Defence

The march of GaN for RADAR arrays

New RADAR systems have increasingly utilised active electronically scanned arrays (AESAs) for their radiating and receiving functionality. AESAs offer several advantageous features compared with other radar systems designs, such as jam resistance, frequency agility and graceful degradation.
Design methodology is also pertinent to other applications such as communications.
Legacy systems developed prior to the AESA evolution often require a single or a few high-power transmitters feeding passive or semi-active arrays or antenna structures. These transmitters have often been vacuum electronic devices such as traveling wave tube amplifiers (TWTAs), klystrons, magnetrons, or cross-field amplifiers, as this has historically been the only method to obtain high power RF efficiently

High-power gallium nitride (GaN) monolithic microwave integrated circuit (MMIC) technology in conjunction with broadband, low-loss power combining methods has enabled solid-state alternatives. GaN MMICs can be implemented in an amplifier platform to achieve power levels from hundreds of watts to over 1,000 watts. Several of these high-power modules can then be combined into a transmitter configuration incorporating power supplies, command and control circuitry and thermal management to achieve power levels in the tens of kilowatts. The ability to replace legacy vacuum transmitters with solid-state replacements enhances the reliability of these systems and potentially results in some enhancement of characteristics of the RADAR system.
Vacuum technology requires high voltage power supplies and can suffer from short life – especially in exacting environments. GaN semiconductor MMICs, on the other hand, exhibit mean-time-to-failure of greater than 10 million hours at junction temperatures of 200 °C.
The structure of the high-power modules, because they combine several devices to achieve their composite power output, has an inherent graceful degradation characteristic. The failure of a single device in a single amplifier of this type typically results in less than 0.7 dB loss of power, with approximately 0.7 dB additional reduction for each subsequent device removed from service. In a typical very-high-power application, with several GaN MMIC amplifiers, the transmitter performance acts very similar to an AESA in that a single device failure has a generally inconsequential effect on overall performance and power.
The solid-state transmitter is also found to output generally less thermal noise and fewer spurious signals than a vacuum device. This significantly better performance enables output filtering requirements and associated power handling requirements to be reduced, with associated cost, reliability and performance benefits.
Vacuum Devices typically operate from very high voltage power supplies, generally anywhere from a minimum of several hundred volts to more than 10 kilovolts. This operating range presents significant challenges to the power supply implementation. GaN-based devices operate at much lower voltages, typically between 20 and 48 volts DC. The power supplies operating at these voltages offer significant size, weight, operating life, and cost savings.
Critics of solid-state solutions often point out apparent deficiencies of the technology with respect to its efficiency when compared with a Vacuum Device, correctly claiming that in some applications, VED-based power amplifiers can achieve efficiencies close to 70 percent. High-efficiency GaN devices are now capable of power levels of >100 watts from a single device, which can be combined with a low-loss combiner structure with less than 0.5 dB of loss. Efficiencies of >80% have been claimed for GaN but figures of 50 to 70% are common depending on the amplifier class etc.
While parametric performance for the application is a requirement that either technology must meet to be accepted for use, the opportunity for volume manufacturing capacity and associated cost reduction, along with significant design reuse offers yet another compelling reason to replace legacy VED transmitters. The structure of the GaN devices is inherently broadband, and can be populated with devices that operate across all, some, or just a tiny portion of its frequency coverage. This enables leverage of the myriad of GaN MMIC devices that are commercially available.
While they are not able to replace every application where vacuum devices prevail, solid-state alternatives can be considered where practicable for increasingly high-power microwave signal amplification.