lmh2120umx National Semiconductor Corporation, lmh2120umx Datasheet - Page 18

• Manufacturer Part Number: lmh2120umx
• Description: Lmh2120 6 Ghz Linear Rms Power Detector With 40 Db Dynamic Range
• Manufacturer: National Semiconductor Corporation

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For more complex waveforms it is not always easy to deter-
mine the exact relationship between RMS value and peak
value. A peak measurement can therefore become impracti-
cal. An approximation can be used for the V
relationship, but it can result in a less accurate average power
estimate.
Depending on the detection mechanism, power detectors
may produce a slightly different output signal in response to
the earlier mentioned waveforms, even though the average
power level of these signals are the same. This error is due
to the fact that not all power detectors strictly implement the
definition for signal power, being the root mean square (RMS)
of the signal. To cover for the systematic error in the output
response of a detector, calibration can be used. After calibra-
tion a look-up table corrects for the error. Multiple look-up
tables can be created for different modulation schemes.
TYPES OF RF DETECTORS
This section provides an overview of detectors based on their
detection principle. Detectors that will be discussed are:
•
•
•
Peak Detectors
A peak detector is one of the simplest type of detector, storing
the highest value arising in a certain time window. However,
a peak detector is typically used with a relatively long holding
time when compared to the carrier frequency and a relatively
short holding time with respect to the envelope frequency. In
this way a peak detector is used as AM demodulator or en-
velope tracker
A peak detector usually has a linear response. An example of
this is a diode detector
input voltage; subsequently, the RC filter determines the av-
eraging (holding) time. The selection of the holding time con-
figures the diode detector for its particular application. For
envelope tracking, a relatively small RC time constant is cho-
sen such that the output voltage tracks the envelope nicely.
In contrast, a configuration with a relatively large time con-
stant measures the maximum (peak) voltage of a signal.
Peak detectors
LOG Amp detectors
RMS detectors
FIGURE 2. Peak detection vs. envelope tracking
(Figure
2).
(Figure
3). The diode rectifies the RF
RMS
30055780
to V
PEAK
18
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 unman-
ageable.
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 piece-
wise linear approximation. Consequently, the LOG amp de-
tector does not implement an exact power measurement,
which implies a dependency on the signal shape. In systems
using various modulation schemes calibration and lookup ta-
bles 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 oper-
ation is based on exact determination of the average power,
i.e. it implements:
RMS detectors are particularly suited for the newer commu-
nication 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 ap-
plication due to different applied modulation schemes.
LMH2120 RF POWER DETECTOR
For optimal performance, the LMH2120 needs to be config-
ured correctly in the application. The detector will be dis-
cussed by means of its block diagram
the electrical interfacing are separately discussed for each pin
below.
FIGURE 3. Diode Detector
(Figure
30055788
4). Details of
(3)