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Measurement of Antenna Load Impedance for Power Amplifiers Dongjiang Qiao, Tsaipi Hung, David Choi, Feipeng Wang, Mingyuan Li, and Peter Asbeck The Department of Electrical and Computer Engineering University of California, San Diego Outline Introduction Antenna load impedance measurement using sectioned transmission line Measurement results and discussion Future work and summary Introduction PA Duplexer Antenna Switch Antenna TX RX VSWR = 4:1 VSWR at antenna ref. plane can vary to 10:1 with any phase. Typical PA to antenna path loss ~ 1dB, which results in 4:1 VSWR at PA ref. plane. It is a challenge to maintain operation of the amplifier with such a wide range of impedances. -1 dB VSWR = 10:1 Performance of PA with Mismatched Load ADS simulation results for a class AB amplifier 45 40 10 0 -10 -30 -40 10 -50 0 5 10 Pout 15 VSWR = 1:1 20 -60 PAE 20 0 -10 -20 20 -30 -40 10 0 -50 0 5 10 Pout 15 20 -60 VSWR = 4:1 Mismatch causes: PAE Pout : need to change bias to maintain the necessary Pout. This may hurt the linearity. Possible oscillation of power amplifier thus damaging the amplifier IMD3 -20 10 30 IMD3 PAE 30 0 45 40 Voltage on Transmission Line for Unmatched Load 1.0 ts(Vout), V 0.5 0.0 -0.5 -1.0 0.0 0.5 1.0 1.5 2.0 2.5 2.0 2.5 2.0 2.5 time, nsec 1.0 ts(V1), V 0.5 0.0 -0.5 -1.0 0.0 0.5 1.0 1.5 time, nsec 1.0 0.5 ts(V2), V Z0 = 50 ohm ZL = 15 ohm 815 MHz 90o Transmission Line 0.0 -0.5 -1.0 0.0 0.5 1.0 1.5 time, nsec Measurement of Load Impedance Using Transmission Line Vo e jz V(z), I(z) V z Vo e jz e jz IL Vo e jz + Z0, Z L Z0 Z L Z0 ZL - z3 z2 z1 w LC z 0 l Vz1 e jz1 e jz1 r Vz 2 e jz2 e jz2 cos z1 j sin z1 real jimag cos jz1 j sin z1 cos z2 j sin z2 real jimag cos jz2 j sin z2 Dependence of Voltage Ratio Along Transmission Line on Load Impedance 4 3 2 V1/V2 Vout/V2 1 0 abs(Vout[1])/abs(V2[1]) abs(V1[1])/abs(V2[1]) 4 abs(Vout[1])/abs(V2[1]) abs(V1[1])/abs(V2[1]) abs(Vout[1])/abs(V2[1]) abs(V1[1])/abs(V2[1]) 4 3 2 Vout/V2 1 V1/V2 50 100 150 Zreal Im(ZL) = 0 ohm 200 2 Vout/V2 1 V1/V2 0 0 0 3 0 50 100 150 Zreal Im(ZL) = 50 ohm 200 0 50 100 150 Zreal Im(ZL) = -50 ohm 200 Measurement of Load Impedance Using Transmission Line Procedure Voltages are measured at 3 different points on a 90o transmission line Two voltage ratios are obtained from the 3 voltages r1 Vz 1 Vz 2 r2 Vz 3 Vz 2 Numerically solve equations for r1 and r2 to obtain the real and imag and thus ZL Characteristics Measurement results depend on voltage ratio, not the voltage Measurement results are independent of input power and the source impedance It is found there is only one solution for the equations for || <= 1 Measurement Setup Network Analyzer Fabricated on PCB board ¼ transmission line 815 MHz Single tone and CDMA IS-95 Loss caused by the setup is ~ 0.4 dB at 815 MHz Z 0 = 50 ohm 1.92 k ohm Impedance Tuner RF Power meters Measurement Results Single Tone CDMA 815 MHz Three input power levels (15, 18 and 20 dBm) 815 MHz Three input power levels (12, 14 and 16 dBm) Observations: Error increases with increasing VSWR Results do not depend on the input power Accuracy Analysis 4 abs(Vout[1])/abs(V2[1]) abs(V1[1])/abs(V2[1]) Use the voltages obtained from ADS simulation to calculate the load impedance 3 2 1 0 -1 1 real(gamma) When Zl is too low or too high, one of the voltages is too small , thus affecting the accuracy. Other possible error sources: Non-perfect soldering positions for resistors Lossy transmission line Non-identical resistors When Zl is high, the measurement circuit is comparable with Zl Reduce the Dimension of Transmission Line 4 abs(Vout[1])/abs(V2[1]) abs(V1[1])/abs(V2[1]) -Using shorter transmission line L = 45o ZL imag = 50 ohm 3 2 1 0 0 -Using lumped elements L 50 100 150 200 Zreal jZ0 sin L A B cos L C D jY sin L cos L 0 jX L 1 0 1 jX L 1 0 1 X L BC jBC 1 0 1 jBC 1 jBC 2 X L BC 1 X L BC Z0, 45o 45o Z0 abs(Vout[1])/abs(V2[1]) abs(V1[1])/abs(V2[1]) 4 3 2 1 0 0 50 100 Zreal Simulation shows the transmission line can be replaced by lumped elements 150 200 Summary A simple method has been developed to measure the antenna load impedance based on the measurement of the voltages at three points along a transmission line. The method is independent of input power and source impedance. Scalar voltage measurements give complex load impedance. The size of the measurement setup can be reduced by using shorter transmission line or lumped elements. The complex load impedance information can be used with tunable matching networks or bias control circuits to facilitate compensation of load mismatch. Future Work Correct the mismatch: use impedance measurement technique with tunable matching, bias control or other impedance mismatch correction schemes. PA Antenna Switch Duplexer Antenna Controller PA Duplexer Antenna Switch Controller Controller could be digital or analog. The tunable matching network can be designed together with the PA module. The tunable matching network can be an individual module. Antenna