LMH2110 Datasheet, PDF (16/33 Pages) National Semiconductor (TI) – 8 GHz Logarithmic RMS Power Detector with 45 dB Dynamic Range

LMH2110
SNWS022C – JANUARY 2010 – REVISED MARCH 2013
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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 (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 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.
Typical Application
The LMH2110 can be used in a wide variety of applications like LTE, W-CDMA, CDMA, GSM. This section
discusses the LMH2110 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 desired, since 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,
since it accurately measures transmit power and is insensitive to temperature, supply voltage and modulation
variations.
Figure 46 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 46. 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:
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