Equalizer circuit for use in communication unit
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an equalizer circuit for use in a communication unit. More particularly, the present invention relates to an equalizer circuit utilizing a transversal filter theory. 
     2. Description of the Prior Art 
     FIG. 1 is a conceptual drawing showing an example of a time division multiple access (TDMA) communication which is a background of the present invention. The TDMA communication is used for a satellite communication, for example, which comprises a plurality of earth stations ES, ES&#39; . . . and a common communication satellite CS. The earth station ES comprises a transmitting apparatus TRA and a receiving apparatus REA. A signal modulated by a modulator MOD included in the transmitting apparatus TRA is sent from an antenna AE through an equalizer EQL and a transmitter TR toward an antenna AS of the communication satellite CS. The signal is frequency converted and the frequency-converted signal is sent to other earth station ES&#39;. Similarly, a signal from other earth station ES&#39; is received by the antenna AE of the earth station ES through the communication satellite CS and the received signal is provided to the receiving apparatus REA. In the receiving apparatus REA, the received signal passes through a receiver RE and an equalizer EQL and is demodulated by a demodulator DEM. It is known that the transmitter TR and the receiver RE in the earth station ES and the receiving system and the transmitting system in the communication satellite cause an amplitude distortion and/or group delay distortion, respectively. Particularly, a high power amplifier (not shown) included in the communication satellite CS is used in a relatively saturated state due to size, price and stability thereof. As a result, an AM - PM conversion is caused in the high power amplifier, which causes a phase variation as shown in line A in FIG. 2 in which the line B denotes an output level. Such a phase variation becomes a group delay distortion. 
     The amplitude distortion and group delay distortion are, respectively, equalized for an amplitude frequency response and a group delay frequency response by the equalizer EQL included in the transmitting apparatus TRA and the equalizer EQL included in the receiving apparatus REA. Conventionally, such equalizer EQL is generally structured to include a fixed amplitude equalizer FAE, a fixed group delay equalizer FDE and a variable equalizer ME, as shown in FIG. 3. Either of the fixed amplitude equalizer FAE and a fixed group delay equalizer FDE or both of them may be omitted depending on the amount of the actual amplitude distortion or group delay distortion. 
     In the TDMA communication system which is the background of the present invention, it is impossible that once an operation is initiated, the above described amplitude frequency response and the group delay frequency response are detected by transmitting and receiving test signals so that an optimum amount of equalization is detected. The reason is that the time period when a single earth station occupies a line is extremely short since such communication system is made in a time divisional manner. Therefore, in case where a new earth station joins such a communication satellite system, it is necessary to seek an optimum point where an amplitude distortion and a group delay distortion are minimum and thus a bit error rate (BER) is minimum. To this end, a variable equalizer ME as shown in FIG. 3 may be used. 
     FIG. 4 is a circuit diagram showing an example of a conventional variable equalizer. An input signal inputted to an input terminal 1 branches through a branch circuit 2, a portion of which is applied to an attenuation setting circuit 4 having coefficient -a 3  and the remaining signal being inputted to the next branch circuit 2 through a delay line 3 having a delay amount T. In a similar operation, the respective signals are applied to the respective attenuation setting circuits having respective attenuation amounts. The signals from the attenuation setting circuits 4, 4, . . . all are applied to an adder 5 and thus the synthesized signals are outputted from the adder 5 to an output terminal 6. The attenuation setting circuits 4, 4, . . . includc polarity reverse. In such a way, the attenuation amounts of the attenuation setting circuits 4, 4, . . . are set to a 0  =1 at the center, +a 1  and -a 1  on both sides thereof, which absolute values are equal but the polarities thereof are opposite to each other, +a 2  and -a 2 , . . . , +a n  and -a n . In such a manner, an amplitude frequency response and a group delay frequency response are set through a known transversal filter theory by arbitrarily setting the respective attenuation amounts of the attenuation setting circuits 4, 4, . . . . More particularly, an optimum point is sought by the variable equalizer ME by varying an amplitude frequency response and group delay frequency response while detecting a bit error rate (BER). 
     In the TDMA communication system, the bit error rate is more greatly affected by a group delay distortion rather than an amplitude distortion and thus an operation for seeking such an optimum point is easily made if an optimum equalization amount for the group delay distortion can be set. Nevertheless, a conventional variable equalizer has not been able to vary, for example, only an amplitude or only group delay since the attenuation amounts of the attenuation setting circuits 4, 4, . . . are arbitrarily set. Accordingly, this means that it is difficult to seek an optimum point by a conventional variable equalizer in a TDMA communication system in which the influence of the group delay frequency response is greater than that of the amplitude frequency response. In addition, although attenuation amounts of a conventional variable equalizer determine an amplitude frequency response and the group delay frequency response, respectively, how such frequency response characteristics vary when a signal attenuation amount varies could not be known without a large number of simulation data since such variation is different depending on other coefficients. For this reason, it can not be easily confirmed how the amplitude and group delay are equalized. 
     Another variable equalizer as shown in FIG. 5 has been already proposed by the present applicant, for example. In FIG. 5, an input signal applied from an input terminal is distributed by a distributor 7. The signal distributor 7 distributes a signal into three signals of the same level. A delay line 3 having a delay amount T is interposed in one path for one signal of three signals, a delay line 31 having a delay amount 2T is interposed in another path, and a polarity inverter 8 is interposed in the remaining path. The polarity inverter 8 is structured by a known transformer or transistor and the like and shifts a phase of an applied signal by 180°. The signal from delay line 31 and the signal from the polarity inverter 8 are synthesized by an adder 9 to be applied to a variable attenuation setting circuit 10. The variable attenuation setting circuit 10 comprises a polarity reverse and an output signal therefrom is synthesized with an output signal from the delay line 3 by an adder 11. 
     Let it be assumed that no attenuation of signal is caused except for the variable attenuation setting circuit 10 and that no time delay is caused except for the delay lines 3 and 31 and the delay of a main signal is used as a reference (zero). Then, an output signal B(ω) obtained from an output terminal 6 is represented in the following equation (1). ##EQU1## The response characteristic G B  (ω) of the amplitude with respect to the frequency of the output signal B(ω) and the response characteristic τ B  (ω) of the delay amount with respect to the frequency of the output singal B(ω) are provided by the following equations (2) and (3), respectively. ##EQU2## In the equations, ω is an angular frequency and ω=2πf wherein f is a frequency. Variation of the amplitude frequency response characteristic G B  (ω) and the group delay frequency response characteristic τ B  (ω) where a coefficient l is larger than 0 (l&gt;0), are shown in FIG. 6. FIG. 6(A) shows an amplitude frequency response and FIG. 6(B) shows a group delay frequency response, wherein the amplitude and delay amount varies in the direction of the arrow when the coefficient l is made larger. More particularly, as shown in FIG. 6, in the FIG. 5 example, the delay amount varies if the coefficient l is varies in the attenuation setting circuit 10. However, even in the FIG. 5 example, the amplitude as well as the delay amount is varied according to the variation of the coefficient l and thus it is extremely difficult to utilize the FIG. 5 example as a variable equalizer in the TDMA communication system. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved variable group delay equalizer capable of varying group delay frequency response of a signal without varying an amplitude frequency response of the signal. 
     The variable group delay equalizer in accordance with the present invention comprises a signal inputting portion, a delay portion receiving an input signal from the signal inputting portion for controlling a group delay frequency response of the input signal, the signal from the delay portion, the delay frequency response of which was already controlled, having an amplitude distortion caused during the control thereof, and an amplitude correcting portion receiving the group-delay-frequency-response controlled signal for correcting the amplitude distortion included therein. Such correction is made by varying the amplitude of the group-delay-frequency-response controlled signal into a phase opposite to the above described distortion. 
     Accordingly, a principal object of the present invention is to provide a variable group delay equalizer that amplitude variation becomes very small even if the delay amount is changed, so that the equalizer can be effectively utilized in a TDMA communication system. 
     This object and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a concept of a TDMA communication system which is a background of the present invention; 
     FIG. 2 is a graph showing a characteristic of a high power amplifier included in a communication satellite; 
     FIG. 3 is a block diagram showing an example of an equalizer for use in a TDMA communication system; 
     FIG. 4 is a circuit diagram showing an example of a conventional variable equalizer; 
     FIG. 5 is a circuit diagram showing another example of a conventional variable equalizer; 
     FIG. 6 is a graph of frequency response characteristics of an amplitude and group delay in the example of FIG. 5; 
     FIG. 7 is a block diagram showing a variable group delay equalizer of one embodiment of the present invention; 
     FIG. 8 is a circuit diagram showing an example of a variable attenuation setting circuit; 
     FIG. 9 shows a characteristic of control voltage vs. output voltage and phase of the FIG. 8 embodiment; 
     FIG. 10 is a circuit diagram showing an example of a variable amplitude equalizer; 
     FIG. 11 is a graph showing frequency response characteristic of an amplitude of the FIG. 10 embodiment; 
     FIG. 12 shows respective frequency response characteristics of an amplitude and group delay of the FIG. 7 embodiment; 
     FIG. 13 shows a frequency response characteristic of group delay of the FIG. 7 embodiment; 
     FIG. 14 is a block diagram showing a variable group delay equalizer of another embodiment of the present invention; 
     FIG. 15 is a block diagram showing a variable group delay equalizer of a further embodiment of the present invention; 
     FIGS. 16, 17 and 18 are block diagrams showing a variable group delay equalizer with a wider variable range in accordance with the present invention; 
     FIG. 19 shows a variable range of the FIG. 18 embodiment; 
     FIG. 20 shows a conventional circuit for switching a fixed group delay equalizer to be a flat characteristic; 
     FIG. 21 shows a circuit utilizing the FIG. 7 embodiment, and capable of switching to a flat characteristic; 
     FIG. 22 shows an amplitude and group delay frequency response of the FIG. 21 embodiment; 
     FIG. 23 shows an example of a fixed group delay equalizer which is utilized in the FIG. 18 embodiment and is capable of switching into a flat characteristic; 
     FIG. 24 shows a variable group delay equalizer as a premise of the FIG. 23 apparatus; 
     FIG. 25 shows an amplitude and frequency characteristics of the FIG. 24 embodiment; and 
     FIG. 26 shows an amplitude equalizer capable of switching to a flat characteristic. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 7 is a block diagram showing an equalizer circuit for use in a TDMA communication system as an embodiment of the present invention. In FIG. 7, a signal from an input terminal 1 is applied to an output terminal 6 through a delay portion 100 and an amplitude correcting portion 200. If and when such variable group delay equalizer is used, for example, in a TDMA communication system as shown in FIG. 1, the input terminal 1 is connected to a modulator (MOD) and the output terminal 9 is connected to a transmitter TR in case where the equalizer should be included in a transmitting system, and the input terminal 1 is connected to a receiver RE and the output terminal 9 is connected to a demodulator DEM in case where the equalizer is included in a receiving system. 
     In FIG. 7, the structure of the delay portion 100 is the same as that of the variable equalizer ME in FIG. 5 and hence the explanation thereof is omitted. On the other hand, the amplitude correcting portion 200 connected in cascade to the delay portion 100 comprises a distributor 12 which distribute an input signal to three signals, delay lines 13 and 14 delaying the distributed outputs by 2T and 4T, respectively, adder 16 for synthesizing a signal passing through the delay line 14 and a signal which was not delayed, two attenuation setting circuits 17a and 17b which are connected in series to each other and multiply a coefficient l by the synthesized output, a fixed attenuator 15 for fixedly attenuating the output from the attenuation setting circuit 17b, and an adder 18 for synthesizing the output signal from the fixed attenuator 15 and the signal passing through the delay line 13 and for outputting the synthesized signal to the output terminal 9. 
     Meanwhile, the setting of the respective coefficients l, l (that is, l 2 ) of the attenuation setting circuits 17a, 17b in the amplitude correcting portion 200 is made simultaneously with the setting of the coefficient l of the attenuation setting circuit 10 in the delay portion 100. In other words, the amplitude correcting portion 200 is structured to be set automatically to l 2  if and when l is set in the delay portion 100. 
     FIG. 8 is a circuit diagram showing an example of a variable attenuation setting circuit. A variable attenuation setting circuit includes a double balanced mixer DBM and the voltage generator VG for applying a control voltage thereto. The double balanced mixer DBM includes an input terminal LO, and an output terminal RF and a control terminal IF. The double balanced mixer DBM comprises a transformer connected to the input terminal LO and a transformer connected to the output terminal RF, and four diodes D1 to D4 connected in a bridge manner between the two transformers. The control voltage generator VG comprises a variable resistor VR and a transistor Q1 the base of which receives a voltage from the variable resistor VR. The collector of the transistor Q1 and one end of the variable resistor VR are connected to a power supply +V and the emitter of the transistor Q1 and the other end of the variable resistor VR are connected to a power supply -V. The value of the resistance of the variable resistor VR is changed so that the control current Ic flows from the emitter of the transistor Q1 towards the control terminal IF. An output signal from the adder 9, for example, is applied to an input terminal LO of the double balanced mixer DBM and the output terminal RF is connected to an input of the adder 11. 
     Next, referring to FIG. 9, an operation of the variable attenuation setting circuit as shown in FIG. 8 will be described. An emitter voltage Ve of the transistor Q1 is set within the range from +Ve to-Ve by adjusting the variable resistor VR. The voltage Ve causes a control current Ic to flow into the double balanced mixer DBM through the terminal IF, the direction of the current Ic depending on the polarity of the voltage Ve. If and when the polarity of the voltage Ve is plus, the diodes D1 and D3 become conductive and the diodes D2 and D4 are cut off. Reversely, if and when the polarity of the voltage Ve is minus, the diodes D2 and D4 become conductive and the diodes D1 and D3 are cut off. If and when the voltage Ve is 0V, all of the diodes D1 to D4 are cut off. Accordingly, in the range of plus or minus of the voltage Ve, the polarity of a signal is inverted and thus an output is withdrawn from an output terminal RF. The value of the resistance of the diodes D1 to D4 changes depending on the current Ic and thus, as shown in FIG. 11, an amplitude of the output voltage changes according to the change of the voltage Ve. Thus, it should be understood that the variable attenuation setting circuit as shown in FIG. 8 not only inverts the polarity of a signal passing therethrough, but also varies the amplitude thereof. 
     Now, a principle and an operation of the amplitude correcting portion 200 in FIG. 7 will be described using a known variable amplitude equalizer. FIG. 10 is a circuit diagram showing one example of such a known variable amplitude equalizer and is mostly the same as the delay portion 100 in FIG. 7 except that a polarity inverter 8 is omitted. More particularly, an input signal from an input terminal 1 is distributed by a distributor 7. A signal which passes through a delay line 31 is synthesized by an adder 9 with a signal which does not pass through the delay line and thus synthesized signal is applied to an adder 11 through a variable attenuation setting circuit 10 having an attenuation amount or coefficient k. In such a way, the adder 11 synthesizes a main signal passing through the delay line 3 and a subsignal passing through a variable attenuation setting circuit 10 and outputs the synthesized signal to an output terminal 6. 
     Now, let it be assumed that no attenuation of signals is caused in whole elements other than the attenuation setting circuit 10 and no time delay is caused in elements other than delay lines 3 and 31 and the delay of the main signal is used as a reference which is set to zero. Then, an output signal A(ω) withdrawn at the output terminal 6 is given in the following equation (4). ##EQU3## The frequency characteristic G A  (ω) of an amplitude of the output signal A(ω) is given in the following equation (5). 
     
         G.sub.A (ω)=20log (1+2kcosωT)                  (5) 
    
     However, the group delay frequency response characteristic τ A  (ω) is flat. A variation for the coefficient k of the amplitude frequency response characteristic G A  (ω) is shown in FIG. 11. If the coefficient k is made larger, the amplitude varies in an arrow direction. More particularly, in FIG. 10, a variable amplitude equalizer can be obtained which is capable of varying only an amplitude frequency response without varying the group delay frequency response by varying the coefficient k of the variable attenuation setting circuit 10. 
     As shown in FIG. 11, a period of repetition of amplitude in the variable amplitude equalizer is 1/T and period of repetition of amplitude in the delay portion 100, that is, in the group delay equalizer in FIG. 5 is 1/2T as shown in FIG. 6(A). 
     Accordingly, it should be understood that an amplitude distortion caused in the delay portion 100 can be corrected in the amplitude correcting portion 200 if and when the period of the repetition of amplitude frequency response of the amplitude correcting portion 200 is made to be half so that such period is made to be the same as that in the delay portion 200 and the polarity of the amplitude is made to be reversed. 
     Now, an operation of a variable group delay equalizer which is a preferred embodiment of the present invention shown in FIG. 7 will be described. In FIG. 7, the delay lines 3 and 31 in the delay portion 100 are set to have a delay amount T and 2T, respectively and the delay lines 13 and 14 in the amplitude correcting portion 200 are set to have 2T and 4T, respectively, which are two times of the delay amounts in the delay portion 100. 
     With such a state, we assume that no attenuation of signal is caused in all elements other than the attenuation setting circuits 17, 17a, 17b and the fixed attenuator 15 and no time delay is caused in all elements other than the respective delay lines. In addition, let it be assumed that the coefficient of the attenuation setting circuit 10 is l, and the summed coefficient of circuits including the fixed attenuator 15 and the attenuation setting circuits 17a and 17b are k, then the 
     amplitude frequency response characteristics G B  (ω) and GA(ω) are represented by the above described equations (2) and (5). In this case, since the delay portion 100 and the amplitude correcting portion 200 are connected in cascade, the total amplitude frequency response characteristic G C  (ω) is represented by a sum of these amplitude frequency response characteristics and hence is represented in the following equation. ##EQU4## The attenuation setting circuits 10, 17a and 17b gang with each other and have the same coefficient l. Let it be assumed that the fixed attenuator 15 has an attenuator amount 6dB, that is, the coefficient is 0.5, the whole coefficient k is given in the following. ##EQU5## Substituting the above described equation for the equation (7), the following equation (8) is obtained. ##EQU6## Comparing the above described equation (8) with the conventional equation (2), it is understood that the term in which an amplitude varies with respect to a frequency becomes very small, since |l|&lt;1 and hence the power of l in the equation (8) is larger than that in the equation (2). 
     On the other hand, the group delay frequency response characteristic τ c  (ω) at this time is the sum of those in the delay portion 100 and the amplitude correcting portion 200. However, since the amplitude correcting portion 200 has no group delay frequency response characteristic, the total group delay frequency response characteristic τ c  (ω) is τ b  (ω) shown in the above described equation (3). That is, τ C  (ω)=τ B  (ω). 
     FIG. 12 shows an amplitude frequency response characteristic G C  (ω) and a group delay frequency response characteristic τ C  (ω) in the FIG. 7 embodiment. Although FIG. 12 shows a variation in case of l&gt;0, in the range of coefficient l&lt;0, a sign of τ B  (ω) in the equation (3), that is, τ C  (ω) is reversed and thus the leading and the lag of the delay amount are reversed with respect to the reference. G C  (ω) in the equation (8) is not reversed if the absolute value of the coefficient l is equal even in case of the coefficient l&lt;0. More particularly, if and when the coefficient l changes from plus to minus, the delay amount reverses and changes as shown in an arrow shown in FIG. 13, but the amplitude frequency response characteristic G C  (ω) simply repeats the variation such as in FIG. 12(A). 
     As described in the foregoing, it will be understood that the FIG. 7 embodiment can vary only the group delay frequency response without any amplitude variation. Accordingly, since, if such a variable group delay equalizer is employed as a variable equalizer in the TDMA communication system, only deterioration of a bit error rate due to group delay distortions can be independently equalized, an operation for seeking an optimum point is extremely readily made as compared with a conventional case where an amplitude frequency response and a group delay frequency response change together. 
     Meanwhile, in the above described embodiments, a double balanced mixer including a polarity reverse is used as the attenuation setting circuits 10, 17a and 17b. However, it will be readily understood by those skilled in the art that such attenuation setting circuit can be achieved by a variable attenuator or the like excluding a polarity reverse in case where a variable range is restricted depending on the usage. In addition, not only a 180° phase shifter but also a 180° adder, a 180° distributor, a 90° adder or a 90° distributor can be utilized as the polarity inverter 8. For example, a 180° adder can be disposed in the adder 9 so that both of functions of the polarity inverter 8 and the adder 9 may be obtained. Similarly, a 90° adder and distributor may be employed in the input of the adder 9 and the output of the distributor 7. 
     Furthermore, the attenuation setting circuits 10, 17a and 17b and a fixed attenuator 15 may be adapted such that the ratio of the amplitudes of the signals input to the adders 11 and 18 can be made constant and the position to be inserted may be arbitrarily selected as long as such requirement is satisfied. For example, the fixed attenuator 15 may be provided in two lines, respectively, between the distributor 12 and the adder 16. 
     Although in the FIG. 7 embodiment, a delay line is provided after distribution of an input signal, distributors 7 and 12 and delay lines 3 and 13 can be alternately provided such as another embodiment shown in FIG. 14. In addition, the delay lines 3, 4, 13 and 14, the polarity inverter 8 and the adder 9 in FIG. 7 can be structured in a two-stage (or n-stage) manner such as a further embodiment as shown in FIG. 15. In the FIG. 15 embodiment, an attenuation amount of a fixed attenuator 32 is set to a square of that of the fixed attenuator 31. Such a restriction is needed for achieving a principle of the present invention as well as the restriction of the coefficient l of the attenuation setting circuit. The delay lines 33-40 have delay amounts indicated in the blocks, respectively. 
     In practically using a variable group delay equalizer, a wider variable range is sometimes needed. In such a case, a cascade connections of the delay portion 100 and the amplitude correcting portion 200 are connected in cascade manner of two or n stages as shown in FIG. 16, and the attenuation setting circuits may gang with each other, although in the FIG. 7 embodiment, a cascade connection of the delay portion 100 and the amplitude connecting portion 200 is in a single stage. According to the FIG. 16 embodiment, a variable group delay equalizer having an extremely good amplitude frequency response characteristic and a wider variable range can be obtained, while the structure of circuit becomes complicated. 
     For the purpose of simplifying a structure of circuit, a single stage of amplitude correcting portion 200 may be provided with respect to n-stages of the delay portions 100, as shown in FIG. 17. In this case, a minimum amplitude deflection (an amplitude variation with respect to frequency) can be always maintained irrespective of variation of coefficient of an attenuation setting circuit by setting a fixed attenuator 15 in the amplitude correcting portion 200 such that an amplitude deflection of the amplitude frequency response characteristic becomes minimum. Although the FIG. 17 circuit includes a single stage of amplitude correcting portion 200, it may include m stages of amplitude correcting portions 200. In such a case, the value of a fixed attenuator 15 may be set in the above described manner. According to the FIG. 17 embodiment, a variable group delay equalizer having a wider variable range can be obtained with a relatively simply circuit, although an amplitude frequency response characteristic deteriorates. 
     Then, a practically desired example for obtaining a wider variable range will be described in the following. In this example, a fixed group delay equalizer and a variable group delay equalizer are employed. A fixed group delay equalizer without any variation of amplitude frequency response characteristic can be obtained as a modification of the FIG. 7 apparatus, by removing the attenuation setting circuits 10, 17a an 17b in the delay portion 100 and the amplitude correcting portion 200 of FIG. 7 embodiment. However, the attenuation setting circuit in the delay portion 100 may be left so that a switchable apparatus may be made among three modes such as the coefficient 0 (infinite loss) and the constant coefficients (in phase and out of phase). 
     FIG. 18 shows a preferred embodiment of the present invention for obtaining a wider variable range. As shown in FIG. 18(A), in this embodiment, two fixed group delay equalizers 51 and 52 and a single variable group delay equalizer 53 are connected in series. The FIG. 7 apparatus may be used as a variable group delay equalizer 53, for example, and the modification of the FIG. 7 apparatus as explained in the foregoing may be employed as fixed group delay equalizers 51 and 52, for example. As shown in FIG. 18(B), each of the fixed group delay equalizers may comprise a delay portion and an amplitude correcting portion and, as shown in FIG. 18(C), the amplitude correcting portion may be provided commonly in the delay portion of each of the fixed group delay equalizers. A number of fixed group delay equalizers and a number of variable group delay equalizers may be properly increased or decreased. 
     FIG. 19 shows a variable range of the FIG. 18 embodiment. In FIG. 19, X represents an output of two fixed group delay equalizers 51 and 52 connected in series. A variable range shown in the region I is obtained by varying a group delay frequency response characteristic of a variable group delay equalizer 53 so that the characteristic is made to be in phase of the group delay frequency response characteristic of the fixed group delay equalizers 51 and 52. A variable range shown in a region II is obtained by varying the group delay frequency response characteristic of the variable group delay equalizer 53 so that such characteristic may be made to be out of phase with respect to the group delay frequency response characteristic of the fixed group delay equalizers 51 and 52. A variable range represented in a region III is obtained by removing the fixed group delay equalizers 51 and 52 from the lines and only using the variable group delay equalizer 53. 
     FIG. 20 is a conventional circuit for making a fixed group delay equalizer a flat characteristic. In FIG. 20, one of the fixed group delay frequency response characteristic and a flat characteristic can be selected by switching the fixed group delay equalizer 33 by means of relays 34 and 35. 
     In the event that there is a requirement that one of the state having a certain group delay frequency response characteristic and the flat state is selected without changing an electrical length and without interrupting signal, the FIG. 20 structure can not satisfy such requirement, since a signal is interrupted at the time of switching the relays and the electrical path in the loop changes. 
     Although it is possible to select one of the flat state and the state having a certain group delay frequency response characteristic by using an attenuation setting circuit, the characteristic deteriorates rather than a theoretical curve due to a little change of electrical length and the frequency characteristic, when the coefficient of the attenuation setting circuit is changed. In addition, an insertion loss of the attenuation setting circuit becomes large and there is change of return loss, and hence amplifiers and the like are needed for compensation thereof, which makes the apparatus expensive. 
     An example will be described in the following, which eliminates the above described defects and can select one of the flat state and the state having a certain characteristic without any interruption due to a switching operation and without any change of an electrical length. 
     FIG. 21 shows an apparatus capable of selecting one of the flat state and the state having a certain characteristic, utilizing the FIG. 7 embodiment. In FIG. 21, the same reference numerals as those in FIG. 7 denote the same or similar elements. The different points from the FIG. 7 structure are that (1) an attenuation setting circuit 10 in a delay portion 100 is replaced by an attenuation selecting circuit 23 which can select one of the coefficient 0 (infinite loss) and constant coefficients (in phase and out of phase), and (2) the amplitude correcting portion 200 first distributes an input in a distributor 24 and a main signal is connected to an adder 18 through a delay line 13 and a subsignal passes through a relay 25, one contact of the relay 25 being connected to an ending terminal 26 and the other contact being connected to a conventional distributor 12, and the attenuation setting circuits 17a, 17b are removed, the output of the fixed attenuator 15 is coupled to an adder 18 through a relay 27, the other contact of the relay 27 being connected to an ending terminal 28. A ganging switch 29 has three contacts a, b and c and provides to the attenuation selecting circuit 23 a coefficient of in - phase at the state a, a coefficient of OFF at the state b and a coefficient of out-of-phase at the state c. For the relays 25 and 27, the switch 29 is connected to the distributor 24 and the adder 18 at the positions a and c, respectively and is connected to the ending terminals 26 and 28 at the position b. 
     Now an operation will be described. If and when the device of FIG. 21 is adjusted at the position A of the ganging switch 29 so that it has a fixed group delay frequency response characteristic arbitrarily determined by the attenuation selecting circuit 23 and the fixed attenuator 15, the device has a characteristic shown in FIG. 22(i) at the position A of the ganging switch 29, a characteristic shown in FIG. 22(ii) at the position B and characteristic shown in FIG. 22(iii) at the position C which is a reverse characteristic of FIG. 22(i). It is clear from the theory of the FIG. 7 circuit that the characteristics (i) and (iii) have a characteristic opposite to each other and it is also clear that the characteristic (ii) becomes flat since a signal passes through only a main signal line. Meanwhile, since a signal is switched while keeping the main signal line being connected, an interruption of signal such as in FIG. 20 device never occurs. In addition, an electrical length never changes since such electrical length is determined based on a main signal line. 
     Although the present apparatus utilizes the relays 25 and 27, a switching circuit and the like comprising transistors having the same function may be used. In addition, although the present apparatus uses two relays 25 and 27, one of these relays can be omitted only if an isolation of a relay is excellent. Furthermore, if and when a subsignal line in an amplitude correcting portion can be made to be interrupted, the position where the relay is to be disposed is not critical. For example, the relay may be disposed in two lines after the distributor 12. 
     The attenuation setting circuit 23 may be a double balanced mixer or the like including a polarity reverse. However, in case where a reverse characteristic (iii), for example, is not needed depending on the usage, a structure without polarity reverse may be possible, such as relay. 
     A separate apparatus different from the above described apparatus in FIG. 21 may be used as the fixed group delay equalizers 51 and 52 in FIG. 18. FIG. 23 shows such an example of the separate apparatus. The FIG. 23 apparatus can achieve substantially the same function as that of the FIG. 21 apparatus. 
     An apparatus shown in FIG. 24 is of an apparatus as a premise of the FIG. 23 apparatus and can vary a group delay frequency response characteristic without varying an amplitude frequency response characteristic, just like the FIG. 7 apparatus which is a premise of the FIG. 21 apparatus. In an operation of the FIG. 24 apparatus, a signal inputted to an input terminal 101 is divided into five by a distributor 102. The first one is applied to an adder 107 as a main signal which is delayed by a delay amount 2T in a delay circuit 111. The second one is applied to an adder 106 which is delayed by a delay amount 3T in a delay circuit 112. The third one is applied to adder 106, which is delayed by T in a delay circuit 113 and the polarity thereof is inverted by the polarity inverter 105. A synthesized signal of the second and the third ones is applied to an adder 118 as a signal to a delay portion 122. The fourth one is directly applied to an adder 115 and the fifth one is applied to adder 115, which is delayed by 4T in a delay circuit 114. A synthesized signal of the fourth and fifth ones attenuates through the fixed attenuator 116 and passes through an attenuation setting circuit 117 and then is applied to an adder 118 as a signal to an amplitude correcting portion 12. Such synthesized signal passes through an attenuation setting circuit 108 and then is synthesized with a main signal in an adder 107, which synthesized signal is applied to an output terminal 109. The attenuation setting circuits 108 and 117 have the same attenuation characteristic and gang with each other. 
     Let it be assumed that no attenuation of signal is caused in all the elements except for the attenuation setting circuits 108 and 117 and the attenuator 116 and no time delay is caused in all the elements except for the delay lines. Assuming that the coefficient of the attenuation setting circuit 108 is l and a total coefficient of the attenuator 116 and the attenuation setting circuits 108 and 117 is k, the output signal is represented by the following equation. ##EQU7## The amplitude frequency response characteristic of C(ω) is represented by the following equation (11). ##EQU8## Now, assuming that the attenuation setting circuit 108 and gang with each other and have the coefficient l and the fixed attenuator circuit 116 has an attenuation amount 6 dB, that is, the coefficient thereof is 0.5, ##EQU9## Substituting the above described equation (12) for the equation (11), the second term within the root sign becomes 0 and thus the above described equation (11) is given in the following equation (13). ##EQU10## A group delay frequency response characteristic τ C  (ω) is represented by the following equation (14). ##EQU11## The variation characteristic of the amplitude frequency response characteristic G C  (ω) and the group delay frequency response characteristic τ C  (ω) with respect to the coefficient l are shown in FIG. 12, just like the FIG. 7 apparatus. If the coefficient l is made larger, the amplitude and the delay amount vary, respectively, in an arrow direction. If and when the coefficient l changes from plus to minus, the group delay frequency response characteristic reverses and changes in an arrow direction as shown in FIG. 13, but the amplitude frequency response characteristic simply repeats the variation such as in FIG. 12(a). 
     Returning to FIG. 23, the different points from FIG. 24 are as follows. An input is distributed by a distributor 123 and the distributed signal passes through a relay 124. One contact of the relay 124 is connected to a terminal 125 and the other contact of the relay 124 is connected to a conventional distributor 102. The coefficient setting circuits 108 and 117 in FIG. 24 are removed in FIG. 23. Thus, an output from a fixed attenuator 126 is connected to a adder 107 through a relay 127. The opposite side of the relay 127 is connected to a terminal 128. 
     If and when the fixed attenuators 116 and 126 are adjusted so as to have a fixed group delay frequency response characteristic arbitrarily determined, the characteristic as shown in FIG. 25(i) is obtained when the relays 124 and 127 are connected to the distributor and the adder as shown in FIG. 23. If and when the relays 124 and 127 are switched so that the relays are connected to the terminals 125 and 128, respectively, then the characteristic becomes flat such as in FIG. 25(ii). The reason is that such characteristic is determined by only a main signal line which is structured by the distributor 123, the delay line 111 and the adder 107. In addition, in this embodiment, a signal is switched while the main signal line is kept being connected, and hence an interruption of signal such as in the FIG. 20 apparatus never occurs and an electrical length never changes since such length is determined by the main signal line. 
     Although the above described apparatus uses the relays 124 and 127, any switching circuit such as transistor having the same function may be used. In addition, although two relays are used, one of these can be omitted if an isolation of the relay is good. Any position where the relay is disposed will do as long as both of the delay portion and the amplitude correcting portion are simultaneously interrupted. For example, the relay may be disposed in the four lines after the distributor 102. 
     Although FIGS. 21 and 23 show examples in which the above described concept applies to a group delay equalizer, such concept can apply to an amplitude equalizer such as in FIG. 26. In FIG. 26, the same reference numerals as those in FIG. 10 or FIG. 23 denote the same or similar elements. In addition, generally, the above described principle can apply to a transversal type of variable equalizer as an ON/OFF type of switch. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.