# ECT 2036 Circuits and Signals

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```					ECT2036: Circuit and Signals                                                                SIG2

Experiment SIG2: Circuit analysis using ORCAD PSpice

PRECAUTIONARY STEPS:
1. Read this experiment sheet thoroughly and carefully before coming to your lab session.
2. Take precautions for safety. Also handle equipment carefully to prevent any damages.
3. Try to get as much of the analysis done during the lab session.
4. Close all running programs other than the OrCAD-PSpice program.
5. Seek the approval of your results for each part of the experiment from your lab supervisor
before moving to the next one.
6. The lab report should include the graphs, calculations of the resistance values of Part 4.0,
comments and analysis of the findings.
7. Appendix A on page 5 provides some guidelines and examples on how to solve Part 4.0.

1.0    Objectives:
To perform the following circuit analysis using OrCAD-PSpice A/D, Release 9.1:
(i)    Variable components sweep analysis.
(ii)   The design of passive low pass filter.
(iii) The design of active Butterworth and Chebyshev low pass filter

2.0     Introduction:
Methods of circuit analysis vary widely depending on the complexity of the problem. Whereas
some circuits require nothing more complicated than the writing of a single equation for their
solution, others may require several equations to be solved simultaneously. When the response of
a circuit is to be performed over a wide range of frequencies, the work is often both tedious and
time consuming. In many cases the problem to be solved requires that the students have an
understanding of which basic laws and principles are involved in the solution. In some cases, if
the topology of a network is known, along with complete descriptions of the circuit elements,
computer programs can be used to perform the analysis. Such programs have been under
programs that are capable of solving many types of electrical networks under a variety of
conditions.

3.0     Procedure:
Click start  Programs  OrCAD demo  Capture CIS demo

Part I: Variable Component Sweep Analysis
Step 1: From the menu bar click: File  New  project
Step 2: Put name as “Project I”  select “Analog Mixed-signal circuit wizard” OK
(Analog ; Source ; Special & Sourcstm) .
Step 4: Draw the circuit as shown in Figure 1: Components needed are as follows:
 Analog (R-var, L , C)
 Special (PARAM)
 Ground (0/source). {from the vertical menu}
 Output (Offpageleft-R). {from the vertical menu “<<C” }

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ECT2036: Circuit and Signals                                                              SIG2

Step 5: Key in values:
 Key in “Resistance” as value for R1
 Capacitor “2u”  double click capacitor  key in 10V into IC block (10V as initial
condition)  highlight IC  click display  click “Display name and value”  close
sub-window
 Inductor “10mH”  double click inductor  key in –90mA into IC block (-90mA as
initial condition)  highlight IC  click display  click “Display name and value” 
close sub-window
 Change the text “Offpageleft-R” to “Out”
Step 6: Key in reference
 Double click “PARAMETERS” (A new window will be pop up)
 Click new  name “Resistance”  key in “20” into resistance block  Highlight
resistance block  click “display”  click “Display name and value”  close sub-
window
Step 7: Simulation
 Click “Pspice” from menu  new simulation profile  give any name  click “create”
 choose “Time domain”  Run to time “2ms”  click the box parametric sweep 
choose global parameter  parameter name “Resistance”  start value “20”  End
value “100”  increment “20”  click “OK”
 Click the small triangular icon to run the simulation
 Click add trace Icon  click Vout
Step 8: Results
 A simulation result shows the transient response with difference value of resistance R
Step 9: OPTIONS
 Click FFT icon to see the frequency domain picture
 Change the increment to a small value to see more patterns

Part II: Design of Passive Low Pass Filter
Step 1: Open a new project named project 2 for the circuit in Figure 2.
Step 2: Instruct OrCAD to perform AC Sweep analysis with a frequency sweep variable that is to
be varied from 1Hz to 1KHz at 100 points.
Step 3: Run simulation.
Step 4: Click trace  click Vout.
Step 5: A result of low pass filter is displayed.

Part III: Design of 1st Order Butterworth Low Pass Filter
Step 1: Open a new project named project 3 for the circuit in Figure 3.
Step 2: Instruct OrCAD to perform AC Sweep analysis with a frequency sweep variable that is to
be varied from 500Hz to 500KHz at 1000 points.
Step 3: Run simulation.
Step 4: Click trace  click Vout.
Step 5: A result of low pass filter is displayed.

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ECT2036: Circuit and Signals                                                                                  SIG2

PARAMETERS:
Resistance = 20
R1                             L1
out
Resistance                     10mH
IC = -90mA

C1
2u , IC = 10v
0

Figure 1

R1                             L1
Out
10                            10mH

V1            10Vac                                            C1
1000u

0
Figure 2

VCC+

R1

820
1Vac         V1                                     7 V+         5
+                                       VCC+               VCC-
3
A741             6
0                                                                  out
2                         1
-    4 V-
15Vdc          V2   15Vdc          V3

C1    0.01u
0                  0
VCC-
0
Figure 3

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ECT2036: Circuit and Signals                                                                       SIG2

4.0       Laboratory Assignment

Second order low pass filter

Table 1: Higher order low pass filter parameters.
1st stage         2nd stage             3rd stage         Overall pass-
Order
RB/RA         f’ RB/RA           f’      RB/RA        f’     band gain (dB)
Butterworth
3         -             1   1.000          1        -            -           6.0
4         0.152         1   1.235          1        -            -           8.2
5         -             1   0.382          1       1.382         1          10.3
6         0.068         1   0.586          1       1.482         1          12.5
2dB Chebyshev
3         -          0.322 1.608       0.913        -            -           8.3
4         0.924      0.466 1.782       0.946        -            -          14.6
5         -          0.223 1.437       0.624       1.862     0.964          16.9
6         0.879      0.321 1.637       0.727       1.901     0.976          23.2
Note: Normalized cut-off frequency, f’ = 1/[2RC  desired cut-off frequency]

(i)   Design a 1st, 3rd and 5th order Butterworth low pass filter where the cut-off frequency is
20kHz.
(ii) Design a 1st, 3rd and 5th order 2dB roll-off Chebyshev low pass filter where cut-off
frequency is 20kHz.

Important:
    You are given one week to prepare, write and submit your lab report.
    All reports must be neatly handwritten. Neatness and carefulness will be taken into account
in the marking of your report.
    Write your own report and use your own findings and results, similar reports will not
be given marks for both the original and the copied ones.
    Late submission of your lab report will not be entertained.
    This lab report carries 5% of the total coursework marks.

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ECT2036: Circuit and Signals                                                                        SIG2

Appendix A

Guidelines for Part 4.0:

 Higher-order filters may be constructed by cascading a combination of 1st and 2nd order filter
sections (stages). The basic structure of 1st order and 2nd order low pass filter sections is
shown in Figure A1:

RA           RB
Vo
Vo         Vi
Vi

Figure A1a: 1st order low pass filter network.            Figure A1b: 2nd low pass filter network

 The block diagrams in Figure A2 illustrate the schemes for a higher-order low pass filter.
Odd-order filters are obtained by cascading a 1st order section with one or more 2nd order
sections. For example, a 5th order low pass filter can be built by cascading a 1st order section
with two 2nd order sections.

1st order                            2nd order

3rd Order Filter

.
1st order                               2nd order                     2nd order

5th Order Filter

Figure A2: Block diagram illustrating the higher-order low pass filters

 The values and parameters in Table 1 can be used to design the required filters in your
assignment. Apply the following relationship:
1
f
2RC  f c
where, fc, is the given cut-off frequency.

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ECT2036: Circuit and Signals                                                                    SIG2

 Design examples:

1) Third order Butterworth filter:
To design a third order Butterworth low pass filter with a cut-off frequency of 19.4kHz.
Set C = 0.01F for all the calculations.
By referring to Table 1, it is simple to determine that the selected resistance value should
be 820 for all 1st and 2nd stages of filters.
1
R                        820 
2  0.01  19 .4k

2) Fifth order Butterworth filter:
To design a fifth order Butterworth low pass filter with a cut-off frequency of 19.4kHz.
Set C = 0.01F in all the calculations.
By referring to Table 1, you can determine the suitable resistance values.

3) Third order Chebyshev filter
To design a third order Chebyshev low pass filter with a cut-off frequency of 21.4kHz.
Set C = 0.01F in all the calculations.
The first stage of the resistance value is calculated as follows:
1
R                                 2310   2200 
2  0.01  21 .4k  0.322
By referring to Table 1, you can determine the suitable resistance values.

.

4) Fifth order Chebyshev filter
To design a fifth order Chebyshev low pass filter with a cut-off frequency of 20.1kHz.
Set C = 0.01F in all the calculations.
The first stage of the resistance value is calculated as follows:
1
R                                 3551   3600 
2  0.01  20 .1k  0.223
By referring to Table 1, you can determine the suitable resistance values.

.

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