lmh2120umx National Semiconductor Corporation, lmh2120umx Datasheet - Page 17

• Manufacturer Part Number: lmh2120umx
• Description: Lmh2120 6 Ghz Linear Rms Power Detector With 40 Db Dynamic Range
• Manufacturer: National Semiconductor Corporation

Application Information
The LMH2120 is a 40 dB Linear RMS power detector partic-
ularly 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 ap-
plications. 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-pow-
er level insensitive to power amplifier (PA) inaccuracy. This is
ACCURATE POWER MEASUREMENT
Detectors have evolved over the years along with the com-
munication standards. Newer communication standards like
LTE and W-CDMA raise the need for more advanced accu-
rate 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 mea-
surement 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 aver-
age power content.
The average power can be described by the following formu-
la:
where T is the time interval over which is averaged, v(t) is the
instantaneous voltage at time t, R is the resistance in which
FIGURE 1. Transmit-Power Control System
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17
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 inac-
curacy 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 ap-
plications, since it accurately measures transmit power and is
insensitive to temperature, supply voltage and modulation
variations.
Figure 1
power control system. The output power of the PA is mea-
sured 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 con-
trol 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 fur-
ther reduce the ripple.
the power is dissipated, and V
age.
According to aforementioned formula for power, an exact
power measurement can be done by measuring the RMS
voltage (V
Implementing the exact formula for RMS can be difficult how-
ever. A simplification can be made in determining the average
power when information about the waveform is available. If
the signal shape is known, the relationship between RMS
value and, for instance, the peak value of the RF signal is also
known. It thus enables a measurement based on measuring
peak voltage rather than measuring the RMS voltage. To cal-
culate the RMS value (and therewith the average power), the
measured peak voltage is translated into an RMS voltage
based on the waveform characteristics. A few examples:
•
•
•
Sine wave: V
Square wave: V
Saw-tooth wave: V
shows a simplified schematic of a typical transmit-
RMS
) of a signal. The RMS voltage is described by:
RMS
RMS
= V
RMS
= V
PEAK
= V
PEAK
/
RMS
PEAK
30055770
√
2
is the equivalent RMS volt-
/
√
3
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