Solid-state technology suitable for radar applications
OpenVPX for EW applications is not only a trend in the U.S., as companies in Europe and Africa are also leveraging the standard for EW signal processing.
New radar systems have increasingly utilized active electronically scanned arrays (AESAs) for their radiating and receiving functionality. AESAs offer several desirable features compared with other radar implementations, including jam resistance, frequency agility, graceful degradation, and use for other applications such as communications, electronic self-protect (ESP), or electronic attack (EA).
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 (VEDs) such as traveling wave tube amplifiers (TWTAs), klystrons, magnetrons, or crossfield amplifiers, having been the only available technologies to achieve the required high levels of RF or microwave power at a sufficiently high operating efficiency.
Recently, the confluence of high-power gallium nitride (GaN) monolithic microwave integrated circuit (MMIC) technology in concert with novel broadband, low-loss power combining methods has enabled solid-state alternatives to these high power vacuum devices. GaN MMICs can be implemented in an amplifier platform to achieve power levels from hundreds of watts to greater than 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. (Figure 1.)
The ability to replace legacy VED transmitters with solid-state replacements potentially results in incidental enhancement of some characteristics of the radar system, and, perhaps more importantly, improves the reliability and availability of the systems.
VEDs and their associated high-voltage power supplies often suffer from short lifetimes, varying from as few as several hundred hours to ten thousand hours for relatively benign 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. In addition to the relatively short lifespans, VEDs are prone to gassing up and may need to be operated periodically to maintain their functionality. Solid-state amplifiers can effectively exist indefinitely in an unpowered state, yet be ready to perform instantly.
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. (Figure 2.)
The solid-state transmitter also is found to output generally less thermal noise and fewer spurious signals than a vacuum device. Broadband TWTAs are particularly notorious for their ability to output substantial power as harmonic content when operating at their full-rated capacity, in some cases putting out as much or more power at the second harmonic than is realized at the fundamental frequency of interest. Intermodulation characteristics are often equally poor. Comparatively, the typical harmonic content of the solid-state amplifier is usually more than 15 dB below the primary signal. This significantly better performance enables output filtering requirements and associated power handling requirements to be reduced, with associated cost and performance benefits.
VEDs 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 as well as a potential personnel safety hazard should exposure to the supply voltages occur. Because a GaN-based device typically operates at much lower voltage, 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 VED, correctly asserting that in some applications, VED-based power amplifiers can achieve efficiencies close to 70 percent. Historically, solid-state high-power amplification was only able to be achieved by circuits combining very large number of gallium arsenide (GaAs)-based amplifiers. The circuit-combining methods often resulted in diminishing returns as accommodations for phase and amplitude matched paths over increased bandwidth and device numbers resulted in ever higher losses following the amplifiers. These losses directly degraded both the output power availability and effective operating efficiency of the amplifier. 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.
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 these 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.