LMV221_14 Datasheet, PDF (25/41 Pages) Texas Instruments – 50 MHz to 3.5 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

LMV221
www.ti.com
SNWS018B – DECEMBER 2006 – REVISED MARCH 2008
• A formula for the detector transfer function.
• Values for the parameters in this formula.
The values for the parameters in the model can be obtained in various ways. They can be based on
measurements of the detector transfer function in a precisely controlled environment (parameter extraction). If
the parameter values are separately determined for each individual device, errors like part-to-part spread are
eliminated from the measurement system.
Obviously, errors may occur when the operating conditions of the detector (e.g. the temperature) become
significantly different from the operating conditions during calibration (e.g. room temperature). Subsequent
sections will discuss examples of simple estimators for power measurements that result in a number of
commonly used metrics for the power measurement error: the LOG-conformance error, the temperature drift
error, the temperature sensitivity and differential power error.
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)
(3)
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, i.e.
P_EST = F_EST(V_OUT) =
V_OUT
K_SLOPE
+ P_INTERCEPT
(4)
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
(5)
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, i.e. 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 75.
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