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AN-653
APPLICATION NOTE
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ABSTRACT
Root-mean-square (rms) power detection is required to
measure and control transmitted power in a multicarrier
wireless infrastructure. Traditional power detectors using
diode detection or log amps do not accurately measure
power when the peak-to-average ratio of the transmitted
signal is not xed. Temperature stability of measurement
circuitry is critical as is the linearity of the detector’s trans-
fer function. This application note presents techniques to
improve the temperature stability of an rms power detector
and the linearity of its transfer function to less than ±0.3 dB
over a dynamic range of more than 50 dB.
INTRODUCTION
Modern wireless transmitters generally require strict
control of transmitted radio frequency (RF) power. In
wireless cellular networks, strict power control allows
the size of cells to be precisely set to enhance coverage.
Precise power control also avoids the need for excessive
thermal dimensioning of the RF power amplier (PA),
which is required when there is uncertainty about the
actual transmitted power. For example, if a 50 W (47 dBm)
power amplier has a transmit power uncertainty of just
Improving Temperature, Stability, and Linearity of
High Dynamic Range RMS RF Power Detectors
By Eamon Nash
1 dB, the PA must be dimensioned so that it can safely
(i.e., without overheating) transmit 63 W (48 dBm).
Power measurement and control is also used in receivers,
usually at intermediate frequencies (IFs). Here the objec-
tive is to measure and control the gain of the received
signal so that IF ampliers and analog-to-digital convert-
ers (ADCs) are not overdriven. While precision in the
measurement of the received signal (commonly referred
to as the received signal strength indicator or RSSI) is
useful for maximizing the signal-to-noise ratio, it is less
important than on the transmit side; the goal is to merely
keep the received signal under a certain limit.
RMS RF power detectors can measure RF power indepen-
dently of signal peak-to-average ratio or crest factor. This
capability is critical when the peak-to-average ratio of a
measured signal is changing. This is common in wireless
cellular networks due to the ever changing number of
calls being carried by a cellular base station. The changing
peak-to-average ratio results both from the transmission
of multiple carriers at varying power levels and from the
variations in code-domain power in a single code division
multiple access (CDMA) carrier.
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Figure 1. Modern wireless transmitters use RF power measurement and control to tightly regulate the
transmitted power. In receivers, power measurement is used to prevent overdrive of IF and baseband
components while maximizing signal-to-noise ratio.
REV. A
