High Frequency Design From June 2004 High Frequency Copyright © 2004, Summit Technical Media, LLC REDUCTION

The RFAL Technique for Cancellation of Distortion in Power

By Romulo (Ray) Gutierrez Micronda LLC

eflected signals This patented linearization have always been technique uses the infor- Rconsidered a curse mation present in the by circuit designers, FORWARD reflected input signal to wasting energy, reducing create a correction gain and adversely affect- OUTPUT signal that significantly ing system performance. REFLECTED reduces the distortion of This article describes a a power new technique that uses this normally trouble- some reflected signal to cancel output distor- tion, significantly increase linearity, and dou- ble the usable output power of single stage amplifiers. Figure 1 · A ’s nonlinear frequency spectrum. The RFAL Technique The basis for this technique is the behavior of a transistor amplifier operating under non- predistortion techniques. The RFAL can also linear conditions. At the input, the reflected improve the performance of feedforward sys- signal contains both the reflected input sig- tems when used in nested loop configuration. nals and the distortion products that are found at its output, as illustrated in Figure 1. Summary of the RFAL features: In the Reflect Forward Adaptive Linearizer • Efficient low intermodulation distortion (RFAL) amplifier, the forward signals and the configuration that doubles the output power reflected signals are sampled at the input of the Main Amplifier over wide input power using a directional coupler. These signals are and frequency ranges. then attenuated and phased properly before • Provides distortion cancellation up to the 1 combining them at a summing coupler. The dB compression point of the main amplifier. “correcting signal” formed at the output of the This is important for operation under multi- summing coupler C3 is amplified and injected tone signals or complex wave- into the amplifier’s output using another cou- forms that have high peak-to-average power pler. The injected signal cancels the distortion ratios. products at the amplifier output, while simul- • Allows use of more economical and readily taneously doubling the power output (see available to achieve the high Figure 2). level of linearity required by signal formats The RFAL is ideal for systems requiring such as Multi-tone, CDMA, WCDMA, gain levels of less than 20 dB. It offers high EDGE, and Wi-Fi. linearity, high efficiency and lower costs than • Compatible with different transistor types other configurations such as feedforward or such as GaAs or silicon; FET or bipolar; any

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ing RFAL operation. To provide a high performance level, both Main amplifiers should be of the same con- struction and performance character- istics. The Booster amplifier should have low distortion over the drive range of the Main 2 amplifier with sufficient gain to overcome the for- ward and reflected path losses. Coupling values for the directional coupler affect the amount of gain required by the Booster amplifier. C1 and C3 coupling values should be selected carefully to achieve the prop- er fundamental drive and signal level cancellation to the Main 2 amplifier. Output coupler C2 can be an in-phase type Wilkinson or a 3 dB quadrature hybrid. If a 3 dB quadrature is used for C2, the DC or thru port should be Figure 2 · Block diagram of an RFAL amplifier. connected to the Main 1 amplifier’s path to reduce the overall Main Delay line length by 90 degrees and type of single stage, parallel and arrival at the output combiner C2 thus reduce the output losses. Table 1 configurations; and differ- for maximum fundamental power lists the power level relationship for ent biasing such as Class A and and real-time cancellation of the the various signal paths. Class AB. distortion products. • Uses no error signal or feedback. Amplitude and Phasing All signals move in a forward direc- Circuit Description Characteristics for RFAL Amplifier tion to reach the same time of Figure 2 is a block diagram show- (Refer to Figures 1 and 2) The fre-

(Calculations are for one of the two-tone signals. Except were noted, use power ratios and watts in all equations. Losses refer to attenuator, delay lines, couplers, and connection losses)

Path 1 (Main 1 Path) Fundamental Signal: Pout1 = (Pin-Pref)*(Sum of losses in input path)*(Main 1 Gain)*(Sum of Output Losses) 3rd Order IMD Power: IMPout1 = (Sum of Output Losses)*(Main 1 Pout) /(10^(2*(IP3dBm-PoutdBm)/10)) Path 2 (Reflected Path) Fundamental Signal: Pout2 = (Reflected Power)*(C1 Coupled Port)*(Sum of Losses)*(Booster Gain)*(Main 2 Gain) 3rd order IMD power: IMPout2 = (Input IM3)*(C1 Coupled Port)*(C3Thru Path Loss)*(Sum of Losses)*(Booster Gain)*(Main 2 Gain) Path 3 (Forward Path) Fundamental Signal: Pout 3 = (Pin*C1 Coupled Port)*(Sum of Losses)*(C3 Coupled Port)*(Booster Gain)*(Main 2 Gain) 3rd order IMD power: [There are no 3rd Order IMDs in this path] Output Signal Fundamental signal: Pout Final = Pout1+Pout2+Pout 3 [Single tone output power] 3rd order IMD Power: IMPout final = IMPout1–IMPout2=0 [Assumes perfect amplitude and phase match for optimum cancellation]

Table 1 · Power level relationships for the various signal paths in the RFAL amplifier.

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Figure 3 · Two-tone IMD performance of the GaAs FET Figure 4 · IMD Performance of the GaAs FET RFAL Main 1 amplifier. amplifier under the same test conditions as Figure 3. quency spectrum drawn in the referenced figures show the Main 2 amplifier to the same output level of the Main vector signals with arrows to denote the relative phasing 1 amplifier. The missing level of the fundamental drive of signals, “0 degree” (arrow-up) or “180 degree” (arrow- can be achieved by using the forward signal path. With down). To simplify the analysis of the phasing character- the proper selection of the directional coupler C1, attenu- istics assume operating frequencies <

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Figure 5 · Pout and IMD delta improvement for the GaAs FET RFAL amplifier vs. the Main 1 amplifier. Figure 6 · Two-tone IMD performance of the LDMOS FET Main 1 amplifier. of 10 dB for C1 and C3, and a Wilkinson Xinger combiner for C2 using an effective Booster gain level of 21 dB. The resulting RFAL amplifier assembly provided a flat gain of 15.5 dB with a >20 dB return loss over the 850 to 890 MHz frequency range. Comparison of the spectrum output performance of the RFAL in Figure 4 to the Main 1 amplifier in Figure 3 shows that the RFAL has +2.5 dB higher level at the fun- damental frequency with an IMD improvement of >34 dBc at 875 MHz at 28.5 dBm composite output power. Over the band edges at 865 to 884 MHz the IMD improve- ment was >22 dBc.

Figure 5 shows the RFAL vs. Main 1 amplifier’s Pout and IMD performance over the RFAL output power range from +26 dBm to +31 dBm. The fundamental power of the RFAL is approximately 2.5 dB higher than the Main amplifiers. At +28.5 dBm Pout the RFAL has an IMD3 cancellation improvement of >34 dB. The IM3 cancella- tion improvement degrades to 23 dB at +30 dBm Pout and Figure 7 · IMD Performance of the LDMOS FET RFAL goes down to 2.5 dB as the Main amplifiers approach the amplifier under the same test conditions as Figure 6. 1 dB compression point. The RFAL also achieves signifi- cant IMD cancellation improvement for the IM5 and IM7 distortion products. gain of 19 dB (±0.3 dB) and an input return loss of >17 dB from 881 to 896 MHz. Comparing the performance of the Class-AB LDMOS Amplifier RFAL in Fig. 7 to the Main 1 output in Figure 6, the RFAL Two Class-AB Main amplifiers were built using increases the fundamental frequencies output level by Motorola MRF9030 (30 watts PEP) LDMOS transistors. approximately 3 dB, with an intermod reduction of >21

Each amplifier has 19 dB of gain with a 31 dBc IMD at a dBc at the center frequency of 888 MHz at an average Pout Pout of 16 watts PEP (8 watts average) over the frequency composite level of 15 watts. range of 881 to 896 MHz. The two-tone frequency spec- Over the entire band of 881 to 896 MHz at an average trum performance of each amplifier is shown in Figure 6. Pout = 15 watts, there was a >13 dBc IMD improvement The two main amplifiers were connected in the RFAL over the Main 1 amplifier’s IMD. The reason for the lower configuration with Anaren Xinger couplers using values level of improvement is attributed to the circuit used for of 10 dB for C1 and C3, and a 3 dB Quad Hybrid for C2 the Motorola MRF9030 transistors. The PCB used is a with an effective Booster gain level of 23 dB. basic narrowband tuned circuit with a bandwidth slight- The Class-AB LDMOS RFAL amplifier provided a flat ly better than 10 MHz. A better broadband Main amplifi-

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Figure 9 · Pout and IMD delta improvement for the LDMOS RFAL amplifier vs. Main 1 amplifier.

Figure 8 · RFAL amplifier performance with 8 carrier signals spaced 300 kHz apart. tion cancellation. The goal is to use the minimum delay in the reflected path (D2) to achieve good cancellation over the desired bandwidth. The Main 1 path and path 2 for- er circuit would have provided better cancellation across ward path are adjusted to match the overall reflected the full frequency band. path delay to achieve the same time of arrival at the out- Figure 8 shows the RFAL performance improvement put coupler C2. At the end of the alignment sequence the (19 dBc) relative to the Main 1 amplifier with 8 carrier values of attenuation and delays are fixed and do not signals 300 kHz apart (input signals from RDL MTG- require further adjustment. 2000 multitone generator were peaked phase, spectrum The alignment is somewhat more difficult for RFAL analyzer display on max hold) at a composite average out- using Class AB amplifier types that require a tight range put power level of 5 watts. of RF signal drive to reach the desired bias, output power,

Figure 9 shows the two-tone Pout and intermod delta and impedance conditions for optimum intermod cancel- improvement of the RFAL vs. Main1 amplifiers over a lation and output power. range of RFAL average output power from 15 watts to 30 watts. The IMD3 cancellation improvement is >21 dBc at Advantages of the RFAL Amplifier a Pout of 15 watts, while the IM3 cancellation improve- The RFAL configuration when used in applications ment degrades to 13 dBc at an output of 22.5 watts and to requiring high linearity, with gain levels under 20 dB, >5 dBc as the Main amplifiers approaches the 1 dB com- offers a higher level of performance and numerous eco- pression point. Fundamental output of the RFAL is nomical advantages when compared to available tech- approximately 3 dB higher than Main1 throughout the niques such as parallel combining, predistortion, and full power range. The RFAL also achieves significant IMD feedforward configurations. cancellation improvement for the IM5 and IM7 distortion Amplifiers that are used in parallel configurations products. provide double the output power of a single stage (minus the combining losses), with intermodulation products Electrical Alignment improving by 6 dBc over the single stage amplifier. With The Main 1 amplifier is driven up to its highest nor- the RFAL technique the RF power combining efficiency of mal operating power level and tuned for optimum flat- the two main amplifiers approaches that of the parallel ness and return loss. Next the forward path is adjusted in amplifiers while the intermodulation products improve amplitude and phase to drive the Main 2 amplifier to the from 20 to 30 dBc at the center band. The cancellation same output level as the Main 1 amplifier. The reflected characteristics over wider bandwidths is determined by path is then adjusted for optimum cancellation at center the amplifiers and couplers used and how well the ampli- frequency. The forward path level is backed-off to main- tude and phase response of the three signal paths can be tain the same output level while the reflected path is controlled over the full bandwidth of the RFAL. The over- increased to achieve IMD cancellation. Phase and ampli- all efficiency of the RFAL is less than the parallel com- tude are adjusted incrementally for optimum perfor- bining approach by the amount of DC power that the mance over a frequency range on each of the three signal Booster amplifier requires. paths while trying to achieve the optimum intermodula- Prior art cancellation techniques like predistortion

26 High Frequency Electronics use independently created distortion products ahead of the system. The Booster amplifier should be selected and the amplifier to provide cancellation. Because of the diffi- temperature compensated correctly to provide linear sta- culty in creating the right transfer characteristics over all ble operation over the temperature range and input input conditions, a typical predistorter normally provides power levels of the RFAL. For very high cancellation efficient cancellation of 3rd order products and reduces requirements over wide temperature and drive conditions the gain of the overall amplifier by the amount of loss an adaptive feedback network can be used to monitor the caused by the predistorter. In contrast, the RFAL uses its relative phase and amplitude of the Main amplifier’s out- own transistor’s input distortion for cancellation, includ- puts and drive the voltage variable attenuators and phase ing higher order products, and thus adapts to different shifters to keep the optimum conditions. signal conditions to provide a higher level of cancellation The RFAL Amplifier concept is protected under US for all distortion products. Gain losses are minimized Patent 6,573,793 B1 issue date June 3, 2003. The inven- because the Booster amplifier makes up for any circuit tion is now available for demonstration and licensing. losses. In the feedforward configuration, significant IMD per- Acknowledgment formance degradation occurs when the Main amplifier is This project could not have been possible without the used above the start point of . At this support of James Patterson, formerly Avnet VP/Director point the first loop becomes unbalanced. The loop imbal- North American Sales, now with RF Electronic Sales, ance causes the level of error signal to increase signifi- Inc., who provided help with the component selection, cantly, driving the error amplifier to become non-linear samples from Motorola and Anaren, test fixtures, engi- and feed new distortion products back into the output. To neering and sales contacts. Also acknowledgment is given prevent these undesirable conditions a very high lineari- to Joseph J. Diesso and Elias Neno for their technical help ty, power-hungry error amplifier needs to be used with with this project. complex controlling circuitry to maintain a high level of performance. This is not the case for the RFAL since it References continues to provide significant cancellation of IMDs up 1. US Patent 6,573,793, “Reflect Forward Adaptive to the compression point of the Main amplifiers using Linearizer,” June 3 2003. simpler circuitry. 2. Nick Pothecary, Feedforward Linear Power The simplicity of the RFAL configuration allows fixed Amplifiers, Artech House Inc., 1999. values of components and delays. Over the same limited operating conditions the RFAL can provide a higher level Author Information of performance without the need of complicated circuitry Romulo (Ray) Gutierrez is President of Micronda LLC as required with classical feedforward or predistorter consulting services. Previously, he was a founder and VP of techniques. The configuration can be manufactured eco- Engineering of Phoenix Microwave Corp. He is author or nomically and has the potential for dense packaging co-author of eleven publications and two patents. Ray can using microcircuit hybrid assembly. be reached by telephone at: 908-391-6235 or by e-mail at: The use of two identical single stage main amplifiers [email protected] provides good tracking over the operating environment of