LMH2100 Datasheet, PDF (30/49 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

LMH2100
SNWS020C – NOVEMBER 2007 – REVISED OCTOBER 2015
8 Application and Implementation
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NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
8.1.1 Functionality and Application of RF Power Detectors
This section describes the functional behavior of RF power detectors and their typical application. Based on a
number of key electrical characteristics of RF power detectors, Functionality of RF Power Detectors discusses
the functionality of RF power detectors in general and of the LMH2100 LOG detector in particular. Subsequently,
Typical Applications describes two important applications of the LMH2100 detector.
8.1.1.1 Functionality of RF Power Detectors
An RF power detector is a device that produces a DC output voltage in response to the RF power level of the
signal applied to its input. A wide variety of power detectors can be distinguished, each having certain properties
that suit a particular application. This section provides an overview of the key characteristics of power detectors,
and discusses the most important types of power detectors. The functional behavior of the LMH2100 is
discussed in detail.
8.1.1.1.1 Key Characteristics of RF Power Detectors
Power detectors are used to accurately measure the power of a signal inside the application. The attainable
accuracy of the measurement is therefore dependent upon the accuracy and predictability of the detector transfer
function from the RF input power to the DC output voltage.
Certain key characteristics determine the accuracy of RF detectors and they are classified accordingly:
• Temperature Stability
• Dynamic Range
• Waveform Dependency
• Transfer Shape
Generally, the transfer function of RF power detectors is slightly temperature dependent. This temperature drift
reduces the accuracy of the power measurement, because most applications are calibrated at room temperature.
In such systems, the temperature drift significantly contributes to the overall system power measurement error.
The temperature stability of the transfer function differs for the various types of power detectors. Generally,
power detectors that contain only one or few semiconductor devices (diodes, transistors) operating at RF
frequencies attain the best temperature stability.
The dynamic range of a power detector is the input power range for which it creates an accurately reproducible
output signal. What is considered accurate is determined by the applied criterion for the detector accuracy; the
detector dynamic range is thus always associated with certain power measurement accuracy. This accuracy is
usually expressed as the deviation of its transfer function from a certain predefined relationship, such as ”linear in
dB" for LOG detectors and ”square-law" transfer (from input RF voltage to DC output voltage) for Mean-Square
detectors. For LOG-detectors, the dynamic range is often specified as the power range for which its transfer
function follows the ideal linear-in-dB relationship with an error smaller than or equal to ±1 dB. Again, the
attainable dynamic range differs considerably for the various types of power detectors.
According to its definition, the average power is a metric for the average energy content of a signal and is not
directly a function of the shape of the signal in time. In other words, the power contained in a 0-dBm sine wave is
identical to the power contained in a 0-dBm square wave or a 0-dBm WCDMA signal; all these signals have the
same average power. Depending on the internal detection mechanism, though, power detectors may produce a
slightly different output signal in response to the aforementioned waveforms, even though their average power
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