EED3014 Analog Communication - 2013 Spring
Group No
Experiment 2
Grade
Student number
and name
Student number
and name
Student number
and name
Experiment 2
Design of Envelope Detector
Preliminary Work
1. Read and understand the working principle of the envelope detector.
2. Design an envelope detector circuit and simulate it in PSpice. Apply a tone modulated
AM signal to the input terminals of your circuit. You may use the part MULT in PSpice
in order to generate an AM wave.
3. You are free to choose the frequencies of the message and carrier signals. But keep in
mind that the signal generators and oscilloscopes in the laboratory works in the frequency
range 0 to 20 MHz.
4. You are free to choose the amplitudes of the message and carrier signals. Remember that
the signal generators in the laboratory can provide AC amplitudes up to 10 V and DC
offsets up to 5 V.
5. Be careful while choosing the capacity and resistance values. You should be able to get
them in the market. You must use a variable capacitor and/or a variable resistor in order
to observe their effects in the circuit.
6. Observe the result for µ=0.5, µ=1, and µ=1.2.
7. Your preliminary report should include
• Your design decisions
• Your circuit
• Waveforms at the input and output of the circuit for µ=0.5, µ=1, and µ=1.2 generated by PSpice. Be careful about the values of modulation index. Wrong
modulated waves don’t get partial credit!
ATTENTION
• You are not allowed to join the laboratory if you don’t submit a preliminary
report.
• Each group will submit only one preliminary report.
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EED3014 Analog Communication - 2013 Spring
1
Experiment 2
THEORY
The process of demodulation is used to recover the original modulating wave from the incoming
modulated wave; in effect, demodulation is the reverse of the modulation process. As with
modulation, the demodulation of an AM wave can be accomplished using various devices; here,
we describe a simple and yet highly effective device known as the envelope detector. Some
version of this demodulator is used in almost all commercial AM radio receivers. For it to
function properly, however, the AM wave has to be narrow-band, which requires that the carrier
frequency be large compared to the message bandwidth. Moreover, the percentage modulation
must be less than 100 percent. An envelope detector of the series type is shown in Fig.1, which
Rs
C
Rl
output
AM wave, s(t)
Figure 1: Envelope detector
consists of a diode and a resistor-capacitor (RC) filter. The operation of this envelope detector
is as follows. On a positive half-cycle of the input signal, the diode is forward-biased and the
capacitor C charges up rapidly to the peak value of the input signal. When the input signal
falls below this value, the diode becomes reverse-biased and the capacitor C discharges slowly
through the load resistor Rl The discharging process continues until the next positive half-cycle.
When the input signal becomes greater than the voltage across the capacitor, the diode conducts
again and the process is repeated. We assume that the diode is ideal, presenting resistance rf to
current flow in the forward-biased region and infinite resistance in the reverse-biased region. We
further assume that the AM wave applied to die envelope detector is supplied by a voltage source
of internal impedance RS . The charging time constant (rf + RS )C must be short compared
with the carrier period 1/fc , that is,
c(t) = Ac cos(2πfc t)
(1)
where Ac is the carrier amplitude and fc is the carrier frequency. Let m(t) denote the baseband
message signal. Amplitude modulation is defined as a process in which the amplitude of the
carrier wave c(t) is varied about a mean value, linearly with the baseband message signal m(t)
An amplitude modulated (AM) wave may thus be described as a function of time as follows:
(rf + Rs ) C <<
1
fc
(2)
so that the capacitor C charges rapidly and thereby follows the applied voltage up to the
positive peak when the diode is conducting. On the other hand, the discharging time constant
Rl C must be long enough to ensure that the capacitor discharges slowly through the load resistor
Rl between positive peaks of the carrier wave, but not so long that the capacitor voltage will
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EED3014 Analog Communication - 2013 Spring
Experiment 2
not discharge at the maximum rate of change of the modulating wave, that is
1
1
<< Rl C <<
fc
W
(3)
where W is the message bandwidth. The result is that the capacitor voltage or detector output
is nearly the same as the envelope of the AM wave.
2
PROCEDURE
a. Construct the circuit you designed in your preliminary work. Note the frequency of the
message signal fm and carrier signal fc below.
fm =
fc =
b. Write the necessary calculation steps to determine the value of Rl and C
Rl (calculated)=
C(calculated)=
c. Check your result and determine if your demodulator should work.
d. Connect the signal generator outputs to the oscilloscope. Measure the exact frequency of
message and carrier signals.
fm (measured)=
fc (measured)=
e. Adjust the amplitudes of message and carrier signal in order to obtain the modulation
index µ = 0.5.
f. Measure the output voltage of the circuit. Draw the waveform with AM signal in dual
mode below. Determine the peak amplitude and frequency of the output signal.
fdemod (measured)=
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EED3014 Analog Communication - 2013 Spring
Experiment 2
Volt/div=
time/div=
Figure 2: AM and demodulated signal for µ = 0.5
g. Adjust the amplitudes of message and carrier signal in order to obtain the modulation
index µ = 1.
h. Measure the output voltage of the circuit. Draw the waveform below with AM signal in
dual mode. Determine the peak amplitude and frequency of the output signal.
fdemod (measured)=
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EED3014 Analog Communication - 2013 Spring
Experiment 2
Volt/div=
time/div=
Figure 3: AM and demodulated signal for µ = 1
i. Adjust the amplitudes of message and carrier signal in order to obtain the modulation
index µ = 1.2.
j. Measure the output voltage of the circuit. Draw the waveform with AM signal in dual
mode below. Determine the peak amplitude and frequency of the output signal.
fdemod (measured)=
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EED3014 Analog Communication - 2013 Spring
Experiment 2
Volt/div=
time/div=
Figure 4: AM and demodulated signal for µ = 1.2
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EED3014 Analog Communication - 2013 Spring
3
3.1
Conclusion
Student Name and ID:
7
Experiment 2
EED3014 Analog Communication - 2013 Spring
3.2
Student Name and ID:
8
Experiment 2
EED3014 Analog Communication - 2013 Spring
3.3
Student Name and ID:
9
Experiment 2