New and increasingly sophisticated threats are driving requirements for radar systems that deliver more instantaneous bandwidth, greater resolution, longer range and multi-beam function.
Traditionally, radar systems have had short pulse widths, narrow instantaneous bandwidths and relatively small duty cycles (e.g., 100 us pulse width and 10% transmit duty cycle). Today, there are requirements across all radar bands for 3x to 5x longer pulse widths and ≥ 50% duty cycle. In some cases, the requirements have been for near-continuous wave operation. Radar system requirements are pushing for more RF output power per element with minimal change in cooling requirements.
To support these system level requirements and reduce system operating costs, the RF hardware must have higher transmit output power and power-added efficiency (PAE), better thermal dissipation and lower receive path noise figure.
How These System Requirements Relate to the RF Module
A challenge for the component supplier is translating system-level needs to component-level capabilities. Generally, higher transmit power and lower receive noise figures equate to greater range and higher resolution—the radar can “see” a smaller target farther away and give the operator more time to react.
The downside of higher power along with near-continuous wave operation is more dissipated heat. In order to reduce the impact of increased heat on performance, the components must have better thermal dissipation and higher PAE. The desire for increased instantaneous bandwidth means more complex design, increased losses and sacrifices in performance that could be achieved in a narrower frequency band application.
A mix of LDMOS (lower frequency operation), GaAs, SiGe and GaN products are being used in radar systems. LDMOS technology is mature, has high PAE and power density for the transmit path, and good thermal dissipation, but generally only supports relatively narrowband operation at S-band and below. LDMOS generally has lower recurring costs for the component level, but requires board-level matching and additional surface mount components. Fully matched LDMOS components for radar applications are rare on the open market.
SiGe technologies allow for large-scale integration of RF and DC functions, low power operation, small component size and wide bandwidths, higher frequency bands and lower recurring costs. In contrast, SiGe has low power density and high non-recurring costs. In the receive path, SiGe components have higher noise figures than both GaN and GaAs technologies. The SiGe technology is well suited for low-power, shorter-range radar applications, signal control functions, large-scale phased arrays and/or high-volume applications.
Mature GaAs HEMT technologies can support the bandwidth and the higher frequency bands, but have lower power density compared to GaN. GaAs HEMT remains a viable solution for both transmit and receive components where lower transmit power per element is viable and receive chain noise figure is key.
GaAs HEMT gate lengths continue to decrease, allowing for lower noise figures, which improves the radar resolution, range and sensitivity. Smaller gate lengths may improve RF performance, but the cost is ESD sensitivity and input power survivability. When higher transmit power is required, the GaAs HEMT device solution is to increase gate periphery by adding gates or by increasing the individual gate width. Increasing individual gate widths will limit the operating band of the device and stress the manufacturability of the product. Adding gates to the FET stack complicates the matching circuit, increases design and manufacturing risks and adds insertion loss.
GaN-on-SiC HEMT technologies have higher power density compared to GaAs HEMT, support wider bandwidths in all the operating bands of interest, use a thermally superior substrate (SiC) and are quickly being adopted by the radar markets as the PA solution of choice.
With the higher power density, matching circuit combining structures are simpler and lower loss vs. GaAs HEMTs. Like LDMOS, GaN operates at higher voltages compared to GaAs and SiGe. GaN-on-SiC HEMTs are viable solutions for high input power, robust LNAs. The noise figure of the GaN device is similar to a GaAs LNA with an input protection limiter.
The Need for a New Approach
To meet new radar system demands, the products being designed today must meet tougher Size, Weight, Power and Cost (SWaP-C) requirements, and have higher PAE, lower channel temperatures and lower noise figures. SWaP-C is especially important in air and space-based systems where physical space and weight are at a premium.
Higher PAE translates to less prime power, lower cooling requirements and lower operating costs. Specifically, in X-band, there are products now on the open market that are pushing >40% higher efficiency with 3 GHz of bandwidth. At L- and S-band radar frequencies, PAEs were in the 50% to 60% range for a discrete GaN HEMT—now products are pushing to 75% and higher. Multi-stage S-band MMICs are approaching 60% efficiency. In the past, these products were not available commercially due to technologies and design capabilities.
The SiC substrate used for GaN is the ideal enabler for the transmit portion of next-generation radars. SiC has higher thermal conductivity when compared with GaAs or Si. GaN-on-SiC transistors can operate at a higher channel temperature than GaAs or LDMOS transistors for the same Mean-Time-to-Failure value.
GaN transistors have operating voltage capability that is 2-5X higher than GaAs transistors. GaN operates between 20V and 50V. Higher operating voltages mean lower I^2*R losses are possible. Higher voltage operation also means fewer voltage step conversions between the power supply and the RF devices. These advantages translate to size reductions, less weight, fewer components, lower cost and increased system performance.
GaN offers a 3-5X increase in power density versus GaAs and an even greater increase vs. SiGe. Increased power density means fewer components and size reductions.
From a radar antenna pattern perspective, when the Equivalent Isotropically Radiated Power (EIRP) remains constant, the PA power per channel increases as the number of elements in the array decrease. More transmit power can translate to fewer elements, smaller size and less complexity.
Use of GaN-on-SiC technology also opens the door to a range of manufacturing options with SWaP-C benefits:
- High thermal conductive materials for die attach and assembly, for increased thermal dissipation (weight and power)
- Direct Cu attach vs. CuMoly or CuW composites, for the highest thermal conductivity possible
Integration of the PA and LNA into a single, front-end module (FEM) further reduces unit size and number of components. For example, designers working with traditional radar architectures often have five to seven components per channel plus all of the associated peripheral resistors and capacitors.
Replacing the traditional RF Tx/Rx module with an integrated GaN-only FEM or GaN/GaAs FEM reduces the number of components to one. This is a significant change in the BoM complexity of the board, simplifying designers’ efforts to place the component closer to the antenna to reduce loss and provide higher dynamic range. Designers can create higher-density arrays and achieve greater range for the same power budget.
When the FEM is combined with a SiGe or GaAs “core” circuit, further component count reduction can occur. The core chip can replace the phase, attenuation and control circuits for one or more radar elements, depending on the radar architectures. It is feasible to see a greater than 50% component reduction for a 4-channel sub-array with the right combination of GaN, GaAs and SiGe.
Below are a few examples from Qorvo of newer GaN-on-SiC products where high PAE, continuous wave operation and SWaP-C were primary design requirements in order to support next-generation military and civilian radar applications:
The Qorvo QPA1022 high-performance power amplifier supports X-band phased arrays. Built on the company’s 0.15 um GaN on SiC process (QGaN15), this amplifier is an integrated, 4 x 4 x 0.85 mm package that can support tight lattice spacing requirements for phased array radar applications. In this PA, GaN technology enables best-in-class PAE of 45% at 4 watts RF power in the 8.5-11 GHz range. This is an increase in efficiency by 8% over previous products while providing 24 dB large signal gain.
The Qorvo QPD1006 supports 1.2-1.4 GHz frequency L-band applications and is capable of continuous wave operation at high voltages—45V CW and 50V pulsed. Since it is fully matched to 50-ohms at the input and output, it supports a smaller module design and fewer components on the system board vs. the typical footprint for an unmatched FET design. The design has 55% drain efficiency for CW operation; 62.2% pulsed operation, with > 300 W CW power and > 450 W pulsed power.
To meet and defeat a new generation of threats, defense radars must leverage sophisticated RF technology to achieve higher efficiencies and greater bandwidth.
Designers are turning to GaN technology to deliver these operational enhancements, as well as SWaP-C improvements that are critical in the harsh, space-constrained environments where these systems operate. While GaN may be a relatively young technology compared to others, it continues to mature as a process and adopted for radar applications.
Further, marrying GaN PAs with GaAs LNAs into single front-end modules creates highly integrated, multifunction components that provide advanced capabilities. System operating and manufacturing costs decrease as component count reduces and PAE increases. These capabilities translate into maximum power with minimum heat, higher reliability and lower cost of operation.
About the Author
Greg Clark has more than 25 years of experience in the Defense and Aerospace industry, with 15+ years in microwave IC design with a focus on the development of highly integrated, multifunction circuits that include amplifier, phase, attenuation and switch design. Mr. Clark is presently a Senior Account Manager for Defense and Aerospace Markets at Qorvo. He has a BS in Electrical Engineering from Texas A&M University and an MS in Electrical Engineering from Southern Methodist University.