utilized also for control of loads and confirmation of the control. An embodiment of such a modified system will be described hereunder with reference to the block diagram shown in FIG. 9 and waveforms shown in FIG. 10.
In FIG. 9, the components designated by reference numerals 701 to 708 are equivalent respectively to those designated by numerals 101 to 108 in FIG. 3. Also, numerals 711 to 718 correspond respectively with lll to 118 in FIG. 3. Therefore, description about such components is spared. Numeral 709 designates a switch which is connected in series with another switch in thesame manner as the switch 109 shown in FIG. 3 but which is controlled by a relay 809. Numerals 801, 802 and 803 respectively designate series resonant circuits tuned to an identical frequency; 804 a timer; 805
7 an auxiliary circuit which causes the timer 804 to start in response to the signal received through the resonant circuit 801 and stops the timer 804 after the completion of one operation cycle; 806 a switch for keeping -'on the output relay' 809 in response to the signal received through the resonant circuit'802; 811 another control circuit for turning-off the same relay in response to the signal received through the resonant circuit 803; and 813 the load',(for example, street lights) to be controlled by this system which is situated at a place near the particular consumers load 713.
In FIG. 10, index (s) indicates an example of a sequence ofsignal injectedto the line 714 through the signal injection terminal 701 and the signal injector 702, and index (t) 'a signal appearing at the output terminal of the signal detector 711. Indexes (u), (v), (w) and (x) indicate the operation of the switches 807, 808, 717 and 707 respectively.-
In the operation, a high frequency signal (s) as shown in FIG. 10 is injected into the distribution line 714 by means of the signal injector 702. The injected signal is picked'upby the resonant circuit 703 of each consumer and starts the rotation of the motor (that. is, a timer) 704 by means of the auxiliary circuit 705 in the same manner as described in connection with FIG. 1 or FIG. 3. In the present embodiment, however, the signal is also picked up by the resonator 801, and thetimer 804 is'started by means of the auxiliary circuit 805 substantially at'the same time as. the start of the timer 704. Signal 901 shown in FIG. 10 indicates this starting signal. The simultaneously started timers 704 and 804 close, and openthe respective switches 707, 717 .and 807, 808 in the timing shown in FIG. 10. (It is assumed that the circuit consisting ofcomponents 801 to 812 is controller No. 2) That is, the switch 807 is first closed to connect the resonant circuit 802 across the distribution line 714. If there is a signal 902 injected through the signal injector 702 whilethe switch 807 is'closed, as shown in F IG. 10, that signal 902 is picked up by the resonant circuit 802 and conveyed to the control circuit 810 which turns on the output relay 809 in response to the signal. Upon the actuation of the relay 809, the switch (or contacts) 812 is closed to thereby turn on the load 813. At the same time the switch 709 alsois closed. As willbe clear from the above description, a'command signal 902 intended to this controller No. 2 is'detected exclusively by this controller by preselecting the timings of the injection of the command signal 902 and the closure of the switch 807 by the timer 804 so that both timings coincide with each other. During the ensuing period 903, the switch 808 is closed ((v) in FIG. 10) to connect the resonant circuit 803 acrossthe line. In this example, as shown in FIG. I
10, there is no command signal during the period 903. Therefore, the control circuit 811 receives no signal from the resonant circuit 803, and the output relay 809 maintains the closed state. It will be understood, however, that if there is injected a command signal during the period corresponding to the period 903 in another control cycle, then the signal will be picked up by the resonant circuit 803, and the relay 809 will be turned off by means of the control circuit 811 thereby to cut the load 813. As described above, a particular period in a monitoring and control cycle is allotted to a particular command signal for a particular controller, and the signal is received exclusively by the particular controller by making a standby period at the said controller coincide with the said particular period. Returning to the ensuing period, the switch 707 is closed either to connect the resonant circuit 7061across the line or not depending .on the state of the switch 709 also as described previously. In this embodiment, however, the switch 709 represents the state of the output relay 809 or the load switch 812. Therefore, the readout signal 906 detected in the detector 711 reflects the state of the relay 809 or the switch 812. Thus, by comparing the reference signal'905 and the readout signal 906,
response of the relay'809 to the above-mentioned com-.
mand signal 902 can be confirmed at the site of the signal injection. Similarly, if another controller, for example, controller No. 1, is connected to another branch 715 of the distributionline, thecontroller No. l is controlled by a command signal which is injected in the period 907 or 908, and the operation of the controller can be confirmed by comparing a reference signal 909 with the ensuing readout signal 910. In this example, the period ensuing from the end of the response No. 2 is allotted to asequential readoutof watt-hour meters of a number of consumers, the readout being performed in a manner such as described in connection to FIGS. 3 and 4.
In the above embodiment, it has been assumed for the convenience of the explanation that a plurality of timers (704, 804), resonant circuits (703, 706, 801, 802, 803), control circuits (810, 81 1) and auxiliary circuits (705, 805) are provided in each controller. I-Iowever, it will be understood that the plurality of components can be respectively substituted by asingle component, since those plurality of components perform similarfunctions in different periods respectively.
Summarizing the above description, a beginning portion of a sequence of injected signal is allotted to control (or command) signalsand time divided among a plurality of controllers, and the next portion to the response function in a similar manner, and the last portion to the readout function for the consumers watthour meters. Therefore, control of loads and the confirmation thereof can be achieved in a single operation cycle including the readout of consumers meters without impairing the meter-readout function of the system.
Further, as the function of response or confirmation is similar to that of meter-readout, the circuit components for the response function can be replaced by those for the meter-readout. Moreover, a single set of such components can be commonly used for both functions simply by providing additional switches (not shown) connected in parallel with the respective switches (for example, switches 107, 109 and 117 in FIG. 3) and by setting the operation periods of the additional switched differently from those of the inherent switches (for example, switches 107 and 117). Thus,v
the system is made simple and economical.
it will be understood that the order of the signal channel s allotted as shown in FIG. 10 can be changed if it is desireable.
Moreover, though the object of the monitoring has been assumed to be the meter-readout in the previous embodiments, it will be clear that the same principle is applicable to other monitoring functions.
What we claim is:
1. Apparatus for remotely monitering at least one consumer load sensing means connected to low voltage commercial power distribution lines for measuring a load connected across said distribution lines, comprising:
injecting means for injecting a high frequency signal having at least one frequency component into said distribution lines, wherein the portion of said high frequency signal alloted to a given consumer has a reference signal portion and a measuring signal portion;
flow path circuit means providing a signal flow path for said reference signal portion; switching means for selectively connecting said flow path circuit means across said distribution lines in parallel with said load;
load sensing means operativein response to the state of said load to create a flow path for said measuring signal portion through one of said flow path circuit means and said load, depending upon the state of said load;
detector means for detecting said reference signal portion and said measuring signal portion; and comparing means for comparing the levels of the detected reference and measuring signal portions to obtain information relating to the state of said load. 2. Apparatus as defined in claim 1, wherein said high frequency signal comprises one frequency component; and said flow path circuit means comprises a series resonant circuit tuned to said high frequency signal.
3. Apparatus as defined in claim 1, wherein said high frequency signal comprises first and second frequency components to be concurrently injected serving as said reference signal portion and said measuring signal portion, respectively; and said flow path circuit means comprises first and second series resonant circuit means tuned to said first and second frequency components, respectively.
4. A system as defined in claim 1, wherein said load sensing means comprises a watt-hour meter, and. said means operative in response to the state of said device comprises a switch contact which operates once for each time said watt-hour meter reads a predetermined amount of power consumption in said load.
5. Apparatus as defined in claim 1, wherein: said high frequency signal comprises:
a starting signal portion for putting a second load into a controllable condition, first and second command signal portions for controlling said second load, 7 a second reference signal portion, and a second measuring signal portion; and
the apparatus further comprises: starting means responsive to said starting signal portion of said high frequency signal for initiating the control operation of said second load; first control circuit means responsive to said first command signal portion of said high frequency signal for generating a first control signal; second control circuit means responsive to said second command signal portion of said high frequency signal for generating a second control signal; further switching means for selectively connecting said first and second control circuit means in parallel with said starting means; and
relay means connected to the outputs of said first and second control circuit means for connecting said second load and said flow path circuit means to said distribution lines, wherein said relay means connects said second load and said flow path circuit means to said distribution linesin response to said first control signal and disconnects said second load and said flow path circuit means from said distribution lines in response to said second control signal and wherein said flow path circuit means provides a signal flow path for said second reference signal portion;
wherein said detecting means and said comparing means respectively detect and compare said second reference and measuring signal portions in the same manner as the first reference and measuring signal portions.