4. MEASUREMENT RESULTS

With the supply voltage of 3.6 V, the quiescent currents of PS1

and PS2 were 60 and 12 mA, respectively. The measured power

performance of the MBPA at the center frequency of each band

(band-1, -2, -3, -5, -8) for a 10-MHz bandwidth (BW) 16 quadra-

ture amplitude modulation (QAM) LTE signal is shown in Figures

5 and 6. Figure 5 shows the gain, evolved universal terrestrial

radio access adjacent channel leakage ratio (ACLRE-UTRA), collec-

tor efficiency (CE), and error vector magnitude (EVM) as a func-

tion of the output power for different frequency bands. Figure 6

shows the power added efficiency (PAE) and CE of the MBPA

for different power mode operations (LPM and HPM) at different

frequency bands. The MBPA in the HPM delivered a gain of 9.1–

12.3dB, a PAE of 30.2–33.8%, and a CE of 31.8–38.5% at an

average output power of 26.5 dBm. The linearity specifications for

the LTE signal include an ACLRE-UTRA less than 230 dBc and

an EVM smaller than 4%. The ACLRE-UTRA of the MBPA was

from 232.5 to 230.2 dBc and the EVM was 3.2–4% at an aver-

age output power of 26.5 dBm. In the LPM, the MBPA delivered

a gain of 7.8–10.4 dB, a PAE of 18.5–32.2%, and a CE of 21.8–

35.4%, an ACLRE-UTRA from 241.4 to 232.3 dBc, and EVM of

0.95–2.45% at an average output power of 18 dBm. The experi-

mental results show that the efficiency of the MBPA at LPM can

be improved significantly using the reconfigurable output matching

network.

5. CONCLUSION

An MBPA using a switch-based reconfigurable matching net-

work is proposed and implemented. The collector efficiency of

the MBPA was higher than 31.8 % at an average output power

of 26.5 dBm (HPM) and higher than 21.8% at average output

power of 18dBm (LPM) for E-UTRA frequency band-1, -2, -3,

-5, and -8. The results show the promise of this proposed tech-

nique for use in implementation of a fully-integrated single-chip

MBPA covering the entire E-UTRA frequency bands.

ACKNOWLEDGMENT

This work was supported in part by grants from the Electronic

Warfare Research Center (EWRC) program and the Bio-imaging

Research Center program at GIST.

REFERENCES

1. E. Neo, Y. Lin, X. Liu, L. Vrede, L. Larson, M. Spirito, M. Pelk, K.

Buisman, A. Akhnoukh, A. Graauw, and L. Nanver, Adaptive multi-band

multi-mode power amplifier using integrated varactor-based tunable

matching networks, IEEE J Solid-State Circuits 41 (2006), 2166–2176.

2. F. Wang, A. H. Yang, F. Kimball, L. E. Larson, and P. M. Asbeck,

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OFDM applications, IEEE Trans Microwave Theory Tech 53 (2005),

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mode/multiband power amplifier with a boosted supply modulator,

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4. N.J. Quintero, Advanced power control for UTRAN LTE uplink,

Master’s thesis, Dept. Electron. Syst., Aalborg Univ., Copenhagen,

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ule with improved low-power efficiency using stage-bypass, In:

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(2010), pp. 163–166.

6. J. Jung, G. Lee, and J. Song, A SiGe HBT power amplifier with

integrated mode control switches for LTE applications, In: IEEE

Radio Wireless Symposium, (2013), pp. 238–240.

VC 2014 Wiley Periodicals, Inc.

ACCURATE BROADBAND MULTIPORTREFLECTOMETER

Kamil Staszek, Slawomir Gruszczynski, and Krzysztof WinczaFaculty of Computer Science, Electronics and Telecommunications,AGH University of Science and Technology, 30-059, Cracow,Poland; Corresponding author: slawomir.gruszczynski@agh.edu.pl

Received 29 April 2014

ABSTRACT: A novel broadband multiport reflectometer, consisting ofa standard 4 3 4 Butler matrix and an additional directional coupler is

proposed. It is shown that such a reflectometer provides highly uniformtunable measurement conditions allowing for measurement precisionenhancement. The impact of the applied directional coupler’s imperfec-

tions on the measurement conditions has been theoretically analyzed.The high measurement accuracy of the proposed reflectometer has beenproved experimentally by reflection coefficient measurements over the

frequency range of 1–5 GHz, which show a good agreement with theresults obtained using VNA. VC 2014 Wiley Periodicals, Inc. Microwave

Opt Technol Lett 56:2884–2887, 2014; View this article online at

wileyonlinelibrary.com. DOI 10.1002/mop.28729

Key words: multiport measurement technique; multiport reflectometer;

reflection coefficient measurement; Butler matrix

1. INTRODUCTION

Measurements of a complex reflection coefficient using multi-

port reflectometers have been a subject of extensive research

over last few decades [1]. The measuring system being com-

posed of a multiport network, signal source, and several power

detectors features a great simplicity and can be realized in both

microstrip and waveguide techniques [2]. Today, such systems

are applied not only to scattering parameters measurements but

also in microwave receivers [3] and in complex permittivity

measurements [4,5]. Apart from the application, the key issue

related to such a measurement technique, which defines the

measurement accuracy, is a signal distribution provided by the

applied multiport network in terms of both magnitude and phase

[6,7]. A very helpful tool in the measurement accuracy analysis

is the geometric interpretation of multiport measurements, in

which the measured complex value is an intersection point of

several circles on a complex plane. The analysis based on that

approach has shown that a high degree of symmetry of the

circle centers’ distribution provides maximum measurement

accuracy [6,7].

Figure 1 A schematic diagram of the proposed multiport reflectometer

composed of a single 4 3 4 Butler matrix and an additional directional

coupler

2884 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 12, December 2014 DOI 10.1002/mop

Recently, it has been shown that a standard 4 3 4 Butler

matrix with a reflective element connected to the one of its out-

put ports can serve as a multiport reflectometer, being capable

of reflection coefficient measurements over a wide frequency

range [8]. It has been revealed that a reflectometer involving a

single 4 3 4 Butler matrix provides three circles having centers

located on the circumference of a unity circle with the angular

distance of 90�. Since there are only three circle centers, their

distribution with respect to all possible reflection coefficients,

being located inside the unitary circle, is not uniform. This dis-

advantage can be overcome by the application of an 8 3 8 But-

ler matrix [9], which can provide a uniform distribution of four

circles having centers on the circumference of a unity circle

with angular distance of 90�. Such uniform measurement condi-

tions are, however, obtained at the expense of complexity of the

utilized 16-port network [9]. Moreover, in that case only one

half of the 8 3 8 Butler matrix is effectively utilized.

In this article, a novel broadband multiport reflectometer is

proposed. It is composed of a standard 4 3 4 Butler matrix and

an additional directional coupler. The proposed modification

introduces the fourth circle, ensuring a uniform circle centers’

distribution. Simultaneously, such a system configuration allows

to scale the circle centers’ distribution, providing an enhanced

measurement accuracy with the preserved simplicity of a mea-

surement setup. The influence of the coupler’s parameters on the

measurement conditions has been analyzed and the results are

given. The presented broadband multiport reflectometer has been

verified experimentally by measurements of the reflection coeffi-

cient of several reflective elements in a wide frequency range 1–

5 GHz. The results compared with the measurements performed

with the use of a commercial VNA prove the high accuracy of

the proposed reflectometer in terms of both magnitude and phase.

2. THEORETICAL ANALYSIS

The advantages of multiport reflectometers utilizing 4 3 4 But-

ler matrices and 8 3 8 Butler matrices can be combined in the

measuring system presented in Figure 1. It utilizes a single 4 3

4 Butler matrix with an additional directional coupler. Such a

modification introduces a fourth circle providing a highly uni-

form circle centers’ distribution, as in case of 8 3 8 Butler

matrix application. Simultaneously, the system complexity is

significantly decreased. It can be observed that the system

arrangement is similar to the system involving a single 4 3 4

Butler matrix presented in [8]. The distinctive differences are:

(i) the system is excited through the external directional coupler

and (ii) the additional power measurement is made at Port #2 of

the external directional coupler. The use of the remaining Butler

matrix’s ports is the same as in [8].

Since the 4 3 4 Butler matrix is fed at Port #1, the relations

between power measured at Ports #2–#4, and the measured

reflection coefficient C remain the same as in [8]. Therefore,

Ports #2–#4 of the utilized 4 3 4 Butler matrix are related to

the circle centers located on the circumference of a unitary

circle with angular distance being equal to 90�. Such circle cen-

ters’ distribution is restricted by a proper choice of the meas-

uring port and port with the reflective element connected, which

has been a subject of a comprehensive analysis in [8]. Having

known the phase relations in a 4 3 4 Butler matrix one can pre-

dict, that Port #1, which in [8] has been used for exciting the

measurement system, is related to the fourth circle. The external

coupler allows to excite the 4 3 4 Butler matrix and simultane-

ously provides an additional port, at which the power reflected

from the measured device can be measured introducing the

fourth circle. It can be shown that if the proper measuring port

and port with a reflective element are chosen (according to the

procedure shown in [8]), one can obtain four circle centers dis-

tributed uniformly on a unitary circle.

Figure 2 Signal flow for the transmission between Ports #1 and #2 of

the directional coupler connected to a 4 3 4 Butler matrix. Signal paths

corresponding to imperfections of the Butler matrix are marked gray

Figure 3 Circle centers’ distribution obtained during the calibration

procedure in the frequency range 1–5 GHz: results obtained with a short

circuit (a) and with a shorted 2-dB attenuator as a reflective element (b).

The additional circle center resulting from the use of the additional cou-

pler is marked gray

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 12, December 2014 2885

The theoretical investigation reveals that all four circle cen-

ters are located uniformly on a unitary circle. The imperfect

impedance match as well as nonideal isolations of the 4 3 4

Butler matrix affect the circle centers’ location, however, as it

has been shown in [8], their influence on the measurement accu-

racy is not significant, therefore, the requirements related to the

Butler matrix’s parameters are not excessive. Nevertheless, the

impact of the additional directional coupler on the location of

the circle center related to Port #2 of the directional coupler has

to be investigated. The signal flow between Ports #1 and #2 of

the added directional coupler is shown in Figure 2. It is seen

that the imperfect isolation I and return losses R of the direc-

tional coupler are assumed. To increase the clarity of this inves-

tigation the imperfect isolations and nonideal impedance match

of the 4 3 4 Butler matrix (marked gray in Fig. 2) have been

neglected [8]. Therefore, the relation between the power meas-

ured at Port #2 of the coupler and the measured reflection coef-

ficient C, assuming reciprocity (Sij 5 Sji) is as follows:

p15P1

PIN

5

�����I1TCSgm

2C1Srm2CC

12R Sgm2C1Srm

2CC

� �����2

(1)

where T and C are transmission and coupling coefficients of the

coupler, respectively, Sgm and Srm are the particular S-parame-

ters of an ideal 4 3 4 Butler matrix and CC is the reflection

coefficient of the reflective element connected to Port 6. Assum-

ing that |R| << 1 (1) can be approximated as follows:

p15P1

PIN

ffi���I1aTC Sgm

2C1Srm2CC

� ���2 (2)

where

a511R Sgm2C1Srm

2CC

� (3)

It is seen that the introduced coefficient a depends on both

return losses R and on the measured reflection coefficient C.

Assuming the return losses of the utilized directional coupler

R 5 10 dB and an ideal 4 3 4 Butler matrix, for which

jSgm2j5jSrm

2j50:25, the magnitude of coefficient a does not

exceed the range of 0.921–1.079, and for R 5 20 dB this range

narrows to 0.975–1.025. It is seen that even in case of relatively

poor impedance match of the applied directional coupler, the

coefficient a is very close to the ideal value aideal 5 1, which

corresponds to the perfect impedance match of the utilized

directional coupler. Therefore, it can be said that the imperfect

impedance match of the external directional coupler has a negli-

gible impact on the circle center distribution of the proposed

reflectometer.

Apart from the impedance match of the utilized external cou-

pler, its imperfect isolation has to be investigated. Analyzing (1)

one can observe that the coupler’s imperfect isolation has a

direct impact on the power reading. Since the analysis shown in

the previous paragraph has revealed insignificance of the cou-

pler’s imperfect impedance match, in further consideration the

external coupler is assumed to be ideally matched. Therefore,

(1) can be rewritten:

p1 jTCSgm2j2jC2c1j2 (4)

where c1 is the circle center related to Port 1 of the Butler

matrix, expressed as follows:

c152I

TCSgm2

1Srm

2

Sgm2CC

�(5)

As it is seen, if the perfect coupler’s isolation is assumed,

the added directional coupler does not influence c1 and its loca-

tion results only from the utilized 4 3 4 Butler matrix. How-

ever, in case of imperfect isolation, the circle center c1 can be

deteriorated proportionally to the coupler’s isolation. Moreover,

this deterioration depends on the coupling coefficient of the

used coupler, therefore, the optimum coupling value has to be

determined. In general, the chosen coupling coefficient is a

compromise between two requirements. On one hand it is

desired to provide the maximum power to the Butler matrix,

which can be ensured by the minimum coupling, but on the

other hand the maximum power delivered to the power meter is

Figure 4 Measured magnitude (a) and phase (b) of the reflection coef-

ficient of shorted attenuators. Solid lines indicate the results of measure-

ments with the use of the proposed measuring system and dashed lines

represent the reference values obtained with the use of commercial VNA

TABLE 1 Measurement Inaccuracy of the Proposed Reflec-tometer vs. the Magnitude of the Measured Reflection Coeffi-cient |C|

|C| (dB) Magnitude error (dB) Phase error (deg)

22 0.15 0.6

24 0.18 0.7

26 0.19 0.8

212 0.27 1.3

220 0.7 3.2

226 1.2 6.5

232 2.4 12.0

2886 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 12, December 2014 DOI 10.1002/mop

also crucial (maximum coupling required). Both these require-

ments result from high dynamics of the measured power with

respect to the average dynamics of applied power detectors.

Taking into account both aspects it can be stated, that a 3-dB

directional coupler is the optimum solution. Assuming an ideal

Butler matrix and an ideal 3-dB directional coupler, it can be

shown that:

jTCSgm2j5 1

8(6)

Therefore (5) becomes:

c158Iej/1 1CCej/2 (7)

where u1 and u2 are the angles resulting from phase relations in

the 4 3 4 Butler matrix. It can be observed that the application

of a 3-dB directional coupler maximizes (6), which ensures the

minimum influence of the coupler’s isolation on the location of

circle center c1. Analyzing (7) one can notice, that the circle

center c1 is also directly scalable with |CC|, exactly in the same

way as three remaining circle centers [7], which allows to scale

the entire circle centers’ distribution in order to increase the

measurement accuracy. It has to be underlined, however, that

the location of c1 is sensitive to the isolation of the applied

direction coupler, which can result in a greater displacement of

circle center c1 in a practical realization in comparison to the

remaining circle centers.

3. EXPERIMENTAL RESULTS

The proposed measuring system has been built with the use of

the broadband 4 3 4 Butler matrix, presented previously in [10]

and the directional coupler shown in [11]. Both components

operate in a broad frequency range 1–5 GHz. As it has been dis-

cussed in the previous section, the relation between the meas-

ured reflection coefficient and the measured power at Port #2 of

the added directional coupler is similar to the ones related to the

remaining ports at which the power is measured. Therefore, it

can be described with general, well known form [1]:

pi5Pi

PREF

5qi

���� 11AiC11A0C

����2

(8)

where Pi (i 5 1–4) is the power measured by ith power meter,

PREF is the reference power measured at 8th port of the Butler

matrix and qi, Ai, and A0 are the calibration constants.

The proposed reflectometer has been calibrated using the cal-

ibration procedure shown in [12], which allows for calibration

of a reflectometer with an arbitrary number of ports, in the

entire operational frequency range of the used components, that

is, 1–5 GHz. To increase the measurement accuracy, the reflec-

tometer has been calibrated for four attenuation values (from 0

to 3 dB) of the shorted attenuator connected to 6th port of the

Butler matrix. Such an approach has allowed to scale the

obtained circle centers’ distribution for measurements of the

reflection coefficient [7]. The circle centers’ distribution

obtained for two attenuation values of the shorted attenuator are

shown in Figure 3, and distinct scalability is seen.

To verify the measurement accuracy, the reflection coeffi-

cients of seven shorted attenuators having attenuation 1-, 2-, 3-,

6-, 10-, 13-, and 16-dB have been measured. The reference

measurements have been performed using Agilent N5224A vec-

tor network analyzer. The measurement results together with the

reference measurements are presented in Figure 4. A very good

agreement in terms of both magnitude and phase is seen over

the entire frequency range. The measurement inaccuracy with

respect to the magnitude of the measured reflection coefficient

is presented in Table 1.

4. CONCLUSION

In this article, a novel broadband multiport reflectometer con-

sisting of a standard 4 3 4 Butler matrix and an additional

directional coupler has been proposed. The presented measuring

system features a great simplicity and provides a uniform distri-

bution of four circles, which can additionally be scaled for

enhancement of the measurement accuracy. The theoretical anal-

ysis of the imperfect directional coupler’s parameters is given.

The proposed reflectometer has been verified by reflection coef-

ficients’ measurements of a set of reflective elements. The

obtained results are very close to the measurements performed

using commercial VNA, proving the enhanced measurement

accuracy of the proposed multiport reflectometer.

ACKNOWLEDGMENT

This work was supported by the National Science Centre under

grant no. UMO-2013/09/N/ST7/01219.

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VC 2014 Wiley Periodicals, Inc.

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 12, December 2014 2887