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Frequency Response of RC Filter

Load the PSPICE file rc-tac.sch, Figure 25.

  

Figure 21: RC circuit.

Exercise:

1.

Simulate the circuit and first obtain the transient response. Note carefully the transient and AC steady state portions of the waveform. How long (approximately) does the transient period last?

2.

Now enable AC sweep, disable transient, and simulate again. Note the AC response.

The AC sweep has been set to decade type, with a start frequency range of 0.0001 Hz up to 1.0 kHz.

The graph will be a plot of capacitor voltage versus frequency. It can be used to determine the AC response (magnitude) at any frequency on the x-axis (for phase, see below). What is the magnitude of the capacitor voltage at 5 Hz?

3.

The graph just produced will be linear-log. Let's now draw a magnitude Bode diagram, which is dB-log.

(a)

Clear the PROBE display by Trace $\to$ Delete All.

(b)

Now bring up the menu of available waveforms Trace $\to$ Add.

(c)

In the right side of the window that appears, click on DB() for dB. It will appear in the bottom Trace Expression portion of the window.

(d)

In the left portion of the window, click on V(VC) for the capacitor voltage. It will appear in the bottom Trace Expression portion of the window.

(e)

Click OK. The magnitude Bode diagram showing capacitor voltage should now appear.

4.

Determine from the graph the 3 dB point. Compare with theory.

Lecture Notes: Frequency Response : Bode Diagram

5.

Now let's plot the phase response.

(a)

For illustration, let's add another plot to the probe display Plot $\to$ Add Plot. A new space for plotting should appear above the magnitude Bode diagram. Ensure that the new graph is selected.

(b)

Bring up the menu of available waveforms Trace $\to$ Add.

(c)

In the right side of the window that appears, click on P() for phase. It will appear in the bottom Trace Expression portion of the window.

(d)

In the left portion of the window, click on V(VC) for the capacitor voltage. It will appear in the bottom Trace Expression portion of the window.

(e)

Click OK. The phase Bode diagram showing capacitor voltage should now appear.

6.

Determine the magnitude (dB) and phase at 0 Hz, 1 Hz, and at the 3 dB point.

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