LMH2100 Datasheet, PDF (32/49 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
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Application Information (continued)
8.1.1.1.2 Types of RF Power Detectors
Three different detector types are distinguished based on the four characteristics previously discussed:
• Diode Detector
• (Root) Mean Square Detector
• Logarithmic Detectors
8.1.1.1.2.1 Diode Detector
A diode is one of the simplest types of RF detectors. As depicted in Figure 78, the diode converts the RF input
voltage into a rectified current. This unidirectional current charges the capacitor. The RC time constant of the
resistor and the capacitor determines the amount of filtering applied to the rectified (detected) signal.
Z₀
D
VREF
R_S
C_S V_OUT
Figure 78. Diode Detector
The advantages and disadvantages can be summarized as follows:
• The temperature stability of the diode detectors is generally very good, since they contain only one
semiconductor device that operates at RF frequencies.
• The dynamic range of diode detectors is poor. The conversion gain from the RF input power to the output
voltage quickly drops to very low levels when the input power decreases. Typically a dynamic range of 20 dB
to 25 dB can be realized with this type of detector.
• The response of diode detectors is waveform dependent. As a consequence of this dependency for example
its output voltage for a 0-dBm WCDMA signal is different than for a 0-dBm unmodulated carrier. This is due to
the fact that the diode measures peak power instead of average power. The relation between peak power and
average power is dependent on the wave shape.
• The transfer shape of diode detectors puts high requirements on the resolution of the ADC that reads their
output voltage. Especially at low input power levels a very high ADC resolution is required to achieve
sufficient power measurement accuracy (See Figure 76).
8.1.1.1.2.2 (Root) Mean Square Detector
This type of detector is particularly suited for the power measurements of RF modulated signals that exhibits
large peak to average power ratio variations. This is because its operation is based on direct determination of the
average power and not – like the diode detector – of the peak power.
The advantages and disadvantages can be summarized as follows:
• The temperature stability of (R)MS detectors is almost as good as the temperature stability of the diode
detector; only a small part of the circuit operates at RF frequencies, while the rest of the circuit operates at
low frequencies.
• The dynamic range of (R)MS detectors is limited. The lower end of the dynamic range is limited by internal
device offsets.
• The response of (R)MS detectors is highly waveform independent. This is a key advantage compared to other
types of detectors in applications that employ signals with high peak-to-average power variations. For
example, the (R)MS detector response to a 0-dBm WCDMA signal and a 0-dBm unmodulated carrier is
essentially equal.
• The transfer shape of R(MS) detectors has many similarities with the diode detector and is therefore subject
to similar disadvantages with respect to the ADC resolution requirements (see Figure 77).
8.1.1.1.2.3 Logarithmic Detectors
The transfer function of a logarithmic detector has a linear in dB response, which means that the output voltage
changes linearly with the RF power in dBm. This is convenient since most communication standards specify
transmit power levels in dBm as well.
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