Specifications

1.4 Ideal, real, and measured values
When you determine an impedance parameter value for a circuit component (resistor, inductor, or
capacitor), it is important to thoroughly understand what the value indicates in reality. The para-
sitics of the component and the measurement error sources, such as the test fixture’s residual
impedance, affect the value of impedance. Conceptually, there are three sorts of values: ideal, real,
and measured. These values are fundamental to comprehending the impedance value obtained
through measurement. In this section, we learn the concepts of ideal, real, and measured values, as
well as their significance to practical component measurements.
• An ideal value is the value of a circuit component (resistor, inductor, or capacitor) that
excludes the effects of its parasitics. The model of an ideal component assumes a purely resis-
tive or reactive element that has no frequency dependence. In many cases, the ideal value can
be defined by a mathematical relationship involving the component’s physical composition
(Figure 1-6 (a).) In the real world, ideal values are only of academic interest.
• The real value takes into consideration the effects of a component’s parasitics (Figure 1-6 (b).)
The real value represents effective impedance, which a real-world component exhibits. The real
value is the algebraic sum of the circuit component’s resistive and reactive vectors, which come
from the principal element (deemed as a pure element) and the parasitics. Since the parasitics
yield a different impedance vector for a different frequency, the real value is frequency dependent.
• The measured value is the value obtained with, and displayed by, the measurement instrument;
it reflects the instrument’s inherent residuals and inaccuracies (Figure 1-6 (c).) Measured
values always contain errors when compared to real values. They also vary intrinsically from
one measurement to another; their differences depend on a multitude of considerations in
regard to measurement uncertainties. We can judge the quality of measurements by comparing
how closely a measured value agrees with the real value under a defined set of measurement
conditions. The measured value is what we want to know, and the goal of measurement is to
have the measured value be as close as possible to the real value.
Figure 1-6. Ideal, real, and measured values
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