LMV221_16 Datasheet, PDF (38/48 Pages) Texas Instruments – LMV221 50-MHz to 3.5-GHz 40-dB Logarithmic Power Detector for CDMA and WCDMA

LMV221
SNWS018D – DECEMBER 2006 – REVISED JUNE 2016
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8.2.2 Application With Voltage Standing Wave Ratio (VSWR) 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, however, 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 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 - _*_}
where
• Γ = V_REFLECTED / V_FORWARD denotes the reflection coefficient.
(17)
This means that to determine the VSWR, both the forward (transmitted) and the reflected power levels must be
measured. This can be accomplished by using two LMV221 RF power detectors according to Figure 82. 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 microcontroller 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 _LMV221
R_P1
EN
REF
43 5
GND
ADC1
C_P1
RF_IN
2
1
OUT
6
TRANSMITTED
POWER _LMV221
R_P2
EN _{4 3}
REF
5
GND
ADC2
C_P2
Copyright © 2016, Texas Instruments Incorporated
Figure 82. VSWR Application
9 Power Supply Recommendations
The LMV221 is designed to operate from an input voltage supply range from 2.7 V to 3.3 V. This input voltage
must be well regulated. Enable voltage levels lower than 400 mV below GND could lead to incorrect operation of
the device. Also, the resistance of the input supply rail must be low enough to ensure correct operation of the
device.
38
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