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Date: September 17, 2008 Page 1 Time: 10:34:30
(A) simplesweep (active)
Frequency
1.0Hz 10Hz 100Hz 1.0KHz 10KHz 100KHz
V(OUTPUT)
100uV
1.0mV
10mV
100mV
1.0V
Figure 28: Amplitude of the output from a high-pass filter as a function of frequency. Both axes
are logarithmic. This is from a printout rather than a screenshot, hence the white background.
which shows that voltage ∝ frequency here. I made the line thicker by selecting the trace, right-
clicking and choosing Properties. . . . Thicker lines (I usually choose the third) look ugly on the
screen but give a much better printout.
The response of filters is traditionally plotted in decibels (dB). These are defined by
voltage gain in decibels = 20log
10
output voltage
input voltage
. (1)
The output of a filter often has a smaller magnitude that its input, in which case the ‘gain’ is
less than unity and becomes negative in decibels.
Here is the easiest way of getting a plot in decibels. Return to Capture, remove the existing
voltage marker, and choose PSpice > Markers > Advanced > dB Magnitude of Voltage from
the menu bar. Place the marker on the output, run the simulation again and you will get a plot
in decibels. (This works correctly only if the input voltage is 1 Vac.)
You should know that AC signals also have a phase. A filter changes the phase of a signal
as well as its magnitude and it can be useful to plot this too. Again Capture has a built-in tool.
Choose PSpice > Markers > Plot Window Templates. . . from the menu bar. This gives a list of
plots that can be produced automatically. Try Bode Plot dB - separate and place the marker on
Output as usual. Run the simulation and you will see a pair of plots for magnitude and phase
as in figure 29 on the next page.
The scales of both graphs are poorly chosen by default. Make the plot clearer by changing
them. Either double-click on the axis or choose Plot > Axis Settings. . . from the menu bar.
Select User Defined for the Data Range and enter more appropriate numbers. You might like
to change the grid as well.
21
(19 pages)
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