LMH2100 Datasheet, PDF (24/49 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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Feature Description (continued)
This section shows how calibration techniques can be used to improve the accuracy of a power measurement
system beyond the intrinsic accuracy of the power detector itself. The main focus of the section is on power
measurement systems using LOG-detectors, specifically the LMH2100, but the more generic concepts can also
be applied to other power detectors. Other factors influencing the power measurement accuracy, such as the
resolution of the ADC reading the detector output signal will not be considered here since they are not
fundamentally due to the power detector.
7.3.2.1 LOG-Conformance Error
Probably the simplest power measurement system that can be realized is obtained when the LOG-detector
transfer function is modelled as a perfect linear-in-dB relationship between the input power and output voltage:
V_OUT,MOD = F_DET,MOD(P_IN) = K_SLOPE(P_IN ± P_INTERCEPT)
(1)
in which K_SLOPE represents the LOG-slope and P_INTERCEPT the LOG-intercept. The estimator based on this model
implements the inverse of the model equation, that is:
P_EST = F_EST(V_OUT) =
V_OUT
K_SLOPE
+ P_INTERCEPT
(2)
The resulting power measurement error, the LOG-conformance error, is thus equal to:
E_LCE = P_EST
-
P_IN
=
V_OUT
K_SLOPE
- (P_IN - P_INTERCEPT )
=
V_OUT - V_OUT,MOD
K_SLOPE
(3)
The most important contributions to the LOG-conformance error are generally:
• The deviation of the actual detector transfer function from an ideal Logarithm (the transfer function is
nonlinear in dB).
• Drift of the detector transfer function over various environmental conditions, most importantly temperature;
K_SLOPE and P_INTERCEPT are usually determined for room temperature only.
• Part-to-part spread of the (room temperature) transfer function.
The latter component is conveniently removed by means of calibration, that is, if the LOG slope and LOG-
intercept are determined for each individual detector device (at room temperature). This can be achieved by
measurement of the detector output voltage - at room temperature - for a series of different power levels in the
LOG-linear range of the detector transfer function. The slope and intercept can then be determined by means of
linear regression.
An example of this type of error and its relationship to the detector transfer function is depicted in Figure 71.
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