ET 304A Laboratory Tutorial Pspice For Transient and Frequency Analysis All circuit simulation packages that use the Pspice engine allow users to do complex analysis that were once impossible to perform by hand Two of these types of analysis are transient analysis and frequency response analysis Transient analysis is the theoretical calculation of the circuit response as a function of time The results are displayed much like oscilloscopes traces on plots that graph voltages and currents on the y axis and time on the x axis Voltages and currents can then be compared and magnitude phase measurements made from the individual traces on the graphs The frequency response analysis gives the exact results that are approximated in the Bode plots This analysis lets users check the frequency response between the input and output points of active with IC and transistors and passive circuits User can specify the range of frequencies and the input output voltages for use in graphing the results Transient Analysis Before any type of analysis can be performed using a Pspice based simulation package the types of analysis must be enabled To enable a analysis choose from the menu Analysis then Setup This will display the dialog box below Figure 1 Analysis setup showing the transient and bias point detail functions enabled Clicking on the Enabled checkbox activates the selected analysis for use Clicking on the type of analysis button will bring up another dialog box that is specific for each selection and gives more details and parameters for the selected study Fall 2001 pstut doc Figure 2 shows the details for the transient analysis When Pspice solves the transient Figure 2 Details for Transient Analysis Setup analysis of a circuit it generates the dynamic equations of the circuit These equations are differential equations that is they use derivatives to represent the response of the inductors and capacitors in the circuit For simple circuits functions can be found that describe the time response of the circuit For most practical circuit simulations the Pspice program solves the dynamic equations by numerical approximation over a user specified time interval All calculations start at time zero The following analysis parameters are important to the operation of the program Print Step Controls when the program saves the output of the solution The algorithms used in transient simulations can produce thousands of points This controls how often the results are saved for printing or plotting It has the units of time seconds mS etc Final Time Controls how long the simulation will last Units are time Step Ceiling Controls the size of time increment taken between calculations When differential equations are solved numerically calculations are performed iteriatively at discreet time intervals This parameter sets the maximum distance between the calculations The units are also time Correct values for these parameters will make the display of the results easier to understand For simple ac sources the frequency of the source is used to determine the proper settings Fall 2001 2 pstut doc Final Time Set this so 10 complete cycles are computed Determine the time by finding the period of the sine source frequency and multiplying it by the number of cycles The period of the sine wave is found from T 1 f seconds The number of cycles can be changed by the user but remember a large number of cycles gives a large value of final time which produces simulations that take lots of computer time and will not show anything of interest Final times computed with 5 to 10 cycles will usually show all the interesting points of the circuit response Print Step Set the print step time so that the graphs will show smooth curves For sine wave analysis the print step should produce at least 20 points per period The value of print step time is found by tps T 20 where tps print step time seconds T period of input signal seconds Experimenting with the number of points per period maybe necessary More points will give smoother curves but take more time to compute and display Step Ceiling Set this value to the same value as print step Making this value smaller will cause the program to compute more points and take more time To perform transient analysis an ac source must be in the circuit The source need not be sinusoidal but for these beginning labs only sine waves are used To insert a sinusoidal source go to the analog parts library and locate the part with the name VSIN and insert it into the circuit Double clicking on the part should bring up the following dialog box Figure 3 Parameters For an Sinusoidal AC Source The following parameters are important and must be assigned correctly to display valid results Fall 2001 3 pstut doc VAMPL The peak value of ac voltage used in the simulation FREQ Frequency of the ac source Hz VOFF Set the condition of the ac source off 1 or on 0 Set this to zero to activate the source AC Activates the source for frequency plot analysis Set this to 1 so Bode plots can be run After setting these parameters to the values needed click on OK to save them in the schematic editor A transient simulation can now be run and the results displayed using the Probe software included with Pspice software Example Simulate the transient response of a simple RC circuit that has a 10 V rms 5000 Hz source Determine the time setting for the transient simulation and the source settings Find the period T 1 5000 2x10 4 S or 200 S Set the value of print step time tps T 20 200 S 20 10 S Set the step ceiling 10 S equal to print step time Set the source parameters VAMPL 10 0 707 14 14 Vp FREQ 5000 VOFF 0 AC 1 AC Sweep Analysis Frequency Response Plots To generate a frequency response plot of a circuit the AC Sweep analysis must be enabled See Figure 1 Clicking on the AC Sweep will display the following dialog box Figure 4 AC Sweep Analysis Parameters Fall 2001 4 pstut doc The only selections that are necessary to perform the AC Sweep analysis are the type of sweep the total number of points were the calculation is made and the frequency limits For the work in the lab select a sweep type of Decade The number of points per decade should be 5 10 for the work being done Larger numbers of points will only slow down the simulation The starting and ending frequencies are give in the lab handouts If these are not know before hand a range of interest can be found from the expected response of the simulated circuit Example bandpass filter with