Addition of solid-state linearizer circuitry to vacuum-electronics-based MPMs can dramatically improve the performance of a high-power amplifier in commercial systems.
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.
Figure 6 shows the measured two-tone carrier-to-intermodulation (C/I) ratios corresponding to these linearized and nonlinerized responses. Linearization provides more than a 15-dB increase in C/I for output-powerback off (OPBO) of greater than 4 dB at C-, X-, and Ku-band and more than 10 dB at DBS frequencies. For all bands, a more than 6-dB increase in effective power is provided by the linearizer for the minimum C/I of 26 dB required by most satellite operators, and a more than 6-dB increase in effective power was achieved for C/I ratios greater than 30 dB.
Next, the reduction in spectral regrowth (SR) or ACLR resulting from linearization was investigated. SR measurements were made at X-band, and later confirmed at C- and Ku-bands. At 0.5-dB OPBO, the linearizer provides a SR of better than 26 dB. At 2 dB OPBO, the linearizer can provide a SR of 30 dB. Figure 7 shows not only the SR of the MPM and linearized MPM, but also the spectral response of the modem/upconverter. In addition, Figure 7 reveals that at some frequency points, the linearizer actually improves the input signal's spectrum. Figure 8 shows a plot of SR versus OPBO with and without the linearizer. The observed improvement in SR was very close to results obtained with conventional TWTAs.6 Similar results would be expected for OQPSK. It is expected that BPSK would provide about 1 dB poorer SR performance while 8PSK would provide about 1-dB improvement with the linearizer.5
The performance of the L-MPM with multiple carriers and with wideband-code-division-multiple access (WCDMA) signals was measured. The performance of HPAs with multiple carriers (more than 10) is normally tested using a noise-power-ratio (NPR) measurement.10 In this test, white noise is used to simulate the presence of many carriers of random amplitude and phase. The white noise is passed through a bandpass filter (BPF) to produce an approximately square spectral pedestal of noise about the same bandwidth as a signal of interest. This signal is then passed through a narrowband band-reject filter to produce a deep notch at the center of the noise pedestal. The depth of the notch at the output of the test HPA is the measure of the NPR. NPR can be considered a measure of multicarrier intermodulation ratio (C/I). To evaluate the multicarrier performance of the MPM, a 40-MHz X-band noise pedestal was employed, a bandwidth typical of most satellite transponders. The results of this measurement are shown in Fig. 9. For an NPR of 16 dB, the linearizer achieves almost a 3-dB increase in effective output power, and for an NPR of 20 dB more that a 4.5-dB power advantage.
In addition to its use in cellular telephones, CDMA technology is also finding its way into satellite-based and the communications systems. SR (ACLR) is a major concern in these applications.11 The SR produced by an L-MPM in response to a 3G WCDMA signal was investigated. Figure 10 shows the resulting SR levels produced by the MPM and the L-MPM at 2.5- and 5-MHz offsets. For a 2.5-MHz WCDMA channel bandwidth and an SR of 30 dB, more than 6 dB of additional power is provided.
Amplifier power consumption is a major concern in many communications applications. It can be a major cost driver and in some instances determine a project's feasibility. The efficiency of the test MPM was not optimum and varied with frequency because of its wideband design. In X-band the overall efficiency (TWT, driver amplifier, power supply) of the MPM at saturation was measured as more than 35 percent, but varied with frequency. For a two-tone C/I of 26 dB, linearization provides more than 3-to-1 improvement in efficiency! Linearization boosts efficiency from less than 7 percent to more than 22 percent in the high-efficiency case.
These results clearly show the enormous value of combining an MPM with a linearizer. This combination makes linearized MPMs attractive for commercial and military communications applications. A linearized MPM provides significantly higher power, efficiency, and linearity than other options. A linearizer helps an MPM achieve four times the output power for C/Is greater than 30 dB, and more than a 10-dB improvement in C/I over much of its power range. A linearizer allows QPSK operation with an SR of more than 30 dB at 50 W output power with overall efficiency of 25 percent, with similar advantages for WCDMA. All this is provided in a box that can be less than 50 in. (127 cm)3 in volume, 5 lbs. (2.25 kg) in weight, and operate over multioctave bandwidths.
ACKNOWLEDGMENTS
The authors would like to acknowledge and thank Rob Elmore, Tom Ninnis, Carter Armstrong, and the engineering department at L3 Communications, Inc. (San Carlos, CA) for providing the MPM amplifier used in this research, and for their valuable suggestions and support in writing this article.
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