Linearizing MPMs For Communications

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[Devices & ICs]
Linearizing MPMs For Communications
Addition of solid-state linearizer circuitry to vacuum-electronics-based MPMs can dramatically improve the performance of a high-power amplifier in commercial systems.

Robert Gray, Allen Katz | ED Online ID #6860 | November 2003

Microwave power modules (MPMs) have been widely used in military applications, including radar and electronic-warfare (EW) systems, but less so in communications applications because of their limited linearity. By combining an MPM with a linearizer, however, it is possible to use these robust microwave and millimeter-wave power amplifiers for communications applications requiring as much as 250 W output power.

An MPM combines a solid-state driver amplifier with a miniature traveling wave tube (TWT) and an electronic power converter in a single compact housing.^1,2 It blends the small size, high gain, and low noise of solid-state devices at low power levels with the high efficiency and small size of TWT technology at higher power levels. MPMs offer a ten-to-one improvement in power density (power per unit weight) and a four-to-one power conversion efficiency over comparable solid-state power amplifiers (SSPAs).³ MPMs also offer a significantly lower noise figure than conventional TWTAs, and can provide all these advantages with superior reliability and at a lower cost than comparable power SSPAs.⁴

Unfortunately, MPMs are limited in linearity performance, a key parameter in modern communications systems.⁵ Complex modulation schemes, often referred to as bandwidth-efficient modulation (BEM), are employed in these systems to maximum the amount of information that can be transmitted over relatively narrowband channels. BEM requires amplifiers with high linearity to reduce errors and minimize adjacent-channel leakage ratio (ACLR). Fortunately, by combining a solid-state linearizer with an MPM, comparable or superior linearity to an SSPA be achieved.⁶

MPMs operate from below 2 GHz to past 45 GHz. Standard wideband models cover bands of 2 to 6, 6 to 18, and 18 to 40 GHz with RF output power levels up to 250 W, noise figures of less than 10 dB, and efficiency to 50 percent.^7,8 MPMs are used for military applications, including unmanned aerial vehicles (UAVs), decoys, radars, and phased array systems. They are designed to be modular and can be reconfigured for different form factors and formats to meet changing systems needs. MPMs are available from a variety of suppliers, including CPI, L3 Communications, NEC, Northrop Grumman, and Triton.

Linearization improves the performance of an MPM by systematically reducing distortion.⁹ Linearization approaches vary, but usually extra components are added to a conventional high-power amplifier (HPA). Often these extra components are configured as a subassembly or box that is referred to as a linearizer. Predistortion (PD) linearizers have been favored at microwave and higher frequencies because of their wideband performance, low power overhead, and ability to function as stand-alone units, and that approach is the basis for the study presented in this article. PD generates transfer characteristics exactly opposite in magnitude and phase to those of the power amplifier. The gain increase of the linearizer cancels the amplifier's gain decrease. Likewise, the phase change of the linearizer cancels the phase change of the amplifier. The desired result is an ideal limiter transfer characteristic (Fig. 1). PD can provide large benefits, especially as output power is backed off from saturated levels. Alternately, PD can be viewed as a generator of intermodulation-distortion (IMD) products. If the linearizer-generated IMD is equal in amplitude and 180 deg. out phase with the IMD produced by the HPA, both groups of IMD signals will cancel. Linearizers are available for all frequency bands from UHF through Ka-band; in addition, several bands can be combined into a single unit.

Engineers at Linearizer Technology have tested linearizers with MPMs from both L3 Communications (formerly Litton Electron Devices, San Carlos, CA) and Triton although, for the purpose of this article, only the results for the L3 Communications MPMs are shown. The linearizer was mounted external to the MPM (Fig. 2), but could have been integrated easily into the MPM housing. The test MPM was designed for wideband operation from 6 to 18 GHz and rated for minimum saturated power of 80 W, although capable of providing more than 100 W output power at some frequencies. It was designed for use with an aircraft buss and operated from a +270-VDC power source but is available with a wide range of operating voltages including +28 VDC and 120 VAC.

The performance of the linearized MPM (L-MPM) was tested at C-band (5.85 to 6.65 GHz), X-band (7.9 to 8.4 GHz), Ku-band (13.75 to 14.5 GHz, and DBS (17.3 to 18.4 GHz) uplink satellite bands. A single triband linearizer was used for the C-, X-, and Ku-band tests, with a separate K-band unit used for 18-GHz measurements.

The L-MPM was first power swept using a vector network analyzer and adjusted for flat gain and phase versus RF input drive. Testing was then conducted with different signal sources on each band. Figure 3 shows the C-band transfer response of the L-MPM compared to the MPM by itself. The 1-dB compression point was moved from about 5 dB from saturation to within 2 dB. The phase change between small signal and saturation was reduced from more than 45 deg. to less than 1 deg.

Figure 4 shows the X-band transfer response. The 1-dB compression point was moved from about 6 dB from saturation to within 2.5 dB. The phase change between small signal and saturation was reduced from more than 45 deg. to less than 2.5 deg. At Ku-band, the 1-dB compression point was also moved from more than 6 dB from saturation to within 2.5 dB. The phase change between small signal and saturation was reduced from more than 52 deg. to less than 8 deg.

At 18 GHz, the MPM displayed some gain overshoot, but was still easily linearized (Fig. 5). With the addition of the linearizer, the 1-dB compression point moved from about 4 dB from saturation to within 0.5 dB, and the phase change was reduced from more than 60 deg. to less than 5 deg.

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