SIX-PORT REFLECTOMETER TEST ARRANGEMENT

Abstract: A test arrangement for testing high frequency electrical devices consists of two 6-port reflectometers (40, 41) which are both connected to a device under test (2). As well as being applied to the device under test, a reference sample of an excitation signal is routed to both 6-port reflectometers simu­ ltaneously. The test arrangement is capable of very wide op­ eration and is able to fully characterise the electrical prop­ erties of a device.

Description: This invention relates to a test arrangement which is suitable for performing electrical measurements on high frequency electrical devices. The invention is intended for operation in the radio frequency or microwave frequency range. One of the most powerful techniques used to characterise radio frequency (R.F.) and microwave components is the Scattering Parameter (or S-Parameter) model. The R.F. device is treated as a "black box" with various input and output terminals. Each pair of terminals is termed a "port". The device's performance is quantified by applying a wave to one port and recording the emergent waves at each port caused by that wave. The relationships between the waves are expressed in a matrix of complex coefficients, called the S-parameter matrix. These complex coefficients can be measured by means of a network analyser which separates out the required waves and evaluates their complex relationship. Automatic network analysers have been developed in which the frequency of a variable frequency oscillator is controlled over a particular frequency band. These analysers are very complex involving directional couplers or bridges to sample the incident and reflected waves. The samples are then ratioed to find the magnitude and phase of the reflection coefficient. This is a complicated step in practice, as phase angles cannot be directly measured at frequencies higher than about 500 kHz. Therefore, the system has to convert the RF samples down to a suitable frequency before measuring them. To do this while accurately maintaining their amplitude and phase relationships is both difficult and expensive. Mixers or samplers used in such a heterodyne system must be extremely linear. An arrangement known as a Six-Port Network Analyser relies on the use of amplitude measurements, without the need to measure phase information, and is able to use relatively simple RF circuitry, providing that the Six­ Port Network Analyser can be accurately calibrated. As previously known, such arrangements have severe operational drawbacks; they are slow and cumbersome to use, and are regarded primarily as laboratory instruments requiring the use of skilled operators. To fully characterise a device under test (DUT) the use of a dual configuration of Six-Port Network Analysers is needed. It can be shown that the DUT must be such that its characteristics are independent of power applied to both ports simultaneously. This is generally true only for passive devices. In particular, measurements of amplifiers would be unreliable because of the unrepresentative operating conditions. There is a strong possibility that the amplifier could oscillate instead of amplify. Broadband operation is not possible because directional isolators are needed to prevent the RF source signal circulating around a signal loop which includes the DUT. A previously proposed test arrangement for a two-port device is disclosed in IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. MTT-25, NO. 12, DECEMBER 1977, pp. 1070-1074 and comprises two six-port networks and a high frequency signal source, a signal from said source being fed to a first port of each network, a second port of each network being connected via a respective test port to the device via a respective coupler. The present invention seeks to provide an improved test arrangement. According to a first aspect of the invention, there is provided a test arrangement for a two-port device (2), said arrangement comprising two six-port networks (40, 41) and a high frequency signal source (3), a signal from said source (3) being fed to a first port (26) of each network (40, 41), a second port (25) of each network (40, 41) being connected via a respective test port (42, 43) to the device (2) via a respective coupler (44, 45), characterised in that a switch arrangement (55) is provided to direct said signal only to one or the other of the test ports (42, 43) via the respective coupler (44 or 45) as well as to said first port (26) of each network (40, 41). This avoids the need to use any narrow band components, and enables the test arrangement to be used automatically over a very wide frequency range. The invention is further described by way of example with reference to the accompanying drawings, in which:­ Figure 1 is an explanatory diagram. Figure 2 is a diagrammatic representation of a 6-port network, and Figure 3 shows an embodiment of the invention. As mentioned previously, the invention utilises a pair of six-port networks. The term "six-port" originates from the classification of types of network analyser - it is an analyser which has six ports used for measuring one- port devices, by way of contrast with a network analyser of the conventional heterodyne type mentioned previously and which has four ports. These four ports are used for : i, the incident signal applied by an RF source to the device under test (DUT); ii, an output sample ′a′ of the incident wave upon the DUT; iii, an output sample ′b′ of the reflected wave from the DUT; and iv, the test port to which the device under test is connected. In a six-port reflectometer, the six ports are used for i, the incident signal applied by an RF source to the device under test, ii, the test port iii, samples mainly the signal ′a′; and ports iv; v and vi sample different combinations of the signals ′a′ and ′b′, where ′a′ and ′b′ are as previously defined. A simple system with a six-port network 1 connected to a device under test 2 (DUT) is shown in Figure 1, in which an RF source 3 supplies the test signal. It can be shown that the six-port equations for each port reduce to : where Pi is the reading of a power meter connected to the ′i′ th port, and port P3 samples the ″a″ vector only. The parameter r is defined as being: r = reflected wave (b) incident wave (a) The derivation of equations in this form is shown in the article by C.A.Hoer "Using Six-port and Eight-port Junctions to Measure Active and Passive Circuit Parameters" NBS Techn. Note 673.1975. Calibration of the six-port reflectometer involves solving several simultaneous equations of the form given above. Different known DUTs are placed on the test-port, and the power meter readings are recorded from which the eleven unknown coefficients can be found. It has been proposed by G.F.Engen, IEEE Trans. MTT-26, No. 12 pp 951-957, Dec. 1978, to separate the calibrations of a six-port network into two stages. The first stage - the "Six-to-Four-port reduction"- establishes the internal constraints of the six-port. These account for five coefficients of the eleven, and arise because the "sixth port" must provide a consistent indication with the others, although it was only introduced to resolve a solution choice which would otherwise result in an ambiguity. This stage can be carried out by observing the power meter measurements for a number of unknown terminations. The second stage of the calibration is identical to that used to calibrate a four-port reflectometer of the heterodyne network analyser type. This requires the use of three standards, and finds the remaining six coefficients. One example of a six-port network which gives a particularly good theoretical performance is that shown in Figure 2, as it produces a useful choice of "a" and "b" wave combinations that emerge from ports P3 to P6. These combinations are such as to hold inherent uncertainties in the measurement system to a minimum. Figure 2 shows a circuit configuration which utilises hybrid couplers which is capable in theory of giving an almost ideal performance. This configuration was proposed by G.F.Engen in "An improved circuit for implementing the six-port technique of microwave measurements", IEE Trans. MTT-25, No. 12, Dec.77 pp 1080-1083. In Figure 2, the source 3 and the device-under-test (DUT) 2 are shown in addition to the six-port network which consists of four interconnected hybrid couplers 11, 12, 13, 14. Each hybrid coupler is a four port device having two input ports 15, 16 and two output ports 17, 18 and is constructed such that an input signal applied to input port 15 is split equally so that half of the input power emerges from each of ports 17 and 18 (neglecting any internal losses) and the signal at port 18 lags the signal at port 17 by 90°. This is represented diagramatically in the left top corner of Figure 3. Conversely, the signal applied to input port 16 is split equally between the two output ports 17 and 18, but this time the signal on port 17 lags by 90° the signal at port 18. It is usual to assume that there is no net phase lag across the hybrid coupler, but in practice this will not be so but will cancel out in the configuration of interconnected hybrid couplers which is illustrated. By each of the four output ports 21, 22, 23, 24 of the six-port network is shown the output signal value which results from the application to the two input ports 25, 26 of the indicated proportions of the reflected wave b and the incident wave a , it being noted that r = b/a. The conventional designation P1 to P6 of the six-port network is also shown on Figure 2. The use of the two six-port networks enables the magnitude and phase characteristics of two-port DUT to be inferred from magnitude measurements alone. Previously the use of a dual six-port network analyser has incurred serious drawbacks. One drawback has already been mentioned, i.e. that the DUT must be such that its characteristics are independent of power applied to both input and output ports simultaneously. Another drawback is that the calibration procedure by which the S- parameters are determined involves the use of a switched phase shifter and switched attenuator to provide three or more different but repeatable relationships between the two waves at the two test ports (the ports to which the DUT is connected). Also for wide-band operation many narrow band precision air-line standards are needed. Furthermore it is invariably necessary to provide narrowband isolators to prevent power circulating between the two six-port networks and through the DUT more than once. An embodiment of the present invention is shown in Figure 3 which allows these drawbacks to be overcome and permits broadband operation as it does not require the use of narrowband components or standards. Referring to Figure 3, two six-port networks 40 and 41 are connected to two test ports 42 and 43 between which, in operation, the DUT 2 is connected. Each network 40 and 41 takes the form shown in Figure 2, and two 6dB directional couplers 44, 45 are connected as shown. These couplers 44, 45 are linked to the RF source 3, which generates the excitation signal at a required frequency, via a 16dB directional coupler 60, a switching network 48 and a further power splitter 49 which form part of a termination generator 46, the latter being used to calibrate the networks 40 and 41. The small fraction of the incident power which is split off by the coupler 60 is that regardless of the switch position (and hence regardless of which test port 42 or 43 is receiving power from the RF source 3) both six-port networks 40 and 41 receive a sample of the RF power via coupler 52 so that both six-port networks operate simultaneously. The sample is taken off after the switching network 48 to minimise the effects of non-repeatability of the switch arrangement 55 and the effects of any non-repeatability can be ratioed out during calculation of the wanted parameters. The configuration of the switching network 48 allows DUT's which are active devices to operate into a matched load, so no problems are caused for amplifier measurement. It will be noted that isolators are not required, so broadband operation is possible. The two six-port networks are of the vector-voltmeter type; a vector-voltmeter finds the vector ratio of the two vector voltages. Each of the six-port networks has two inputs (corrresponding to points 25 and 26 of Figure 2), the signals at which are ratioed together. With the switch 55 in the forward position shown in Figure 3, six­ port network 40 measures the reflection wave of the DUT 2 at test port 42, and six-port network 41 measures the transmission of the DUT 2 at test port 43. When the switch 55 is moved to the reverse position, six-port network 41 measures reflection at test port 43, and six­ port network 40 measures the transmission at test port 42. Hence all four of the Scattering parameters (S­ routed back to the source 3 as a negative feedback signal to control the level of the output power of the source 3. A control circuit (not shown) is included in the feedback loop to compensate for the power variations that would occur as the frequency of the RF source is swept over a wide band. The switching network 48 consists of three interconnected hybrid couplers 50, 51, 52, the fourth ports of couplers 50, 51, being terminated at matched loads 53 in the usual way, and a 3 dB attenuator 54 being interposed at each of the two input ports of coupler 52. The switching network 48 also includes a switch arrangement 55 which can connect either coupler 50 or coupler 51 to the RF source 3. The termination generator 46 has a variable phase shifter 56 and a variable attenuator 57 which are used to alter the phase and magnitude of the RF source signal which is fed to output port 58. In operation, power from the RF source 3 is fed to the device under test either via the test -port 42 or via the test port 43, and the incident wave and the reflected waves are coupled via the couplers 44 and 45 to the six­ port networks 40 and 41. The power emerging at the four output ports of the networks 40 and 41 is measured and the expressions (which are shown in Figure 2) are calculated to determine the characteristics of the DUT 2. The configuration of the switching network 48 is such parameters) can be measured individually, provided that the six-port networks are accurately calibrated. To achieve correct calibration, the arbitrary termination generator 46 is used. Calibration of the dual six-port networks is carried out in two stages. In the first stage the six­ port network is reduced to four-ports, that is to say the number of unknown coefficients is reduced to the number associated with a four-port network. In the circuit shown in Figure 3, the reduction coefficients remain constant for a particular six-port network irrespective of whether it is being used to make reflection or transmission measurements. The six-port networks have twelve error paths which are accounted for by calibration, and the circuit shown in Figure 3 has extra leakage paths associated with the configuration of hybrids 50, 51, 52 used to feed reference samples of the RF source signal to the two six-port networks. These leakage paths are excited and quantified using the termination generator 46. The power splitter 49 divides the RF source power between the two six-port networks on the one hand, and the switchable phase shifter and switchable attenuator on the other hand. In accordance with the first mentioned aspect of the invention it is envisaged that the switch 55 may be dispensed with so that the RF signal is always routed to the same test port, but the reference signals from couplers 52 are still each routed to a respective six-port network 40 and 41. This configuration is a rather less convenient to use as it is necessary to repetitively reverse the connections to the DUT 2 during the test sequence.

Claims: 1. A test arrangement for a two-port device (2), said arrangement comprising two six-port networks (40, 41) and a high frequency signal source (3), a signal from said source (3) being fed to a first port (26) of each network (40, 41), a second port (25) of each network (40, 41) being connected via a respective test port (42, 43) to the device (2) via a respective coupler (44, 45), characterised in that a switch arrangement (55) is provided to direct said signal only to one or the other of the test ports (42, 43) via the respective coupler (44 or 45) as well as to said first port (26) of each network (40, 41). 2. A test arrangement as claimed in claim 1, wherein the couplers (44, 45) comprise directional couplers, a portion of any signal fed thereto being provided to the second port (25) of each associated six-port network (40, 41). 3. A test arrangement as claimed in claims 1 or 2, wherein the switch arrangement (55) forms part of a switching network (48) which allows the signal to be fed to only one or the other of the test ports (42, 43), said signal also being fed as a reference to both networks (40, 41). 4. A test arrangement as claimed in claim 3, wherein the switching network (48) includes two power splitters (50, 51) which are switchably connected to said high frequency signal source (3), with one output of each power splitter (50, 51) being coupled to a respective test port (42, 43), and the other output of each power splitter (50, 51) being coupled via a power attenuator (54) to both six-port networks (40, 41). 5. A test arrangement as claimed in claim 4 wherein said power splitters (50, 51) are hybrid couplers, and a ganged two path switch (55) is provided so as to switchably connect said high frequency signal source (3) to either of the two power splitters (50, 51) such that the switch (55) is operative to connect a characteristic impedance (53) to the input of that power splitter (50, 51) which is not coupled to the high frequency signal source (3). 6. A test arrangement as claimed in claim 5, wherein the switching network (48) includes a third hybrid coupler (52) having two inputs connected respectively to the two first-mentioned hybrid couplers (50, 51) via said power attenuators (54).

Inventors: HJIPIERIS, GEORGE, POTTER, CHRISTOPHER MALCOLM
Applicants: MARCONI INSTRUMENTS LIMITED
Application year: 1986
Main classification: G01R27
IPC codes: G01R27, G01R35