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926 ⏐⏐⏐ RESONANCE
cursor established on the screen. The Cursor Peak pad was then chosen
to find the peak value of the curve. The result was A1 � 319.45 mV
at 28.94 kHz which is a very close match with the calculated value of
318.68 mV at 28.57 kHz for the maximum value of V C. The bandwidth
is defined at a level of 0.707(319.45 mV) � 225.85 mV. Using the
right-click cursor, we find that the closest we can come is 224.72 mV
for the 10,000 points of data per decade. The resulting frequency is
34.69 kHz as shown in the Probe Cursor box of Fig. 20.45.
We can now use the left-click cursor to find the same level to the left
of the peak value so that we can determine the bandwidth. The closest
that the left-click cursor can come to 225.85 mV is 224.96 mV at a frequency
of 23.97 kHz. The bandwidth will then appear as 10.72 kHz in
the Probe Cursor box, comparing very well with the longhand solution
of 10.68 kHz in Example 20.7.
It would now be interesting to look at the phase angle of the voltage
across the parallel network to find the frequency when the network
appears resistive and the phase angle is 0°. First use Trace-Delete All
Traces, and call up P(V(C:1)) followed by OK. The result is the plot
of Fig. 20.46, revealing that the phase angle is close to �90° at very
high frequencies as the capacitive element with its decreasing reactance
takes over the characteristics of the parallel network. At 10 kHz the
inductive element has a lower reactance than the capacitive element,
and the network has a positive phase angle. Using the cursor option, we
can move the left click along the horizontal axis until the phase angle is
at its minimum value. As shown in Fig. 20.46, the smallest angle available
with the determined data points is 49.86 mdegrees � 0.05° which
is certainly very close to 0°. The corresponding frequency is 27.046 kHz
which is essentially an exact match with the longhand solution of
FIG. 20.46
Phase plot for the voltage v C for the parallel resonant network of Fig. 20.44.
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