A new design approach for low-cost multiprobe reflectometers is presented. While traditional circuits adopt equally-spaced probes, the presented solution provide a method to greatly enhance the bandwidth of the measuring system by a proper choice of each probe position. As example, a five-probe 0.6-16 GHz system has been designed

Ferrero, Andrea Pierenrico

Madonna, G.L.

Rostagno, M.

English

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A new approach to the design of wide-band multiprobe reflectometers

PORTO Publications Open Repository TOrino

WEPB4-2. 
A NEW APPROACH TO THE DESIGN OF 
WIDE-BAND MULTIPROBE REFLECTOMETERS 
G i d u i g i  Madonna, Andrea Ferrero and Mauro Rostagno 
Dipartimento di Elettronica 
Politecnico di Torino 
Corso Duca degli Abruzzi 24 
10129 Torino, Italy 
Phone: (xx3911) 5644082 Fax: (xx3911) 5644099 E-Mail: ferrero@polito.it 
A new design approach for low-cost multiprobe 
reflectometers is presented. While traditional circuits 
adopt equally-spaced probes, the presented solution 
provide a method to greatly enhance the bandwidth of 
the measuring system by a proper choice of each probe 
position. As example, a five-probe 0.6-16 GHz system 
has been designed. 
Introduction 
Modern automatic network analyzers use 
complex and expensive heterodyne and mixing circuitry 
to determine amplitude and phase of unknown reflection 
coefficients at microwave frequencies. The 
developement of six-port reflectometers have provided 
an alternative method, which reconducts the problem to 
the measurements of poweir ratios. Six-port systems [l] 
still require many directional couplers and hybrids and 
they are quite complicated. The simplest method is 
probably the multiprobe reflectometer, which consists in 
sampling with fixed probes the standing wave pattern on 
a transmission line which feeds the device under test. 
This solution can be easily built in microstrip and 
monolithic version 121, providing a really low-cost way 
to have reflection coefficient informations inside larger 
subsystems. Equally-spacedl probes can be easily design 
to operate at a single frequency or for small bandwidth 
applications [3]. In order to extend the operating range, 
Chang et al. [4] proposed to raise the number of probes 
and to mod@ the spacing between them, but a general 
design method has not been presented yet. 
Here a novel approach is investigated, which 
allows to obtain the probe ]positions as a function of the 
required bndwidth. To validate the method, comparisons 
were made with the results already published in the case 
of equally-spaced probes. 
Theorv and desien apuroach 
Figure 1 shows a simplified scheme of the 
measurement setup. Probes, transmission line and 
coupler can be seen as a generic multiport junction, 
whose theory has already been developed for six-port 
system [l]. For each probe, a simple equation can be 
written to relate the detected power P, with the unknown 
reflection coefficient l-, 141. It can be proved that the 
obtained set of equations, that we regard as the 
measurement equations, is a linear system provided that 
l- = [ILi' 
is chosen as the vectors of unknowns. 
Probes 
source 
Coefficient < 2 3  
Fig. 1. Simplified measurement scheme 
The performance of the multiprobe system was 
studied analyzing the condition number of the 
measurement equations, as defined in [ 5 ] .  In particular, 
given a specified bandwidth, the adopted algorithm 
minimizes a proper mean condition number computed 
over the required range of frequencies. 
424 
Simulation results 
The proposed approach gives results that are 
coherent with what found in literature for equally- 
spaced probes. Figure 2 shows the condition number for 
a three-probe-reflectometer as a function of the constant 
spacing (expressed as electrical length). The curve 
minimum is for AI$ = 120”, as well-known from the six- 
port theory. Chang et al. [4] regard as acceptable a shift 
greater than 60” and slightly less than 180” over the 
operating bandwidth; since in this range the condition 
number is less than 7, th is  value was assumed as a 
reasonable specification for our design. 
By using more than three probes and optimising 
their positions for a minimum condition number, a 
broader bandwidth is obtained. Figure 3 shows the 
condition number for the optimum equally-spaced five- 
probe reflectometer (spacing AI$ = 72”) as a function of 
the normalised frequency. Figure 4 shows the condition 
number of the unequally-spaced five-probes 
reflectometer (spacing: 219”, 648”, 772O, 950’). 
Bandwidth performance appears to be greatly enhanced 
in the latest case with respect to both three and five 
equally-spaced probes. A 600 MHz - 16 GHz microstrip 
prototype has been realised and measurement results 
will be available at conference time. 
REFERENCES 
G.F.Engen, “The six-port reflectometer: an 
alternative network analyzer,” IEEE Trans. 
Microwave Theorv Tech., v01 .m-25,  pp. 1075- 
1080, Dec.77. 
RE.Neidert, “Monolithic circuit for reflection 
coeficient measurement,” IEEE Microwave 
Guided Waves Lett., vol.1, pp.195-197, Aug.91. 
C.J.Hu, “A novel approach to the design of 
multiple-probe high-power microwave automtic 
impedance measuring system,” IEEE Trans. 
Microwave Theorv Tech., v01 .m-28,  pp. 1422- 
1428, Dec.80. 
K.Chang, M.Li and T.Sauter, “Low cost 
microwavehillimeter-wave impedance scheme 
using a three-probe microstrip circuit,” 
Trans. Microwave Theorv Tech., vol.M‘IT-38, 
G.Golub and C.Van Loan, Matrix Comuutations, 
2nd ed., Baltimore, John Hopkins University Press, 
1989. 
pp. 1455-1460,Oct.90. 
25 
20 
15 iz 
s 
U 
10 
5 
0 
Spacing A+ 
Fig.2. Conditioning for a three equally spaced probe 
reflectometer as a function of the probe spacing. 
Normalized frequency 
Fig.3. Conditioning for the optimum equally spaced 
five-probe reflectometer. 
lo“ 10’ 
Fig.4. Conditioning for the optimum five-probe 
reflectometer designed according to the presented 
criteria. 
Normalized frequency 
425 


M.Li and T.Sauter, “Low cost microwavehillimeter-wave impedance scheme using a three-probe microstrip circuit,” Trans.

RE.Neidert, “Monolithic circuit for reflection coeficient measurement,”

G. L. Madonna

M. Rostagno

FERRERO, ANDREA PIERENRICO

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

04 August 2020POLITECNICO DI TORINORepository ISTITUZIONALEA new approach to the design of wide-band multiprobe reflectometers / G. L. Madonna; A. Ferrero; M. Rostagno. -STAMPA. - (1998), pp. 424-425. ((Intervento presentato al convegno CPEM, Conference on Precision ElectromagneticMeasurements tenutosi a Washington, DC (USA) nel 06-10 Jul 1998.OriginalA new approach to the design of wide-band multiprobe reflectometersPublisher:PublishedDOI:10.1109/CPEM.1998.699984Terms of use:openAccessPublisher copyright(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/2497916 since:WEPB4-2. A NEW APPROACH TO THE DESIGN OF WIDE-BAND MULTIPROBE REFLECTOMETERS G i d u i g i  Madonna, Andrea Ferrero and Mauro Rostagno Dipartimento di Elettronica Politecnico di Torino Corso Duca degli Abruzzi 24 10129 Torino, Italy Phone: (xx3911) 5644082 Fax: (xx3911) 5644099 E-Mail: ferrero@polito.it A new design approach for low-cost multiprobe reflectometers is presented. While traditional circuits adopt equally-spaced probes, the presented solution provide a method to greatly enhance the bandwidth of the measuring system by a proper choice of each probe position. As example, a five-probe 0.6-16 GHz system has been designed. Introduction Modern automatic network analyzers use complex and expensive heterodyne and mixing circuitry to determine amplitude and phase of unknown reflection coefficients at microwave frequencies. The developement of six-port reflectometers have provided an alternative method, which reconducts the problem to the measurements of poweir ratios. Six-port systems [l] still require many directional couplers and hybrids and they are quite complicated. The simplest method is probably the multiprobe reflectometer, which consists in sampling with fixed probes the standing wave pattern on a transmission line which feeds the device under test. This solution can be easily built in microstrip and monolithic version 121, providing a really low-cost way to have reflection coefficient informations inside larger subsystems. Equally-spacedl probes can be easily design to operate at a single frequency or for small bandwidth applications [3]. In order to extend the operating range, Chang et al. [4] proposed to raise the number of probes and to mod@ the spacing between them, but a general design method has not been presented yet. Here a novel approach is investigated, which allows to obtain the probe ]positions as a function of the required bndwidth. To validate the method, comparisons were made with the results already published in the case of equally-spaced probes. Theorv and desien apuroach Figure 1 shows a simplified scheme of the measurement setup. Probes, transmission line and coupler can be seen as a generic multiport junction, whose theory has already been developed for six-port system [l]. For each probe, a simple equation can be written to relate the detected power P, with the unknown reflection coefficient l-, 141. It can be proved that the obtained set of equations, that we regard as the measurement equations, is a linear system provided that l- = [ILi' is chosen as the vectors of unknowns. Probes source Coefficient < 2 3  Fig. 1. Simplified measurement scheme The performance of the multiprobe system was studied analyzing the condition number of the measurement equations, as defined in [ 5 ] .  In particular, given a specified bandwidth, the adopted algorithm minimizes a proper mean condition number computed over the required range of frequencies. 424 Simulation results The proposed approach gives results that are coherent with what found in literature for equally- spaced probes. Figure 2 shows the condition number for a three-probe-reflectometer as a function of the constant spacing (expressed as electrical length). The curve minimum is for AI$ = 120”, as well-known from the six- port theory. Chang et al. [4] regard as acceptable a shift greater than 60” and slightly less than 180” over the operating bandwidth; since in this range the condition number is less than 7, th is  value was assumed as a reasonable specification for our design. By using more than three probes and optimising their positions for a minimum condition number, a broader bandwidth is obtained. Figure 3 shows the condition number for the optimum equally-spaced five- probe reflectometer (spacing AI$ = 72”) as a function of the normalised frequency. Figure 4 shows the condition number of the unequally-spaced five-probes reflectometer (spacing: 219”, 648”, 772O, 950’). Bandwidth performance appears to be greatly enhanced in the latest case with respect to both three and five equally-spaced probes. A 600 MHz - 16 GHz microstrip prototype has been realised and measurement results will be available at conference time. REFERENCES G.F.Engen, “The six-port reflectometer: an alternative network analyzer,” IEEE Trans. Microwave Theorv Tech., v01 .m-25,  pp. 1075- 1080, Dec.77. RE.Neidert, “Monolithic circuit for reflection coeficient measurement,” IEEE Microwave Guided Waves Lett., vol.1, pp.195-197, Aug.91. C.J.Hu, “A novel approach to the design of multiple-probe high-power microwave automtic impedance measuring system,” IEEE Trans. Microwave Theorv Tech., v01 .m-28,  pp. 1422- 1428, Dec.80. K.Chang, M.Li and T.Sauter, “Low cost microwavehillimeter-wave impedance scheme using a three-probe microstrip circuit,” Trans. Microwave Theorv Tech., vol.M‘IT-38, G.Golub and C.Van Loan, Matrix Comuutations, 2nd ed., Baltimore, John Hopkins University Press, 1989. pp. 1455-1460,Oct.90. 25 20 15 iz s U 10 5 0 Spacing A+ Fig.2. Conditioning for a three equally spaced probe reflectometer as a function of the probe spacing. Normalized frequency Fig.3. Conditioning for the optimum equally spaced five-probe reflectometer. lo“ 10’ Fig.4. Conditioning for the optimum five-probe reflectometer designed according to the presented criteria. Normalized frequency 425 

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A new approach to the design of wide-band multiprobe reflectometers

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