AC Parametric Sweeps:
In order to learn about the frequency response of circuits, an AC parametric sweep can be performed. This is a different type of analysis than transient analysis as far as PSPICE is concerned, because instead of calculating the instantaneous values for voltages and currents, PSPICE instead calculates the steady-state effective currents and voltages.
Any source with an AC parameter can be used as a swept-frequency AC voltage source. The object “VSRC” is a generic AC signal source. The “VSIN” device also has the capability to be used during AC sweeps. It is usually more convenient to use “VSIN” since the same signal source can be used for both steady state and transient analysis.
PSPICE can present the results of an AC sweep in many useful ways. We’ll first look at raw voltage data, then have PSPICE display the data in decibel form.
1. Change the schematic diagram of the project to look like this:
This modification raises the resonant frequency of the circuit to 10.065 Hz, and the Q becomes 100.
2. Change the simulation profile to look like this:
3. Run the simulation. Notice that the circuit takes a little while to build its voltage up due to the high Q of the tank circuit!
4. Let’s find the frequency response of this filter circuit. First, let’s create a new simulation profile (remember, a project can contain multiple simulation profiles.)
• At the PSPICE menu, choose “New Simulation Profile”
• Fill in the resulting dialog box as follows and click the CREATE button.
• A new simulation profile will appear. Set it up as follows:
5. To do an AC frequency sweep, there must be at least one signal source with an “AC” parameter in the circuit. Double click the body of the voltage source and you’ll get a detailed screen like this one:
This screen is available for all components in a design. Not all of the parameters display on your drawing because the display isn’t turned on for them. To fix that, highlight one of the items, then click the “Display…” button. Choose “Name and Value” from the dialog to make the parameter show on the drawing.
To exit the property editor, click the lower “X” button at the upper-right portion of the display.
6. The circuit should now simulate. Press the F11 key to do so. But wait, there’s a problem — why doesn’t the output voltage rise all the way to 1 V (Vin) at resonance? The circuit doesn’t work! (Try more points per decade in the simulation setup — 1000 is a good number.)
7. You can easily switch back to the previous simulation profile. Simulation profiles are stored with the project.
• From the Window menu of the schematic editor, choose “boing.opj”. You’ll see the
• By expanding the “Simulation Profiles” folder, you can see that your project has two different profiles.
• You can enable any profile by highlighting it, then selecting “Make Active” from the “PSPICE” menu.
• The active profile is always highlighted with a red “P!” (for Probe).
8. Let’s look at the data with a decibel display. Switch back to the schematic (making sure the “frequency response” profile is active).
PSPICE provides special probes that read in dBV units. By definition:
Place two of these probes onto the circuit (and delete the other voltage probes). The circuit should look like this:
9. With these special probes in the circuit, run the analysis again. Notice that we played a dirty little trick. By setting the input voltage to 1 V (0 dBV), we get output voltage magnitude readings that correspond to the voltage gain. It is possible to mathematically compute gain by adding a trace to the plot. See if you can figure out how it works.
#PSPICE Seminar / DeVry KC – Spring 2002
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