Transcript Document

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 jz
V(z), I(z)

V z   Vo e jz  e jz
IL
Vo e jz
+
Z0, 
Z L  Z0

Z L  Z0
ZL
-
z3
z2
z1
  w LC
z
0
l
Vz1 e  jz1  e jz1
r

Vz 2 e  jz2  e jz2


cos z1   j sin  z1   real  jimag cos jz1   j sin z1 
cos z2   j sin  z2   real  jimag cos jz2   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 
Vz1
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
jZ 0 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