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694 ⏐⏐⏐ SERIES AND PARALLEL ac CIRCUITS
frequencies, the phase angle associated with the input impedance
should be very close to 90°.
Now for the computer analysis. The current source, the resistor element,
and the inductor are all placed and connected using procedures
described in detail in earlier chapters. However, there is one big difference
this time that the user must be aware of: Since the output will be
plotted versus frequency, the Analysis Setup heading must be selected
in the AC Current dialog box for the current source. When selected,
the AC Magnitude must be set to the value of the ac source. In this
case, the default level of 1A matches that of the applied source, so we
were set even if we failed to check the setting. In the future, however, a
voltage or current source may be used that does not have an amplitude
of 1, and proper entries must be made to this listing.
For the simulation the sequence Simulate-Analyses-AC Analysis is
first applied to obtain the AC Analysis dialog box. The Start frequency
will be set at 10 Hz so that we have entries at very low frequencies,
and the Stop frequency will be set at 1MHz so that we have
data points at the other end of the spectrum. The Sweep type can
remain Decade, but the number of points per decade will be 1000 so
that we obtain a detailed plot. The Vertical scale will be set on Linear.
Within Output variables we find that only one node, 1, is defined.
Shifting it over to the Selected variables for analysis column using the
Plot during simulation key pad and then hitting the Simulate key will
result in the two plots of Fig. 15.112. The Show/Hide Grid key was
selected to place the grid on the graph, and the Show/Hide Cursors
key was selected to place the AC Analysis dialog box appearing in Fig.
15.112. Since two graphs are present, we must define the one we are
working on by clicking on the Voltage or Phase heading on the left side
of each plot. A small red arrow will appear when selected to keep us
aware of the active plot. When setting up the cursors, be sure that you
have activated the correct plot. When the red cursor is moved to 10 Hz
(x1), we find that the voltage across the network is only 0.251 V (y1),
resulting in an input impedance of only 0.25 �—quite small and
matching our theoretical prediction. In addition, note that the phase
angle is essentially at 90° in the other plot, confirming our other
assumption above—a totally inductive network. If we set the blue cursor
near 100 kHz (x2 � 102.3 kHz), we find that the impedance at
219.2 � (y2) is closing in on the resistance of the parallel resistor of
220 �, again confirming the preliminary analysis above. As noted in the
bottom of the AC Analysis box, the maximum value of the voltage is
219.99 � or essentially 220 � at 1 MHz. Before leaving the plot, note
the advantages of using a log axis when you want a response over a
wide frequency range.
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