LMH2120_15 Datasheet, PDF (16/34 Pages) Texas Instruments – LMH2120 6 GHz Linear RMS Power Detector with 40 dB Dynamic Range

LMH2120
SNWS021C – JULY 2010 – REVISED FEBRUARY 2013
APPLICATION INFORMATION
www.ti.com
The LMH2120 is a 40 dB Linear RMS power detector particularly suited for accurate power measurements of
modulated RF signals that exhibit large peak-to-average ratios (PAR’s). The RMS detector implements the exact
definition of power resulting in a power measurement insensitive to high PAR’s. Such signals are encountered,
e.g, in LTE and W-CDMA applications. The LMH2120 has an RF frequency range from 50 MHz to 6 GHz. It
provides an output voltage that relates linearly to the RF input power in volt. Its output voltage is highly
insensitive to temperature and supply variations.
TYPICAL APPLICATION
The LMH2120 can be used in a wide variety of applications like LTE, W-CDMA, CDMA and GSM. This section
discusses the LMH2120 in a typical transmit power control loop for such applications.
Transmit-power-control-loop circuits make the transmit-power level insensitive to power amplifier (PA)
inaccuracy. This is desirable because power amplifiers are non-linear devices and temperature dependent,
making it hard to estimate the exact transmit power level. If a control loop is used, the inaccuracy of the PA is
eliminated from the overall accuracy of the transmit power level. The accuracy of the transmit power level now
depends on the RF detector accuracy instead. The LMH2120 is especially suited for transmit-power control
applications, since it accurately measures transmit power and is insensitive to temperature, supply voltage and
modulation variations.
Figure 54 shows a simplified schematic of a typical transmit-power control system. The output power of the PA is
measured by the LMH2120 through a directional coupler. The measured output voltage of the LMH2120 is
digitized by the ADC inside the baseband chip. Accordingly, the baseband controls the PA output power level by
changing the gain control signal of the RF VGA. Although the output ripple of the LMH2120 is typically low
enough, an optional low-pass filter can be placed in between the LMH2120 and the ADC to further reduce the
ripple.
RF
B
A
S ^GAIN
E
B
A
N ADC
D
EN
VGA
OPTIONAL
R_S
C_S
PA
V_DD
COUPLER
ANTENNA
50:
OUT
A1
RF_IN
A2
B1
LMH2120
EN
C2
B2, C1
GND
Figure 54. Transmit-Power Control System
ACCURATE POWER MEASUREMENT
Detectors have evolved over the years along with the communication standards. Newer communication
standards like LTE and W-CDMA raise the need for more advanced accurate power detectors. To be able to
distinguish the various detector types it is important to understand what the ideal power measurement should
look like and how a power measurement is implemented.
Power is a metric for the average energy content of a signal. By definition it is not a function of the signal shape
over time. In other words, the power content of a 0 dBm sine wave is identical to the power content of a 0 dBm
square wave or a 0 dBm W-CDMA signal; all these signals have the same average power content.
The average power can be described by the following formula:
16
Submit Documentation Feedback
Product Folder Links: LMH2120
Copyright © 2010–2013, Texas Instruments Incorporated