LMH2100 Datasheet, PDF (19/32 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

Application Information
The LMH2100 is a versatile logarithmic RF power detector
suitable for use in power measurement systems. The
LMH2100 is particularly well suited for CDMA and UMTS ap-
plications. It produces a DC voltage that is a measure for the
applied RF power.
This application section describes the behavior of the
LMH2100 and explains how accurate measurements can be
performed. Besides this an overview is given of the interfacing
options with the connected circuitry as well as the recom-
mended layout for the LMH2100.
1.0 FUNCTIONALITY AND APPLICATION OF RF POWER
DETECTORS
This first section describes the functional behavior of RF pow-
er detectors and their typical application. Based on a number
of key electrical characteristics of RF power detectors, section
1.1 discusses the functionality of RF power detectors in gen-
eral and of the LMH2100 LOG detector in particular. Subse-
quently, section 1.2 describes two important applications of
the LMH2100 detector.
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 distin-
guished, 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.
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 de-
tectors and they are classified accordingly:
• Temperature Stability
• Dynamic Range
• Waveform Dependency
• Transfer Shape
Each of these aspects is discussed in further detail.
Generally, the transfer function of RF power detectors is
slightly temperature dependent. This temperature drift re-
duces 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) oper-
ating 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 ap-
plied criterion for the detector accuracy; the detector dynamic
range is thus always associated with certain power measure-
ment accuracy. This accuracy is usually expressed as the
deviation of its transfer function from a certain predefined re-
lationship, 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 at-
tainable 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 pow-
er contained in a 0 dBm square wave or a 0 dBm WCDMA
signal; all these signals have the same average power. De-
pending on the internal detection mechanism, though, power
detectors may produce a slightly different output signal in re-
sponse to the aforementioned waveforms, even though their
average power level is the same. This is due to the fact that
not all power detectors strictly implement the definition for-
mula for signal power, being the mean of the square of the
signal. Most types of detectors perform some mixture of peak
detection and average power detection. A waveform inde-
pendent detector response is often desired in applications
that exhibit a large variety of waveforms, such that separate
calibration for each waveform becomes impractical.
The shape of the detector transfer function from the RF input
power to the DC output voltage determines the required res-
olution of the ADC connected to it. The overall power mea-
surement error is the combination of the error introduced by
the detector, and the quantization error contributed by the
ADC. The impact of the quantization error on the overall
transfer's accuracy is highly dependent on the detector trans-
fer shape, as illustrated in Figure 1.
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