Low Noise Microwave amplifiers with improved input matching applicable in active array antennas

Transcription

1 JAE, VO. 17, NO.1, 15 JOURNA OF AIED EECTROMAGNETIM ow Noise Microwave amplifiers with improved put matchg applicable active array antennas M.. Tonev Technical University of ofia, Bulgaria Faculty of Tlecommunication, TU-ofia, Kl.Ohridsky str. 8, 1 ofia, Bulgaria mihail_tonev@abv.bg Abstract The difference between the optimal put reflection coefficient for achievg mimal noise figure and those needed for perfect matchg is usually a challenge for low noise amplifiers design. Normally amplifiers designed to achieve lowest possible noise figure have very poor put reflection coefficient. This may cause a lot of problems case of their application as a component modern active phased array antennas and can be a reason for significant performance degradation. One of the possible solutions is to use balanced amplifier configuration order to improve put and output matchg. The ma disadvantages of this solution are degradation of the noise figure due to the losses the put hybrid and a bigger occupied space. Another approach for solvg the problem is presented this paper. The non unilateral properties of the transistors are used order to obta mimum noise figure and at the same time conjugate matchg. ractical circuits and achieved performance are discussed details. 1. INTRODUCTION As known general for every radio-communications receiver first stage of put amplifier is matched to achieve mimal noise figure. This often leads to poor put return loss of the receiver and when it is directly connected to the receivg antenna may cause problems [5]. Usually microwave antennas are designed to be matched at their output to 5 ohms. When they see different impedance, for example as those of poorly matched low-noise amplifier, their characteristics might be changed. This is true especially concerng phased antenna arrays. Good example for such mismatch sensitive system is active antenna arrays used modern mobile satellite communications. uch type of antennas must provide dynamic polarization control while on move. In order to support such functionality antennas must comprise dual port antenna elements receivg signals with two orthogonal lear polarizations and two dependent summation feed circuits. In order to achieve good isolation between these summation circuits and to avoid phase and amplitude errors antenna aperture it is need to ensure good matchg between all feed le components side the 3

2 OW NOIE MICROWAVE AMIFIER M.. TONEV antenna. o if low noise amplifiers at the antenna outputs are not well matched to their characteristic impedance it may cause significant degradation of the most important antenna parameters, such as ga and cross-polarization isolation. Two possible solutions of the mentioned above problem are known practice. The first possible solution is to connect isolator between antenna and the receiver []. This completely solves the problem with poor put matchg of the receiver but degrades its nose figure performance because of the sertion loss troduced by the isolator. From another side usg isolator with low sertion loss will significantly crease price of the device, which is another factor that restrict usg isolators. Another less expensive solution is to use balanced amplifiers [6]. Balancg technique is well known method for improvg return loss of low noise amplifiers, but it also degrades noise performance due to the put hybrid device loss. Other disadvantages of balanced amplifiers are twice bigger power consumption and twice bigger occupied space compared to sgle ended amplifier. These disadvantages sometimes can make them not applicable. Another approach, usg a novel matchg technique allowg achievg simultaneously low noise figure and good matchg for such type of amplifiers is described the presented paper.. MICROWAVE TRANITOR A TWO ORT NETWORK At microwave frequencies transistors are usually presented as two port networks, which are characterized with -parameters [1]. uch two port network with defed load and source impedances is presented schematically on Fig.1. Figure.1 Two-port network with specific source and load impedances. The three types of power gas for an arbitrary two-port network connected to source and load impedances, and may be defed as follows [3]: 31

3 JAE, VO. 17, NO.1, 15 JOURNA OF AIED EECTROMAGNETIM ower Ga = G = / is the ratio of power dissipated the load to the power delivered to the put of the two-port network. This ga is dependent of. Available Ga = G A = avn / avs is the ratio of the power available from the two-port network to the power available from the source. It depends on but not on. Transducer ower Ga = G T = / avs is the ratio of the power dissipated the load to the power delivered to the put of two port network. This depends on both and. These defitions differ primarily the way the source and load are matched to the two-port device. If the put and output are both conjugate matched, then the ga is maximized and G = G A = G T. Equations for these gas and reflection coefficients terms of the parameters of the active device can be defed, referrg to Fig.1, as follows [1]: (1) () (3) where G G G A T 1 avn avs out (4), and (5a) (5b) out out out In general to obta good put matchg it is necessary put reflection coefficient of two port network to be close to zero. From equation 5a can be seen that 3

4 OW NOIE MICROWAVE AMIFIER M.. TONEV depends on active component parameters ( 11, 1, 1 ) and on load reflection coefficient. This means that we can manipulate put reflection coefficient of two port network by changg load impedance. 3. DEIGN OF MICROWAVE OW NOIE AMIFIER WITH IMROVED INUT MATCHING The ma goal designg low nose amplifiers is obtag proper reflection coefficient from put matchg network. This coefficient must cocide with optimum reflection coefficient for obtag mimum noise figure of active component. Usually this coefficient is different from conjugate machg and this is the reason of NA s not to be zero. It is possible to design proper put matchg network and then to choose load impedance, which will mimize put reflection coefficient of the two port network. In this case amplifier will be simultaneously matched for obtag mimum nose figure and conjugate matched with source. In general for sgle stage amplifier this will worse output matchg. The problem can be solved by addg second stage amplifier configuration. Then usg ter-stage matchg circuit we can chose proper reflection coefficient to make and then output matchg circuit can be designed to make out. The approach, presented above was proven by designg microwave NA, suitable for usg Ku band satellite communications workg frequency band from 1.7GHz to 1.8GHz. It was designed on Arlon 5N substrate with.51mm thickness, dielectric constant 3.38 and loss tangent.5. Used transistors are NE31 low noise hetero-junction FET, manufactured by NEC. The -parameters of transistors and capacitors, provided by manufacturers were putted simulator and circuit simulation was performed. The first step design is to defe put matchg circuit. In Fig. reflection coefficients of put matchg circuit and optimum reflection coefficient of transistor are shown. As can be seen, there is cocidence between them which means that transistor is matched for obtag mimum noise figure. Once put of the amplifier is matched, the ter-stage and output matchg circuit can be designed, usg optimization procedure order to achieve good put and output matchg. imulated -parameters and noise figure of NA model are shown fig.3 and fig4. 33

5 JAE, VO. 17, NO.1, 15 JOURNA OF AIED EECTROMAGNETIM Figure. Reflection from put matchg circuit and transistor opt Figure.3 imulated -parameters of NA Figure.4 imulated noise Figure of NA 34

6 OW NOIE MICROWAVE AMIFIER M.. TONEV The put and output reflection coefficients are lower than.1 and noise figure is lower than.5db. imulated amplifier has low noise figure and good matchg with source and load. To ga real side how circuit works the simulated layout of amplifier was manufactured and measured. Measurement results are presented graphically Fig.5 and Fig.6. ome difference between measured and simulated results may be caused by the tolerances component parameters and by reflections from put and output connectors used for measurements. Measured put reflection coefficient is lower than.13 and measured noise figure is about.65db. Figure.5 Measured -parameters of NA Figure.6 Measured noise Figure of NA 35

7 JAE, VO. 17, NO.1, 15 JOURNA OF AIED EECTROMAGNETIM 4. CONCUION A design of microwave low-noise amplifier with improved put matchg was presented this paper. As was explaed this is critical parameter for some communication systems. ractically the proposed method for enhancg this parameter, by usg non-unilateral characteristic of microwave transistors was proved by model measurements. REFERENCE [1] ozar, Microwave Engeerg, 4th edition. [] Vendel Rohde, avio, Microwave circuit design usg lear and nonlear techniques. [3] Gilmore, ractical RF Circuit Design for Modern Wireless ystems, vol.. [4] Rohde, Microwave Circuit Design for Wireless Applications. [5] ozar, Microwave and RF Design of Wireless ystems. [6] Guillermo Gonzalez, Microwave Transistor Amplifiers Analysis and Design. [7] Engberg, Noise Theory of ear and Nonlear Circuits. [8] Butterworth, High Frequency and Microwave Engeerg. 36