Technical memorandum. six port reflectometer for the 75¿105 GHz band
5
Technical memorandum SIX PORT REFLECTOMETER FOR THE 75-105 GHz BAND Indexing term: Waveguides Abstract: The construction, using standard direc- tional couplers, of a six-port reflectometer for measurement of scattering parameters in the 75- 105 GHz band is reported. The broad band performance is assessed in terms of relative Q-centre positions. Ultimately performance is limited by the repeatabil- ity of waveguide flanges. Introduction This paper describes, with reference to earlier work by the author [1], a six-port reflectometer employing manual source tuning to operate over the 75 to 105 GHz band (WG27). To achieve operation over this range, it was necessary to compensate for departure from 90° of the phase between the coupled and transmitted paths of the waveguide directional couplers forming the six-port junc- tion. Measurement of this phase relationship and appro- priate compensation are described. Results of measurement of voltage reflection coefficient T L , to quantify the minimum power needed to achieve random measurement errors comparable with variations arising from WG27 flange reconnection, are also presented. Measurement and adjustment of directional couplers The six-port junction employed is shown in Fig. 1. It com- prises a 6 dB directional coupler (1) and three 3 dB four microwave input port directional coupler 1 measurement port Fig. 1 WG27 six-port junction Inset: four-port coupler port couplers (2-4). Its theory of operation, given in earlier work [1], assumes a quadrature phase relationship between the coupled and transmitted paths in couplers 2-4. In this Section, the measurement of this phase and the compensation for its departure from 90° are described. Using a previously reported narrowband six-port junc- tion [2, 3], the electrical lengths of the coupled and trans- mitted paths in the couplers were determined from F L measurements carried out at three frequencies. Each path in turn was terminated by a known load, the remaining ports being terminated by nominally matched loads. The results from measurements on one of the couplers (used for coupler 3 in Fig. 1) are given in Table 1 (see also inset Fig. 1). For this coupler to have a quadrature phase relation- ship, its coupled path length must be 111.41—y ff /4 = 110.36 mm (using midband values). Therefore its side arm was shortened by 0.94 mm. Measurements, after machin- ing, showed that the quadrature phase relationship held within 13° over the frequency band. The reverse side arm of coupler 2 was also machined, but the adjustments for the forward coupled paths of couplers 2 and 3 were accounted for in the compensating length of waveguide. Performance of the six-port junction Referring to Fig. 1, an approximate analysis, neglecting the finite directivity and mismatch in the couplers, included in an earlier paper [1] shows that, given phase quadrature in couplers 2, 3, 4, then T L is related to vectors of amplitude \W K \ (where K = 1, 2, 3 and \W K \ 2 is proportional to (i) (2) (3) detector output ratios ^ + 3/^3) by eqns. 1-3 as follows: r -4- —- — i —- L t 2 J c f 3 ^4 W-,1 = = i r L -i Where C n and t n are the voltage coupling and transmission coefficients of coupler n (= 2, 3, 4), such that \t n \ 2 + \jC n \ 2 = 1. Eqns. 1-3 describe these circles in the F plane having centres (Q-centres) dependent on the coup- ling factors of coupler n. For 3 dB couplers, and assuming 0.05 dB/cm attenuation for the compensating waveguide (t,) of angular length 6,, the Q-centres should be as shown in Fig. 2. Comparison of the relative positions of the Q- centres, derived from measurement with these theoretical centres, illustrates the effectiveness of the phase adjustment to the couplers already described. The junction has been calibrated at 2 GHz intervals from 75 to 105 GHz. Q-centres derived from measure- ments on a nominally matched load are shown in Fig. 3. Comparison with Fig. 2 shows that the phase relationship of the Q-centres is maintained (within reasonable limits) over this frequency range. The variation in magnitude is probably due to resonances caused by the significant reflection coefficient of the detectors and any mismatch in the couplers. All of the Q-centres are stable, and the junc- Table 1: Results of measurements of coupler 3 Frequency GHz 80 92.5 105 Transmitted path ports 1 -* 2 Phase deg -39.35 -172.85 -103.48 Elect length mm 111.55 111.40 111.41 Length used mm 111.41 Coupled ports 1 — Phase deg -22.27 -164.16 -101.64 path • 4 Elect length mm 111.29 111.30 111.39 Length used mm 111.30 Phase relationship deg +17.08 +8.69 + 1.84 Correction for quadrature relationship mm -0.94 IEE PROCEEDINGS, Vol. 132, Pt. H, No. 2, APRIL 1985 141
Transcript of Technical memorandum. six port reflectometer for the 75¿105 GHz band
Technical memorandumSIX PORT REFLECTOMETER FOR THE 75-105GHz BAND
Indexing term: Waveguides
Abstract: The construction, using standard direc-tional couplers, of a six-port reflectometer formeasurement of scattering parameters in the 75- 105GHz band is reported. The broad band performanceis assessed in terms of relative Q-centre positions.Ultimately performance is limited by the repeatabil-ity of waveguide flanges.
Introduction
This paper describes, with reference to earlier work by theauthor [1], a six-port reflectometer employing manualsource tuning to operate over the 75 to 105 GHz band(WG27). To achieve operation over this range, it wasnecessary to compensate for departure from 90° of thephase between the coupled and transmitted paths of thewaveguide directional couplers forming the six-port junc-tion. Measurement of this phase relationship and appro-priate compensation are described. Results of measurementof voltage reflection coefficient TL, to quantify theminimum power needed to achieve random measurementerrors comparable with variations arising from WG27flange reconnection, are also presented.
Measurement and adjustment of directional couplers
The six-port junction employed is shown in Fig. 1. It com-prises a 6 dB directional coupler (1) and three 3 dB four
microwaveinput port
directionalcoupler 1
measurementport
Fig. 1 WG27 six-port junction
Inset: four-port coupler
port couplers (2-4). Its theory of operation, given in earlierwork [1], assumes a quadrature phase relationshipbetween the coupled and transmitted paths in couplers2-4. In this Section, the measurement of this phase and thecompensation for its departure from 90° are described.
Using a previously reported narrowband six-port junc-tion [2, 3], the electrical lengths of the coupled and trans-mitted paths in the couplers were determined from FL
measurements carried out at three frequencies. Each pathin turn was terminated by a known load, the remainingports being terminated by nominally matched loads. Theresults from measurements on one of the couplers (used forcoupler 3 in Fig. 1) are given in Table 1 (see also inset Fig.1). For this coupler to have a quadrature phase relation-ship, its coupled path length must be 111.41—yff/4 =110.36 mm (using midband values). Therefore its side armwas shortened by 0.94 mm. Measurements, after machin-ing, showed that the quadrature phase relationship heldwithin 13° over the frequency band. The reverse side armof coupler 2 was also machined, but the adjustments forthe forward coupled paths of couplers 2 and 3 wereaccounted for in the compensating length of waveguide.
Performance of the six-port junction
Referring to Fig. 1, an approximate analysis, neglecting thefinite directivity and mismatch in the couplers, included inan earlier paper [1] shows that, given phase quadrature incouplers 2, 3, 4, then TL is related to vectors of amplitude\WK\ (where K = 1, 2, 3 and \WK\2 is proportional to
(i)
(2)
(3)
detector output ratios ^ + 3/^3) by eqns. 1-3 as follows:
r -4- —- — i —-L t2 J c
f 3 ^4
W-,1 =
= i r L - iWhere Cn and tn are the voltage coupling and transmissioncoefficients of coupler n (= 2, 3, 4), such that\tn\
2 + \jCn\2 = 1. Eqns. 1-3 describe these circles in the
F plane having centres (Q-centres) dependent on the coup-ling factors of coupler n. For 3 dB couplers, and assuming0.05 dB/cm attenuation for the compensating waveguide(t,) of angular length 6,, the Q-centres should be as shownin Fig. 2. Comparison of the relative positions of the Q-centres, derived from measurement with these theoreticalcentres, illustrates the effectiveness of the phase adjustmentto the couplers already described.
The junction has been calibrated at 2 GHz intervalsfrom 75 to 105 GHz. Q-centres derived from measure-ments on a nominally matched load are shown in Fig. 3.Comparison with Fig. 2 shows that the phase relationshipof the Q-centres is maintained (within reasonable limits)over this frequency range. The variation in magnitude isprobably due to resonances caused by the significantreflection coefficient of the detectors and any mismatch inthe couplers. All of the Q-centres are stable, and the junc-
Table 1 : Results of measurements of coupler 3
Frequency
GHz8092.5
105
Transmitted pathports 1 -* 2
Phase
deg-39.35
-172.85-103.48
Elect length
mm111.55111.40111.41
Length used
mm
111.41
Coupledports 1 —
Phase
deg-22.27
-164.16-101.64
path• 4
Elect length
mm111.29111.30111.39
Length used
mm
111.30
Phaserelationship
deg+17.08
+8.69+ 1.84
Correctionfor quadraturerelationship
mm
-0.94
IEE PROCEEDINGS, Vol. 132, Pt. H, No. 2, APRIL 1985 141
a centre W2-1.J2.336
\
W2\
KW1 /
/
-1.-J2.336Q centre W1
J Q centre W3
Table 2: Theoretical values for the network con-stants
Frequency GHz
Theoretical values75.076.078.080.082.084.086.088.090.092.094.096.098.0
100.0103.0104.0105.0
Network constants
P
0.8870.0370.1510.2730.4190.7990.9950.8270.8000.9571.1881.1401.1470.9130.5710.9400.7450.632
Q
0.8871.7851.2630.8040.4950.7060.5500.4050.6780.1760.4880.1690.3690.2600.2490.3190.2120.190
R
2.0472.2131.7871.2451.0031.6621.9981.4361.9961.2522.3561.5451.8251.1781.0961.5491.2701.134
A2
1.0000.9431.1740.8700.9361.5471.5681.3631.7021.6631.3511.1821.6641.1640.6641.2900.8150.628
B2
0.2500.4490.0900.2040.1260.1690.3200.1670.3390.0700.2310.2020.1830.0920.0780.1310.1010.048
Fig. 2 Predicted relative Q-centre positions
Q-centres for W2
Q-centres for W1
Q-centres for W3
+ 98
142
Fig. 3 Measured relative Q-centre positions
IEE PROCEEDINGS, Vol. 132, Pt. H, No. 2, APRIL 1985
tion gives accurate TL measurements at all of these fre-quencies.
The five real constants P, Q, R, A2 and B2, which areindependent of TL and characterise the junction, are listedin Table 2. They are produced by the six to four portreduction during calibration [1]. Theoretical values forthese constants (assuming 0.05 dB attenuation in the com-pensating waveguide) are also given. Comparison showsthat above 75 GHz all five constants are consistent withthese values within the range X0.17 to X1.7.
Measurement results
A load consisting of an E-plane bend terminated by anoffset short circuit was measured repeatedly at 94 GHz, theload being disconnected and turned through 180° betweeneach measurement. This enabled the repeated mating ofcommercial WG27 flanges (type UG 387/U) to be assessed.Ten reconnections gave a spread in TL of 0.002 in magni-tude and 0.4° in phase.
Table 3 lists the results of measurements carried out todetermine the minimum input power to the junction,
Table 3: Measured reflection coefficient of test load at94 GHz. Mean and standard deviation of 5 measurements
Input power,mW
Meanmag phase
Calculated-measuredphase
Standard dev.mag phase
510.50.20.1
0.99870.99840.99920.99820.9983
62.7562.8862.9962.9562.96
-0.22-0.35-0.46-0.42-0.43
0.000150.000620.001120.002500.00605
0.0190.0760.0450.2080.828
(Calculated phase = +62.53°)
which would give random measurement errors comparablewith the variation obtained for WG27 flange reconnection.The test load comprised a precision waveguide spacer andoffset short circuit. The calculated value in the results wasnot corrected for any attenuation in the device. However,this effect was also ignored for similar items used for cali-bration standards, therefore any discrepancy should besmall. It can be seen from the results that the minimumpower to meet the above conditions is 500 JIW. This ismainly due to the sensitivity of the thermistor mounts used
for detectors. However, if enough measurements are madethe junction will continue to give accurate TL measure-ments, to within 0.5° in phase, with input power down to100
Conclusion
A six-port reflectometer which operates from 75 to105 GHz has been described. The method of construction[1, 4] required adjustment to the waveguide directionalcouplers which provided quadrature phase tracking within±13° over the waveguide band. The means for achievingthis have also been described.
It has been shown that the repeatability of commercialWG27 flange connections gives a variation in reflectioncoefficient within +0.001 in magnitude and ±0.2 in phase.The reflectometer gave random measurement errors com-parable with this, using an input power of only 500 ;uW,even though thermistor power sensors were used for detec-tion.
Acknowledgment
The author gratefully acknowledges the support andencouragement of Mr E.J. Griffin in preparing this paper.
Acknowledgment is made to the Controller of HM Sta-tionary Office for permission to publish this paper.
References
1 HILL, L.D., and GRIFFIN, E.J.: 'An automatic stepped frequency six-port reflectometer for WG22', IEE Proc.
2 GRIFFIN, E.J., BRADSTREET, D.W.G., HILL, L.D., andHODGETTS, T.E.: 'A six-port reflectometer for millimetric wave-lengths', CPEM 82, IEE Cat No. 82CH 1737-6, pp. 5-3 to 5-5, June1982
3 GRIFFIN, E.J.: 'Six-port reflectometer circuit comprising three direc-tional couplers', Electron. Lett., 1982, 18, (12), pp. 491-493
4 HILL, L.D.: 'Implementing waveguide bandwidth six-port reflec-tometer circuits at millimetric wavelengths'. RSRE Memorandum No.3678, May 1984
5 UK Patent Application No. 8311 170, May 1983.
28th August 1984
Clifton Angus Technical Services LimitedBarons Cross LodgeLeominsterHertfordshire
L.D. HILL
IEE PROCEEDINGS, Vol. 132, Pt. H, No. 2, APRIL 1985 143
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