RF MEMS phase shifters

RF MEMS phase shifters

We recently spoke with Dr. Koen Van Caekenberghe, author of several articles on RF MEMS technology for radar sensors, about RF MEMS phase shifters. Koen shares his thoughts on the small but growing RF MEMS phase shifter market including applications, market developments, pricing and vendors of RF MEMS phase shifters as well as alternative technologies.

The radar sensor market has a global turnover of about $6.25 billion annually according to Defense Industry Daily. In Koen's opinion, approximately 50% of the budget is spent on airborne, ground-based, and naval AESA radar sensors, and approximately 25% of the budget is spent on mechanically scanned radar sensors -- and during the next decade, 20% of the mechanically scanned radar sensors might be replaced by PESA radar sensors based on RF MEMS shifters, resulting in a potential global market of $300 million annual

MEMS Investor Journal: Please provide a general description of RF MEMS phase shifters.

Koen: RF MEMS phase shifters alter the phase of an RF signal by means of RF MEMS switches, switched capacitors, and varactors [1, 2]. Phase shifters are used in radars based on electronically scanned arrays.

MEMS Investor Journal: How do radars work?

Koen: Radars sense angle, range and velocity of (moving) scatterers in the environment. Radar figures of merit include field of view in terms of solid angle and maximum unambiguous range and velocity, as well as angular, range and velocity resolution. The angle of a target is detected by scanning the field of view with a directive beam. Scanning is done electronically, by scanning the beam of an array, or mechanically, by rotating an antenna. The range and radial velocity of a target are detected through frequency modulation (FM) ranging and range differentiation (frequency modulated continuous wave radar), or through pulse delay ranging and the Doppler effect (pulse-Doppler radar). The angular resolution is inversely related to the half power beamwidth of the antenna or the array, whereas the range resolution is inversely related to the signal bandwidth.

MEMS Investor Journal: As you mentioned, RF MEMS phase shifters are used in radars based on electronically scanned arrays. What are the main advantages of using them?

Koen: Electronically scanned arrays, or phased arrays, offer several advantages over mechanically scanned antennas such as multiple agile beams and interleaved radar modes. Figures of merit of an electronically scanned array, as shown in Fig. 1, are the bandwidth, the effective isotropically radiated power (EIRP) times the Gr/T product, the field of view, the half-power beamwidth, the pointing error, the polarization purity and the sidelobe level. EIRP is the product of the transmit gain, Gt, and the transmit power, Pt. Gr/T is the quotient of the receive gain and the antenna noise temperature. Gr and Gt are linearly related to the aperture area, whereas the half power beamwidth is inversely related to the largest aperture dimension. The field of view is limited by the antenna element spacing, d, and the pointing error is inversely related to the phase shift resolution (number of effective bits of the phase shifter).

Figure 1: Figures of merit of an electronically scanned array set the radar sensor's ability to search and track targets.

MEMS Investor Journal: What is the history of RF MEMS phase shifters and where were they first developed?

Koen: RF MEMS phase shifters were pioneered by HRL, Malibu, CA [3], Raytheon, Dallas, TX [4], Rockwell Science, Thousand Oaks, CA [5], and the University of Michigan, Ann Arbor, MI [6], during the nineties. Since then loaded-line, reflection, switched LC network and switched-line phase shifter designs have been implemented using RF MEMS switches, switched capacitors and varactors, as shown in Fig. 2. The switched LC network phase shifter is the most common phase shifter. RF MEMS distributed loaded-line and switched-line true-time-delay phase shifters will enable ultra wideband (UWB) radar sensors, whereas RF MEMS reflection phase shifters will find application in reflect arrays; a reflect array is a particular embodiment of a PESA.

Figure 2: Loaded-line, reflection, switched LC network, and switched-line phase shifter designs have been implemented using RF MEMS switches, switched capacitors and varactors.

MEMS Investor Journal: Are RF MEMS phase shifters an extension of or an improvement on an existing technology? If so, can you describe for our readers the features and benefits as compared with existing systems?

Koen: While most RF MEMS switches, switched capacitors and varactors are biased electrostatically instead of magnetostatically, RF MEMS technology can be thought of being a microscopic extension of electromechanical relay and switch technology, which dates back to the 19th century [7]. The application of electromechanical relay technology is limited to the VHF band (30-300 MHz), which confines its application to tunable filters for multi-band VHF communication equipment such as used in public safety 2-way radio networks. RF MEMS technology enables the use of a broader RF spectrum, ranging from the VHF band to the W-band (75-110 GHz), with a corresponding increase in communication and sensing applications.

RF MEMS phase shifters offer lower insertion loss, and higher linearity and power handling than semiconductor phase shifters, enabling passive electronically scanned arrays (PESAs) with higher EIRP x Gr/T product and longer range detection. They do not consume prime power, but require a high control voltage and wafer-level packaging.

MEMS Investor Journal: How are RF MEMS phase shifters used today and what are the various markets in which they find application?

Koen: RF MEMS phase shifters will find application in airborne and space-borne PESA radar sensors, which require low prime power consumption but do not require long-range search and track capability. The low-altitude unmanned aerial vehicles (UAV) radar sensor market, for example, offers potential for RF MEMS phase shifters.

In general, the choice between an active electronically scanned array (AESA) and a PESA is determined by the range requirement. An AESA has distributed power amplification because every antenna is connected to a T/R module. An AESA therefore has a higher EIRP x Gr/T product (dynamic range) and better search and track capabilities than a PESA. A PESA has centralized power amplification, but offers cost, prime power consumption, size and weight savings, as shown in Fig. 3.

Some airborne platforms, such as fighter jets, have a dual need. For example they have a high-performance nose-cone AESA radar sensor to search and track agile targets, and a low-power pod-mounted PESA radar sensor underneath to measure the height, to follow (avoid) terrain, or to map the ground during a low fly over. The use depends on the range of the envisioned target.