Frequency Analysis with SPICE Phasors Impedance Frequency Sweep

Frequency Analysis with SPICE Phasors, Impedance, Frequency Sweep, and SPICE Kevin D. Donohue, University of Kentucky 1

Recall Loop Analysis Example Determine the steady-state response for vc(t) when vs(t) = 5 cos(800 t) V + vc(t) 3 k. W vs(t) 114. 86 n. F 6 k. W Result: Kevin D. Donohue, University of Kentucky 2

SPICE Solution Steady-State Analysis in SPICE is performed using the. AC (frequency sweep) option in the simulation set up. It will perform the analysis for a range of frequencies. You must indicate the: 1. Scale for uniform frequency increments 2. Starting frequency 3. Ending frequency 4. Number of frequencies used in the given range. Sources in the AC analysis must be set up in “edit simulation model” menu to: 1. Identify source as sinusoidal 2. Check the “use AC” option 3. Provide a magnitude and phase Kevin D. Donohue, University of Kentucky 3

SPICE Example Find the phasor for vc(t) for vs(t)= 5 cos(2 ft) V in the circuit below for f = 100, 200, 300, 400, 500, …. . 1000 Hz ex 16 -Small Signal AC-2 -Table FREQ MAG(V(IVM)) (Hz) (V) +100. 000 +3. 299 +200. 000 +3. 203 +300. 000 +3. 059 +400. 000 +2. 887 +500. 000 +2. 703 +600. 000 +2. 520 +700. 000 +2. 345 +800. 000 +2. 182 +900. 000 +2. 033 +1. 000 k +1. 898 Kevin D. Donohue, University of Kentucky PH_DEG(V(IVM)) (deg) -8. 213 -16. 102 -23. 413 -30. 000 -35. 817 -40. 893 -45. 295 -49. 107 -52. 411 -55. 285 4

Plotting Frequency Sweep Results Choices for AC (frequency sweep simulation) Ø For frequency ranges that include several orders of magnitude, a logarithmic or Decade (DEC) scale is more practical than a linear scale Ø The magnitude results can also be computed on a logarithmic scale referred to a decibels or d. B defined as: Kevin D. Donohue, University of Kentucky 5

Plot of Magnitude Linear Magnitude, Log Frequency Linear Magnitude, Linear Frequency d. B Magnitude, Log Frequency d. B Magnitude, Linear Frequency Kevin D. Donohue, University of Kentucky 6

Plot of Phase Linear Frequency, in Degrees Log Frequency, in Degrees Kevin D. Donohue, University of Kentucky 7

Recall Nodal Analysis Example Find the steady-state value of vo(t) in the circuit below, if vs(t) = 20 cos(4 t): 10 vs 0. 1 F ix 1 H 2 ix + vo 0. 5 H - Show: v 0(t) = 13. 91 cos(4 t + 198. 3º) Kevin D. Donohue, University of Kentucky 8

SPICE Example Find the phasor for vc(t) when vs(t)= 20 cos(4 t) V in the circuit below (note f = 2/ =0. 6636) FREQ MAG(I(VAM)) PH_DEG(I(VAM)) MAG(V(IVM)) PH_DEG(V(IVM)) +108. 440 +13. 912 -161. 560 (Hz) +636. 600 m +7. 589 Kevin D. Donohue, University of Kentucky 9