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MILLIMETER WAVE VECTOR NETWORK
BACKGROUND OF THE INVENTION
The present invention arose out of research sponsored by the Naval Ocean Systems Center under Grant No. N66001-83-C-0363. The United States Government may have rights under this invention.
With the expansion of communications into higher frequency bands in order to provide greater resolution for systems such as radar, data transmission, and the like, interest has extended into the millimeter wavelength range of frequencies, and particularly into frequencies in the range of 20 to 50 gigaHertz (GHz). Recent developments indicate a strong future for such frequencies.
The current activities in millimeter wave research has stimulated a demand for fast, low-cost, reliable and accurate equipment for the measurement of complex reflection and transmission coefficients, or impedance characteristics, of components and devices used in millimeter wave systems. These characteristics must be measured over a wide range of frequencies with accuracy and reliability.
Several millimeter wave network analyzers are presently available, such devices including an automatic scaler network analyzer, an impedance bridge device, a six-port network analyzer, and a down-converter network analyzer. (See J. A. Paul, "Wide Band Millimeterwave Impedance Measurements," Microwave Journal, pages 95-102, April 1983.) The automatic scaler network analyzer is commercially available for operation up to 100 GHz and although the unit can measure the magnitude of both reflection and transmission coefficients accurately and quickly on a swept frequency basis, phase information on the coefficients cannot be obtained.
Thus, there is a need for a low cost system for accurately measuring complex reflection and transmission coefficients on a swept frequency basis over a wide range of millimeter wave frequencies.
SUMMARY OF THE INVENTION
The present invention is directed to a swept-fre- 45 quency network analyzer which is constructed of standard microwave passive components and a controlled frequency sweeper and is used to make both reflection and transmission measurements with only two power meters. The simplicity of this device is based on the use 50 of a 180° hybrid, or "magic-tee" waveguide device, and allows accurate point-by-point measurements of device coefficients over a broad frequency band.
The network analyzer of the present invention includes a swept frequency source of millimeter wave 55 energy which is supplied through first and second directional coupler main arms to an impedance element under test. The coupled arms of the directional couplers provide samples of the signal incident on and the signal reflected from (or transmitted through) the impedance 60 element, respectively, the first directional coupler providing a reference voltage which is supplied to one arm of a magic tee device. An electronically controlled phase shifting structure consisting of a combination of a PIN diode with a second magic tee is inserted in the 65 reference channel to provide a selectable 90-degree phase shift to eliminate a phase measurement ambiguity. Other forms of PIN phase shifters could be used in
which a circulator or a 3 dB hybrid replaces the magic tee. A mechanically movable wave guide short circuit could also be used in place of the PIN diode, if desired. The second coupler samples the signal reflected from or transmitted through the impedance device under test and provides a test signal which is supplied to a second arm of the magic tee device. The magic tee device mixes the reference and the test signals and delivers power from each of its 180° colinear output arms to corresponding power detectors. Any type of power detector such as a thermistor or square-law crystal detector can be used.
The output voltages from the two power detectors are related to the reflection coefficient or the transmission coefficient of the device under test in accordance with known mathematical formulas, and accordingly these equations can be solved, preferably by a small digital computer, to obtain the magnitude and phase of the reflection or transmission coefficients, as well as the impedance of the device under test. The results of these computations may be presented on a CRT screen in the form of a Smith Chart or in tabular form, or hard copies may be provided using a suitable pen recorder and/or printer.
This device is low cost, yet is capable of providing accurate measurements of the characteristics of devices under test over a wide range of frequencies, using low cost, commercially available hardware and a very simple computer control for providing the frequency sweeping and calculations of the values being measured.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and additional objects, features and advantages of the present invention will become apparent from a consideration of the following detailed description of preferred embodiments thereof, taken in conjunction with the accompanying drawings, in which:
FIG. I is a schematic diagram of a vector network analyzer for reflection coefficient measurements in accordance with the present invention;
FIG. 2 is a modified schematic diagram of the vector network analyzer of FIG. 1, adapted for measurement of either reflection or transmission coefficients;
FIG. 3 is a flow chart showing the calibration and testing procedures used in the present network analyzer; and
FIGS. 4, 5 and 6 are charts of results obtained with the analyzer of the present invention.
DESCRIPTION OF PREFERRED
Turning now to a detailed consideration of the present invention, there is illustrated in FIG. 1 in schematic diagram form, a magic tee vector network analyzer 8 in accordance with the present invention. The analyzer, which is constructed from standard millimeter wave passive components and which uses a computer controlled signal generator which is swept across a frequency band of interest, can be used to make both reflection and transmission measurements, but for simplicity, FIG. 1 illustrates reflection measurements only. As shown, a controllable signal generator 10 generates an output signal of a selected millimeter wavelength frequency and supplies it to an output waveguide 12. The output signal is fed through a first directional isolator 14, an alternator 16, and a second directional isolator 18,
and through a pair of directional couplers 20 and 22 to a device 24 to be tested, connected to a test port 25. The device under test may be any desired load such as a crystal detector or some other microwave component, the characteristics of which are to be tested at various 5 frequencies.
The signal generator 10 is a conventional controllable generator which produces a complex output voltage at selected frequencies in the range of between about 18 and 100 GHz, with the specific frequency produced at 10 any time being selected by an input signal on line 26 which may be supplied, for example, by a small digital computer 28, such as a PDP-11/04. A frequency meter 30 may be provided, for example, at a waveguide section 32 connected between the isolators 14 and 18 to 15 provide an independent measurement of the output frequency of generator 10. Attenuator 16 may be placed in the waveguide 32 to permit adjustment of the amplitude of the output signal from the signal generator 10. The isolators 14 and 18 prevent reflected signals from a 20 device under test from enetering the signal generator output.
The directional couplers 20 and 22 are connected in the waveguide section 34 which extends from isolator 18 to the device 24 under test, and are conventional, 25 passive waveguide devices. Coupler 20 couples a part of the output voltage from signal generator 10 to provide a reference voltage, while the coupler 22 couples a portion of the signal reflected from the device under test to provide a test signal. The output from the signal 30 generator 10 which reaches the device under test is a complex voltage V,-, while the microwave signal reflected from the device under test is a complex voltage TV,-where T is the complex reflection coefficient of the device 24. The reflected signal is coupled by the direc- 35 tional coupler 22 into a test channel waveguide 36, passes through an adjustable attenuator 38 and is incident on the E-arm of a magic tee waveguide component 40. This incident voltage is the test signal, V,. The magic tee 40 is a conventional, passive waveguide com- 40 ponent having a pair of inlet arms E and H at right angles to each other, both inlet arms being perpendicular to the colinear outlet arms 42 and 44 of the device.
The directional coupler 20 supplies a portion of the incident voltage V/to a reference channel waveguide 46 45 which includes an extension arm 48 to equalize the path lengths of the reference and test signals. The signal in the reference channel 46 is fed through an adjustable attenuator 48 and is supplied to the E arm of a magic tee device 50. The colinear arms 52 and 54 of the magic tee 50 50 are connected to a movable waveguide short circuit 56 and to a terminating attenuator 58, respectively, to provide a 90° phase shift structure generally indicated at 60. The phase shifter 60 produces a reference signal Vr at the output arm H of magic tee 50, Vrbeing selectively 55 shiftable 90 degrees from the incident voltage V/ supplied to the device under test. The signal Vr is supplied to the H arm of magic tee 40 and is also 90 degrees out of phase with the test voltage V,. The reference and test voltages are mixed vectorially in the magic tee 40, 60 which then delivers power from each of its colinear arms 42 and 44 through corresponding directional isolators 62 and 64 to corresponding power detectors 66 and 68. Any type of power detector, such as a thermistor or a square-law crystal detector, can be used at 66 and 68. 65
It should be noted that although an adjustable waveguide short circuit 56 is illustrated in the phase shifter 60, other forms of phase shifters could be used. For
example, a circulator or a 3 dB hybrid could replace the magic tee, or an electronic phase shifting structure consisting of a combination of a PIN diode with the magic tee could be used.
The outputs from the power detectors 66 and 68 are supplied by way of lines 70 and 72, respectively, to the computer 28 for calculation of the impedance characteristics of the device under test. These calculations are made by solving the mathematical equations (algorithms) for these values. Thus, the reference signal Vr and the test signal V; are expressed in the following general form:
where Tr and Tt are transmission coefficients of the reference and test channels, respectively, the D's are the directivity of the corresponding couplers and T(= | T | eie) is the complex reflection coefficient of the device under test 24. The f s in parentheses are included to emphasize that the quantities are frequency dependent.
The second terms in each of the above equations represent coupler imperfections which cause errors in measurement. Note that the second term of the first equation becomes important compared to the first term when | rDr| is comparable to or greater than unity. The second term in the second equation becomes significant when |T| becomes comparable to |D([. These second terms will limit the accuracy of measurement if they are not included in the formulation.
For simplicity in the following formulation, however, these second terms are excluded. In this case the output dc voltage Vj from power detector 66 is expressed as follows:
V\ = Si(f)Pi (2)
= ... ■ v, + bnwm ■ fx12
= SiV)\ \briTrVi] + \btlT,Vi\ ■ ...
= ... ... ...
where Sn(f)=Vs^(f)\briTrVi\, StiV)=VSXfi\btlTtVi\, 0i=£>rt-0wl
The coefficients bri, bti represent the power splitting characteristics of the magic tee and ideally take a value of 1/V2 in magnitude. Note that in the last line of Eq. (2) a sign for absolute value has been removed from T for brevity.
The dc output voltage V2 from detector 68 is written in a similar form with the positive sign in front of the second term being replaced by a negative sign in Eq. (2). This is because a phase inversion occurs in the magic tee when the signal is incident on its E-arm. Thus
It should be noted that any deviation from an ideal phase inversion, in a practical magic tee, is accounted for in 02
Equations (2) and (3) can be written in the following form:
... cos (e+«o
tion reduces the measurement error in 0 as compared to a direct solution of Eqs. (5) or (6).
The equations for transmission coefficient measurements are obtained by simply replacing T, the transmission coefficient, for T in all the equations derived above.
The computer 28 performs the foregoing calculation to determine the complex reflection coefficient T and the phase shift 0 produced by the device under test for any given incident frequency. The calculated values are supplied by the computer through line 74 to suitable display devices such as a cathode ray tube 76, a pen recorder 78, or a printer 80. Upon completion of a calculation, the signal generator 10 is shifted to the next desired frequency by a signal from the computer on line 26 and the measurements and calculations repeated.
A modification of the system of FIG. 1 is illustrated in FIG. 2, wherein a variable frequency signal generator 90 supplies output signals of selected frequencies in the millimeter wavelength range on output waveguide 92. These output signals are supplied through a directional isolator 94 and a waveguide 96, which incorporates a pair of directional couplers 98 and 100, to a test port 101, to which is connected a device under test 102, the signal generator 90 supplying an incident voltage V,to the device 102. Reflections from the device 102 are represented by TV,-, while signals transmitted through the device under test are represented at its output at waveguide 104 by the signal TV/. In the test system, the output signal is fed through a directional coupler 106, to a terminating attenuator 108.
The first directional coupler 98 couples a portion of the incident signal V; appearing on waveguide 96 to a reference channel waveguide 110, which includes an adjustable extension 112 to permit equalization of the path lengths of the reference and test channels to minimize errors due to any frequency instability in the signal generator 90. The reference channel waveguide also includes an adjustable attenuator 114 to permit adjustment of the amplitude of the signal in the reference channel 110. The signal from waveguide 110 is supplied to the E arm of a magic tee device 116 which is a part of a 90° phase shifter 118. In this embodiment, a PIN diode 120 is connected in the arm 122 of the magic tee 116, the conductivity of the diode being controlled by the computer 28. Thus, the PIN diode 120, which is backed by a short circuit 124, is shiftable between on and off conditions to provide a 90° phase shift in the signals appearing on the H arm of the magic tee 16. The colinear arm 128 incorporates a terminating attenuator 130.
The output signal from the H arm of magic tee 116 is the reference voltage Vr which is supplied by way of waveguide 132 to the H arm of a magic tee 134 in the test channel of the system.
The device under test 102 in FIG. 2 may be tested for both reflection and transmission coefficients, and for this purpose the directional coupler 100 samples the reflected signals TVi reflected from the device under test and couples those signals to a reflection test arm which includes waveguide 140. Signals that are transmitted through the device under test 102 are represented by voltage TV/ and are sampled by the directional coupler 106 to supply test signals to the transmission test arm which includes waveguide 142. A switch 144 allows the signal on either of the waveguides 140 or 142 to be directed through a test channel waveguide 145, which includes an adjustable attenuator 146, to the