Abstract:
The characteristic evaluating system of the present invention includes: a cable-driving transmitter transmitting a signal to one end of a cable to be measured; a load connected to the other end of the cable; a probe detecting a common mode current of the cable; a receiver receiving a signal detected by the probe; and a controller controlling the cable-driving transmitter, the load, and the receiver. The cable-driving transmitter is constructed such that a plural transmission condition is selectable when transmitting the signal. The load is constructed such that plural termination conditions corresponding to the signals transmitted to the cable is selectable. The characteristic of the cable is measured by scanning relative positions of the probe and the cable in a longitudinal direction of the cable.

Description:
The present application claims priority from Japanese application JP2005-248712 filed on Aug. 30, 2005, the content of which is hereby incorporated by reference into this application. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a characteristic evaluating system and a characteristic evaluating method. In particular, it relates to an evaluating system suitable for evaluating EMC performance of cables and an EMC performance evaluating method. 
   2. Description of the Related Art 
   In recent years, there has been a growing demand for evaluating EMC (Electro-Magnetic Compatibility) performance of various products. 
   In order to predict EMI (Electro-Magnetic Interference) generated from a cable, it is necessary to measure a common mode current in the cable or to measure a magnetic field distribution in the vicinity of the cable. Patent Reference 1 discloses a current probe which is operated with high frequency and has little influence on electric characteristics of signals to be measured. Further, Patent Reference 2 discloses an apparatus for measuring a high frequency magnetic field in the vicinity of a cable. 
   [Patent Reference 1] Japanese Patent Unexamined Publication No. 2000-147002 
   [Patent Reference 2] Japanese Patent Unexamined Publication No. Hei 8-68837 
   SUMMARY OF THE INVENTION 
   However, according to technologies disclosed in patent documents 1 and 2, it is not possible to simulate cable implementations, which differ from product to product, at one time. Therefore, it is necessary to prepare various samples having different terminal load conditions, drive voltages, and cable lengths, and to evaluate EMC performance of each cable as a single unit by measuring the above characteristics. 
   It is an object of the present invention to provide a characteristic evaluating system and a characteristic evaluating method capable of simulating cable implementations being different from product to product and easily measuring overall EMC performance of cables. 
   The above object can be achieved by a characteristic evaluating system including a probe, a transmitter, a load, a receiver, and a controller, and a characteristic evaluating method therefor. The probe clamps a cable and measures a current flowing in the cable. The transmitter transmits a signal to one end of the cable and drives the cable. The load is connected to the other end of the cable. The receiver receives a signal from the probe. The controller controls the transmitter, load, and receiver. Further, the controller selectively controls a transmission condition of the transmitter and a termination condition of the load, scans the probe, and measures a characteristic of the cable. 
   According to the present invention, there is provided an EMC performance evaluating system and an EMC performance evaluating method capable of evaluating EMC performance of cables by a simple operation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing the configuration of an EMC performance evaluating system for cables; 
       FIG. 2  is a circuit diagram wherein a single end voltage is applied to one signal line of a differential cable; 
       FIG. 3  is a circuit diagram wherein a common mode voltage is applied to the differential cable; 
       FIG. 4  is a circuit diagram wherein a differential mode voltage is applied to the differential cable by using a transformer; 
       FIG. 5  is a circuit diagram wherein a differential mode voltage is applied to the differential cable by using a differential driver; 
       FIG. 6  shows a T-type load circuit of variable resistance; 
       FIG. 7  shows a T-type load circuit of variable capacitance; 
       FIG. 8  shows a T-type load circuit of variable inductance; 
       FIG. 9  shows a π-type load circuit of variable resistance; 
       FIG. 10  shows a π-type load circuit of variable capacitance; 
       FIG. 11  shows a π-type load circuit of variable inductance; 
       FIG. 12  illustrates a common mode current flowing in the cable wherein there exist both differential driving components and common mode driving components; 
       FIG. 13  illustrates a common mode current flowing in the cable during the differential mode drive; and 
       FIG. 14  illustrates a common mode current flowing in the cable during the common mode drive. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the accompanying figures, some preferred embodiments of the present invention will be described.  FIG. 1  is a diagram showing the configuration of an EMC performance evaluating system for cables. In  FIG. 1 , a subject cable  10  is fixed to a U-shaped cable fixing jig  310 . A current probe  140  which clamps the subject cable is attached to a movable stage  320  being movable in the longitudinal direction of the subject cable. The common mode current flowing in the subject cable  10  is detected by the current probe  140  at each point of the subject cable  10 . In this regard, the maximum common mode current detected during the scan in the longitudinal direction of the subject cable  10  is simply called common mode current. 
   A transmitter  110  and a switching unit  120  controlled by a controller  210  is connected to a transmission end of the subject cable  10 , and a signal is applied to the subject cable  10 . A variable load  130  controlled by the controller  210  is connected to the other end of the subject cable  10 . The current detected by the current probe  140  is measured by a receiver  150  including a spectrum analyzer controlled by the controller  210 . 
   The EMC performance evaluating system  500  has a computing part  220  to compute EMC performance of the cable from the measured current, amplitude of the transmitter  110 , a load value of the variable load  130 , and route information of the switching unit  120 . The EMC performance evaluating system  500  is also equipped with a storage unit (not shown) for storing a result computed by the computing part  220  and a display  230  for displaying the computed result. 
   The EMC performance evaluating system  500  evaluates EMC performance of the cable by the following operation. Also, although the spectrum analyzer is used as a receiver, a network analyzer may be used in place of the transmitter and receiver. Alternatively, a spectrum analyzer with a built-in tracking generator may be used. 
   By choosing between the transmitter  110  and the switching unit  120 , a signal in an arbitrary driven state is applied to the subject cable  10 . Namely, an arbitrary transmission condition is set. Further, an arbitrary termination condition of the cable is set by the variable load  130 . The current probe  140  fixed to the movable stage  320  measures an electric current at an arbitrary point of the cable. The signal detected by the current probe is measured by the receiver  150 . 
   The transmitter  110 , switching unit  120 , variable load  130 , stage controller  240 , and receiver  150  are all controlled by the controller  210 . The stage controller  240  controls the movable stage  320 . The receiver  150  transmits the measured signal to the controller  210 . 
   The signals associated with the amplitude of the transmitter  110 , route information of the switching unit  120 , load value of the variable load  130  and measured by the receiver  150  are processed in the computing part  220  and stored in the storage unit which is a part of the controller  210 . Also, the computed result is shown on the display  230 . 
   Further, the controller  210  and the computing part  220  may not be provided separately, and a single central processing unit (CPU) may perform control and computing. Also, a single computer may serve as the controller  210 , computing part  220 , and display  230 . The transmitter  110  and switching unit  120  are altogether called cable-driving transmitter. The cable-driving transmitter may simply be called transmitter. Although the stage controller  240  moves the probe  140 , the cable fixing jig  310  may be fixed to the movable stage  320  and the cable  10  may be moved. 
   The subject cables  10  which can be measured by the evaluating system shown in  FIG. 1  are coaxial cables, parallel two-wire cables, twisted pair cables, flat cables, and multicore cables. Each of them can be measured whether it has a screening shield or not. 
   Referring to  FIGS. 2 to 5 , the configuration of the transmitter  110  and switching unit  120  will be described. In this regard,  FIG. 2  is a circuit diagram wherein a single end voltage is applied to one signal line of a differential cable.  FIG. 3  is a circuit diagram wherein a common mode voltage is applied to the differential cable.  FIG. 4  is a circuit diagram wherein a differential mode voltage is applied to the differential cable by using a transformer.  FIG. 5  is a circuit diagram wherein a differential mode voltage is applied to the differential cable by using a differential driver. 
   In  FIG. 2 , the transmitter  110  includes a voltage source  111  and a drive source impedance  112 . The switching unit  120  includes two switches  121  being operated in cooperation and two matching impedances  122 . By choosing between two switches  122  alternately in cooperation, the switching unit  120  connects one signal line of the differential cable with the transmitter  110  and applies a signal. The other signal line is connected to the matching impedance  112 . A signal is applied independently to each of the two signal lines of the differential cable and each common mode current is measured. The EMC performance evaluating system  500  can compute common mode currents flowing in the cable during the differential mode drive and common mode drive by performing addition and subtraction on two obtained common mode currents in the computing part  230 . When signals are inputted to both the signal lines of the cable at a time, it is difficult to observe common mode currents. The EMC performance evaluating system  500 , however, computes a common mode current during the common mode drive (the current flowing in the signal line is in the same direction) by applying a signal separately and by addition. On the other hand, the common mode current during the differential drive (the current flowing in the signal line is in the reverse direction) is obtained by subtraction. 
   In  FIG. 3 , the transmitter  110  includes a voltage source  111  and a drive source impedance  112 . The switching unit  120  includes two matching impedances  122 , and the two matching impedances  122  are both connected to the transmitter  110 . If the configuration shown in  FIG. 3  is applied to the EMC performance evaluating system  500 , the common mode current during the common mode drive can be measured. 
   In  FIG. 4 , the transmitter  110  includes a voltage source  111  and a drive source impedance  112 . The switching unit  120  includes a transformer  123 , whose middle point tap on the secondary side is grounded, and two matching impedances  122 . The single end voltage is converted to a differential mode voltage by the transformer  123 . 
   In  FIG. 5 , a differential driver is formed by two voltage sources  111  in the transmitter  110 . The switching unit  120  includes two matching impedances  122 . The two matching impedances  122  are connected to the voltage sources  111 , respectively. If the configuration shown in  FIG. 4  or  FIG. 5  is applied to the EMC performance evaluating system  500 , the common mode current during the differential mode drive can be measured. 
   If the matching impedance  122  used in  FIG. 2  to  FIG. 5  includes a parameter variable element, an arbitrary driving condition can easily be achieved. Further, although the switching unit  120  in  FIG. 3  to  FIG. 5  does not perform switching operation, it is a switching unit in a broad sense. 
   Now, referring to  FIGS. 6 to 11 , the configuration of the variable load  130  will be described. In this regard,  FIG. 6  shows a T-type load circuit of variable resistance.  FIG. 7  shows a T-type load circuit of variable capacitance.  FIG. 8  shows a T-type load circuit of variable inductance. Further,  FIG. 9  shows a π-type load circuit of variable resistance.  FIG. 10  shows a π-type load circuit of variable capacitance.  FIG. 11  shows a π-type load circuit of variable inductance. 
   The load circuit of the variable resistance in  FIGS. 6 and 9  is the most commonly used load circuit capable of matching termination. Although the load circuit of the variable capacitance shown in  FIGS. 7 and 10  is non-matching, it is a load circuit wherein a CMOS-type receiver may be used and, also, the use of the variable inductance load shown in  FIGS. 8 and 11  may be considered. 
   It is possible to provide an arbitrary loaded state by changing the characteristic value of the parameter variable element in  FIGS. 6 to 11 . Further, the variable load may be formed by combining the constructions shown in  FIGS. 6 to 11 . 
   Referring to  FIGS. 12 to 14 , EMC performance of the cable will be defined. In this regard,  FIG. 12  illustrates a common mode current flowing in the cable wherein there exist both the differential driving components and common mode driving components.  FIG. 13  illustrates a common mode current flowing in the cable during the differential mode drive. Also, FIG.  14  illustrates a common mode current flowing in the cable during the common mode drive. 
   The ordinary common mode current shown in  FIG. 12  is the sum of a common mode current made up of differential driving components and a common mode current made up of common mode driving components. However, when there exist both the common mode current made up of differential driving components and common mode current made up of common mode driving components, it is difficult to handle them. Therefore, as shown in  FIG. 13  and  FIG. 14 , respectively, the current made up of differential driving components alone and the current made up of common mode driving components alone are separated. 
   In  FIG. 13 , an amplitude of the drive voltage is shown as Vi_diff and an amplitude of the common mode current during the differential drive is shown as I diff_com. In this regard, the coefficient Ydc of conversion from the differential drive voltage to the common mode current is defined by use of the expression (1) below:
 
 Ydc=I diff   —   com/Vi   —   diff   (1)
 
   The conversion coefficient Ydc can be used as a first parameter for evaluating EMC performance of the cable. 
   On the other hand, in  FIG. 14 , an amplitude of the drive voltage is shown as Vi_com and an amplitude of the common mode current during the common-mode drive is shown as I com_com. In this regard, the coefficient Ycc of conversion from the common mode drive voltage to the common mode current is defined by use of the expression (2) below:
 
 Ycc=I com   —   com/Vi   —   com   (2)
 
   The conversion coefficient Ycc can be used as a second parameter for evaluating EMC performance of the cable. 
   The EMC performance evaluating system  500  computes the above evaluation parameters Ydc and Ycc in the computing part  220 .  FIGS. 13 and 14  show results of the cases wherein mode separation is performed in the embodiment of  FIG. 12 . Therefore, the coefficient of conversion from the drive voltage on an arbitrary drive condition to the common mode current can be expressed by linear combination of Ydc and Ycc. Thus, by evaluating Ydc and Ycc, the common mode conversion coefficient, or the EMC performance, of the cable can be expressed. 
   The EMC performance evaluating system  500  for cables applies differential drive and common mode drive to a cable to be evaluated, computes Ydc and Ycc in the computing part  220 , stores the computed result in the controller  210 , and shows the result on the display  230 . 
   According to the present embodiment, there is provided an EMC performance evaluating system capable of evaluating EMC performance of cables by a simple operation. Further, an EMC performance evaluating method for the cable can be provided through evaluating Ydc and Ycc. 
   Although the maximum common mode current during the scan in the longitudinal direction of the cable is called common mode current, the evaluation may be performed by using an average common mode current during the scan or by using a half value of the maximum common mode current, and the value which may be used is not limited to the above. 
   While we have shown and described several embodiments in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to those skilled in the art, and we therefore do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as the encompassed by the scope of the appended claims.