Specifications

5-15. Equivalent circuit analysis and its application
Agilent’s impedance analyzers are equipped with an equivalent circuit analysis function. The pur-
pose of this function is to model the various kinds of components as three- of four-element circuits.
The values of the component’s main elements and the dominant residuals can be individually deter-
mined with this function.
Many impedance measurement instruments can measure the real (resistive) and the imaginary
(inductive or capacitive reactance) components of impedance in both the series and parallel modes.
This models the component as a two-element circuit. The equivalent circuit analysis function
enhances this to apply to a three- or four-element circuit model using the component’s frequency
response characteristics. It can also simulate the frequency response curve when the values of the
three- or four-element circuit are input.
Impedance measurement at only one frequency is enough to determine the values of each element in
a two-element circuit. For three- or four-element circuits, however, impedance measurements at
multiple frequencies are necessary. This is because three (four) equations must be set up to obtain
three (four) unknown values. Since two equations are set up using one frequency (for the real and
imaginary), one more frequency is necessary for one or two more unknowns. The equivalent circuit
analysis function automatically selects two frequencies where the maximum measurement accuracy
is obtained. (This is at the frequency where the √
—
2 × minimum value or √
—
2 × maximum value is
obtained). If the equivalent circuit model (described later) is properly selected, accuracy for
obtained values of a three- or four-element circuit is comparable to the measurement accuracy of
the instrument.
The equivalent circuit analysis function has five circuit modes as shown in Figure 5-44, which also
lists their applications. The following procedure describes how to use the equivalent circuit analy-
sis function.
1. Perform a swept frequency measurement for the unknown DUT using the |Z|-θ or |Y|-θ func-
tion. The sweep mode can be either linear or logarithmic.
2. Observe the frequency response curve. See the typical frequency response curve given in
Figure 5-44. Choose the circuit mode that is most similar to the measured curve.
3. Calculate the equivalent parameters by pressing the “Calculate Parameter” key (or the key with
the same function.) Three or four values for selected circuit mode are calculated and displayed.
4. Check the simulated frequency response curve. The simulated curve is calculated from the
obtained equivalent parameters. If the fitting quality between the simulated curve and the actu-
al measurement results is high, the proper circuit mode was selected. If not, try one of other
circuit modes.
5-34