THE LINEARIZATION TECHNIQUE FOR MULTICHANNEL WIRELESS SYSTEMS WITH THE INJECTION OF THE SECOND HARMONICS

Transcription

1 THE LINEARIZATION TECHNIQUE FOR MULTICHANNEL WIRELESS SYSTEMS WITH THE INJECTION OF THE SECOND HARMONICS N. Males-Ilic#, B. Milovanovic*, D. Budimir# #Wireless Communications Research Group, Department of Electronic Systems, University of Westminster, London W1W 6UW, UK, *Faculty of Electronic Engineering, University of Nis, Nis, Yugoslavia, ABSTRACT A linearisation technique for reducing the third-order intermodulation products with the injection of the second harmonics together with the fundamental signals at the amplifier input has been considered in this paper. The second harmonics are either led from output through a feedback loop to the amplifier input or generated from the separated nonlinear source and injected at the amplifier input. The amplifier including the components (band pass filter, phase shifter, attenuator ets.) in the path of the second harmonics was designed as a hybrid microwave integrated circuit by using programs Libra or ADS. The adjustable parameters are the phase and amplitude of the second harmonics. Various improvements in this linearization technique have been presented in this paper in order to overcome some shortcomings of the linearization technique such as the sensitivity to the deviation of the second harmonic phase from an optimal value as well as the existence of the group delay in the injection path of the second harmonics. INTRODUCTION In multicarrier communications systems, the intermodulation products (IM), especially the third-order (IM3), represent the most serious problem. Many different techniques for IM reducing can be found in literature such as predistortion, feedforward, feedback and combination of them, [1,2]. However, the application of these techniques requires the circuitry that may be complex, expensive and large in size. The linearization technique for reducing the IM3 products with the injection of the difference frequency between the fundamental signals together with fundamental signals at amplifier input is one way to reduce IM3 product levels, [3]. The other way is that the second harmonics of the input signals are fed together with the fundamental signals to the amplifier input. Both approaches of the linearization technique satisfy the reduction of IM3 product levels without affecting the fundamental signal power levels. Additionally, the required circuitry is simple, inexpensive and small in size. This work presents analyses of a multicarrier amplification with the effects of the carrier second harmonics to the thirdorder intermodulation in microwave amplifiers. The published results preceding the investigation of the authors of this paper were based on two ways for the injection of the second harmonic signals. In one approach the second harmonics were led to the amplifier input across the feedback loop which components (band pass filter, phase shifter, attenuator and isolator) were modeled by ideal elements from the library of commercial programs such as Libra or MDS [4]. In the other, instead of feeding back the second harmonics, they were generated and injected into the amplifier input together with fundamental signals in the simulations as well as in the experiments [5]. Our works [6, 7] extend the previous analyses concerning the linearization technique with the effects of the second harmonics to the first kind of IM3 power level. Additionally, the influence of the other second order products at frequencies that are the sum of pairs of the fundamental signal frequencies (IM2) to the second kind IM3 products that exist for more than two fundamental signals at the amplifier input has been considered as well [8]. The published results are based on an amplifier design with additional second harmonics that are either extracted from the output and led through the feedback loop to the amplifier input [6-9] or generated out of the amplifier and injected together with fundamental signals [10-12]. In our work, the components (bandpass filter, phase shifter, attenuator, Wilkinson s combiner and divider), in the second harmonic path (feedback or injection) in contrast to above-mentioned approaches, were designed for the application in a hybrid microwave integrated circuit (HMIC) of the amplifier. Simulation and design of a single stage amplifier as well as the second harmonic components were performed by the microwave circuit simulator Libra and ADS. Designed amplifier was tested for analog as well as digitally modulated signals. However, these approaches are very sensitive to the second harmonic phase variation from optimal values. In order to overcome this problem, two modifications of the basic approach have been suggested [10-11]. Also, the reducing in IM3 power levels is decreasing if differences between phases of the second harmonics become greater. Procedure that makes the linearization technique insensitive to the group delay of the second harmonics has been proposed in [12].

2 DIFFERENT VERSIONS The proposed technique uses the amplifier non-linear characteristic to generate an additional third-order distorted signal that is used to cancel the original third-order intermodulation products generated by the cubic term in the amplifier characteristic. The analysis applied in [8] shows that the injection of the second harmonics reduces the first kind of IM3 and the injection of the sum of pairs of the fundamental signal frequencies reduces the second kind of IM3. Therefore, reducing of a particular first kind IM3 can be achieved by a proper selection of the phase and amplitude of the second harmonics while for reducing of the second kind IM3 product, the amplitudes and phases of appropriate injected IM2 signals should be adjusted on optimal values. Fundamental version The amplifier circuit with the feedback loop of the second harmonics is presented in Fig. 1. The amplifier including additional components in feedback path of the second harmonics was designed as a hybrid microwave integrated circuit. Fig. 1. Amplifier with the second harmonics feedback loop The bandpass filter that can be conveniently fabricated in microstrip is the capacitive-gap coupled resonator filter, was designed at 5 GHz centre frequency with 10% and 20% bandwidth, 0.5 db equal-ripple response, with 3 and 5 sections. For the phase shifter, a 360 o reflection-type analog phase shifter with a single 90 o branch-line coupler was designed. Also, in order to provide broadband phase shift at second harmonic frequencies, two analog and reflection-type phase shifters with Lange coupler at input and two types of varactor diodes are connected in cascade to achieve a 240 o phase shift. Varying bias voltage of the varactor diodes, the phase shift can be adjusted on a required value. Variable PIN diode attenuator was designed to accomplish an appropriate attenuation of the second harmonics by changing a diode forward current. Wilkinson s combiner and divider were designed in order to combine or divide the fundamental and second harmonic signals. This configuration of circuit has been simulated for two (2.5 and 2.51 GHz) and three (2.5, 2.51 and GHz) analog fundamental signals with the power 2 dbm at the amplifier input as well as for OQPSK digitally modulated signal at 2.5 GHz carrier frequency with input power 0 dbm and 1.23 MHz spectrum width. The results obtained during analyses show that a maximum reduction of approximately 40 db of each IM3 product can be obtained separately without reduction of the fundamental signals. However, the values of the phase shift and attenuation at which maximum reduction is obtained do not coincide. The second harmonic or IM2 spectral components must have approximately the same power level so that all of them can be controlled within a fraction of db in amplitude and of a few degrees in phase in order to attain maximum reduction in all IM3 products. On the other hand, it is difficult to obtain a maximum reduction in all IM3 products with the same amplitude and phase adjustment due to these products have slightly different amplitudes and phases. The results obtained in [6,7] for two analog signals at amplifier input refer to the compromise between the maximum reducing in each IM3 signal obtained separately, yielding 15 db improvement in both. The improvement achieved in [8] for three analog fundamental signals are 18 db for the first kind IM3 and 13 db for the second kind IM3. For digitally modulated signal at amplifier input, optimizing phase shift and attenuation of the feedback loop signals 16 db improvement in the adjacent channel power ratio (ACPR) for ±900 KHz offset from carrier frequency over 30 KHz bandwidth was obtained in simulation [9]. However this approach can lead to unstable operation of the amplifier at the second harmonic frequencies. In that case the group delay of the second harmonics increases and lowers the level of IM3 reducing. In order to avoid this problem, the second harmonics injected together with the fundamental signals at the amplifier input are generated out of the amplifier circuit. The improvement in IM3 power levels achieved could be up to 25 db depending on the differences between phases of the second harmonics. Apart the good results, both approaches are very sensitive to the deviation of the second harmonic phase characteristics from an optimal value. For instance, the change of the phase of second harmonics from an optimal value by ±5 o will lead to the 5 db lower reductions in the third-order intermodulation products. Correction of the sensitivity to the phase of the second harmonics In further work [10], second harmonics of the fundamental signals are injected together with fundamental signals at the amplifier input and feedforwarded at the amplifier output, Fig. 2. RF in second harmonics bandpass filter variable attenuator phase shifter bandpass filter amplifier variable attenuator Fig. 2. Amplifier circuit with injection and feedforwarding of the second harmonics The linearization technique proposed allows altering the phase of the injected second harmonics at amplifier input by approximately ± 40 o from an optimal value with appropriate

3 phase shift of the feedforwarded second harmonics which has two different values for plus and minus deviation. Then, in order to obtain the same improvement in IM3 power levels as for the optimal phase, the adjustable parameters are only amplitudes of the second harmonics in injecting and feedforwarding paths. As the result, the improvement of 20 db was achieved in third-order intermodulation products for the phase variation of the injected second harmonics up to ±40 o. Also, the same improvement can be obtained for ±15 o variation of an appropriate phase in the feedforwaded second harmonics. In comparison with earlier applied second harmonic linearization technique, this approach makes the improvement in intermodulation more flexible to the phase variation of the second harmonics. Another solution concerning this technique sensitivity to the second harmonic phase deviation has been presented in [11]. The proposed correction procedure combines the second harmonic signals (main and corrective) with 90 o differences in phases at the amplifier input, Fig. 3. Varying the phase shifter characteristic from an optimal value in main path, this approach provide that adjusting only amplitudes of the injected second harmonic signal in two paths, the same improvement can be obtained as for the case without deviation from an optimal second harmonic phase. As the result, the improvement of 22 db was achieved in the adjacent channel power ratio (ACPR) for OQPSK modulation at 2.5 GHz carrier requency at 0 dbm input power, 1.23 MHz spectrum width for the phase variation of the injected main second harmonic up to ±80 o. Fig. 3 Amplifier with the injection of the second harmonics with phase correction Additionally, the same results can be obtained for ±20 o variation of appropriate phase in the corrective second harmonic signal. For analog signals, the same results were obtained as in the previous presented configuration of the amplifier, Fig. 2. Advantage of the second approach is that the amplifier in corrective path of the second harmonics is not necessary comparing with the first proposed solution with the amplifier in the feedforwarding path of the second harmonics. Correction of the group delay of the second harmonics Applying the linearization techniques proposed, the maximal reducing in all IM3 products could be accomplished simultaneously, if the fundamental signals at amplifier input are with equal amplitudes and phases as well as the same condition is valid for their second harmonics. According to the circuit topologies proposed by the linearization techniques applied so far [4-11], the equal amplitudes and phases of the fundamental signals as well as the equal amplitudes of their second harmonics are real to expect. In contrast to this, the different phases of the second harmonics are unavoidable. As a result, it is impossible to reduce maximally all IM3 products with the same phase of the second harmonics. In order to overcome this problem, it has been proposed in [12] that the fundamental signals with equal amplitudes have a phase characteristic with the same slope as the second harmonics have across their injection path, Fig. 4. Fig. 4 Amplifier with phase slope adjustment between fundamental and second harmonics. Four input fundamental signals at frequencies 2.5, 2.51, and GHz were chosen with power levels 8 dbm at the amplifier input. The spectrums that include fundamental signals, the first and second kinds of the thirdorder IM products obtained at the amplifier output before and after applying linearization technique are shown in Fig. 5 and Fig. 6, respectively. The IM3 signal power levels are reduced by 40 db for the optimized values of the phase shifter and variable attenuator. This result is much better than 18 db improvement obtained without phase slope adjustment and comparable with one obtained for an ideal case with the equal amplitudes and phases of the fundamental signals as well as the second harmonics. Varying phase shifter for ±10 o from the optimal value, the improvement in IM3 power is reduced to 18 db that represents satisfied results too, in comparison with only few db improvements in the case without phase slope adjustment but for the same deviation from optimal phase of the second harmonics. The designed amplifier was tested for OQPSK digitally modulated signal at various carriers from frequency range GHz, 1.23 MHz spectrum width and carrier power 0 dbm at the amplifier input. The results for the mean output power and improvement in ACPR for ±900 KHz offset from various carriers (2.5, 2.55 and 2.6 GHz) over 30 KHz bandwidth are compared in Table I. The improvement in ACPR is approximately 22 db for lower and 24 db for upper channel. All results relate to optimised parameters of the phase shifter and attenuator that are the same for each carrier. It should be pointed out that differences between results obtained for various carrier can be neglected in contrast to the results obtained by linearization technique without correction in phase slope of the fundamental signals where each carrier demands new optimisation of the phase shifter to reduce spectral regrowth [9]. CONCLUSION A linearization technique for reducing the third-order intermodulation, that proposes the injection of the fundamental signals and their second harmonics (means second harmonics and IM2 products for more than two fundamental signals) at the amplifier input has been presented in this paper. A few modification of this linearization technique has been described concerning the

4 dbm(pout) dbm(pout) sensitivity of the technique to the deviation of the phase of the second harmonic from an optimal value and to the differences between phases of the second harmonics. Application of this linearization technique does not affect the output fundamental signal power levels and accomplishes the satisfied reducing of IM3 products freq, GHz Fig. 5 The simulated fundamental powers and IM3 powers for four analog signals before applying the linearization technique freq, GHz Fig. 6 The simulated fundamental powers and IM3 powers for four analog signals after applying linearization technique with phase slope adjustment. Table I: The results for digitally modulated signals at various carriers after applying linearization technique with phase slope adjustment. Carrier frequency (GHz) Mean output power (dbm) Improvement in ACPR for lower channel (db) Improvement in ACPR for upper channel (db) REFERENCES [1] J.G. Mc Rory, R.H. Johnson, An RF Amplifier For Low IM Distortion, IEEE MTT-S Digest, pp , [2] Daniel Myer, Design Linear Feedforward Amplifiers For PCN Systems, Design Feature, Microwaves & RF, pp , September [3] D. Budimir, M. Modeste, and C. S. Aitchison, "A Difference Frequency Technique for Improving IM Performance of RF Amplifiers", Microwave and Optical Technology Letters, February 2000 [4] M.R. Moazzam, C.S. Aitchison, A Low Third Order Intermodulation Amplifier With Harmonic Feedback Circuitry, IEEE MTT-S Digest, pp , [5] C. S. Aitchison, M. Mbabele, M. R. Moazzam., D. Budimir, and F. Ali, "Improvement of Third Order Intermodulation Products of RF and Microwave Amplifiers by Injection", IEEE MTT Transactions on Microwave Theory and Techniques, Vol. 49, No. 6, pp , June [6] N. Males-Ilic, B. Milovanovic, Dj. Budimir, Design of Low Intermodulation Amplifiers for Wireless Multichannel Applications, Proceedings of International Conference EUMC 01, pp [7] N. Males-Ilic, B. Milovanovic, Dj. Budimir, Low Intermodulation Amplifiers for RF and Microwave Wireless Systems, Proceedings of International Conference APMC 01, pp [8] N. Males-Ilic, B. Milovanovic, Dj. Budimir, A Multicarrier Amplifier Design Linearized Trough Second Harmonics and Second-Order IM Feedback, Facta Universitatis, Series: Electronics and Energetics, 2001, Vol. 14, pp [9] N. Males-Ilic, B. Milovanovic, Dj. Budimir, Amplifiers with Improved IMD Performance for Multi-Channel Wireless Systems, Proc. Int. Conf. European Microwave Conference, September 2002, Milan, Italy. [10] Dj. Budimir, N. Males-Ilic, B. Milovanovic, A New Low Intermodulation Amplifier Using Simultaneous Injection and Feedforwarding of Second Harmonics, Proc. Int. Conf. Mediterranean Microwave Conference, Jun 2002, Caceres, Spain, pp. cp: [11] N. Males-Ilic, B. Milovanovic, Dj. Budimir, Diminished Sensitivity to Phase Variation in Injected Second Harmonic Linearization Technique for Multychannel Amplifiers, Accepted for presentation at Int. Conf. Asia pacific Microwave Conference, November 2002, Kyoto, Japan. [12] N. Males-Ilic, B. Milovanovic, Dj. Budimir, Improved Second Harmonic Injection Linearization Technique with Enhanced Reducing in Third-order Intermodulation, Proc. Int. Conf. ICEST, October 2002, Nis, Yugoslavia, pp..

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