LMH2120 Datasheet, PDF (18/34 Pages) National Semiconductor (TI) – Linear RMS power detector particularly suited for accurate

LMH2120
SNWS021C – JULY 2010 – REVISED FEBRUARY 2013
ENVELOPE
PEAK
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CARRIER
Figure 55. Peak Detection vs. Envelope Tracking
A peak detector usually has a linear response. An example of this is a diode detector (Figure 56). The diode
rectifies the RF input voltage; subsequently, the RC filter determines the averaging (holding) time. The selection
of the holding time configures the diode detector for its particular application. For envelope tracking, a relatively
small RC time constant is chosen such that the output voltage tracks the envelope nicely. In contrast, a
configuration with a relatively large time constant measures the maximum (peak) voltage of a signal.
Z_{0 D}
V_REF
C
R
V_OUT
Figure 56. Diode Detector
Since peak detectors measure a peak voltage, their response is inherently dependent on the signal shape or
modulation form as discussed in the previous section. Knowledge about the signal shape is required to
determine an RMS value. For complex systems having various modulation schemes, the amount of calibration
and look-up tables can become unmanageable.
LOG Amp Detectors
LOG Amp detectors are widely used RF power detectors for GSM and the early W-CDMA systems. The transfer
function of a LOG amp detector has a linear-in-dB response, which means that the output in volts changes
linearly with the RF power in dBm. This is convenient since most communication standards specify transmit
power levels in dBm as well. LOG amp detectors implement the logarithmic function by a piecewise linear
approximation. Consequently, the LOG amp detector does not implement an exact power measurement, which
implies a dependency on the signal shape. In systems using various modulation schemes calibration and lookup
tables might be required.
RMS Detectors
An RMS detector has a response that is insensitive to the signal shape and modulation form. This is because its
operation is based on exact determination of the average power, i.e. it implements:
³ ^{VRMS =}
1
T
v(t)²dt
(3)
RMS detectors are particularly suited for the newer communication standards like W-CDMA and LTE that exhibit
large peak-to-average ratios and different modulation schemes (signal shapes). This is a key advantage
compared to other types of detectors in applications that employ signals with high peak-to-average power
variations or different modulation schemes. For example, the RMS detector response to a 0 dBm modulated W-
CDMA signal and a 0 dBm unmodulated carrier is essentially equal. This eliminates the need for long calibration
procedures and large calibration tables in the application due to different applied modulation schemes.
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