PSPICE 计算机仿真

1. PSPICE计算机仿真 Simulation Program with Integrated Circuit Emphasis

2. CH12 FREQUENCY RESPONSE 频率响应

3. 12.1 specifying frequency variation and number

4. Sinusoidal • Linear, Logarithmic • Decade, octave

5. 12.2 frequency response output • Rectangular, polar, decibel

6. Example 18 illustrates how to analyze the frequency response of a parallel RLC circuit with PSpice.

7. Fig. 104 ddb

8. Example 18 • a) The current source in the circuit shown in Fig. 104 is 50cosωt mA. Use Probe to plot Vo versus f from 1000 to 2000 Hz in increments of 10 Hz on a linear frequency scale. • b) From the Probe plot, estimate the resonant frequency, the bandwidth, and the quality factor of the circuit. • c) Compare the results obtained in b) with an analytic solution for f0, β, and Q.

9. Solution a & b

10. Fig. 105 sch

11. Fig. 106 setting

12. Using the Probe Cursor, we note that the peak amplitude(峰值振幅) of about 400 V occurs at a frequency of 1590 Hz in Fig. 107a. • Thus we estimate the resonant frequency(衰减频率) at 1590 Hz.

13. Fig. 107a probe

14. To estimate the bandwidth, we use both cursors to find the frequencies where Vo=399.697/1.414=282.67 V. • The closest values are 281.992 at 1552 Hz and 1632.1 Hz. (Fig. 107b) • Thus we estimate the bandwidth to be 1632.1- 1552, or about 80 Hz.

15. Fig. 107b

16. We calculate the quality factor from the relationship,

17. Solution c) • A direct analysis of the circuit yields,

18. Comparison

19. Example 19

20. Example 19 • Modify the PSpice schematic for Example 18 to step the capacitor values from 0.15 μF through 0.35 μF. • Then use Probe to display the frequency response characteristics for all values of capacitance. • Comment on the effect of the changing capacitance.

21. Solution

22. Fig. 108 sch

23. Fig. 108a param property

24. Fig. 108b setting

25. Fig. 108c Param_setting

26. Fig. 109 Probe

27. Result • The smallest value of capacitance produced the plot farthest to the right. • We expect this result because the equation for resonant frequency for an RLC circuit is:

28. Furthermore, as the capacitance increases, the resonant peak becomes sharper. • This result, too, comes as no surprise because the equation for Q in a parallel RLC circuit is:

29. 12.3 Bode plots with probe

30. Fig. 110 ddb

31. Example 20 compares the exact dB voltage magnitude versus log frequency and phase angle versus log frequency plots to the Bode straight-line approximation using Probe.

32. Example 20 • Construct a PSpice schematic and associated analysis to generate the frequency response of the circuit shown in Fig. 110 for three different values of resistance: 5 Ω, 50 Ω, and 500 Ω. • Then use Probe to plot the output voltage magnitude in dB and output voltage phase angle versus log frequency. • Finally, use the Label tool in Probe to overlay a straight-line Bode approximation plot and comment.

33. Fig. 111 sch

34. Because this is a series RLC circuit, we know that the centre frequency and the bandwidth are given by:

35. For the circuit shown in Fig. 110, the centre frequency is:

36. The bandwidth ranges:

37. Fig. 111a param_property

38. Fig. 111b vac_property

39. Fig. 111c setting1

40. Fig. 111d setting2

41. Fig. 112a Plot/Add plot to window

42. 选中上面窗口(SEL>>表示选中!) • Trace/Add trace • 先选择右侧DB() • 再选择左侧V(out) • 底部出现: DB(V(out))

43. Fig. 112b select DB(V(out))

44. 选中下面窗口(SEL>>表示选中!) • Trace/Add trace • 先选择右侧P() • 再选择左侧V(out) • 底部出现: P(V(out))

45. Fig. 112c select P(V(out))

46. Fig. 112 probe

47. 12.4 Filter design

48. Example 21 demonstrates the use of PSpice and Probe in verifying the behavior of a high-Q bandpass filter.

49. Fig. 113 ddb

50. Example 21 • The circuit in Fig. 113 is an active high-Q bandpass filter. • Using 1 nF capacitors and an ideal op amp, design values for the three resistors to yield a centre frequency of 10 KHz, a quality factor of 10, and a passband gain of 3. • Use Probe to verify that the resistor values you compute produce a filter that satisfies the three frequency response specifications.