International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 Single-ended FET Mixer Design for 36MHz Bandwidth C-band Satellite Transponder Yi Yi Aye, Chaw Myat Nwe Abstract—The design and performance of an active single-ended FET mixer is presented. A single gate GaAs MESFET (Gallium Arsenide Metal Epitaxial Semiconductor FET) mixer is used for 36MHz bandwidth C-band satellite transponder between 6GHz and 4GHz frequency down conversion. The mixer is designed 6GHz RF input signal frequency with 2GHz local oscillator (LO) frequency and 4GHz IF (intermediate frequency) frequency. In this mixer design, DC biasing, input and output matching are considered. The IF filter is designed at the output of mixer to get the desired frequency and reject the unwanted signal frequency. IF filter is designed with 5th order Chebyshev response. Agilent’s Advance Design System (ADS 2009) software has been used for simulation and optimization of the circuits. Index Terms—single-ended mixer, GaAsFET, DC biasing, Input/output matching and IF filter. I. INTRODUCTION Mixers are key devices for front-end components in any transceiver of a communication system. A mixer is a three-port device that uses a nonlinear or time varying element to achieve frequency conversion. Firstly choose passive or active and down or up frequency conversion. The down conversion mixer is used to convert the RF signal down to an intermediate frequency by mixing the RF signal from the Low Noise Amplifier (LNA) with the local oscillator (LO) signal. Active mixers can supply a conversion gain instead of loss. They require less LO drive power and much less sensitive to port terminations. Active mixers also possess a more noise figure than passive mixer [1]. this paper, mixer and filter designs are presented. The input of frequency converter is low noise amplifier (LNA) and the output is high power amplifier. The input signal frequency is 6GHz and 2GHz (LO frequency) at the output of (LNA). The IF filter is designed at the output of mixer at 4GHz frequency. II. PROCEDURE FOR MIXER DESIGN A. Design Selection Mixer types are single-ended (unbalance), single balance, double- balance and image rejection. Single-ended mixer is presented in this paper. Single-ended mixer type can have spurious outputs than other types. This type is simple, low cost, low power consumption and low isolation. Balance mixers have high power, high isolation and additional components [2]. B. Component Selection Microwave mixer can be designed by using Schottky barrier diode or FET, either MESFET or HEMT. Using a FET rather than a diode as the non-linear element in a mixer has several advantages. Some of these include the possibility of achieving a conversion gain, using lower LO drive power and obtaining isolation between the signal ports of the FET. This mixer design uses Gallium Arsenide Metal Semiconductor FET (GaAsMESFET) component. The MESFET is biased by the two sources, the drain-to-source voltage and the gate-to-source voltage. These voltages control the channel current by varying the width of the gate-depletion region and the longitudinal electric field. This device provides low noise and higher gain in established solid state application. It also provides high frequency characteristics unavailable from bipolar transistors. The electron mobility of gallium arsenide is five to seven times than of silicon [3]. III. MIXER PERFORMANCE PARAMETERS A. Conversion Loss Fig.1 Block diagram of C-band satellite transponder C-band satellite transponder model is shown in Fig.1. Band Pass Filter, Low Noise Amplifier, Frequency Converter and High Power Amplifier sections are included in this model. In Yi Yi Aye, Department of Electronic Engineering, Mandalay Technological University, Mandalay, Myanmar, (yiyiaye57@gmail.com), Myanmar, +95-94300272. Chaw Myat Nwe, Department of Electronic Engineering, Mandalay Technological University, Mandalay, Myanmar,+95-9259034924, (chawmyatnwe77@gmail.com). Mixer design requires impedance matching at three ports, complicated by the fact that several frequencies and their harmonics are involved. Each mixer port would be matched at its particular frequency (RF, LO, or IF), and undesired frequency products would be absorbed with resistive loads, or blocked with reactive terminations. There are inherent losses in the frequency conversion process because of the generation of undesired harmonics and other frequency products. An important figure of merit for a mixer is therefore the conversion loss, which is defined as the ratio of available RF input power to the available IF output power, expressed in dB. 1 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 Conversion loss applies to both up-conversion and down-conversion. Practical diode mixers typically have conversion losses between 4 and 7 dB in the 1–10 GHz range. Transistor mixers have lower conversion loss, and they may even have conversion gain of a few dB. The conversion gain of the FET mixer can be found as I D I DSS (1 VGS 2 ) Vp (2) VDD =5V (supply voltage) I DSS =60mA and Vp =-2V (from datasheet) VDS =3V, I D =30 mA 2 Gc gm R d 2 2 4 ω RF Cgs R i (1) VGS VP (1 The quantities g m , R d , R i and C gs are all parameters of the selected FET. The g m value is 60mS (max) and 20mS (min), R d is 2Ω and R i is 2 Ω for selected component in this design. Conversion gain is -6dB (min) and 3dB (max). One factor that strongly affects conversion loss is the LO power level; minimum conversion loss often occurs for LO powers between 0 and 10 dBm. This power level is large enough that the accurate characterization of mixer performance often requires nonlinear analysis. ID 2 ) I DSS (3) VGS =-0.6V VDD R D I D VDS (4) R D =66 Ω R G = 100 K Ω B. Noise Figure Noise is generated in mixers by the diode or transistor elements, and by thermal sources due to resistive losses. Noise figures of practical mixers range from 1 to 5 dB, with diode mixers generally achieving lower noise figures than transistor mixers. The noise figure of a mixer depends on whether its input is a single-sideband signal or a double sideband signal [4]. C. Port Isolation The isolation between LO and RF of the mixer is important as LO-to-RF feedthrough results in LO signal leaking through the antenna. The leaked LO signal should be small enough to avoid corrupting the desired signals of other RF systems. LO-to-IF and LO-to-RF isolation are not important because the high-frequency feedthrough signals can be rejected by the high-Q IF filter easily [5]. Fig.2 Schematic diagram of DC biasing B. For input and output matching In the input matching, RF and LO ports matching are designed at 6GHz and 2GHz. The input impedance can be estimated from S-parameters as follows: Γ in S11 IV. DESIGN CALCULATION S21S12 1 S22 (5) There are several FET parameters that offer nonlinearities that can be used for mixing, but the strongest is the transconductance g m , when the FET is operated in a common source configuration with a negative gate bias. When the gate bias is near the pinch-off region, where the transconductance approaches zero, a small positive variation of gate voltage can cause a large change in transconductance, leading to a nonlinear response. Thus the LO voltage can be applied to the gate of the FET to pump the transconductance to switch the FET between high- and low-transconductance states, thus providing the desired mixing function. The S-parameters of the transistor for 6 GHz (RF frequency) and 2GHz (LO frequency) are selected and input impedances are calculated. For RF matching, Γ in =0.81<-99.8 A. For DC Biasing Generally, two methods (dual power source and self-bias) can be used to bias a GaAs FET. Dual power source method is used in this design for DC biasing. This method is appropriate for use in the higher frequencies. When directly connecting the source to the ground terminal, source inductance can be made relatively small. By using this method, higher gain can be obtained and a lower noise factor anticipated in the higher frequencies [6]. Zin =0.32-2.8j. Zin 1 Γ in 1 Γ in (6) Zin =0.2-0.8j For LO port matching, Γ in =0.93<-39.39 The values are set on combined Smit-Chart and series and shunt reactive components are read from Chart. The actual component values are gained by using equation (7) and (8). 1 ωXN N L1 ωB C1 (7) (8) 2 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 Where, ω 2 πf X =the reactance as read from the chart B =the susceptance as read from the chart N =the number used to normalize the original impedances that are to be matched The series capacitor and shunt inductor values for RF port matching are 0.53pF and 1.1nH. C=0.88pF and L=11.36nH are the impedance values for LO port matching. The output impedance can be estimated from S-parameters as follows: Γ out S22 S21S12 1 S11 R D are used for DC bias. L1 , C1 and L 3 , C3 are used for input matching. L 2 and C2 are used as output matching. The extra capacitors C 4, and C 5 are used IF short resistance and bias. Supply voltage VDC is 5V. A capacitor C6 is used at IF output port to suppress the high frequency feedthrough signals. At mixer output, IF frequency is (RF freq-LO freq) and IF filter is designed at 4GHz. The expected results are gained by designing IF filter at mixer output. (9) The S-parameters from the datasheet of the selected transistor at 4 GHz (IF frequency) are achieved and calculate the output impedance. Γ out =0.72 -19.7 Z out 1 Γout 1 Γout (10) Zout =2.96-2.7j This value is set on Smit chart. Series inductance can be read from Z-chart and shunt capacitance can be get from Y-chart. The actual component values are found using eq (11) and eq (12). This completes the out put matching network. L2 XN ω (11) =4.38nH C2 B ωN (12) Fig.4 Complete Mixer Circuit with Bias and matching circuit C. IF filter design IF filter is designed at mixer output. This filter is designed with 5th order Chebyshev response. In this design, centre frequency is 4GHz, 0.5dB ripple and 500MHz bandwidth. =0.44pF Firstly the impedance ratio ( output. Load resistance Fig.3 Input and Output matching circuit diagram The schematic diagram for matching is shown in Fig.2. The shunt inductor L1 and series capacitor C1 are used for RF matching. The values are 1.1nH and 0.53pF respectively. At the output, the shunt inductor does short the RF signal feed-through. For output matching, establish an equivalent LC parallel circuit with L 2 and C2 . At IF frequency, the values are 4.38nH and 0.44pF [7]. For LO matching, L 3 =11.36nH and C 3 =0.88pF are used at the gate of the transistor. In the complete mixer circuit, DC bias tees, input and output matching circuit are included. Gate resistance R G , drain Rs ) is gained at the mixer RL R L is 50 Ohm and R s is the source impedance from the output of mixer. The element values of Chebyshev low-pass prototype are gained from table 3-6B in “RF Circuit Design”(second edition) book [8]. The actual transformation from the low-pass to the band-pass configuration is accomplished by resonating each low-pass element of the opposite type and the same value. All shunt elements of the low-pass prototype circuit become parallel resonant circuits, and all series elements become series-resonant circuits [8]. To complete the filter design, the transformed filter is then scaled using the following formulas. For the parallel-resonant branches, C Cn 2 πRB (13) L RB 2 2 πf 0 L n (14) For the series-resonant branches, 3 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 C B 2 2 πf 0 C n R (15) L RL n 2 πfB (16) power is (-10dBm) and maximum is +10dBm. The mixer performances (Conversion gain, port-to-port isolation and output spectrum) are shown with RF frequency and LO power sweep. Where, in all cases, R=the load impedance, B=the 3-dB bandwidth of the final design, f 0 =the center frequency of the final design, Ln =the normalized inductor band-pass element values, Cn = the normalized capacitor band-pass element values. Fig.8 Mixer complete circuit without IF filter Fig.5 Fifth-order Chebyshev Low pass prototype Fig.6 Fifth-order Chebyshev Band-pass prototype Fig.7 Fifth-order Chebyshev Bandpass Filter for 4 GHz Center frequency V. SIMULATION RESULTS The present design is simulated using the Advanced Design System 2009. An ADS (Advanced Design System) is a fast general purpose RF and microwave circuit design. Harmonic balance simulation makes possible the simulation of circuits with various types. Out put spectrums of mixer without IF and with IF filter are shown in Fig.9. In mixer output spectrum include spurious signal outputs and neglect them. Conversion gains with RF frequency and LO power sweep are described in Fig. 11(a) and (b). The maximum conversion gain of single-ended FET mixer can gain 2dB with RF power of -20dBm. The calculated conversion gain is between -6dB and 3dB, so this simulated result is good result. The results of Port-to-Port isolation versus RF frequency and LO power are described in Fig.12 (a) and (b). Port-to-Port isolation (dB) between RF and IF is between 10 and 5dB. LO-to-RF and LO-to-IF isolation is between -10 and -20 dB. Isolation of single-ended mixer type is lower than balance mixer design. This isolation results are satisfied. In the simulation, RF frequency range is (5 to 6) GHz with -20dBm power and LO frequency is 2 GHz. Minimum LO Fig.9 Mixer output spectrum without and with IF filter 4 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 Fig.12(b) Port-to-Port isolation with RF frequency Fig.10 Mixer complete circuit with IF filter VI. CONCLUSION A single gate GaAs MESFET mixer using a NE72000 microwave transistor was designed. The performances of mixer were presented with RF frequency and LO power. Calculated conversion gain of single-ended FET mixer with RF frequency is between -6dB and 3dB. Down conversion gain versus RF frequency obtained from simulation is between 2 and -2dB. So simulation results are found with good performances. The performance of the mixer was simulated and compared with the required specifications. Good agreement was found. Fig.11(a) Mixer conversion gain versus RF frequency Typically Port-to-Port isolation of single-ended mixer type is poorer than balance type. In this design, good isolation is found between RF-to-IF port. So port-to-port isolation is also good performance. ACKNOWLEDGMENT The author would like especially thank to Dr.Chaw Myat Nwe, for her encouragement and suggestions. The author wishes to express her special thanks to, Dr. Kyaw Soe Lwin for his kindness, helpful suggestions for this paper. I would like to thank all teachers in MTU. Especially, I would like to express my special thanks to my parents for their noble support and encouragement. REFERENCES [1] Nuriha Abd Rahman, Burhanuddin Yeop Majlis, “A GaAs PHEMT single-ended mixers for 28 GHz applications”, The 4th Annual Seminar of National Science Fellowship 2004. [2] steve Long, “Fundamentals of Mixer Design” Agilent EEsoft customer Fig.11(b) Mixer conversion gain with LO power [3] [4] [5] [6] [7] [8] Fig. 12(a) Port-to-Port isolation with LO power Education and Applications, Design Seminar, April 2001. Stephen A. Maas, “Nonlinear Microwave and RF Circuits”, Second Edition. David M.Pozar, University of Massachusetts at Amherst “Microwave Engineering” Fourth Edition. Keng Leong Fong, Member, IEEE and Robert G. Meyer, Fellow,IEEE, “Monolithic RF Active Mixer Design,” IEEE Transaction on circuits and Systems-II: Analog and Digital signal processing, vol.46,No.3,March 1999. Application Note, “Application of Microwave GaAs FETs”, California Eastern Laboratories. Esmat A.F. Abdalah, “Computer Aided Analysis and Design of Single Gate MESFET Mixer”, Electronics research Institute, Dokki, Cairo,Egypt. Chris Bowick with John Blyler and Ajluni, “RF Circuit Design” Second Edition. 5 All Rights Reserved © 2012 IJSETR