ENB6060
TELECOMMUNICATIONS A
LABORATORY SHEET 5
Amplitude Demodulation (Experimental)
Name:- (Type Your Name Here)
Student ID :- (Type Your Student ID Here)
Objective
This lab sheet will investigate double sideband full carrier demodulation. As part of this lab different
parameters of the demodulation process will be reviewed, and the results will be obtained and observed
experimentally.
Apparatus






AD633JNZ Multiplier
Range of different capacitors (100pF 10nF 100nF)
1KOhm resistor
1N4148 (or similar) diode
1N4003 (or similar) diode
Elvis II platform
Method
This lab has been designed to further investigate the Double Side Band (DSB) full carrier demodulation
using an envelope detector by performing this task experimentally. The AM modulation will be carried
out using the AD633JNZ multiplier chip from Analog Devices, where this particular chip has been used in
previous laboratory sessions. The different parameters of the demodulation process will be investigated
and by the end of this lab students will understand the mechanisms involved in the demodulation
process.
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Revised 11th April 2012
The baseband and carrier frequencies used in Section A of this report are the following:




Baseband Frequency = 1 KHz,
Carrier Frequency = 100 KHz,
Use the FGEN on the elvis board to generate the Carrier Signal Grequency, and use a Vpp of 3
Volts,
Use the Arbitrary Waveform Generator on the Elvis board to generate the Baseband signal. Load
the sinewave and set the Gain = 4.
Section A
C
C
2
C
Probe
Oscilloscope
0.1uF
1N4148
Z
2
K
1
AD633JN
D
N
G
-VCC
0.1uF
(C)
Cdetector
R
-
V
5
1
C
Y
Diode
6
1
Y
4
FGEN
3
W
7
2
X
2
D
V
+
8
1
1
X
B
R
A
1
U
V
Setup the circuit as shown below:
Figure 1 Circuit Schematic including AD633 Multiplier and Envelope Detector
Deliverable A.1:- Explain the function of each of the components in this setup,
C1 :C2 :U1 :D :Cdetector :R :Deliverable A.2:- Use a capacitor of 100 nF for Cdetector. Measure the output of the Multiplier (Pin 7)
before the envelope detector. Paste in the signal obtained using either a Screen shot of the Oscilloscope
or the data acquired and plotted using Matlab. Calculate the Modulation index of the resultant signal,
note for this calculation you are required to provide the values for both the carrier and baseband
amplitudes. Add a figure title that includes relevant information on the signal and comment on the
signal obtained.
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Revised 11th April 2012
Deliverable A.3:- Remove the Capacitor Cdetector from your circuit and measure the signal at the output of
the Envelope detector. Paste the signal into this document using either a Screen shot of the Oscilloscope
or the data acquired and plotted using Matlab. Add a title to the figure that includes relevant
information. Comment on the graph obtained, you may compare the signal obtained to that obtained in
your Previous Lab 5 for similar circumstances.
Deliverable A.4:- Place the 100 nF Capacitor Cdetector back into your circuit and measure the signal at the
output of the Envelope Detector. Paste the signal into this document using either a Screen shot of the
Oscilloscope or the data acquired and plotted using Matlab. Add a figure title that includes relevant
information on the signal and comment on the graph obtained you may compare the signal obtained to
that obtained in your Previous Lab 5 for similar circumstances. Measure the peak to peak ripple voltage
at the peak voltage and the minimum voltage.
Deliverable A.5:- Set the Baseband signal to 9 KHz (Cdetector = 100 nF) and measure the signal at the
output of the Envelope Detector. Paste the signal into this document using either a Screen shot of the
Oscilloscope or the data acquired and plotted using Matlab. Comment on the graph obtained.
Deliverable A.6:- Set the Baseband signal back to 1 KHz and replace the 100 nF Capacitor with a 10 nF
capacitor (i.e. Cdetector = 10 nF) and measure the signal at the output of the Envelope Detector. Paste the
signal into this document using either a Screen shot of the Oscilloscope or the data acquired and plotted
using Matlab. Add a figure title that includes relevant information on the signal and comment on the
graph obtained you may compare the signal obtained to that obtained in your Previous Lab 5 for similar
circumstances. Measure the peak to peak ripple voltage at the peak voltage and the minimum voltage.
Deliverable A.7:- With the Baseband signal at 1 KHz, replace the 100 nF Capacitor with a 100 pF
capacitor (i.e. Cdetector = 100 pF) and measure the signal at the output of the Envelope Detector. Paste the
signal into this document using either a Screen shot of the Oscilloscope or the data acquired and plotted
using Matlab. Add a figure title that includes relevant information on the signal and comment on the
graph obtained you may compare the signal obtained to that obtained in your Previous Lab 5 for similar
circumstances. Measure the peak to peak ripple voltage at the peak voltage and the minimum voltage.
Section B
Deliverable B.1:- Set the Baseband signal to a 1 KHz square wave signal (on the ARB function generator)
and use Cdetector = 100 nF. Measure the signal at the output of the Envelope Detector. Paste the signal
into this document using either a Screen shot of the Oscilloscope or the data acquired and plotted using
Matlab. Add a figure title that includes relevant information on the signal and comment on the graph
obtained. Measure the peak to peak ripple voltage at the peak voltage and the minimum voltage.
Deliverable B.2:- Measure the time constant Δτ as shown in Figure 2 below from the signals obtained in
Deliverable B.1. Compare this time constant to the time constant of your envelope detector. Comment
on your answer. NOTE Additional notes have been provided at the end of the document on the Time
Constant.
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Revised 11th April 2012
Δτ
100 %
63 %
Figure 2 Output from Envelope Detector with R = 1KOhm and C = 100 nF, (carrier sinewave 100KHz and Baseband Square
Wave set to 1KHz)
Deliverable B.3 Use again the Baseband signal as a 1 KHz square wave signal (on the ARB function
generator) and use Cdetector = 10 nF. Measure the signal at the output of the Envelope Detector. Paste the
signal into this document using either a Screen shot of the Oscilloscope or the data acquired and plotted
using Matlab. Add a figure title that includes relevant information on the signal and comment on the
graph obtained.
Deliverable B.4 Use again the Baseband signal as a 1 KHz square wave signal (on the ARB function
generator) and use Cdetector = 100 pF. Measure the signal at the output of the Envelope Detector. Paste
the signal into this document using either a Screen shot of the Oscilloscope or the data acquired and
plotted using Matlab. Add a figure title that includes relevant information on the signal and comment on
the graph obtained.
Section C
Deliverable C.1 Set the Baseband signal to a 1 KHz square wave signal (on the ARB function generator)
and use Cdetector = 100 nF replace the diode 1N4148 with the 1N4003 Measure the signal at the output of
the Envelope Detector. Paste the signal into this document using either a Screen shot of the
Oscilloscope or the data acquired and plotted using Matlab. Add a figure title that includes relevant
information on the signal and comment on the graph obtained.
Deliverable C.2 Baseband signal = 1 KHz square wave signal (on the ARB function generator) and use
Cdetector = 10 nF (diode 1N4003) Measure the signal at the output of the Envelope Detector. Paste the
Page 4 of 9
Revised 11th April 2012
signal into this document using either a Screen shot of the Oscilloscope or the data acquired and plotted
using Matlab. Add a figure title that includes relevant information on the signal and comment on the
graph obtained.
Deliverable C.3 Baseband signal = 1 KHz square wave signal (on the ARB function generator) and use
Cdetector = 100 pF (diode 1N4003). Measure the signal at the output of the Envelope Detector. Paste the
signal into this document using either a Screen shot of the Oscilloscope or the data acquired and plotted
using Matlab. Add a figure title that includes relevant information on the signal and comment on the
graph obtained.
Marking Scheme
Marks Assigned
10
10
10
10
10
10
10
10
10
10
10
10
10
10
140
Deliverable A.1
Deliverable A.2
Deliverable A.3
Deliverable A.4
Deliverable A.5
Deliverable A.6
Deliverable A.7
Deliverable B.1
Deliverable B.2
Deliverable B.3
Deliverable B.4
Deliverable C.1
Deliverable C.2
Deliverable C.3
TOTAL
Note :- The late Penalty is 5 % per day
Deliverable date is M6 May (lab 5 : deadline)
to be emailed to muhab.manjood@polytechnic.bh +Printed copy (lecture time)
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Revised 11th April 2012
Time Constant
When an electrical/electronic circuit is switched on by a DC or AC signal a time delay between the input
and the output is observed. This time delay is referred to as the Time Constant (τ). The time constant
that arises is depends on the reactive components, i.e. either Capacitors or Inductors, and is measured
in time constants τ. The information presented in these notes will focus on the Capacitor where a similar
analysis can be performed for inductors.
When a signal (DC or AC) is applied to a discharged “empty” capacitor the capacitor takes the flow of
charge, i.e. current, to “charge up”. Capacitors, in effect, act like a battery because they are able to store
charge. When the source voltage is reduced or removed, the capacitor discharges in the opposite
direction. The charging and discharging of the capacitor takes time, this time is characterized by the time
constant.
If a resistor is connected in series with a capacitor, the capacitor will charge up gradually through the
resistor until the voltage across the capacitor is the same as the source supply. The time for this process
takes close to 5 time constants, i.e. 5τ, where the time constant τ is given by:
𝜏 = 𝑅𝐶
(1)
In equation 1 above R and C are the resistance and capacitor respectively and the units of τ is seconds.
In the following Figure 3 the step response to an RC circuit is presented. Here the value of the resistor
used for the simulation is R = 10 Ω and the Capacitor C = 1 μF. The time constant for this particular setup
would be τ = 10 μS.
1.000
vin
vout
0.900
0.800
Vout
1
Vin
R
0.700
1
F
u
1
0.500
VPULSE
(V)
1KHz
C
0
1
0.600
D
N
G
0.400
0.300
0.200
0.100
0.000
0.000u
6.250u
12.50u
18.75u
25.00u
31.25u
37.50u
Time (s)
43.75u
50.00u
56.25u
62.50u
68.75u
75.00u
Figure 3 Transient analysis of an RC circuit (see RC inlay circuit diagram) where R = 1KOhm and C = 1μF. The input voltage Vin
is presented in red and the output signal Vout is in blue.
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Revised 11th April 2012
The expression used to predict the behaviour of a charging capacitor is given by:
𝑡
𝑉𝑜𝑢𝑡 (𝑡) = 𝑉𝑖𝑛 (1 − 𝑒 −𝜏 )
(2)
Where Vout(t) is the output voltage at time t, Vin is the source voltage and τ is the time constant given by
equation 1. If we let the time 𝑡 = 𝜏 then equation 2 would become:
𝜏
𝑉𝑜𝑢𝑡 (𝜏) = 𝑉𝑖𝑛 (1 − 𝑒 −𝜏 ) = 𝑉𝑖𝑛 (1 − 𝑒 −1 ) = 0.632𝑉𝑖𝑛
(3)
From Equation 3 it can thus be said that the output voltage Vout is equal to 63.2% of the input voltage Vin
after a time τ = RC. In Figure 4 an illustration of the time constant on the RC circuit mentioned earlier is
presented.
Vout
1
1
F
u
1
D
N
G
VPULSE
1KHz
C
0
1
Vin
R
63.2%
10 μS =RC
Figure 4 Illustration of the Time Constant of the RC circuit shown in the inlay of the figure, time elapsed after 1 time constant
is 10 uS which is equal to RC.
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Revised 11th April 2012
Vout
1
1
F
u
1
D
N
G
VPULSE
1KHz
C
0
1
Vin
R
In the following Figure 5 the discharging of the capacitor when the source voltage is 0 V is presented. As
in the previous case for the charging capacitor, here the value of the resistor used for the simulation is
again R = 10 Ω and the Capacitor C = 1 μF. The time constant for this particular remains as τ = 10 μS.
Figure 5 Transient analysis of a discharging RC circuit (see RC inlay circuit diagram) where R = 10Ω and C = 1μF. The input
voltage Vin is presented in red and the output signal Vout is in blue.
The expression used to predict the behaviour of a discharging capacitor is given by an exponential decay
as follows:
𝑡
𝑉𝑜𝑢𝑡 (𝑡) = 𝑉𝑖𝑛 (𝑒 −𝜏 )
(4)
Where Vout(t) is the output voltage at time t, Vin is the source voltage and τ is the time constant given by
equation 1. If we let the time 𝑡 = 𝜏 then equation 4 would become:
𝜏
𝑉𝑜𝑢𝑡 (𝜏) = 𝑉𝑖𝑛 (𝑒 −𝜏 ) = 𝑉𝑖𝑛 (𝑒 −1 ) = 0.368𝑉𝑖𝑛
(5)
From Equation 5 it can thus be said that the output voltage Vout is equal to 36.8% of the input voltage Vin
after a time τ = RC from when the capacitor begins to discharge. In Figure 6 an illustration of the time
constant on the RC circuit mentioned earlier is presented.
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Revised 11th April 2012
Vout
1
R
1
C
0
1
Vin
F
u
D
N
G
1
1KHz
VPULSE
36.8%
10 μS =RC
Figure 6 Illustration of the Time Constant of the RC circuit shown in the inlay of the figure, time elapsed after 1 time constant
after the source voltage returns to 0 V is 10 uS which is equal to RC.
The above notes have been prepared by Dr. Vincent Cunningham at the Bahrain Polytechnic on the 11 th of April 2012. The software circuit
simulations have been performed using Altium Designer Summer 09.
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Revised 11th April 2012