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LMH2120 Datasheet, PDF (17/34 Pages) National Semiconductor (TI) – Linear RMS power detector particularly suited for accurate
LMH2120
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SNWS021C – JULY 2010 – REVISED FEBRUARY 2013
³ P = 1
T
T v(t)2
VRMS2
0 R dt = R
(1)
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 the power is dissipated, and VRMS is the equivalent RMS voltage.
According to aforementioned formula for power, an exact power measurement can be done by measuring the
RMS voltage (VRMS) of a signal. The RMS voltage is described by:
³ VRMS =
1
T
v(t)2dt
(2)
Implementing the exact formula for RMS can be difficult however. 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 calculate 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: VRMS = VPEAK / √2
• Square wave: VRMS = VPEAK
• Saw-tooth wave: VRMS = VPEAK / √3
For more complex waveforms it is not always easy to determine the exact relationship between RMS value and
peak value. A peak measurement can therefore become impractical. An approximation can be used for the VRMS
to VPEAK relationship, but it can result in a less accurate average power estimate.
Depending on the detection mechanism, power detectors may produce a slightly different output signal in
response to the earlier mentioned waveforms, even though the average power level of these signals are the
same. This error is due to the fact that not all power detectors strictly implement the definition for signal power,
being the root mean square (RMS) of the signal. To cover for the systematic error in the output response of a
detector, calibration can be used. After calibration a look-up table corrects for the error. Multiple look-up tables
can be created for different modulation schemes.
TYPES OF RF DETECTORS
This section provides an overview of detectors based on their detection principle. Detectors that will be discussed
are:
• Peak Detectors
• LOG Amp Detectors
• RMS Detectors
Peak Detectors
A peak detector is one of the simplest type of detector, storing the highest value arising in a certain time window.
However, a peak detector is typically used with a relatively long holding time when compared to the carrier
frequency and a relatively short holding time with respect to the envelope frequency. In this way a peak detector
is used as AM demodulator or envelope tracker (Figure 55).
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