|Saturday, 13 January 2007|
A phase shifter is an essential element, which controls the phase of a microwave signal in a phased array antenna. A wide variety of antennas are used to transmit and/or receive signals at microwave or millimeterwave frequencies. These signals (commonly referred to as radio frequency (RF) signals) often pass through phase shifters between a transceiver circuit and the radiating elements of the antenna. Antennas generally fall into two classes including omnidirectional antennas and steerable antennas. Omnidirectional antennas transmit and receive signals omnidirectionally, i.e., transmit signals to and receive signals from all directions. While omnidirectional antennas are inexpensive and widely used in environments where the direction of signal transmission and/or reception is unknown or varies, omnidirectional antennas have a significant disadvantage in that the power signal requirements of omnidirectional antennas are relatively high. Steerable antennas overcome the power requirement problems of omnidirectional antennas. Steerable antennas generally fall into two categories, mechanically steerable antennas and electronically steerable antennas. Mechanically steerable antennas use a mechanical system to steer an antenna structure. Most antenna structures steered by mechanical systems include a parabolic reflector element and a transmit and/or receive element located at the focal point of the parabola. Electronically steerable antennas employ a plurality of antenna elements and are steered by controlling the phase of the signals transmitted and/or received by the antenna elements. Electronically steerable antennas are commonly referred to as phased array antennas. A phased array antenna has a large number of radiating elements that emit phased signals to form a radio beam. The radio signal can be electronically steered by the active manipulation of the relative phasing of the individual antenna elements. The electronic beam steering concept applies to antennas used with both a transmitter and a receiver. Electronically scanned phased array antennas are advantageous in comparison to their mechanical counterparts with respect to speed, accuracy, and reliability. Phased array antennas have the features of high beam scanning (tracking) speed and low physical profile. Furthermore, phased array antennas can provide multiple beams so that multiple signals of interest can be tracked simultaneously, with no antenna movement. Phased array antennas are used in a number of applications, including both terrestrial and airborne radar, and satellite and mobile communications, where fast beam scanning is required or where mechanical rotation of the antenna is not practical and/or desirable. The ability to control these phase shifters determines the speed and accuracy of switching beams. With rapid switching of beams, for example, a radar system is able to perform multiple functions, allowing the radar to track numerous targets. The directivity provided by an antenna array reduces interference generated by a given radio connection with connections made to other access units operating within the same region, or cell, serviced by a particular base station. In order to accomplish the required directivity of the antenna array a number of components may be used to create the antenna beam. This may include switches, delay circuits, or phase shifters; the phase shifters provide the maximum control over the direction and shape of the resulting beam. A good performance and low cost phase shifter can significantly improve performance and reduce the cost of the phased array, which should help to transform this advanced technology from recent military dominated applications to commercial applications. Since the phase shifters are usually controlled electronically, the direction of the antenna main beam can be scanned very rapidly in comparison with a mechanically rotated antenna. In addition, a phased array requires no moving parts and can be constructed as a planar or conformal structure.
A phase shifter is a two ports circuit providing an output having specific phase difference according to an inputted microwave frequency and it is widely used for a phase array antenna, radar and a linearized amplifier. A typical phase shifter for shifting the phase of electromagnetic waves propagating on a transmission line includes a dielectric plate which is positioned with respect to a waveguide so as to be freely insertable into the waveguide. Phase shifter manufacturing technologies include those based on field-effective transistors (FETs), laser diodes, MEMS, interferometry, pin diodes, and ferrites. The phase shifter using semiconductor process is classified into an analogue phase shifter gaining continuous phase difference by using a varactor or a ferrite and a digital phase sifter gaining a binary phase difference by using a field-effective transistor (FET) or a pin diode. A passive phase shifter includes passive elements which provide a phase lag and a phase lead network and includes a pair of signal paths provided between an input terminal RF IN and an output terminal RF OUT with the upper one of the signal paths being through a high pass filter to provide phase lead or positive phase shift to a signal and the lower one of the signal paths being through a low pass filter to provide phase lag or negative phase shift to a signal. Typically, a pair of switches are used to couple a signal between the input and output terminals through a selected one of said filter networks. Often, a pair of field effect transistors are arranged to provide active switching elements of each one of said switches. Field effect transistors are employed in these applications because they are easily formed as part of monolithic integrated circuits unlike other types of active switching devices such as pin diodes. The phase shifting devices in modern phase array antenna systems are typically digital phase shifters. Although PIN diodes have very fast switching speeds relative to FETs and other switching devices, they require a holding current to maintain the PIN diode in a low loss "on" state. In a high power antenna array system, PIN diodes may consume a large amount of aggregate bias power to maintain each forward biased PIN diode at a sufficiently low resistance. PIN diodes and the FETs that are used as switches also have junction capacitances that limit their isolation and, hence, their performance for phase shifter applications at MMW frequencies. For some applications, microelectromechanical systems (MEMS) switch technology has become an attractive alternative to implement the necessary switching functions in phase shifter circuits and systems. A MEMS realized switching module consumes nearly zero bias current, which is much less when compared to PIN diode switching modules, and has significantly better insertion loss performance than the solid-state alternatives. However, MEMS switching devices have significantly lower switching speeds and lower power-handling characteristics compared with their solid-state counterparts, FETs and PIN diodes.
Toroid phase shifters are well-known devices in the field of microwave technology for shifting the phase of a microwave signal propagating along a microwave transmission line. Toroidal phase shifters are non-reciprocal phase shifters that use a toroid geometry consisting of a ferromagnetic toroid located within a section of waveguide. Toroid phase shifters, however, are difficult to fabricate at low cost because of the extruded and machined rectangular ferrite tubes used to provide a closed magnetic latching circuit and due to the tight dimensional tolerances involved. Ferrite toroid phase shifters are also well-known devices and have been extensively used in phased array antennas because their insertion loss is low, RF power handling is high, and drive power is low compared to solid state devices. Two types of electronic phase shifters are currently utilized for modern phased array antenna systems, which are ferrite phase shifters and solid state semiconductor phase shifters. Ferrite phase shifters generally fall into two categories: phase shifters enclosed within a waveguide structure and phase shifters built using transmission line microstrip configurations. Ferroelectric phase shifters are used to control the phase of an electromagnetic wave such as a microwave or a millimeter wave signal. This changing of the phase or phase shifting makes possible the steering of an electromagnetic beam without physically moving an antenna. The ferroelectric phase shifter is a microstrip circuit having a ferroelectric material interposed between a conductor line and a ground plane. The conductor line typically includes an impedance transformer for matching the impedances at the material interface between the nonferroelectric and ferroelectric materials. In this manner, the impedance transformer reduces signal reflections. A microwave signal input to the phase shifter emerges from the transformer and travels through the ferroelectric material between the conductor line and the ground plane. Signal controlled phase shifters are usually analog devices including a variable reactance, such as a varactor, having a value controlled by the amplitude of a voltage. Such phase shifters are frequently employed in voltage controlled variable frequency oscillators, such as are employed in phase locked loops.
The phase shifters for performing the phase shift function are classified into fixed phase shifters for outputting signals by shifting phases of input signals to fixed levels, and tunable phase shifters for shifting the phases of the input signals to desired levels. Tunable phase shifters include a ferroelectric substrate as the phase modulating elements. The permittivity of the ferroelectric substrate can be changed by varying the strength of an electric field applied to the substrate. Tuning of the permittivity of the substrate results in phase shifting when an RF signal passes through the phase shifter. The digital phase shifter is further classified into a switched line type, a reflection delay line type, a loaded line type, a switched network type and a switched coupler type. The switched line variable phase shifter comprises two line segments of different length selectively connected to the transmission line and the differential path length between them determines the amount of phase shift to be introduced. The transmission line is switched over from one line segment of the phase shifter to the other when a phase shift is introduced and switched back to the original line segment when the phase shift is removed. Differential phase shifters are commonly implemented using a switched-line configuration in which switching devices are used to switch a signal between alternate transmission paths. The alternate transmission paths, in turn, have different electrical lengths, and thus there is a difference in relative signal phase between signals propagated through the alternate transmission paths. The loaded line phase shifter comprises a pair of spaced-apart main stubs extending from the transmission line and a pair of extension stubs selectively coupled to the main stubs. The characteristic of the loaded line type has a minimum phase deviation at the center frequency.