LMH2100 Datasheet, PDF (35/49 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
From the expression above it follows that one would design the F_EST transfer function to be the inverse of the
F_DET transfer function.
In practice the power measurement error will not be zero, due to the following effects:
• The detector transfer function is subject to various kinds of random errors that result in uncertainty in the
detector output voltage; the detector transfer function is not exactly known.
• The detector transfer function might be too complicated to be implemented in a practical estimator.
The function of the estimator is then to estimate the input power P_IN, that is, to produce an output P_EST such that
the power measurement error is - on average - minimized, based on the following information:
1. Measurement of the not completely accurate detector output voltage V_OUT
2. Knowledge about the detector transfer function F_DET, for example the shape of the transfer function, the
types of errors present (part-to-part spread, temperature drift) etc.
Obviously the total measurement accuracy can be optimized by minimizing the uncertainty in the detector output
signal (select an accurate power detector), and by incorporating as much accurate information about the detector
transfer function into the estimator as possible.
The knowledge about the detector transfer function is condensed into a mathematical model for the detector
transfer function, consisting of:
• 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.
Errors may occur when the operating conditions of the detector (for example, the temperature) become
significantly different from the operating conditions during calibration (for example, room temperature). Examples
of simple estimators for power measurements that result in a number of commonly used metrics for the power
measurement error are discussed in LOG-Conformance Error, the Temperature Drift Error, the Temperature
Compensation and Temperature Drift Error.
8.2.1.2.1.2 RF Input
RF parts typically use a characteristic impedance of 50 Ω. To comply with this standard the LMH2100 has an
input impedance of 50 Ω. Using a characteristic impedance other then 50 Ω will cause a shift of the logarithmic
intercept with respect to the value given in the 2.7-V DC and AC Electrical Characteristics. This intercept shift
can be calculated according to Equation 13.
©§ ¹· ^{PINT-SHIFT = 10 LOG}
2 R_SOURCE
R_SOURCE + 50
(13)
The intercept will shift to higher power levels for R_SOURCE > 50 Ω, and will shift to lower power levels for R_SOURCE
< 50 Ω.
8.2.1.2.1.3 Output and Reference
The possible filtering techniques that can be applied to reduce ripple in the detector output voltage are discussed
in Filtering. In addition two different topologies to connect the LMH2100 to an ADC are elaborated.
8.2.1.2.1.3.1 Filtering
The output voltage of the LMH2100 is a measure for the applied RF signal on the RF input pin. Usually, the
applied RF signal contains AM modulation that causes low frequency ripple in the detector output voltage. CDMA
signals for instance contain a large amount of amplitude variations. Filtering of the output signal can be used to
eliminate this ripple. The filtering can either be realized by a low pass output filter or a low pass feedback filter.
Those two techniques are depicted in Figure 81 and Figure 82.
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