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Analyzing the AC Characteristics of the example Circuit

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B. Program Schematics Startup.
        1. For Window 95/98/NT users, go to the Start Menu and left click the mouse.
        2. Under the Start Menu, find Programs and left click the mouse again.
        3. Find the folder named either DesignLab Eval 8 or Microsim Eval 8 and again left click.
        4. Finally, left click on Schematics. The Schematics section of the PSPICE program should start up.

C. Load or Build the Example Circuit.

a. To load up the circuit.
If you have already built and saved a schematic file of example circuit, load it up by selecting File - Open in the schematics. If you have not saved the circuit, the example circuit may be downloaded.

       To download Right Click the mouse over the link and choose Save Target As... or Save Link As..., select target directory and then save.

Download Low Pass Filter Example Schematic Circuit (LPFilter.sch)

b. Build the Circuit.
If you have not yet built the circuit, follow the instructions in the section indicated below showing how to build the circuit.

How to Build Example Circuit (p-build.html)

    The example circuit should look something like this:

D. Insert Input Source.

To properly analyze a circuit, a proper input source has to be connected to the input of the circuit. In this case, we have to connect a source to the left end of Resistor R1.

1. For a DC or AC type analysis, one of possible input source is VSRC. So go to Draw - Get New Part and select VSRC. Insert the + end of the source at the input of the circuit and add a GND_ANALOG to the - end of the source.

2. Now, set up VSRC by double left clicking the mouse on it. Change the values of DC to 0 and AC to 1 remembering to click on Save Attr after each change. Finally Click on OK. What has been done here is to provide the circuit with a AC source of 1 Volt biased on a DC point of 0 Volt.

1. Setup the the desired type of analysis by going to Analysis - Setup.

a. The AC analysis will be done on the AC source V3, since it is the only AC source in the circuit.
b. Click on Decade, enter 10 at Pts/Decade, 10 at Start Freq, and 100k at End Freq. This will plot a curve in Decade form with 10 points per decade with the frequency of the AC source swept from 10 Hz to 100 kHz. Remember not to enter any spaces between the characters.
c. Click on OK.

3. Back on the Analysis Setup Screen, make sure AC sweep and Bias Point Detail are Enabled (their corresponding boxes ticked). Bias Point Detail allows the user to examine the operating points (or DC points) of the circuit. Click on Close.

4. Using File-Save As Save the circuit with another name, for example LPfilter1.sch.
 

1. To Start the AC analysis, Go to Analysis - Simulate. After a brief while of calculation, PSPICE will display the MicroSim Probe window (or select it manually, if necessary).If there are any errors, check the circuit, the values for the components.

2. To choose which plot to trace, select Plot - Add Traces. First select DB() from the Functions or Macros window to display the output in decibles. Then select V(U1:OUT) which is the output voltage of the Operational Amplifier. Therefore, V(U1:OUT) will be displayed in decibles. The box Trace Expression at the bottom of the window indicates the outputs that will be traced. Click on OK.

3. We should see the trace of the Gain (Output over Input voltage) displayed. The results seem to be correct as we are looking at a Low Pass Filter.

 
4. If there is a need to examine the slopes in detail, go to Tools - Cursor - Display. PSPICE  will provide 2 cursors for value examination. One cursor is controlled by the Left Hand Mouse Button and the other one by the Right Hand Mouse Button.

a. Left-Click on the Green Square by V(V1:OUT) (at the bottom of the display), now move the mouse across the graph while Holding down the Left Button of the Mouse, the Left Cursor should move along the green line with the coordinates values displayed in the Probe Cursor window.
b. Hold down the Left Button of the Mouse and move the cursor along the green line until the value indicated on the right for A1 in the Probe Cursor window) is approximately -3 dB. Look at the corresponding frequency value for A1. Here the cutoff or 3dB frequency of this circuit is about 1.59 kHz.