LMH2100 Datasheet, PDF (31/49 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

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LMH2100
SNWS020C – NOVEMBER 2007 – REVISED OCTOBER 2015
Application Information (continued)
level is the same. This is due to the fact that not all power detectors strictly implement the definition formula 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 independent 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 resolution of the ADC connected to it. The overall power measurement 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 transfer shape, as shown
in Figure 76 and Figure 77.
2
2
ÂV
ÂV1
ÂV2
0
-60
ÂP
0
ÂP
RF INPUT POWER (dBm)
ÂV
0
-60
ÂP
0
ÂP
RF INPUT POWER (dBm)
Figure 76. Convex Detector Transfer Function
Figure 77. Linear Transfer Function
Figure 76 and Figure 77 shows two different representations of the detector transfer function. In both graphs the
input power along the horizontal axis is displayed in dBm, since most applications specify power accuracy
requirements in dBm (or dB). The figure on the left shows a convex detector transfer function, while the transfer
function on the right hand side is linear (in dB). The slope of the detector transfer function — the detector
conversion gain – is of key importance for the impact of the quantization error on the total measurement error. If
the detector transfer function slope is low, a change, ΔP, in the input power results only in a small change of the
detector output voltage, such that the quantization error will be relatively large. On the other hand, if the detector
transfer function slope is high, the output voltage change for the same input power change will be large, such
that the quantization error is small. The transfer function on the left has a very low slope at low input power
levels, resulting in a relatively large quantization error. Therefore, to achieve accurate power measurement in this
region, a high-resolution ADC is required. On the other hand, for high input power levels the quantization error
will be very small due to the steep slope of the curve in this region. For accurate power measurement in this
region, a much lower ADC resolution is sufficient. The curve on the right has a constant slope over the power
range of interest, such that the required ADC resolution for a certain measurement accuracy is constant. For this
reason, the LOG-linear curve on the right will generally lead to the lowest ADC resolution requirements for
certain power measurement accuracy.
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