LMH2110 Datasheet, PDF (18/33 Pages) National Semiconductor (TI) – 8 GHz Logarithmic RMS Power Detector with 45 dB Dynamic Range

LMH2110
SNWS022C – JANUARY 2010 – REVISED MARCH 2013
ENVELOPE
PEAK
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CARRIER
Figure 47. Peak detection vs. envelope tracking
A peak detector usually has a linear response. An example of this is a diode detector (Figure 48). The diode
rectifies the RF input voltage and 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. A
configuration with a relatively large time constant can be used for supply regulation of the power amplifier (PA).
Controlling the supply voltage of the PA can reduce power consumption significantly. The optimal mode of
operation is to set the supply voltage such that it is just above the maximum output voltage of the PA. A diode
detector with relative large RC time constant measures this maximum (peak) voltage.
Z_{0 D}
V_REF
C
R
V_OUT
Figure 48. Diode Detector
Since peak detectors measure a peak voltage, their response is inherently depended 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)
18
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