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PERFORMANCE OF MICROWAVE AND MILLIMETER WAVE POWER MODULES (MPMs) WITH LINEARIZATION

By Allen Katz, Robert Gray and Roger Dorval

Linearizer Technology Inc.

ABSTRACT Microwave and millimeter wave power modules (MPMs) provide compact, low mass and highly efficient power amplification for frequencies from S through Q-band. When combined with a linearizer they can provide the most efficient compact linear microwave and millimeter wave amplification in the 50 to 200 watt power range. This paper will present the most current performance achieved by combining predistortion linearizers with MPMs. Measured performance will be provided for a variety of linearized MPMs including octave bandwidth and multi-band applications for frequencies through Q-band.

INTRODUCTION MPMs have been called the most significant recent advancement in high power microwave amplifiers [1]. An MPM is a complete microwave amplifier and usually includes a solid-state driver amplifier, a miniature helix traveling wave tube (TWT), and an electronic power converter in a single housing as illustrated in Figure 1. This combination takes advantage of the small size, low noise and high gain of solid-state devices at low power levels and the high efficiency and small size of TWT technology at higher power levels.

Figure 1 Block diagram of a typical linearized MPM/L-MPM MPMs offer a ten-to-one improvement in power density (power per unit weight) and a four-to-one power conversion efficiency advantage over comparable solid-state power amplifiers (SSPAs) [2]. MPMs also offer a 20 dB lower noise figure than conventional TWTAs. Studies indicate MPMs can provide all these advantages with superior reliability and at a lower cost than comparable power SSPAs [3]. Modern communications services require high linearity. In order to send more information over a limited

bandwidth, complex digital modulation, often referred to as bandwidth efficient modulation (BEM), is employed. BEM requires amplifiers with high linearity to reduce errors and adjacent channel leakage ratio (ACLR). It will be shown that by combining a linearizer with an MPM comparable or superior linearity to an SSPA can be achieved even over octave bandwidths. Data on power, efficiency and linearity will be provided. Linearity will be evaluated in terms of two-tone intermodulation levels, noise power ratio (NPR) and ACLR for QPSK, WCDMA and QAM modulated signals. It will be shown that high power, high efficiency and high linearity can all be achieved, making linearized MPMs very attractive for both commercial and military communications applications.

MPMs MPMs have been developed for operation from below 2 GHz to over 45 GHz. Wideband MPMs are available that operate from 2 to 6 GHz, 6 to 18 GHz, and 18 to 40 GHz with RF output power levels of up to 250 watts, noise figures of less than 10 dB, and an efficiency up to 50% [4]. MPMs are used for military applications, including unmanned aerial vehicles (UAVs), decoys, radar and phased array systems. MPMs are designed to be modular and can be reconfigured for a variety of form factors and formats to meet changing systems needs. Figure 2 shows some of the forms in which MPMs can be found. MPMs are manufactured by a multiplicity of TWT producers including CPI, L-3 Communications, NEC, Northrop Grumman, Thales and Triton.

Figure 2. MPMs are available in a variety of forms

LINEARIZATION Linearization is a systematic procedure for reducing distortion. There are many different ways of linearizing an amplifier [5]. Usually extra components are added to a conventional high power amplifier. When these extra components are configured as a subassembly or box they

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are referred to as a linearizer. Predistortion (PD) linearizers are used in MPMs and have been favored at microwave and millimeter wave frequencies because of their wideband performance, low power overhead and ability to function as stand alone units. PD must generate transfer characteristics that are the inverse of the MPM’s in both magnitude and phase. The gain increase of the linearizer cancels the MPM’s gain decrease. Likewise, the phase change of the linearizer cancels the phase change of the MPM. The desired result is the ideal limiter transfer characteristic of Figure 3. This characteristic is the best that can be achieved. PD can provide large benefits; especially as output power is reduced from saturation. Alternately, PD can be viewed as a generator of intermodulation distortion (IMD) products. If the IMDs produced are made equal in amplitude and 180 degrees out of phase with the IMDs generated by the MPM, the IMDs will cancel. Linearizers are available for all bands from UHF through Q-band, can have several bands combined into a single unit, and now can correct distortion over multi-octave bandwidths.

Figure 3. A PD linearizer generates the inverse of the MPM’s response in both magnitude and phase.

LINEARIZED MPMs Linearizer Technology, Inc., has developed linearizers that integrate into MPMs and that can operate over their wide frequency range of operation. These linearizers are also designed to function over the wide thermal range found inside an MPM’s housing. They have been tested with a variety of MPMs including units made by L-3 Communications, NEC, Thalas and Triton. A typical single band MPM linearizer is shown in Figure 4. This linearizer employs an active GaAs FET pre-distorter, and receives its dc power and control voltages from the MPM’s control electronics. Its dimensions are 0.6” x 0.6” x 2.75”, and it is made from nickel-plated aluminum. Daul- and tri-band linearizers have been produced in only slightly larger housings. Figure 5 is a block diagram of this linearizer module. It has integral gain trim and magnitude control attenuators. Typical units have 25 dB of gain and produce an output power of greater than +10 dBm. They can be aligned for various phase and

gain corrections depending on the requirements of the MPM. This module provides performance over the temperature range of –50 to 90 degrees Celsius.

Figure 4. Typical linearizer designed for integration into an MPM.

MMIC Amplifier P re-distorter MMIC Amplifier MMIC Amplifier

Trim P IN Magnitude P IN

Attenuator Attenuator

Figure 5. Block diagram of L-MMPM Linearizer Module

L-MPM PERFORMANCE L-MPM linearity was tested at C-band (5.85 – 6.65 GHz), X-band (7.9 – 8.4 GHz) and Ku-band (12.75 – 14.5 GHz, DBS band (17.3 – 18.4 GHz), and at millimeter wave (26.5 – 31 GHz and 38 – 40 GHz). The C through DBS band results were taken with a single 6-18 GHz wideband MPM. Very similar results were obtained with a variety of narrower band MPMs. The test L-MPMs were first power swept using a Vector Network Analyzer and adjusted for flat gain and phase versus RF input drive at the center of each test band. Testing was then conducted with different signal sources on each band. Figure 6 shows the C-band transfer response of the test L-MPM and of the MPM by itself for comparison. 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 degrees to less than 1 degree. Figure 7 shows an 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 degrees to less than 2.5 degrees. Figure 8 shows a Ku-band transfer response where 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 degrees to less than 8 degrees. (Only the linearized response is shown in Figure 8.)

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Figure 6. C-band transfer response of MPM and L-MPM.

Figure 7. X-band transfer response of MPM and L-MPM.

Figure 8. Ku-Band transfer response of L-MPM. At DBS band (18 GHz) the MPM tested displayed some gain overshoot, but was still easily linearized as shown in Figure 9. With the addition of the linearizer the 1 dB compression point was moved from about 4 dB from saturation to within 0.5 dB, and the phase change was reduced from more than 60 degrees to less than 5 degrees. Linearizers have been produced that continuously cover the 6 to 18 GHz frequency range with only slightly degraded performance to that shown here.

Figure 9. DBS band transfer response of MPM and L-MPM. The measured 2-tone carrier-to-intermodulation ratios (C/I) corresponding to these responses are shown in Figure 10. Linearization provides more than a 15 dB increase in C/I for output power backoff (OPBO) from saturation > 4 dB on C, X and Ku (see Figure 11) and more than 10 dB on DBS band. On all bands more than a 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 more than a 6 dB power increase for C/Is > 30 dB.

Fig 10. Linearization gives a >6 dB power increase for C/Is > 30 dB.

Figure 11. The Linearizer boosts C/I by > 15 dB when optimized.

BANDWIDTH EFFICIENT MODULATION AND MULTI CARRIER EXCITATION

The reduction in spectral regrowth (SR), also known as ACLR, provided by linearization was investigated. The

L-MPM Pout

L-MPM Gain

L-MPM Phase

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SR measurements were made at X-band, and later confirmed at C and Ku. Figures 12, 13 and 14 show the spectral growth (SR) for a single carrier QPSK signal with and without linearization at 0.5, 2 and 3 dB OPBO respectively. At 0.5 dB OPBO the linearizer provides a SR of better than 26 dB.

Figure 12. Spectral Regrowth at 0.5 dB OPBO of MPM and L-MPM.

Figure 13. Spectral Regrowth at 2 dB OPBO of MPM and L-MPM.

Figure 14. Spectral Regrowth at 3 dB OPBO of MPM and L-MPM. At 2 dB OPBO the linearizer can provide a SR of 30 dB. Figure 13 shows not only the SR of the MPM and L-MPM, but also the spectral response of the modem/upconverter. This figure shows that at some frequency points the linearizer actually improves upon the input signal spectrum. Figure 15 shows a plot of MPM SR versus OPBO with and without the linearizer. The observed

improvement in SR was very close to the results obtained with conventional TWTAs [6]. Slightly poorer results would be expected for OQPSK. BPSK would be expected to provide about 1 dB poorer performance and 8-PSK about 1 dB better performance [7].

Linearized MPM SR for QPSK

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Figure 15. Spectral regrowth is reduced by > 20 dB at 6 dB OPBO. The performance of an L-MPM with many carriers and with wideband code division multiple access (WCDMA) signals were measured. The performance of HPAs with many carriers (>10) is normally tested using noise power ratio (NPR) [8]. In this test white noise is used to simulate the presence of many carriers of random amplitude and phase. The noise is passed through a bandpass filter to produce an approximately square spectral pedestal of noise about the same bandwidth as the signals of interest. This signal is then passed through a narrow band-reject filter to produce a deep notch at the center of the noise pedestal. The notch depth at the output of the HPA is the measure of the NPR. NPR can be considered a measure of multi-carrier C/I. NPR can be used as a measure of OFDM performance. To evaluate the NPR of an MPM, a 40 MHz X-band noise pedestal was employed. This bandwidth is typical of many satellite transponders. The results of this measurement are shown in Figures 16 and 17. For an NPR of 16 dB, the linearizer achieves almost a 3 dB increase in effective output power, and at 20 dB more that a 4.5 dB power advantage.

MPM w/o Linearizer =15 dB

MPM w/ Linearizer =23 dB

MPM @ 5 dB OPBO from Sat.

Figure 16. NPR improvement with linearizer at 5 dB OPBO. CDMA, in addition to cellular telephones, is starting to be used for satellite and other communications applications.

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SR (ACLR) is a major concern in these applications [9]. The SR produced by an L-MPM in response to a 3G WCDMA signal was investigated. The resulting SR produced by the MPM and L-MPM at 2.5 and 5 MHz offsets are shown in Figure 18. This figure shows that as in the case of NPR, the linearization provides a huge power advantage. At 2.5 MHz and an SR of 30 dB more than 6 dB of additional power is provided.

Figure 17. Almost a 3 dB power boost is provided for a 16 dB NPR.

Figure 18. At 2.5 MHz more than 6 dB of additional power is provided.

MILLIMETER WAVE OPERATION Figure 19 illustrates the wideband performance of a Ka-band MPM linearizer near 40 GHz.

Figure 19. Gain and Phase correction provided at small and large signal by a Ka-band MPM linearizer from 38 to 40 GHz

The L-MPM’s TWT in the 38 to 40 GHz band typically requires about 4 dB of gain correction across the band, and a phase correction that increases with frequency from 35 to 50 degrees over the band. The non-linear characteristics of the pre-distorter must match these characteristics to maintain distortion correction across the band. The corresponding power transfer responses of the linearizer module at 38, 39, and 40 GHz are shown in Figure 20.

Figure 20. Power Transfer response at 38, 39, and 40 GHz An L-MPM transfer response measured at a lower Ka frequency is shown in Figure 21. Note that the shape of the linearized Pout/Pin curve approaches the desired ideal limiter characteristic. The improvement is easily seen and is typical of what can be achieved across Ka-band.

Figure 21. Power Transfer Response of the MPM and the L-MPM The separation of the 1 dB compression point (CP1dB) from saturation is a good indicator of linearizer performance. Ideally, CP1dB should be located 1 dB in input power beyond saturation. Typically for TWTs CP1dB occurs 6 to 10 dB below saturation. In Figure 21,

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CP1dB moves from about 6 dB below saturation to 1 dB from saturation. The linearizer also reduces the change in phase with power level from > 30 degrees for the MPM alone to < 5 degrees. These excellent results are achieved because the linearizer can realize different magnitude and phase transfer characteristics as required. The gain compression across frequency of the MPM and the L-MPM is depicted in Figure 21 as a function of input backoff (IPBO). The linearizer provided > 3.75 dB improvement across the 2 GHz band.

Figure 22 shows at Ka-band respectively the L-MPM ACPR calculated for 16QAM and 64QAM signals as a function of OPBO with and without linearization. At 7 dB OPBO the linearized case shows > 45 dB ACPR for both modulations, while at 10 dB OPBO the unlinearized case shows < 40 dB ACPR for both modulations.

Figure 22 Calculated ACPR for L-MPM with 16, 64 and 256QAM signal with and without linearization The measured spectral responses at 7 and 10 dB OPBO, shown in Figure 23 a and b, matched the calculated results within 1 dB.

Figure 23a. Spectral response of L-MPM at 7 dB OPBO with 16QAM signal with and without linearization – linearization provides 10 dB improvement in ACPL.

Figure 23b Spectral response of L-MPM at 10 dB OPBO with 16QAM signal with and without linearization – linearization provides 10 dB improvement in ACPL

EFFICIENCY CONSIDERATIONS 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 wideband MPMs tends to not be as optimum as narrower band designs and can vary with frequency. Linearization does not inherently improve an MPM’s efficiency. It enables operation at a higher efficiency by allowing an MPM to provide a greater output power for a given level of linearity. The overall efficiency (dc to RF) of the wideband MPM used for the C to DBS band testing was measured at X-band as > 35 % (at saturation). Figure 24 shows both high and low efficiency bounds for this MPM. For a 2-tone C/I of 26 dB, linearization provides more than 3 to 1 improvement in efficiency! It increases the efficiency from less than 7% to greater than 22% in the high efficiency case.

Overall MPM Efficiency

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C/I = 26dB @ 9dB OPBO

Figure 24. Linearization can provide a > 300 % increase in efficiency. The Ka-band test MPM was narrowband and optimized to achieve the best efficiency at higher backoff levels [10]. TWTs in linear applications are never operated at CW

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saturation. They must be backed-off to achieve the required linearity. In backoff, the TWT beam power remains nearly the same, while the RF level is decreased. This results in reduced efficiency. 16QAM and higher level QAM modulations require amplifiers to be operated at significant backoffs. Figure 25 shows the relationship between test L-MPM overall efficiency and OPBO. To achieve an ACPR of 45 dB with 16QAM modulation requires an OPBO without linearization of about 13.5 dB. With linearization it can be achieved at a 7 dB OPBO. This difference corresponds to an increase in efficiency from 5.5% to about 15% or almost a 3 to 1 improvement as marked on Figure 25. The improvement can be even greater for higher-level QAM signals and increases the importance of employing linearization to minimize the required backoff.

Figure 25. L-MPM efficiency is greatly enhanced by linearization, which allows operation 6.5 dB closer to saturation for a 16QAM ACPR of 45 dB

The L-MPMs are considered the only amplification device capable of meeting the needs of the emerging Ka-band services. SSPAs for the millimeter wave bands generally have poorer linearity than achieved at lower frequencies, but can be enhanced using linearization to provide similar distortion levels. At an OPBO level of 7 dB reported power added efficiency for SSPAs is 5 percent or less [11]. (The SSPAs discussed were of lower power than the L-MPMs. Figures for comparable power units were not available). For comparison these figures must be decreased by the EPC efficiency (~ 80%) for an overall efficiency of about 4 percent. Thus an equivalent SSPA will require about four times the dc power and thermal load as an L-MPM.

SUMMARY The results in this paper clearly show the superiority of MPM technology and illustrate the enormous value of combining an MPM with a linearizer. This combination makes L-MPMs highly attractive for commercial and military communications applications. The linearized MPM provides significantly higher power, efficiency and

linearity than alternate options. The linearizer allows an MPM to provide 4 times the output power for C/Is > 30 dB, and more than a 10 dB improvement in C/I over much of its power range. It allows QPSK operation with an SR of > 30 dB at an output power of 50 watts with an overall efficiency of 25 %. Similar advantages are shown for WCDM. All this has been provided in a box that can be less than 50 cubic inches in volume, 5 lbs in weight and operates over multioctave bandwidths.

ACKNOWLEDGEMENTS The authors would like to acknowledge and thank Rob Elmore, Tom Ninnis and Carter Armstrong at L-3 Communications, Inc. for providing the wideband MPM amplifier used in this research, and to Junichi Matsuoka and NEC Microwave Tube, Ltd for their assistance with the Ka-band L-MPM testing and data collection.

REFERENCES [1] C. R. Smith and et al, “The Microwave Power Module: A Versatile RF Building Block for High-Power Transmitters,” Proc. IEEE, vol. 87, pp 717-737, May 1999. [2] M. C. Smith and et al, “Comparison of Solid State, MPM, and TWT Based Transmitters for Space borne Applications,” IEEE Southeastcon '98 Proceedings, April 24-26,1998. [3] J. Weekley and B. Mangus, “TWTA Versus SSPA: A comparison of On-Orbit Data,” IEEE Trans. on Electron Devices, Vol. 52, pp. 650-652, May 2005. [4] R. H. Abrams, “The Microwave Power Module: A Supercomponet for Radar Transmitters,” IEEE National Radar Conference, pp.1–6, March 29-31,1994. [5] A. Katz, “Linearization: Reducing Distortion in Power Amplifiers,” IEEE Microwave Magazine, pp. 37-49, Dec. 2001. [6] A. Katz, “TWTA Linearization,” Microwave Journal, Vol. 39, No. 4, pp. 78-90, April 1996. [7] A. Katz, "Multi-Carrier 16QAM Over a Linearized TWTA," IEEE, MTT-S International Microwave Symposium Digest, May 2001. [8] A. Katz and R. Gray, “Noise Power Ratio Measurement Tutorial,” [Online], available at http://lintech.com/infodown.htm. [9] J. Staudinger, “Effects of AM/AM and AM/PM Distortion on Spectral Regrowth in 3GPP W-CDMA BS Power Amplification,” Microwave Journal, Vol. 45, No. 11, pp. 90-100, November 2002. [10] K. Tsutaki and et al, “Development of Ka-Band 100-W Peak Power MMPM,” IEEE Trans. on Electron Devices, Vol. 52, pp. 660-664, May 2005. [11] M. Lyons and et al, “Design of Low-Cost 4W and 6W MMIC HPA for Ka-Band Mod,” MTT-S International Microwave Symposium Digest, May 2004.