* *Department of Electrical Engineering and Electronics, Brunel University, Uxbridge, Middlesex UB 8 3PH, UK.

Transcription

1 19 September 1997, University of Leeds Second Harmonic Injecting Technique for Low Intermodulation RF-Microwave Amplifiers T. Nesimoglu, D. Budimir, M.R. Moazzam*, and C.S. Aitchison** Microwave Research Group, Department of Electronic Systems, University of Westminster, London, W 1 M 8 JS, UK. *Nokia Mobile Phones Inc, San Diego, CA , USA. * *Department of Electrical Engineering and Electronics, Brunel University, Uxbridge, Middlesex UB 8 3PH, UK. nesimot@cmsa.westminster.ac.uk and budimid@cmsa.westminster.ac.uk Abstract: An amplifier where the third order IM distortion has been reduced is proposed. If two signals at different carrier frequencies are incident at the input of an amplifier, due to the nonlinearity, these signals will interact with each other and the output will contain additional signals at all combinations of sum and difference of these frequenc:ies, which are Ih4 products. The injection of the second harmonics of the two input in the amplifier together with the fundamental signals will produce additional IM products at the out]put. By proper selection of phase and amplitude of the injected second harmonics, it is possible to make the third order IM products produced by the second harmonics and the original tlhird order IM products out of phase and equal in amplitude. As a result third order IM products will be eliminated, in principle. Theory and simulation results obtained at 2.5 GHz are presented which supports the suggestion. 1. INTRODUCTION The IM products, especially the third order are regarded as the most troublesome in communication systems and they cause some undesired effects [ 11. In many communication systems the ratio of carrier to the third order IM products of the transmitter output is a system figure of merit. The desirability of reducing the IM distortion has led to the devising of a number of techniques. Applying these techniques may prevent the designer from using the full capability of the active device or alternatively the required circuitry may be complex, expensive and large in size [2]. Reducing the third order IM products at the output ideally should not affect the fundamental output power levels. Reducing the fundamental power levels means that the amplifier power efficiency has been reduced which is not a desired result. This paper presents the theory and the simulation results obtained at 2.5 GHz in which the level of the third order IM products is reduced without affecting the fundamental power levels. 2. TECHNICAL DISCUSSION AND CIRCUIT CONFIGURA.TION ?/97/$ IEEE 143

2 19 September 1997, University ofleeds The proposed technique uses the amplifier non-linear characteristic to generate a second third order IM signal which is used to cancel the original third order IM product at the output. In this technique the second order harmonics of the source signals are injected into the amplifier as well as fundamental signals. Non-linearity of the amplifier causes interaction between the source signals and their injected second order harmonics. This interaction results in additional signals at the output of the amplifier at the third order IM fi-equencies. On the other hand there are components of the third order IM product due to the interaction between the fundamental signals. By proper selection of phase and amplitude of the injected second harmonics, it is possible to make the third order IM product produced by the second harmonics and the original third order product out of phase and equal in amplitude. As a result the third order IM will be eliminated, in theory. In order to analyse the technique mathematically an approximate general expression for non-linear transfer function is used with its first three terms [3]: Where Y,, is the input and VO is the output of the non-linear device. This model can be applied to a non-linear amplifier by modifying the output as the drain current and the transconductance as the non-linear parameter of the amplifier which gives [4] : Where his the drain current and Y,, is the input signals at the gate of the transistor. Without injecting the second harmonics, the two input signals which are equal in amplitude and slightly different in frequency are: In order to obtain simple equations the amplitudes of the fundamental signals are assumed to be unity. When (3) is substituded in (2) we obtain the output drain current Id as: Expanding the third term in (4) and using trigonometric identities we obtain : The terms in the form of cos(2@12&t and cos(zw22wi)t are the third order IM products which are located adjacent to the carriers. When we inject the second harmonics together with the fundamental signals the input signal is: Kn COS(UI~) + COS(U~~) + AICOS(~~I~+ 01) + A~cos(~cu~~+ &) (6) 144

3 19 September 1997, University of Leeds where 01 are the phases and Ai and A2 are the normalised amplitudes of the injected second harmonics. Substituding (6) in (2) gives all the components at the output of the amplifier. For simplicity, only the third order IM products in the form of (201-02)t are shown as (7) : The first and the third terms in (7) are the results of the interaction between- the injected second harmonics and the fimdamental signals and they have the same frequency as the original third order IM products which are the second terms in (7) and (5). By the proper selection of the phases and the amplitudes of the injected second harmonics, the original third order IM products and the ones which has been produced by thie second harmonics can cancel each other and the result will be the elimination of the third order IM products. This technique was investigated on the circuit configuration shown in Fig. 1. At the input the amplifier is matched to source by a broadband T-section circuit. Thie input capacitor of the transistor is used as a shunt element in the T-section circuit. This rconfiguration is used to ensure that the input circuit bandwidth does not affect the circuit performance and to enhance the match over the frequency band under investigation. The output circuitry consists of Tee-LC circuit and a transformer which provide the optimum load for the transistor while maintaining a match condition to the load. The circuit configuration shown in Fig.2 shows the second harmonic signal generators connected to the input of the amplifier. Second harmonic signals are injected in the amplifier after passing through the phase adjuster and the hybrid combiner, which combines fbndamentd signals and the second harmonics before the input of the amplifier. 3. CAD INVESTIGATION In the CAD simulation the MESFET model 2 fiom the LIBRA libriuy was used as the nonlinear active device. Figure 1 : The Amplifier Schematic Circuit Diagram. 145

4 I9 September 1997, University ofleeds Figure 2: The Amplifier Schematic Circuit Diagram with Injection of the Second Harmonics. The chosen frequencies of the two main input hndarnental signals are 2.5 GHz and GHz and their input power levels are -2 dbm. The output circuit has been matched to give the highest output fundamental power of approximately 1 dbm and the lowest third order IM power of -54 Bm. This has been done by CAD optimisation facility by changing the values Ll, L2, C1 and the transformer ratio N1. After matching the output circuit successhlly and without applying our technique the spectrum obtained at the output is shown in Fig3 which includes fimdamental signals and the third order IM products at 2.49 GHz and 2.52 GHz. Input and output power relations of hndamental signals and third order IM products are shown in Fig.4 again without applying our technique. After injection of the second harmonics, CAD optbnisation is used to find the right phase and amplitude of the injected signals in order to reduce the third order IM products by keeping the fhdamental power levels constant. The spectrum obtained after simulation is shown in Fig.5 and input-output power relations are shown in Fig6 These figures shows that applying our technique has reduced the third order IM products Eom -54 dsm to -104 dbm. The hndamental power levels is unchanged. 4. CONCLUSIONS A high efficiency low distortion amplifer in which the Second Harmonic Injecting Technique has been applied is presented. Theory and CAD simulation shows that the third order IM level at the output of a non-linear amplifier can be reduced by the injection of the second harmonics in the amplifier together with the fi"ental signals. 146

5 19 September 1997, University ofleeds 10.00, ImDRES =SAY? FREQ-GHz Figure 3: The Simulated Fundamental Powers and the Third1 Order IM Powers Before Employing the Technique. *?ES2.50 xpi(ls2.si 6 PRESL. 49 * PRES2.52 X.s;V? RSSLV? RESA'P RESAMP oco -2. occ PWR-dBm :'.ooo Figure 4: The Fundamental Powers (left hand axis) and the Third Order IM Powers (right hand axis) vs Input Power Before Employing the Technique s " 1 RESAMP , meq-ghz Figure 5: The Simulated Fundamental Powers and the Third Order IM Powers After Employing the Technique

6 ccdigital 1997 High Frequency Postgraduate Student Colloquium 19 September 1997, University ofleeds Figure 6: The Fundamental Powers (left hand axis) and the Third Order IM Powen (right hand axis) Vs Input Power After Employing the Technique. Mer non-linear optimisation and simulation a reduction of 50 de3 has been achieved in the third order IM products without affecting the fundamental power levels. This reduction in the third order IM products can be considered as a huge improvement in the amplifier distortion performance. ACKNOWLEDGEMENTS This work was supported by the Engineering and Physical Sciences Research Council (GM4933 1), UK. REFERENCES [ 11 A. H. Aghvami ~ London, Satellite Communications, Lecture Notes, King s College, [2] J.G. Mc Rory, R.H. Johnson, An RF Amplifier For Low IM Distortion, IEEE MTT-S Digest, pp , [3] Daniel Myer, Design Linear Feedfonvard Amplifiers For PCN Systems, Design Feature, Microwaves & RF, pp , September [4] M.R. Moazzam, C.S. Aitchison, The reduction of Third Order Intermodulation Product in Microwave Ampliiiers, IEE Colloquium on Solid State Power Ampl$mtion and Generation, Digest No: 1996/013, pp.7/1-7/5, Savoy Place London, 25 January [SI M.R. Moazzam, C.S. Aitchison, A Low Third Order Intermodulation Amplifier With Harmonic Feedback Circuitry, IEEE MTT-S Digest, pp ,