JOURNAL OF TELECOMMUNICATIONS, VOLUME 11, ISSUE 2, DECEMBER 2011 27

Nmos Based Variable Phase Shifter for Phased Array Antenna Application Tapas Mondal, Milan Mazumdar, Rowdra Ghatak and Sekhar Ranjan Bhadra Chaudhuri Abstract—In this paper, we propose the design of a NMOS based reflective type variable phase shifter for phased array antenna systems pertaining to Dedicated Short Range Communication Services applications in the field of Intelligent Transport System at 5.88GHz. In this reflective type variable phase shifter, a quadrature microstrip hybrid coupler is used to avoid the undesirable impact of mismatch between the input and output impedances. To achieve the reflections, two symmetrical NMOS based circuits are connected at both the secondary and coupled arms of the branch-line hybrid coupler. The phase shift can be varied by controlling the capacitance value of the NMOS circuits. The phase shift can be controlled by changing the bias voltage up to 190° in the proposed circuit and up to 300° with additional complex circuitry. In either case, the DC power consumption is very less as well as the transmission loss is -7dB ±0.75dB. Hence the proposed circuit is well suited to construct the phased array antenna for Intelligent Transport System applications. Index Terms—DSRCS frequency band, Intelligent Transport System, NMOS circuit, Reflective type Phase Shifter.

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1 INTRODUCTION

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HASE shifters are indispensable components in numerous microwave circuit applications. In the variable phase shifter the insertion phase can be changed by controlling the bias voltage of the circuit. Ideal phase shifters provide low insertion loss and equal amplitude (or loss) in all phase states [1]. Normally phase shifters are reciprocal devices, which mean the phase of the signal is shifted in the same proportion in either direction. Several types of printed board containing the lumped elements as well as fully integrated MMIC type active and passive phase shifter designs have been demonstrated for specific applications. But in the passive phase shifters, they exhibit more insertion loss while providing better properties for large signals. The most common phase shifters are based on vector modulator, distributed, reflective-type, and switched-type topologies [2]. Among these, the reflective type phase shifter is very common due to several advantages like lower loss, better temperature stability and low sensitivity on process tolerances. The first reflective type phase shifter was proposed in [3] which was further improved in [4 – 5]. In this paper a reflective type phase shifter (RTPS) is proposed where the reflection is achieved by using a NMOS based variable reactance circuit. A microstrip quadrature hybrid coupler is used to avoid the input-output impedance mismatch in the circuit. It is intended to study the proposed phase shifter for Dedicated Short Range Communication Service (DSRCS) phased array antenna

application for Intelligent Transport System (ITS); hence the operating frequency is chosen at the frequency band of 5.850 – 5.925 GHz [6 – 8].

2 VARIABLE PHASE SHIFTER The insertion phase shift of any circuit can be modified by varying the real and imaginary part of the load. However, such modifications have a non-desired impact on the input and output impedance matching of the circuit. A quadrature microstrip hybrid coupler can be used to solve this problem. The basic topology of the proposed phase shifter is illustrated in Fig. 1.

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Tapas Mondal is with Dr B C Roy Engineering College, Durgapur, Fuljhore More, Jemua Road, Durgapur 713206, West Bengal, India. Milan Mazumdar is with Dr B C Roy Engineering College, Durgapur, Fuljhore More, Jemua Road, Durgapur 713206, West Bengal, India. Rowdra Ghatak is with National Institute of Technology Durgapur, West Bengal, INDIA. S R Bhadra Chaudhuri is with Bengal Engineering and Science University, Shibpur, West Bengal, INDIA.

Fig. 1. Basic topology of the proposed phase shifter.

© 2011 JOT www.journaloftelecommunications.co.uk

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2.1 Quadrature Microstrip Hybrid Coupler The signals are reflected at the load terminals which are isolated by means of the coupler from the input and output nodes of the phase shifter. A microstrip hybrid coupler is realized on 0.785mm thick RT Duroid substrate with 2.32 dielectric constant and 0.001 loss-tangents. With all ports matched, the signal entering into the input port (port 1) of the coupler is equally divided between the output ports of the through arms (port 2) and coupled arms (port 4), with a phase difference of 90°. As port 2 and port 4 is terminated with the symmetrical NMOS based variable reactance circuits, the signals reflected back from the port 2 and port 4 are added together in the isolated port 3 but no signal returns to port 1. 2.2 Reflective Terminations The phase shift of the RTPS can be controlled by varying the reflective impedance of the reactive load at port 2 and port 4 of the hybrid coupler. A variable capacitor based symmetrical LC network is placed at port 2 and port 4 to reflect the RF signal to the isolated port. The phase of the reflected RF signal can be varied by tuning the LC network. A Schiman-Hodges MOSFET with the drain and source connected together can be used as varactors as shown in Fig. 2. With only capacitance variation, a maximum phase control range of 180° can be reached. By tuning the capacitance value of the varactor with an inductor L, in series, the phase variation can be significantly improved.

Fig. 3. Layout diagram 1 of NMOS based reflective type phase shifter.

To simplify the analysis, it is assumed that the microstrip hybrid coupler and the LC network are both lossless by neglecting the parasitic resistance. Furthermore, by neglecting the fixed time delay and the effect of the finite isolation among port 1 and port 3 of the hybrid coupler, the insertion phase variation of the circuit is found proportional to the reflection coefficient of the reflective loads as obtained using (1).

S31 =

Z LC - Z0 Z LC + Z0

(1)

Where, ZLC and Z0 refer to the LC network impedance and the characteristics impedance of the hybrid coupler, respectively. ZLC is given by ZLC = j2 (ωL – 1/ωC)

(2)

So the total amount of phase shift for a particular bias voltage condition of the varactors is

φ = ∠ S31 = ±π - 2tan -1

ωL – 1 / ωC Z0

(3)

From (3) it is clear that by tuning either L or C in the LC network can change the phase shift. Total amount phase variation of the RTPS can be found as

Δφ = 2 tan-1 Fig. 2. Schematic diagram 1 of NMOS based reflective type phase shifter.

Z max Z - tan-1 min Z0 Z0

(4)

Where, Zmax = j2 (ωL – 1/ωCmin) and Zmin = j2 (ωL – 1/ωCmax). Hence (4) clarifies that to maximize the phase variation, the variation of capacitance value of the varactors should be maximum. The hybrid coupler and the NMOS

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circuit is connected through 1pF DC blocking capacitor and a power divider network helps to feed the varying bias voltage to the NMOS circuit. A high frequency filtering capacitor is also connected in parallel with the bias voltage to bypass the unwanted high frequency signals. Using the above circuit, only 190° phase variation is achieved and hence to increase the phase variation two MOSFETs are connected in parallel with the varactor configuration as shown in Fig. 4. The parallel configuration of two capacitors are added together to increase the impedance variation in between Zmax and Zmin.

3 RESULTS AND DISCUSSION A quadrature microstrip hybrid coupler is designed on 0.785mm thick RT Duroid substrate with dielectric constant 2.32 and 50Ω characteristics impedance at all ports. The operating frequency is chosen at 5.88GHz and the corresponding simulated results are shown in Fig. 6 and Fig. 7. From Fig. 6, it is clear that the hybrid coupler is resonant at 5.88GHz and less than -23 dB return loss has been achieved with more than 27 dB isolation in between port 1 and port 3. Fig. 6 and Fig. 7 are also demonstrated that the output power at port 2 and port 4 are approximately half of input power means -3dB and they are orthogonal to each other. So this branch-line hybrid coupler can be used to isolate the input output port of the reflective type phase shifter as well as it also helps to maintain the 50Ω characteristics impedance at both the ports of the RTPS.

Fig. 4. Schematic diagram 2 of NMOS based reflective type phase shifter. Fig. 6. Scattering parameters of the Microstrip Hybrid Coupler.

Fig. 5. Layout diagram 2 of NMOS based reflective type phase shifter.

Fig. 7. Phase response between the output ports (port 2 and port 4) of the Microstrip Hybrid Coupler.

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The NMOS circuits are connected at the output ends of the microstrip hybrid coupler in two different configurations as shown in Fig. 2 and Fig. 4. The phase variation at a particular bias voltage at different operating frequency and the phase variation at a particular operating frequency at different bias voltage of schematic 1, using two NMOS devices are portrait at Fig. 8 and Fig. 9. From Fig. 8, it is observed that at 210mV bias voltage the phase shift is different for different operating frequencies. Fig. 9 confirms that by varying DC bias voltage from 0mV to 1000mV, the insertion phase shift can be controlled from 55° to -135° with a little variation of insertion loss in the entire range.

In the first case by changing the bias voltage total 190° phase shift is achieved but after increasing the circuit complexity in the second case by changing the bias voltage from -600mV to -100mV total 300° phase change is obtained as shown in Fig. 11. Similarly from Fig. 10, it is found that in the second circuits also at -420mV bias voltage the phase shift is different for different operating frequencies. In both the case, the DC power consumption of the whole circuits is negligible.

Fig. 10. Variation of phase shift at different operating frequency of the schematic diagram 2. Fig. 8. Variation of phase shift at different operating frequency of the schematic diagram 1.

Fig. 9. Variation of phase shift at different bias voltage of the schematic diagram 1.

Fig. 11. Variation of phase shift at different bias voltage of the schematic diagram 2.

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From Fig. 12 and Fig. 13 it is clear that the return loss of whole circuit, schematic diagram 2, is less than -50dB at 5.88GHz and in the entire bias voltage range the transmission loss is -7dB ±0.75dB.

4 CONCLUSION A reflective type phase shifter using NMOS technology is presented in this paper. The RTPS has been designed for smart phased array transceivers at DSRCS band centered around 5.88 GHz; using microstrip quadrature hybrid coupler with the influence of NMOS based reflective loads in two different configurations. By optimizing the reflective loads, the phase control of ranges more than 300° have been achieved. In both the configurations, the DC power consumption of the whole circuit is negligible. The transmission loss of the phase shifter is -7db ± 0.75dB in the entire 300° phase variation range. Hence the proposed circuit is well suited to construct the phased array antenna for Intelligent Transport System applications.

REFERENCES [1]

Fig. 12. Return loss and transmission loss at different operating frequency of the schematic diagram 2.

Wagner J, Mayer U, Ellinger F, “Passive transmission line phase shifter at C-band in CMOS using lumped elements,” International Conference on Microwaves, Radar and Wireless Communications, MIKON, pp. 1- 4. 2008. [2] Ellinger F, Mayer U, Wickert M, Joram N, Wagner J, Eickhoff R, Santamaria I, Scheytt C, Kraemer R, “Integrated Adjustable Phase Shifters,” IEEE Microwave Magazine, vol. 11, Issue 6, pp. 97 – 108, 2010. [3] R N Hardin, E J Downey, J Munushian, “Electronically variable phase shifter utilizing variable capacitance diodes,” Proccedings of IRE, vol. 48, pp. 944–945, May 1960. [4] M E Bialkowski, N C Karmakar, “An L-band 90° hybrid coupled phase shifter using UHF band p-i-n diodes,” Microwave Optical Technology Letter, vol. 21, no. 1, pp. 51–54, Apr. 1999. [5] D M Klymyshyn, S Kumar, “A simple GMSK modulator for microwave and millimeter-wave frequencies,” Microwave Journal, vol. 42, no.3, Feb. 1999. [6] Antony Tang, Alice Yip, “Collision avoidance timing analysis of DSRCbased vehicles,” Accident Analysis & Prevention, Vol. 42, No. 1 pp. 182195, Jan. 2010. [7] Tapas Mondal, J. S. Roy, S. R. Bhadra Chaudhuri, “Phased Array Antenna Design for Intelligent Transport Systems,” IEEE International Workshop on Antenna Technology: Small Antennas and Novel Metamaterial (iWAT), March 2009, p. 1 – 4. [8] Tapas Mondal, R. Ghatak, S. R. Bhadra Chaudhuri, “Design and Analysis of a 5.88GHz Microstrip Phased Array Antenna for Intelligent Transport Systems,” IEEE International Sumposium on Antennas and Propagation and CNC/USNC/URSI Radio Science Meeting, July 2010, p. 1 – 4. Tapas Mondal received the M.E. degree in Microwave Engineering from Birla Institute of Technology, Mesra, Ranchi, in 2003. He is currently working toward the Ph.D. degree at the ETC Department, Bengal Engineering and Science University, Shibpur, West Bengal, INDIA. His research interests include Antenna, Wireless communication and Intelligent Transport Systems applications. Presently, he is Associate Professor in the Department of Electronics & Communication Engineering, Dr B C Roy Engineering College, Durgapur, West Bengal, India. He is a member IEEE and life member of ISTE.

Fig. 13. Variation of transmission loss at different control voltage of the schematic diagram 2.

Milan Mazumdar received the AMIETE degree from IETE, New Delhi in 2008 and M.Tech degree in Modern Communication Engineering from West Bengal University of Technology, in 2011. His research interests include Antenna, Wireless communication and Nonlinear Dynamics. He is a member IETE. Rowdra Ghatak received his MSc (Physics) with specialization in Radio Physics and Electronics, M.Tech (Microwave Engineering) from The University of Burdwan and PhD (Engg) from Jadavpur

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University in ’99, ’02 and 08 respectively. He currently holds a faculty position in the Electronics and Communication Engineering Department of National Institute of Technology Durgapur. He is a recipient of the URSI Young Scientist Award in 2005 and DST Fast Track Research support for Young Scientist in 2010. He has more than 70 publications in various National/International conferences and journals. He is a member IEEE, member IETE and life member ISTE. Sekhar Ranjan Bhadra Chaudhuri was born in Kolkata, West Bengal, India. He received his B.E. & M.E. degree in Electronics & Telecommunication Engineering from the then Bengal Engineering College, Shibpur under Calcutta University (C.U.) in seventies and M.B.A. degree from IISWBM under C.U. & Ph.D. (Engineering) degree from Jadavpur University, Kolkata, India in eighties. He is a Professor in the Department of Electronics & Telecommunication Engineering, Bengal Engineering & Science University (BESU), Shibpur, West Bengal, India. His research interest is in the area of Microwave & Communication in general & Planar & Non Planar Antennas for Wireless Communication in particular. He has guided 3(three) PhD scholars. At present 6(six) doctoral scholars are working at BESU under his guidance. He has 80 research publications in Engineering/Technology in various International/National Journals & Conferences. He is a Life Member of ISTE & Member of IEEE, USA.