Design of Compact Reconfigurable Switched Line ... - IEEE Xplore

2012 1st International Conference on Emerging Technology Trends in Electronics, Communication and Networking

Design of Compact Reconfigurable Switched Line Microstrip Phase Shifters for Phased Array Antenna Puneet Anand, Sonia Sharma*, Deepak Sood, C.C.Tripathi**

ECE Department, University Institute of Engineering and Technology, Kurukshetra University Kurukshetra, Haryana, India [email protected]**, [email protected]*, [email protected] Wireless communication systems are evolving toward multifunctionality. This multi functionality and reconfigurability provides users with options of connecting to different kinds of wireless services for many purposes at different times by providing additional degrees of freedom in the system e.g. cognitive radio, MIMO systems and high performance phased arrays. Although Phase shifters are important component in many RF microwave subsystems e.g. radar, phased array antennas, beam forming networks, frequency translators yet the reconfigurable phase shifter provides additional degree of freedom in these systems by reconfiguring the phase shift at a given frequency. In this paper design and simulation of reconfigurable switched line phase shifter for phased array antenna using Ansoft HFSS 14 is presented. The reconfigurability is achieved by loading phase shifter using high frequency PIN diodes which acts as microwave switches. A compact reconfigurable phase shifter has been designed which gives a very low insertion loss ≈ -1 dB and wide range of linear reconfigurable phase shift over frequency from 1 to 5 GHz. The designed phase shifter gives reconfigurable phase shift for different application i.e. at 1.8 GHz (GSM), 2.4GHz (Wi-Fi), 3GHz (traffic control and collision avoidance radars), 3.6GHz (WLAN) by appropriate settings of the 8 switch positions. As designed phase shifter can be used for 3600 beam steering in phased array antenna.

KEYWORDS: Switched line phase shifter, Phased array antenna, Beam-steering, Reconfigurable. I. INTRODUCTION Beam-steering antennas are the ideal solution for a variety of system applications including traffic control, regulation and collision avoidance radars (S-band 3 GHz) installed on most ocean going ships to provide better detection of ships in rough sea and heavy rain condition[1,2], Beam-steering antennas also used in smart base station antennas for WLAN and cellular communication. Beam-steering is most commonly achieved through phased arrays, where phase shifters are used to control the relative main-beam of antenna array. Many antenna system applications require that the direction of the beam's main lobe be changed with time, or scanned. This is usually done by mechanically rotating a single antenna or an array with fixed phase to the element. However, mechanical scanning requires a positioning system that can be costly and scan too slowly. For this reason, electronic scanning antennas which are known as phased array antennas are used. It can Sweep the direction of the beam by varying electronically the phase of the radiating element, thereby producing a moving Pattern with no moving parts. Phased array antennas are

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electrically steerable, which means the physical antenna can be stationary [3, 4]. For this by placing a phase shifter on each radiating element allows the antenna beam to be electronically steered without physically moving the antenna element as shown Fig1.

Fig1: A Standard phased array antenna system

A. Phase shifter The fundamental function of a phase shifter circuit is to produce a replica of the signal applied at its input, but with a modified phase [6, 7]. Its performance is characterized by its insertion loss, bandwidth, power dissipation, power handling capability, and insertion phase. Depending on the nature of the insertion phase (i.e., whether switchable continuously or in discrete steps), phase shifters are further classified into analog and digital, respectively [7, 8, 9]. Analog phase-shifters change the output phase by means of an analogue signal (e.g. voltage) to provide a continuously variable phase. Digital phase shifters use a digital signal to change the output phase, providing a discrete set of phase states that are controlled by two-state phase bits. The highest order bit is 180 degrees, the next highest is 90 degrees, then 45 degrees, etc, as 360 degrees is divided into smaller and smaller binary steps. For instance, a six bit digital phase shifter would have a 5.625 degree least significant bit. Phase shifters can be controlled electrically, magnetically or mechanically etc. Electrically controlled analog phase shifters can be realized using varactor diodes that change capacitance with voltage, or nonlinear dielectrics such as barium strontium titanate, or Ferro-electric materials such as yttrium iron garnet [5]. A mechanically-controlled analog phase shifter is just a mechanically lengthened transmission line, often called a trombone line. In order to understand various design method to implement a phase shift we need to review different methodology used for phase shifter such as loaded line phase shifter, reflection type phase shifter, switched line phase shifter etc. All the above three techniques have been


tried in past to design phase shifter for various application to achieve system compactness, high quality factor etc. B. Switched Line Phase Shifter Switched Line Phase shifter [8, 10] is the most straight forward approach for microstrip phase shifter because it uses the simple time delay difference between two direct paths to provide desired phase shift. The simplest switched line Phase Shifter is dependent only on the lengths of microstrip line used. One of the two transmission lines is labeled as a “reference” line, and the other as a “delay” line. An important advantage of this circuit is that the phase shift will be approximately a linear function of frequency, getting a wideband frequency range of the circuit. In the switched line phase shifter, the incoming input signal is routed/ switched through one of many alternate paths to the output, so as to introduce specific phase shifts with minimum loss [10, 11]. The fundamental unit (1 bit) of the switched-line phase shifter is shown in Fig 2. Switches S1, S2, S3, S4, are single-pole double-throw (SPDT) switches. By their proper activation, the incoming signal is routed either through the short-length path, via S1 and S3, or through long-length path, via S2 and S4, thus endowing it with commensurate delays. The differential phase shift is given by ∆ . The switching elements in digital phase shifters are: mechanical switches (or relays), PIN diodes, Field Effect Transistors (FET), or microelectromechanical switches (MEMS). PIN diodes are common used in Phase Shifters due to their fast switching time, low loss, and relative simple bias circuits, which provides changes of PIN resistance approximately from 10 kilo-ohms to 1 ohm.

uses eight switches and four transmission lines i.e. upper microstrip line section of length l0 =83.206mm; lower U shaped microstrip line section of length l1 =59.034mm; circular transmission line section of length l2=40.264mm ; straight microstrip line section of length l3=24.18mm. Only one arm should be ON at a time and any line can be chosen as reference line and rest are delay lines. Any reconfigurable differential phase value from 0° to 360° can be achieved by appropriate settings of the 8 switch positions as listed in table 1,2,3,4. Typically, to avoid the phase errors the isolation of the switches must exceed -20dB in the required frequency band. By switching the signal between two pre-determined lengths of transmission lines when the incoming input signal is routed/ switched through the reference and any one of the delay line with appropriate switches position, it is possible to realize a specific phase shift (ΔΦ) at a given frequency where 2 ⁄ and λ is the ∆ guided wavelength.

Fig 3: Reconfigurable switched line phase shifter

III. RESULT AND DISCUSSION

Fig 2: Different configurations of switched-line phase shifters

II. DESIGN The proposed design is simple and compact as compared to conventional cascaded 3stage switched line microstrip phase shifter. In this design phase shifter functionality has been achieved without using any impedance matching network, as entire structure has been built in single stage by using semi circular arc section, U-shaped microstrip line and straight line microstrip line with diode switching reconfigurability. There is restriction in designing the number of microstrip section based upon the consideration of mutual coupling effect between adjacent section and area minimization. Schematic of reconfigurable switched-line microstrip Phase Shifter that is interconnected through RF MEMS switches as shown by Fig 3. Proposed phase shifter is simulated on a substrate having ε =2.6, height h=1.27mm, characteristic permittivity impedance Z0=50 ohm, width of microstrip line =3.513mm. Reconfigurable switched-line microstrip Phase Shifter which

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The value of insertion loss has been observed to nearly -1dB and return loss is below -20dB from the simulated results as shown in Fig 4 and 5.

Fig 4: Insertion loss |S21| (dB) vs. frequency (GHz)

The phase shift obtained from the Fig 6 at 1.8GHz, 2.4GHz, 3GHz, 3.6GHz are tabulated in table 1, 2, 3, 4. In order to obtained reconfigurable phase shift we can choose from any


one of them l0, l1, l2, and l3 as a reference line and rest are the delay line. At 1.8 GHz for GSM application we obtained 12 different reconfigurable phase shift by appropriate setting of switches. By choosing l0 , l1 ,l2, and l3 as a reference line one by one and rest of other are delay line we can obtain no of different phase shift as shown in table 1. At 2.4 GHz, 3GHz and 3.6GHz for WI-FI, Traffic control and collision avoidance Radar and for WLAN applications we obtained 48 different reconfigurable phase shift; 12 for each application by appropriate setting of switches as shown in table 2, 3 and 4. As designed phase shifter can be easily integrated with phased array in order to change the direction of main beam without altering phased array parameters such as gain, resonating frequency, directivity etc.

Ref. line l0 l1 l2 l3

Ref. line l0 l1 l2 l3

TABLE 3 Phase shift obtained at 3GHz l0 l1 l2 (S0,S00=0N) (S1,S11=0N) (S2,S22=0N) --

212.140

0

-212.1 -70.90 -170.20

--

141.20 41.90

170.240 -141.20 --

-99.30

TABLE 4 Phase shift obtained 3.6 GHz l0 l1 l2 (S0,S00=0N) (S1,S11=0N) (S2,S22=0N) --

0

172.5 -15.40 -134.20

-172.50 --187.90 -306.70

134.20 187.90 --

-118.80

l3 (S3,S33=ON)

70.940 -41.90 99.30 --

l3 (S3,S33=ON)

15.40 306.70 118.80 --

IV.CONCLUSION Reconfigurable switched line Microstrip phase shifters has been successfully designed and simulated which have many advantages like compact structure, reconfigurable linear phase shift over wide range of frequency. The designed reconfigurable phase shift is used for beam steering in phased array antenna at 1.8 GHz (GSM), 2.4GHz (Wi-Fi), 3GHz (traffic control and collision avoidance radars), and 3.6 GHz (WLAN) application.

Fig 5: Return loss |S11|(dB) vs. frequency (GHz)

V. References

Fig 6: Phase angle |S21| (degree) vs. frequency (GHz) TABLE 1 Phase shift obtained at 1.8 GHz taking l0,l1, l2, l3 each as reference line Ref. line l0 l1 l2 l3

Ref. line l0

l0 (S0,S00=0N)

l1 (S1,S11=0N)

l2 (S2,S22=0N)

l3 (S3,S33=ON)

TABLE 2 Phase shift obtained 2.4 GHz l0 l1 l2 (S0,S00=0N) (S1,S11=0N) (S2,S22=0N)

l3 (S3,S33=ON)

-86.80 170.40 112.60

--

71.40 -83.60 25.80

l1 l2

116.3 228.80

-116.30 -112.50

l3

150.80

34.50

0

1550 -83.60 --57.80

-150.80 -112.50 --780

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97.20 -25.80 57.80 --

-228.80 -34.50 780 --

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