LMH2100 Datasheet, PDF (38/49 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

LMH2100
SNWS020C – NOVEMBER 2007 – REVISED OCTOBER 2015
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8.2.2 Application With Voltage Standing Wave Ratio Measurement
Transmission in RF systems requires matched termination by the proper characteristic impedance at the
transmitter and receiver side of the link. In wireless transmission systems though, matched termination of the
antenna can rarely be achieved. The part of the transmitted power that is reflected at the antenna bounces back
toward the PA and may cause standing waves in the transmission line between the PA and the antenna. These
standing waves can attain unacceptable levels that may damage the PA. A Voltage Standing Wave Ratio
(VSWR) measurement is used to detect such an occasion. It acts as an alarm function to prevent damage to the
transmitter.
VSWR is defined as the ratio of the maximum voltage divided by the minimum voltage at a certain point on the
transmission line:
1+ _*_
VSWR = _{1 - _*_}
(17)
Where Γ = V_REFLECTED / V_FORWARD denotes the reflection coefficient.
This means that to determine the VSWR, both the forward (transmitted) and the reflected power levels have to
be measured. This can be accomplished by using two LMH2100 RF power detectors according to Figure 88. A
directional coupler is used to separate the forward and reflected power waves on the transmission line between
the PA and the antenna. One secondary output of the coupler provides a signal proportional to the forward power
wave, the other secondary output provides a signal proportional to the reflected power wave. The outputs of both
RF detectors that measure these signals are connected to a micro-controller or baseband that calculates the
VSWR from the detector output signals.
RF
PA
COUPLER
ANTENNA
V_DD
MICRO
CONTROLLER
RF_IN
2
1
OUT
6
REVERSE
POWER _LMH2100
R_P1
EN
4
REF
35
GND
ADC1
C_P1
RF_IN
2
1
OUT
6
TRANSMITTED
POWER _LMH2100
R_P2
EN ₄
3
REF
5
GND
ADC2
C_P2
Figure 88. VSWR Application
38
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