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FIG. 23.93
High-pass R-C filter to be investigated using PSpice.
of the logarithmic axis: a wide range of values on a plot of limited size.
To ensure sufficient data points, the Points/Decade was set at 10k.
When V(R:1) is selected, the resulting SCHEMATIC1 appears as
shown in Fig. 23.94. Note the nice transition from one region to the
other. At low frequencies when the reactance of the capacitor far outweighs
that of the resistor, most of the applied voltage appears across
the capacitor, and very little across the resistor. At much higher frequencies
the reactance of the capacitor drops off very quickly, and the
voltage across the resistor picks up toward a maximum of 1 V.
Select Plot-Add Plot to Window-Trace-Add Trace-P(V(R:1))-
OK, and the phase plot of Fig.23.94 results showing a shift from 90°,
when the network is highly capacitive in nature, to 0°, when it becomes
resistive. If we select the phase plot SEL>> and click on the Toggle
cursor pad, a left click will place a cursor on the screen that can define
the frequency at which the phase angle is 45°. At 45.12°, which is the
closest we can come with the available data points, we find that the corresponding
frequency is 1.585 kHz which is a very close match with the
1.592 kHz calculated above. The right-click cursor can be placed at
100 kHz to show that the phase angle has dropped to 0.91°, which certainly
defines the network as resistive at this frequency.
Double-Tuned Filter Our analysis will now turn to a fairly complexlooking
filter for which an enormous amount of time would be required
to generate a detailed plot of gain versus frequency using a handheld
calculator. It is the same filter examined in Example 23.9, so we have a
chance to test our theoretical solution. The schematic appears in Fig.
23.95 with VAC again chosen since the frequency range of interest will
be set by the Simulation Profile. Again, the attributes for the source
COMPUTER ANALYSIS ⏐⏐⏐ 1081