Abstract:
A multi-port device analysis apparatus is capable of analyzing a multi-port device having three or more with improved efficiency and accuracy. The multi-port device analysis apparatus includes: a signal source for providing a test signal to one of terminals of a multi-port device under test (DUT); a plurality of test ports for connecting all of the terminals of the multi-port DUT to the corresponding test ports; a plurality of measurement units for measuring signals from the corresponding test ports; a reference signal measurement unit for measuring the test signal for obtaining reference data; a plurality of terminal resistors each being assigned to one of the test ports; and switch means for selectively providing the test signal to one of the test ports (input port) and disconnecting the terminal resistor from the input port while connecting the terminal resistors to all the other test ports; wherein parameters of the multiport DUT are acquired without changing the connections between the test ports and the terminals of the DUT, while changing selection of the test port until all of the test port being assigned as the input port.

Description:
FIELD OF THE INVENTION 
     This invention relates to a multi-port device apparatus and method for analyzing the characteristics of the multi-port device having three or more terminals (ports), and more particularly, to a multi-port device analysis apparatus and method and a calibration method of the multi-port analysis apparatus for measuring various parameters of a multi-port device with high efficiency and high dynamic range without changing connections between the multi-port device under test and the analysis apparatus. 
     BACKGROUND OF THE INVENTION 
     In order to analyze the characteristics of the communication devices or communication components (device under test) used in various communication systems, a network analyzer is frequently used. A network analyzer obtains various test parameters, such as a transfer function, reflection characteristics, and phase characteristics (hereafter “scattering parameter S” or “S-parameter”), of a device under test. Such S-parameters are known in the art and determined by observing the frequency response (voltage and phase) of the device under test resulted in response to a sweep frequency signal from the network analyzer. 
     A network analyzer is usually comprised of two ports, one is input port and the other one is output port. The input port sends a sweep frequency signal (test signal) to the device under test and the output port receives the response output signal of the device under test. The input port and the output port of the network analyzer are usually organized such that either port can be switched to the other by a switching operation in the network analyzer. An example of configuration of such a network analyzer is shown in a block diagram of FIG.  1 . 
     The configuration and operation of the network analyzer shown in FIG. 1 is briefly explained. A network analyzer  10  has two input-output ports P 1  and P 2  which are connected to directional bridges (or directional couplers)  11  and  12 , respectively. Each of the bridges  11  and  12  functions as a signal separation circuit. A test signal from a signal generator  15  is sent to one of either the bridge  11  or bridge  12  which is selected by a switch  13 . The test signal (sweep frequency signal) is sent from the selected one of the port P 1  or port P 2  to the device under test. The test signal from the signal generator  15  is also sent to the inside of the network analyzer as a reference signal. Namely, this reference signal and the input signal from the bridge  11  or  12  are respectively provided to frequency converters  17 ,  18  and  19  whereby converted to signals of a lower frequency. 
     The frequency converted input signal and the reference signal are respectively converted to digital signals by AD converter  21 ,  22  and  23 . The digital signals are processed by a digital signal processor (DSP)  25  to determine S-parameters of the device under test. The S-parameters or other data derived from the S-parameters are displayed by a display  29  in various formats under the control of a CPU  28  which controls the overall operation of the system. 
     The devices to be tested, for example, components such as used in communication devices and systems, are sometimes formed with not only two terminals but also three or more terminals (hereinafter may also be referred to as “multi-port device”). In order to measure the S-parameters of the multi-port devices, an S-parameter test set having three or more ports may be used in combination with the network analyzer having two ports. Such an example is shown in FIG. 2 wherein a three port DUT is connected to a three port S-parameter test set having three ports. 
     In using the three port test set of FIG. 2, before connecting the DUT to test ports  90 ,  92  and  94 , the test set is preferably calibrated to test the DUT with high accuracy. Typically, such a calibration process is conducted by using a predetermined two port calibration set between the test ports  90  and  92 , between the test ports  92  and  94 , and between the test ports  94  and  92 . Then the DUT is connected to the test set and the S-parameters are measured. 
     The process for measuring the S-parameters of three port device with use of the conventional network analyzer is described in more detail. FIG. 3 is block diagram showing an example of network analyzer designed for three port device testing. The network analyzer  200  of FIG. 2 includes a three port test set therein, and thus functions in the same manner as the example of FIG.  2 . 
     The network analyzer  200  includes a signal source  210  which is a sweep frequency signal, switches  212 ,  214 ,  216 ,  218  and  220 , each having two switching circuits (designated by circle  1  and circle  2 ), a receiver circuit  222  and three direction bridges (couplers)  230 ,  232  and  234 . The receiver circuit  222  includes three measurement units  224 ,  226  and  228 . The receiver circuit  222  of FIG. 3 thus corresponds to the frequency converters  17 ,  18 ,  19  and the A/D converters  21 ,  22 ,  23  and the DSP  25  of FIG.  1 . The measurement unit  228  is to measure a signal level of the signal source  210 , i.e., a reference level “R”. The other measurement units  224  and  226  are to measure signal levels of output signals (transmission signal and/or reflection signal) from the device under test. In this example, measured results based on the voltage ratio between the measurement units  224  and  228  is denoted as “measurement A” and measured results based on the voltage ratio between the measurement units  226  and  228  is denoted as “measurement B”. 
     FIG. 4 is a table showing between types of S-parameters and switch settings and number of signal sweep operation when testing the S-parameters of the three port device  40  by the network analyzer of FIG.  3 . In FIG. 4, labels SW 1 -SW 5  correspond to the switches  212 - 220 , respectively. When the switching circuit (circle  1  or circle  2 ) in the switch is ON, it is connected to a path to other circuit components, and when the switching circuit is OFF, it is connected to the ground through a terminal resistor. 
     The three port device (DUT)  300  is connected to test ports  240 ,  242  and  244  of the network analyzer  200 . First, the switch setting is made so that the test signal is provided to the DUT  300  through the test port  240 . Under this condition, the network analyzer  200  measures S-parameters S 11 , S 21  and S 31  of the DUT  300 . For example, for measuring S-parameter S 11 , the test (sweep frequency) signal  210  is supplied to the DUT  300  through the switch  212  (SW 1 ) and the test port  240 . At the same time, a reflected signal from an input terminal ( 1 ) of the DUT  300  is received by the measurement unit  224  through the directional bridge  230  and the switch  216  (SW 3 ) to conduct the “measurement A”. Also at the same time, for measuring S-parameter S 21 , a transmission signal from a terminal ( 2 ) of the DUT  300  is received by the measurement unit  226  through the bridge  232  and the switches  218  (SW 4 ) and  220  (SW 5 ) to conduct the “measurement B”. Thus, S-parameters S 11  and S 21  can be measured by a single sweep of the test signal  210 . 
     For measuring S-parameter S 31 , while applying the test signal  210  to the terminal ( 1 ) of the DUT  300  through the test port  240 , a transmission signal from the terminal ( 3 ) of the DUT  300  is measured. Thus, the switch  5  is changed its connection so that the transmission signal from the terminal ( 3 ) of the DUT  300  is received by the measurement unit  226  through the directional bridge  234  and the switch  220 . As in the foregoing, for measuring S-parameters S 1 , S 21  and S 31 , the sweep signal must be applied to the terminal ( 1 ) by two times as shown in the left column of FIG.  4 . 
     In a similar manner, by applying the test signal to the terminal ( 2 ) of the DUT  300 , the network analyzer  200  measures S-parameters S 12 , S 22  and S 32  of the DUT  300  under the settings shown in the center column of FIG.  4 . The network analyzer  200  further measures S-parameters S 13 , S 23  and S 33  of the DUT  300  under the settings shown in the right column of FIG.  4 . Thus, all of the S-parameters are measured in the forgoing procedure and conditions. 
     In the measurement by the three port test set of FIG. 2 or the three port network analyzer  200  of FIG. 3, however, there is a problem in that the measurement accuracy of a three port device under test is not high enough even after conducting the calibration procedure between two test ports (two port calibration). More specifically, two port calibration will be conducted between the test ports  90  and  92  ( 240  and  242 ), the test ports  92  and  94  ( 242  and  244 ), and the test ports  94  and  90  ( 244  and  240 ) before testing the DUT. However, by the calibration procedure above, although error coefficients between the two test ports can be removed, error coefficients in the third test port are not fully calibrated. For example, in the calibration between the test ports  90  and  92  ( 240  and  242 ), the error at the test port  94  under the situation is not measured. 
     Other problem involved in measuring the S-parameters by the conventional test set or network analyzer  200  as noted above is that it requires a considerably long time to complete the measurement. For example, as shown in the table of FIG. 4, for measuring each set of three S-parameters, the sweep test signal must be applied to the DUT by two times. Thus, for obtaining all of the nine S-parameters, the test signal sweep must be repeated six times, resulting in a long time for completing the measurement. 
     A further problem is directed to a signal loss, i.e., a measurement dynamic range. Since the example of FIG. 3 includes the switches  218  which is series connected to the switch  216  or  220  for transmitting the signal from the DUT, a signal loss will be incurred before the signal reaching the measurement units  224  or  226 . Such a signal loss decreases a measurement dynamic range or measurement sensitivity in the network analyzer. 
     In testing the three port device (DUT) by a two port network analyzer (FIG. 5A) or through a two port test set (FIG.  5 B), the third terminal of the DUT is must be terminated through a resistor of known value. Before the S-parameter measurement, the two port calibration is performed between two test ports P 1  and P 2  (Q 1  and Q 2 ). Then, two ports of the DUT are connected to the test ports of the network analyzer (FIG. 5A) or the test set (FIG. 5B) while the remaining port of the DUT is connected to a resistor R. Under this condition, S-parameters of the two ports of the DUT are measured. Then, by connecting the next two ports of the DUT to the test ports and connecting the resistor R to the remaining port of the DUT, S-parameters are measured. By repeating the similar process by one more time, all of S-parameters can be obtained. 
     In the measurement by using the two port network analyzer of FIG. 5A or two port test set of FIG. 5B noted above, connections between the DUT and the network analyzer (test set) and resistor R have to be manually changed many times. Therefore, this test method is disadvantageous in that it is complicated and time consuming. Moreover, in the case where the resistor R is deviated from the ideal value, a reflection at the port of the resistor R may occur, resulting in errors in the measurement of the S-parameters. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a multi-port device analysis apparatus and method which is capable of accurately measuring parameters of a multi-port device having three or more ports with high efficiency and accuracy. 
     It is another object of the present invention to provide a multi-port device analysis apparatus calibration method which is capable of detecting error coefficients of the analysis apparatus and compensating such error coefficients in the measurement of the multi-port device. 
     It is a further object of the present invention to provide a multi-port device analysis apparatus and method for measuring various parameters of a multi-port device with high efficiency and high dynamic range without changing the connections changes between the multi-port device under test and the analysis apparatus. 
     It is a further object of the present invention to provide a three port device analysis apparatus and a calibration method thereof for measuring S-parameters of a three port device with high efficiency and high accuracy and high dynamic range. 
     It is a further object of the present invention to provide a three port device analysis apparatus with use of a two port network analyzer for measuring S-parameters of a three port device with high efficiency and high accuracy. 
     In order to test the multi-port device having three or more ports, the multi-port device analysis apparatus of the present invention is comprised of: a signal source for providing a test signal to one of terminals of a multi-port device under test (DUT); a plurality of test ports for connecting all of the terminals of the multi-port DUT to the corresponding test ports; a plurality of measurement units for measuring signals from the corresponding test ports connected to the corresponding terminals of the multi-port DUT; a reference signal measurement unit for measuring the test signal for obtaining reference data relative to measurement of the signals from the test port by the plurality of measurement units; a plurality of terminal resistors each being assigned to one of the test ports; and switch means for selectively providing the test signal to one of the test ports (input port) and disconnecting the terminal resistor from the test port provided with the test signal (input port) while connecting the terminal resistors to all the other test ports; wherein parameters of the multi-port DUT are acquired without changing the connections between the test ports and the terminals of the DUT, while changing selection of the test port until all of the test port being assigned as the input port. 
     According to the present invention, since the multi-port device analysis apparatus of present invention has the number of ports that can connect all of the ports of the multi-port DUT, once the DUT is fully connected, there is no need to change the connection between the analysis apparatus and the DUT. Further, the multi-port device analysis apparatus is provided with a terminal resistor at each test port (for receiving signal from the DUT), and each terminal resistor is included in both the calibration stage and the S-parameter measurement stage. Thus, the accurate measurement can be achieved even when the terminal resistors are deviated from the ideal value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing an example of structure in a network analyzer having two test ports. 
     FIG. 2 is a schematic diagram showing an example of structure for measuring a three port device by a combination of a network analyzer and a three port test set in the conventional technology. 
     FIG. 3 is a schematic block diagram showing an example of configuration of a network analyzer having a three port test set therein for analyzing a three port device. 
     FIG. 4 is a table showing types of S-parameters and switch settings and the like when testing the S-parameters of the three port device by the network analyzer of FIG.  3 . 
     FIG. 5A is a schematic diagram showing a basic structure for measuring a three port device by a two port network analyzer, and FIG. 5B is a schematic diagram showing a basic structure for measuring a three port device by a two port test set. 
     FIG. 6 is a block diagram showing a three port network analyzer as a first embodiment of the multi-port device analysis apparatus of the present invention. 
     FIG. 7 is a table showing relationship between types of S-parameters and switch settings when testing a three port device by the apparatus of FIG.  6 . 
     FIG. 8 a schematic block diagram showing a three port device analysis apparatus which is a combination of a three port network analyzer and a three port test set in the first embodiment of the present invention. 
     FIG. 9 is a table showing measurement modes in the three port device analysis apparatus of FIG.  8 . 
     FIG. 10 (a) shows a signal flow model for the test port designated by “S” in the table of FIG. 9, and FIG.  10 ( b ) shows a signal flow model for the test port designated by “R” in the table of FIG.  9 . 
     FIG. 11 is a signal flow graph when a device under test is connected to the test set in the three port device analysis apparatus of FIG.  8 . 
     FIG. 12 is a flow chart showing a calibration process in the three port device analysis apparatus of the present invention shown in FIGS. 6 and 8. 
     FIG. 13 shows a signal flow graph of the multi-port analysis apparatus of the present invention in the calibration process where the device under test is disconnected. 
     FIG. 14 is a block diagram showing an example of basic structure of a multi-port device analysis apparatus in the second embodiment having n test ports for measuring a multi-port device having n ports. 
     FIG. 15 is a block diagram showing an example of structure of a multi-port device analysis apparatus in the third embodiment using a two port network analyzer for measuring a three port device. 
     FIG. 16 is a table showing measurement modes in the analysis apparatus of FIG.  15 . 
     FIG.  17 ( a ) shows a signal flow model for the test port designated by “S” in the table of FIG. 16, and FIG.  17 ( b ) shows a signal flow model for the test port designated by “R” in the table of FIG.  16 . 
     FIG. 18 is a signal flow graph for the test port designated by “L” in the table of FIG.  16 . 
     FIG. 19 is a signal flow graph in the measurement mode a of FIG. 16 when the device under test is connected. 
     FIG. 20 is a flow diagram showing a calibration process in the multi-port (three port) analysis apparatus of the present invention shown in FIG.  15 . 
     FIG. 21 shows a signal flow graph of the analysis apparatus of FIG. 15 in the calibration process where the device under test is disconnected. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The preferred embodiments of the present invention will be described with reference to the drawings. The first embodiment of the multi-port device analysis apparatus of the present invention will be described with reference to FIGS. 6-13 which is directed to the three port analysis apparatus. The three port analysis apparatus of FIG. 6 is a network analyzer  100  having a three port test set in the same housing. The network analyzer  100  includes a signal source  112 , a power divider  114 , a receiver circuit  120  having measurement units  122 ,  124 ,  126  and  128 , switches  130  and  132  each having two switch circuits and terminal resistors (normalized impedance), and directional bridges (or couplers)  134 ,  136  and  138 . 
     The signal source  112  generates a test signal whose frequency is linearly changed within a predetermined range in response to a control signal from a sweep controller  116 . The power divider  114  divides the power of the test signal from the signal source  112  and provides the test signal to the selected terminal of the three port DUT  140  through the switches  130  and  132  and to the measurement unit  122  in the receiver circuit  120 . 
     The receiver circuit  120  has four measurement units  122 ,  124 ,  126  and  128 . Each measurement unit may be configured by a frequency converter, an A/D converter, and a signal processor such as shown in FIG.  1 . The measurement unit  122  is to measure a signal level of the signal source  112 , i.e., a reference level “R”. The other measurement units  124 ,  126  and  128  are to measure signal levels of output signals (transmission signal and/or reflection signal) from the three port DUT  140 . In this example, measured results based on the voltage ratio between the measurement units  122  and  124  is denoted as “measurement A” and measured results based on the voltage ratio between the measurement units  122  and  126  is denoted as “measurement B”. Further, measured results based on the voltage ratio between the measurement units  122  and  128  is denoted as “measurement C”. 
     Each of the switches  130  and  132  includes two switching circuits designated by circles  1  and  2  in FIG. 6 to connect the circuit either to an external signal path or to the inner terminal resistor. Each terminal resistor in the switches  130  and  132  is set to characteristic (normalized) impedance of the DUT  140  and the network analyzer which is typically  50  ohms. Thus, the switches  130  and  132  function to supply the test signal to a selected input port of the three port DUT and terminates the other ports of the DUT. 
     The directional bridges (or directional couplers)  134 ,  136  and  138  transmit the test signal from the switches  130  and  132  to the DUT and detect signals from the DUT (transmission signal and/or reflection signal) and provide the detected signals to the receiver circuit  120 . The detected signal from the directional bridge  134  is provided to the measurement unit  124 , the detected signal from the directional bridge  136  is provided to the measurement unit  126 , and the detected signal from the directional bridge  138  is provided to the measurement unit  128 . 
     FIG. 7 is a table showing relationship between types of S-parameters and switch settings and number of signal sweep when testing the S-parameters of the three port DUT  140  by the network analyzer of FIG.  6 . In FIG. 7, labels SW 1  and SW 2  correspond to the switches  130  and  132 , respectively. In the table, when the switch circuit (represented by circle  1  or circle  2 ) is ON, it means that the switch circuit is connected to the external circuit components, and when the switch circuit is OFF, it means that the switch circuit is connected to the ground through the terminal resistor. 
     The three port DUT  140  is connected to test ports  144 ,  146  and  148  of the network analyzer  100 . First, the switch  130  is set so that the test (sweep frequency) signal is provided to a port ( 1 ) of the DUT  140  through the directional bridge  134  and the test port  144 . Under this condition, the network analyzer  100  measures S-parameters S 11 , S 21  and S 31  of the DUT  140 . A reflected signal from the port ( 1 ) of the DUT  140  is received by the measurement unit  124  through the directional bridge  134  to determine S-parameter S 11  (measurement A). For measuring S-parameter S 21 , a transmission signal from a port ( 2 ) of the DUT  140  is received by the measurement unit  126  through the directional bridge  136  (measurement B). For measuring S-parameter S 31 , a transmission signal from a port ( 3 ) of the DUT  140  is received by the measurement unit  128  (measurement C). Thus, three S-parameters S 11 , S 21  and S 31  are measured at the same time by a single sweep of the test signal. 
     Then, the switches  130  and  132  are set as in the center column of FIG. 7 so that the test (sweep frequency) signal is provided to the port ( 2 ) of the DUT  140  through the directional bridge  136  and the test port  146 . Under this condition, the network analyzer  100  measures S-parameters S 12 , S 22  and S 32  of the DUT  140 . A transmission signal from the port ( 1 ) of the DUT  140  is received by the measurement unit  124  through the directional bridge  134  for measuring S-parameter S 12  (measurement A). A reflection signal from the port ( 2 ) of the DUT  140  is received by the measurement unit  126  through the directional bridge  136  for measuring S-parameter S 22  (measurement B). A transmission signal from the port ( 3 ) of the DUT  140  is received by the measurement unit  128  for measuring S-parameter S 31  (measurement C). Thus, three S-parameters S 12 , S 22  and S 32  are measured at the same time by a single sweep of the test signal. 
     Next, the switches  130  and  132  are set as in the right column of FIG. 7 so that the test (sweep frequency) signal is provided to the port ( 3 ) of the DUT  140  through the directional bridge  138  and the test port  148 . Under this condition, the network analyzer  100  measures S-parameters S 13 , S 23  and S 33  of the DUT  140 . A transmission signal from the port ( 1 ) of the DUT  140  is received by the measurement unit  124  through the directional bridge  134  for measuring S-parameter S 13  (measurement A). A transmission signal from the port ( 2 ) of the DUT  140  is received by the measurement unit  126  through the directional bridge  136  for measuring S-parameter S 23  (measurement B). A reflection signal from the port ( 3 ) of the DUT  140  is received by the measurement unit  128  from the directional bridge  138  for measuring S-parameter S 33  (measurement C). Thus, three S-parameters S 13 , S 23  and S 33  are measured at the same time by a single sweep of the test signal. 
     As in the foregoing, the network analyzer of the present invention has the same number of measurement units  124 ,  126  and  128  (other than the measurement unit  122  for the reference test signal) as the number of ports of the DUT  140 . Three signals (one reflection signal and two transmission signals) from the corresponding three ports of the DUT  140  are evaluated at the same time by a single sweep of the test signal. Therefore, all (nine) S-parameters of the DUT  140  can be measured by only three sweeps of the test signal. Further, since each pair of measurement unit and direction bridge is assigned to a port of the DUT, the three signals from the DUT are transmitted to the corresponding measurement units without using switches or changing connections in the transmission paths. Thus, signal loss in the transmission path is substantially reduced, thereby achieving a wider measurement dynamic range. 
     The calibration method of the present invention is described with reference to FIGS. 8-13. FIG. 8 a schematic block diagram showing a three port analysis apparatus which is a combination of a three port network analyzer  10  and a three port test set  30 . Although the example of FIG. 8 has the network analyzer and test set separately, the structure of this apparatus is the same as that of the network analyzer of FIG. 6 having the test set in the same housing. 
     In FIG. 8, the network analyzer  10  includes a signal source  12 , a sweep controller  22 , three measurement units  14 ,  16  and  18 , a measurement controller  24 , a display  26  and a controller  28 . The signal source  12  generates a sine wave test signal under the control of the sweep controller  22 . For example, the signal source  12  and the sweep controller  22  form a frequency synthesizer whereby generating a test signal whose frequency changes linearly (sweep) within a predetermined range. When testing a three port device (DUT)  40 , the test signal is provided to a port of the DUT  40  selected by a switch  32  (in the test set  30 ). The measurement units  14 ,  16  and  18  correspond to the measurement units  124 ,  126  and  128  in the network analyzer of FIG.  6 . 
     The measurement controller  24  controls an overall operation of the network analyzer  10  including performing a calibration procedure to determine error coefficients of the overall analysis apparatus and compensating the error coefficients to obtain the S-parameters of the DUT with high accuracy. The display  26  illustrates various measurement conditions and measurement results of the test parameters. The controller  28  includes various keys and switches and pointing devices to function as an interface with an operator of the apparatus. 
     The test set  30  includes a switch  32 , three directional bridges (directional couplers)  34 ,  36  and  38 , and three test ports  44 ,  46  and  48 . Three ports of the device under test (DUT)  40  are connected to the corresponding test ports through cables. The switch  32  selectively provides the test signal from the signal source  12  to one of the test ports  44 ,  46  or  48 , i.e., one of the ports of the DUT  40 . The directional bridges  34 ,  36  and  38  detect and transmit signals from the corresponding test ports, i.e., the ports of the DUT  40  to the corresponding measurement units  14 ,  16  and  18 . 
     FIG. 9 is a table showing measurement modes in the analysis apparatus of FIG.  8 . This table shows as to which test port of the test set  30  provides the test signal to the DUT and which test ports receive signals from the DUT. For example, in the mode a, the test port  44  functions as a signal source “S” and the test ports  46  and  48  function as receivers “R” to send the received signals to the measurement units  16  and  18 . It should be noted, however, that since the reflection signal from the DUT through the test port  44  is also received by the measurement unit  14 , the label “S” in the table means both signal source and receiver. Thus, S-parameters S 11 , S 21  and S 31  of the DUT are measured in the mode a, S-parameters S 12 , S 22  and S 32  of the DUT are measured in the mode b, and S-parameters S 13 , S 23  and S 33  of the DUT are measured in the mode c. 
     With reference to signal flow graphs of FIGS. 10 and 11, error factors involved in the measurement modes a-c in the table of FIG. 9 are described in the following. FIG.  10 ( a ) shows a signal flow model for the test port designated by “S” in the table of FIG. 9, and FIG.  10 ( b ) shows a signal flow model for the test port designated by “R” in the table of FIG.  9 . Each of the test ports  44 ,  46  and  48  is designated by two nodes, nodes  50  and  52  in FIG.  10 ( a ) and nodes  54  and  56  in FIG.  10 ( b ). 
     As shown in FIG.  10 ( a ), with respect to the test port connected to the signal source  12 , the test signal from the signal source  12  is input to the node  50 . At the same time, a part of the test signal is transmitted to the other test port in the “R” mode such as through the directional bridges in the test set  30  (Ed: directivity). The reflection signal from the DUT  40  is input to the reflection node  52 . At the same time, a part of the reflection signal is transmitted to the “R” mode test port (Er: reflection tracking) and another part of the reflection signal is reflected back by the test port or other components in the test set  30  and returns to the input node  50  (Es: source match). 
     As shown in FIG.  10 ( b ), with respect to the test port connected to only the measurement unit, i.e., in the “R” mode, a signal from the DUT is received by the measurement unit. At the same time, a part of the signal from the DUT is input to the reflection node  54  and is transmitted to the “R” mode test port (Et: transmission tracking), and another part of the signal is reflected back by the test port or other components in the test set  30  and returns to the input node  56  (El: load match). 
     FIG. 11 is a signal flow graph when the DUT is connected to the test set  30  in the measurement mode a of FIG.  9 . For the three port DUT  40 , nine S-parameters S 11 , S 12 , S 13 , S 21 , S 22 , S 23 , S 31 , S 32  and S 33  will be defined where each parameter is an amplitude ratio expressed by complex numbers. S 21  and S 31  respectively represent the transmission coefficients from the test port  44  to the test port  46  and  48 . S 11  represents the reflection coefficient at the test port  44 . Similarly, S 32  and S 12  respectively represent the transmission coefficients from the test port  46  to the test ports  48  and  44 . S 22  represents the reflection coefficient at the test port  46 . S 13  and S 23  respectively represent the transmission coefficients from the test port  48  to the test ports  44  and  46 . S 33  represents the reflection coefficient at the test port  48 . All of the S-parameters in the foregoing can be measured by measuring the voltages by the measurement units through the measurement modes a-c. 
     As noted with reference to FIGS.  10 ( a ) and  10 ( b ), and as also shown in FIG. 11, various error coefficients (terms) are involved in the S-parameter measurement. For example, in the measurement mode a, the test port  44  connected to both the signal source and the measurement unit  14  are associated with three error coefficients Ed, Es and Er. The test port  46  connected to the measurement unit  16  is associated with two error coefficients Et and El, and the test port  48  connected to the measurement unit  16  is associated with two error coefficients Et′ and El′. Further, a part of the test signal from the test source  12  may leak within the test set  30  and reach the measurement units  16  and  18 , thus, these leakage signals are also considered to be error coefficients (Ex, Ex′: isolation). 
     Therefore, these error coefficients (terms) must be detected and compensated for measuring the S-parameters of the DUT with high accuracy. FIG. 12 is a flow diagram showing such a calibration process in the three port device analysis apparatus of the present invention. During the calibration process, the DUT  40  is disconnected from the test ports of the analysis apparatus. FIG. 13 shows a signal flow graph of the analysis apparatus in the calibration process where the DUT is disconnected. In FIGS. 12 and 13, the test port connected to the signal source is represented by a “test port a” and the measurement unit corresponding to the “test port a” is denoted as a “circuit a”. The test ports which are not connected to the signal source are represented by “test port b” and “test port c”, respectively, and the measurement units corresponding to the test ports b and c are denoted by “circuit b” and “circuit c”, respectively. 
     In the calibration process of FIG. 12, when the calibration process is initiated (step  100 ) by an operator through the controller  28 , the switch  32  in the test set  30  selects one of the measurement mode (step  101 ). For example, the measurement mode a may be selected so that the signal source is provided to the test port  44  (test port a). Preferably, a calibration set having three modes of “open”, “short” and “load” is used in the calibration process. 
     For measuring the error coefficients Ex and Ex′, the measurement controller  24  sets the test port a to be “open” and provides the test signal of predetermined frequency from the test source to the test port a (step  102 ). Since the DUT is not connected, no signal is received by the test port  46 , and the circuit b (measurement unit  16 ) can directly measure the error coefficient Ex which is a signal leaked from the signal source to the measurement unit  16  within the test set  30  (step  103 ). Similarly, by measuring a signal received by the circuit c (measurement unit  18 ), the error coefficient Ex′ can be directly determined (step  104 ). 
     In the calibration process of FIG. 12, the error coefficients Ed, Es and Er are then determined as described below. Generally, for determining these error coefficients, the test port a (test port  44 ) is provided with three different conditions when the test signal is supplied thereto. A signal received by the circuit a (measurement unit  14 ) under each condition is evaluated, thereby obtaining three equations. The error coefficients Ed, Es and Er can be determined by solving the three equations. 
     For example, assuming the reflection coefficient of the test port  44  is S 11 , a voltage VR 11  received by the circuit a (measurement unit  14 ) is expressed as follows: 
     
       
           VR   11   =Ed+ErS   11 /(1− EsS   11 )  (1) 
       
     
     Typically, the above noted three different conditions include “open”, “short” and “load” the test port  44 . The “load” means that the test port  44  is connected to a terminal resistor having characteristic (normalized) impedance of the apparatus such as 50 ohms. 
     Thus, in the process of FIG. 12, by maintaining the open circuit of the test port a ( 44 ), a voltage at the circuit a (measurement unit  44 ) is measured (step  105 ). When the test port is open, the reflected signal has the same phase as that of the test signal, i.e., S 11 =1, the equation (1) is expressed as: 
     
       
           VR   11   =Ed+Er /(1− Es )  (2) 
       
     
     In the next step, the test port a ( 44 ) is short circuited (step  106 ), and a voltage at the circuit a (measurement unit  44 ) is measured (step  107 ). When the test port is short, the reflected signal has the phase opposite to that of the test signal, i.e., S 11 =−1, the equation (1) is expressed as: 
     
       
           VR   11   =Ed−Er /(1 +Es )  (3) 
       
     
     In the next step, the test port a ( 44 ) is terminated by the normalized resistor (step  108 ), and a voltage at the circuit a (measurement unit  44 ) is measured (step  109 ). When the test port is terminated by the normalized (ideal) impedance, no reflection signal occurs, i.e., S 11 =0, thus, the equation (1) is expressed as: 
     
       
           VR   11   =Ed   (4) 
       
     
     Thus, by solving the equations (1), (2) and (3) obtained in the above procedure, three error coefficients Ed, Es and Er can be determined (step  110 ). 
     The calibration process of FIG. 12 proceeds to the steps of determining the error coefficients Et and El. In the situation where the test ports  44  and  46  are connected in an ideal manner, a reflection coefficient at each of the test ports is zero, and a transmission coefficient at each of the test ports is one (1). Therefore, under this condition, voltages measured by the measurement units  14  and  16 , respectively, are as follows: 
     
       
           VR   11   =Ed+ErEl /(1− EsEl )  (5) 
       
     
     
       
           VR   21   =Et /(1− EsEl )  (6) 
       
     
     Since the error coefficients Ed, Er and Es are know in the step  110  noted above, the error coefficient El can be determined by the equation (5), and based on this result and the equation (6) , the error coef ficient El can be determined. 
     Thus, in the flow chart of FIG. 20, the test ports a ( 44 ) and b ( 46 ) are connected together (step  111 ), and a voltage VR 11  at the circuit a (measurement unit  14 ) and a voltage VR 21  at the circuit b (measurement unit  16 ) are measured (step  112 ). The calibration process applies the error coefficients Ed, Es and Er obtained in the step  110  noted above to the equations (5) and (6), thereby determining the error coefficients Et and El concerning the test port b ( 46 ) (step  113 ). 
     By the procedure similar to the steps  111 - 113 , the error coefficients Et′ and El′ can also be determined. The test ports a ( 44 ) and c ( 48 ) are connected together (step  114 ), and a voltage VR 11  at the circuit a (measurement unit  14 ) and a voltage VR 31  at the circuit c (measurement unit  18 ) are measured (step  115 ). Under this condition, voltages measured by the circuit a (measurement units  14 ) and the circuit c (measurement unit  18 ), respectively, are as follows: 
     
       
           VR   11   =Ed+ErEl′ /(1− EsEl′ )  (7) 
       
     
     
       
           VR   31   =Et′ /(1− EsEl′ )  (8) 
       
     
     The process applies the error coefficients Ed, Es and Er obtained in the step  110  to the equations (7) and (8), the error coefficients Et′ and El′ concerning the test port c ( 48 ) can be determined (step  116 ). 
     In the foregoing process, the error coefficients involved in the measurement mode a (wherein the test signal is provided to the test port  44 ) are obtained. Then, the process inquires as to whether there is a remaining measurement mode whose error coefficients are not determined (step  117 ). In the foregoing example, since the measurement modes b and c are not evaluated as to the error coefficients, the process goes back to the step  101  to change the switch  32  in the test set  30  so that the test signal is supplied to the test port b ( 46 ). The procedures in the steps  101  to  117  are repeated until all of the error terms are collected for the measurement modes b and c. Then the calibration process ends. 
     As described in the foregoing, the error coefficients in the three port analysis apparatus for all of the measurement modes can be obtained. Thus, when measuring the S-parameters of the DUT by connecting the DUT to the apparatus, such error coefficients can be removed (compensated) during the calculation of the S-parameters. Consequently, the S-parameters of the three port DUT  40  can be obtained with high accuracy. 
     Since the multi-port device analysis apparatus of present invention has the number of ports that can be connected to all of the ports of the multi-port DUT, once the DUT is fully connected, there is no need to change the connection between the analysis apparatus and the DUT. Further, the multi-port analysis apparatus is provided with a terminal resistor at each port (signal receiving port), and each terminal resistor is included in both the calibration stage and the S-parameter measurement stage. Thus, the accurate measurement can be achieved even when the terminal resistors are deviated from the ideal value. 
     FIG. 14 is a block diagram showing a basic structure of the second embodiment of the multi-port device analysis apparatus of the present invention for measuring a multi-port device having n ports. In this example, the multi-port analysis apparatus has n test ports P 1 -Pn and a receiver circuit  120   2  having n measurement units MU 1 -MUn (other than the measurement unit R for the reference test signal) for testing a multi-port device having n terminals (ports). The multi-port analysis apparatus of FIG. 14 further includes n directional bridges (couplers) BRG 1 -BRGn and n switches SW 1 -SWn, n terminal resistors TR 1 -TRn, a signal source  112 , a power divider  114 , and a sweep controller  116 . As seen in FIG. 14, although the number of test ports, measurement units, switches and directional bridges are increased, the basic structure is the same as that of the examples of FIGS. 6 and 8. 
     The signal source  112  generates a test signal whose frequency is linearly changed within a predetermined range in response to a control signal from the sweep controller  116 . The power divider  114  divides the power of the test signal from the signal source  112  and provides the test signal to the selected terminal of the n-port DUT through one of the switches SW 1 -SWn. The measurement unit R is to measure a signal level of the test signal from the signal source  112 . The other measurement units MU 1 -MUn are to measure signal levels of output signals (transmission signal and/or reflection signal) from the corresponding port of the DUT. 
     Each of the switches SW 1 -SWn connects the corresponding test port and directional bridge to either the test source  112  or to the terminal resistor TR. When measuring S-parameters of the n-port DUT, one of the test ports P 1 -Pn is provided with the test signal from the test source  112 , while all the other test ports are connected to the terminal resistors TR. Each of the terminal resistors TR 1 -TRn is set to normalized (characteristic) impedance of the analysis system and the DUT, which is typically 50 ohms. The directional bridges BRG 1 -BRGn transmit the signals (transmission signal and/or reflection signal) from the DUT to the corresponding measurement units MU 1 -MUn. 
     Before measuring the S-parameters of the DUT, the multi-port device analysis apparatus of FIG. 14 is calibrated to determine various error coefficients. The error coefficients and the procedure for determining such error coefficients are basically the same as that of the three port analysis device described in the foregoing. However, the number of the error coefficients and S-parameters will be greater than that of the first embodiment if the number of ports (n) of the DUT and the analysis apparatus is more than three. 
     Since the multi-port device analysis apparatus of present invention has the number of ports that can connect all of the ports of the multi-port DUT, once the DUT is fully connected, there is no need to change the connection between the analysis apparatus and the DUT. Further, the multi-port device analysis apparatus is provided with a terminal resistor at each test port (for receiving signal from the DUT), and each terminal resistor is included in both the calibration stage and the S-parameter measurement stage. Thus, the accurate measurement can be achieved even when the terminal resistors are deviated from the ideal value. 
     FIG. 15 is a block diagram showing a basic structure of the third embodiment of the multi-port device analysis apparatus for measuring a three port device. In this example, the multi-port analysis apparatus is a combination of a two port network analyzer and a three port test set. In FIG. 15, the network analyzer  310  includes a signal source  12 , a sweep controller  14 , two measurement units  14  and  16 , a measurement controller  24 , a display  26  and a controller  28 . The signal source  12  generates a sine wave test signal under the control of the sweep controller  14 . When testing a three port device under test (DUT)  40 , the test signal is provided to a port of the DUT  40  selected by a switch  32  (in the test set  330 ). 
     The measurement controller  24  controls an overall operation of the network analyzer  310  including performing a calibration procedure to determine error coefficients of the overall analysis apparatus and compensating the error coefficients to obtain the S-parameters of the device under test with high accuracy. The display  26  displays various measurement conditions and measurement results of the test parameters. The controller  28  includes various keys and switches and pointing devices to function as an interface with an operator of the apparatus. 
     The test set  330  includes a switch  32 , three directional bridges (directional couplers)  34 ,  36  and  38 , and three test ports  44 ,  46  and  48 , a switch  150  and a terminal resistor  152 . Three ports of the device under test (DUT)  40  are connected to the corresponding test ports through cables. The switch  32  selectively provides the test signal from the signal source  12  to one of the test ports  44 ,  46  or  48 , i.e., one of the ports of the DUT  40 . 
     The directional bridges  34 ,  36  and  38  detect and transmit signals from the corresponding test ports, i.e., the ports of the DUT  40  to the two measurement units  14  and  16 . Since the network analyzer  310  has only two measurement units  14  and  16 , a signal from one of the directional bridges is provided to the terminal resistor  152 . Such a selection is made by the switch  150 . The terminal resistor  152  is normalized (characteristic) impedance of the analysis apparatus (and the DUT) which is typically 50 ohms. 
     FIG. 16 is a table showing measurement modes in the analysis apparatus of FIG.  15 . This table shows as to which test port of the test set  330  provides the test signal to the DUT  40 , and which test ports transmit signals from the DUT to the measurement units, and which test port is connected to the terminal resistor  152 . For example, in the mode a, the test port  44  functions as a signal source “S” to input the test signal to the DUT and send the reflected signal from the DUT to the measurement unit  14 . The test port  46  functions as a receiver “R” to send the received signal to the measurement units  16 , and the test port  48  functions as a load “L” to terminate the corresponding port of the DUT through the terminal resistor  152 . Thus, S-parameters S 11 , S 21  and S 31  of the DUT  40  are measured in the modes a and b, S-parameters S 12 , S 22  and S 32  of the DUT  40  are measured in the modes c and d, and S-parameters S 13 , S 23  and S 33  of the DUT  40  are measured in the modes e and f. 
     With reference to signal flow graphs of FIGS. 17-19, error terms (coefficients) involved in the measurement modes a-f in the table of FIG. 16 are described below. FIG.  17 ( a ) shows a signal flow model for the test port designated by “S” in the table of FIG. 16, and FIG.  17 ( b ) shows a signal flow model for the test port designated by “R” in the table of FIG.  16 . Each of the test ports  44 ,  46  and  48  is designated by two nodes, nodes  50  and  52  in FIG.  17 ( a ) and nodes  54  and  56  in FIG.  17 ( b ). Since the error terms in FIGS.  17 ( a ) and  17 ( b ) are the same as that of FIGS.  10 ( a ) and ( b ), no further explanation is given here. 
     FIG. 18 is a signal flow graph for the test port designated by “L” in the table  16  where the test port is connected to the terminal resistor  152  in the test set  330 . Since the terminal resistor  152  may not be perfect, a part of the signal from the DUT  40  will be reflected back to the test port (error coefficient Ez). 
     FIG. 19 is a signal flow graph when the DUT is connected to the test set  330  in the measurement mode a of FIG.  16 . For the three port DUT  40 , nine S-parameters S 11 , S 12 , S 13 , S 21 , S 22 , S 23 , S 31 , S 32  and S 33  will be defined where each parameter is an amplitude ratio expressed by complex numbers. These S-parameters are known in the art and explained in the foregoing with reference to FIG.  11 . In the multi-port analysis apparatus of FIG. 15, all of the S-parameters are obtained by measuring the voltages by the measurement units through the measurement modes a-f. 
     FIG. 20 is a flow diagram showing a calibration process in the multi-port (three port) analysis apparatus of the present invention. During the calibration process, the DUT  40  is disconnected from the test ports of the analysis apparatus. FIG. 21 shows a signal flow graph of the analysis apparatus in the calibration process where the DUT  40  is disconnected. Since the calibration procedure in FIG. 20 is similar to that of FIG. 12, only a brief description is given in the following. 
     When the calibration process is initiated (step  400 ), the switch  32  selects one of the measurement mode (step  401 ). For measuring the error coefficient Ex, the test port a (test port  44 ) is opened and the test signal is provided to the test port a (step  402 ). The measurement unit  16  measures the error coefficient Ex (step  403 ). 
     For determining the error coefficients Ed, Es and Er, by maintaining the open circuit of the test port a, the measurement unit  14  measures the received signal (step  404 ). The test port a is short circuited (step  405 ) and the measurement unit  14  measures the received signal (step  406 ). The test port a is terminated by a terminal (normalized) resistor (step  407 ), and the measurement unit  14  measures the received signal (step  408 ). By solving the equations (1), (2) and (3) obtained in the above procedure, the error coefficients Ed, Es and Er are determined (step  409 ). 
     The calibration process of FIG. 20 proceeds to the steps of determining the error coefficients Et and El. The test port a (test port  44 ) and the test port b (test port  46 ) are connected together (step  410 ), and the measurement unit  16  measures the received signal voltage (step  411 ). By applying the coefficients Ed, Es and Er and the measured voltage to the equations (5) and (6), the error coefficients Et and El are determined (step  412 ). 
     By the procedure similar to the steps  410 - 412 , the error coefficients Ez can also be determined. The test port a (test port  44 ) and the test port c (test port  48 ) are connected together (step  413 ), and a voltage VR 11  is measured by the measurement unit  14  (step  414 ). Under this condition, the voltage measured by the measurement units  14  is expressed as follows: 
     
       
           VR   11   =Ed+ErEz /(1− EsEz )  (9) 
       
     
     The process applies the error coefficients Ed, Es and Er obtained in the above to the equation (9), the error coefficient Ez concerning the test port c (test port  48 ) can be determined (step  415 ). 
     Then, the process inquires as to whether there is a remaining measurement mode whose error coefficients are not determined (step  416 ), and if there is a mode not yet calibrated, the process goes back to the step  401  to repeat the procedures in the steps  401  to  415  until all of the error terms are collected for the measurement modes b-f. Then the calibration process ends. 
     As described in the foregoing, the error coefficients in the three port device analysis apparatus for all of the measurement modes can be obtained. Thus, when measuring the S-parameters of the DUT by connecting the DUT to the apparatus, such error coefficients are removed (compensated) during the calculation of the S-parameters. Consequently, the S-parameters of the three port DUT can be obtained with high accuracy. 
     Moreover, in the three port device analysis apparatus of present invention, once the DUT is fully connected, there is no need to change the connection between the analysis apparatus and the DUT. Further, the three port analysis apparatus is provided with the terminal resistor  152  for terminating one of the three ports of the DUT, and the same terminal resistor  152  is included in both the calibration stage and the S-parameter measurement stage. Thus, the accurate measurement can be achieved even when the terminal resistor is deviated from the ideal value. 
     In the foregoing description of the present invention, various modifications are possible. For example, the error coefficients Ed, Es and Er are determined by using the three conditions, open, short and load. However, different conditions such as terminating the test port by different terminal resistors of known reflection coefficients Sll can also be used. Further, when determining the error coefficients Et and El, it is not necessary that such connections between two test ports be ideal, i.e, the transmission coefficient can be less than one (1). It is only necessary that these different conditions have to be incorporated in the equations (1)-(9) in the calculation of the error coefficients. 
     Although only preferred embodiments are specifically illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing the spirit and intended scope of the invention.