LMH2110_15 Datasheet, PDF (21/36 Pages) Texas Instruments – LMH2110 8-GHz Logarithmic RMS Power Detector with 45-dB Dynamic Range

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8 Application and Implementation
LMH2110
SNWS022D – JANUARY 2010 – REVISED JUNE 2015
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers must
validate and test their design implementation to confirm system functionality.
8.1 Application Information
The LMH2110 is a 45-dB Logarithmic RMS power detector particularly suited for accurate power measurements
of modulated RF signals that exhibit large peak-to-average ratios (PARs). The RMS detector implements the
exact definition of power resulting in a power measurement insensitive to high PARs. Such signals are
encountered, for exampe, in LTE and W-CDMA applications. The LMH2110 has an RF frequency range from
50 MHz to 8 GHz. It provides an output voltage that relates linearly to the RF input power in dBm. Its output
voltage is highly insensitive to temperature and supply variations.
8.2 Typical Applications
8.2.1 Application With Transmit Power Control
The LMH2110 can be used in a wide variety of applications such as LTE, W-CDMA, CDMA, and GSM. Transmit-
power-control-loop circuits make the transmit power level insensitive to 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 LMH2110 is especially suited for transmit power control applications, because it accurately
measures transmit power and is insensitive to temperature, supply voltage and modulation variations.
Figure 44 shows a simplified schematic of a typical transmit power control system. The output power of the PA is
measured by the LMH2110 through a directional coupler. The measured output voltage of the LMH2110 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 LMH2110 is typically low
enough, an optional low-pass filter can be placed in between the LMH2110 and the ADC to further reduce the
ripple.
RF
B
A
S ^GAIN
E
B
A
N ADC
D
EN
VGA
COUPLER
PA
ANTENNA
50:
OPTIONAL
V_DD
R_S
OUT
A1
RF_IN
A2
B1
C_S
LMH2110
EN
C2
B2, C1
GND
Figure 44. Transmit Power Control System
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