Experiment 2 AM Modulation and Demodulation

Experiment 2
AM Modulation and Demodulation
2.1 Objective
1. To evaluate and analyze an implementation for an AM transmitter.
2. To test an implementation of the superheterodyne receiver and envelope
detector.
2.2 Basic Information
An AM signal is generated by forcing the envelope of a high frequency
sinusoid, carrier, to vary in proportion to the desired low frequency message signal
f(t). The resulting AM spectrum is concentrated in the vicinity of the carrier
frequency. A typical amplitude modulated carrier has the time domain form:
v( t )
[ mf ( t )] cos( t ) g( t )cos( t )
= A 1+
ω C =
Where A is the carrier amplitude, m is the modulation index, ω c is the carrier
frequency, and g(t) is the envelope function of the AM signal. The signal f(t) must
satisfy the condition |f(t)|≤1.
ωC
Figure 2.1 (a) the message signal, (b) the carrier signal, (c) AM (d) Over modulation
If the envelope g(t) ≥ 0, i.e. m ≤ 1, full carrier AM will results. In this case the
modulation index m can be found by the formula (see figure 2.1-c)
C
m =
C
−
+
In case of m>1 over modulation results as in figure 2.1-d. Obviously in this
case the envelope no longer matches the message. Demodulation (separating the
message from the carrier) in this case is not possible.
B
B
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Measurement of the Envelope (The Trapezoidal Pattern)
The Lessajous pattern of v(t) vs. Af(t) for an AM signal formed on the
oscilloscope yields a trapezoidal pattern as in figure 2.3-a.
(a)
(b)
Figure 2.2. The trapezoidal pattern for (a) a normal AM signal and (b) AM with
envelope nonlinearity.
Connecting the message to channel 1 and the modulated signal to channel 2,
and switching the oscilloscope to its XY mode will generate this pattern (figure 2.2).
The resulting trapezoid can be used for several measurements:
1- To measure the modulation index:
( D − E) ( D E)
m =
+
2- To detect any distortion in the envelope of the signal, see figure 2.2-b. This
distortion is exhibited as a departure from straight lines for the upper and lower edges
of the trapezoid.
Experiment AM modulator/transmitter circuit.
Figure 2.4 Block diagram of experiment AM modulator.
The AM modulator/transmitter (figure 2.4) consists of; an audio tone
generator; an audio amplifier; a stable RF oscillator; a mixer; and an RF amplifier
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whose output is connected to the transmitter antenna. The AM transmitter used in this
experiment is prebuilt on a PCB. The purpose of the experiment is to observe the
main signals and operations carried on them in a system approach.
The mixer in the experimental circuit is the analog multiplier IC (MC1496).
Internally, the IC is based on the Gilbert Cell principle. Figure 2.5 shows a simplified
circuit diagram for operating the IC.
Figure 2.5 AM modulation using the MC1496 IC.
The carrier is fed to pin 10 and the message (audio) to pin 1. The Modulation
Level potentiometer adjusts the modulation index. The output is available at pins 6
and 12. The Carrier Suppress controls carrier suppression. Setting at the either end
position yields full carrier AM, while adjusting to the center yields suppressed carrier
AM.
The complete experiment circuit is in figure 2.9. It consists of:
a) Crystal oscillator. Amplifier A with its associated circuit is used to generate
the carrier, 1MHz (TP4), with the output taken from C6. This is fed to the
carrier input at pin 10 of the analog multiplier IC.
b) Audio generator and amplifier. The audio sine wave signal is generated from
an 8038 waveform generator IC. This signal is fed to a pot to provide control
for the amplitude, i.e. the modulation index, before being amplified by the
amplifier B. The final audio output (TP2) is fed to pins 1 & 4 of the
modulator/multiplier.
c) The balance control, R13 potentiometer, is used to cancel the carrier out.
d) The amplifier C is used to amplify the audio signal for generating the
trapezoidal pattern. This is needed for some oscilloscopes.
e) The output of the mixer IC is taken between pins 12 and 6 into a parallel RLC
tank tuned to 1MHz. Then it is fed to transistor amplifier Q1, which drives the
antenna.
The output of this circuit is in the commercial AM band and can be received
by common AM radio receivers.
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AM Demodulation
An AM signal can be demodulated using either synchronous or asynchronous
detection methods. While synchronous methods are more precise and offer
exceptional results, asynchronous methods are simple and economical. Asynchronous,
also known as envelope detectors, can only be used for full carrier AM. A simple
envelope detector circuit and the signals involved are shown in figure 2.6.
Figure 2.6 The envelope detector and its signals.
Assuming that the amplitude of the input signal v i (t) is large enough to turn the
diode on an off, the circuit in figure 2.6 can be used to demodulate AM signals. The
selection of the capacitor C and resistor R should satisfy the formula
1
RC = ω c >> ω m
ω mω
c
where ω m is the maximum signal frequency and ω c is the carrier frequency. The
capacitor C c is used to block the DC component of the output.
The Superheterodyne Receiver
For the detection of commercial AM, more than a simple envelope detector is
required. The desired radio frequency (RF) signal needs to be amplified while other
signals need to be rejected before subjecting the signal to the envelope detector. This
can be done in a tuned RF amplifier. However, when it is desirable to tune to more
than one RF signal, the design of the tuned RF amplifier becomes extremely difficult
and expensive. A simpler approach is to design the amplifier to a fixed intermediate
frequency (IF), and to shift the desired RF signal down or up to that IF. Figure 2.7
shows the block diagram for a superheterodyne receiver.
Figure 2.7 Block diagram of a superheterodyne receiver.
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The antenna feeds the RF waves into the first passive bandpass filter, which
rejects signals outside the desired band; afterwards a wideband RF amplifier is used to
amplify the signal. The output of the RF amplifier is filtered again to reduce the noise
power. A local oscillator (LO) generates a signal with frequency above or below the
desired AM signal by a fixed amount (f IF ). In commercial AM receivers this
frequency is usually 455kHz above the desired AM signal. This LO signal is mixed
with the AM signal producing four signals, the incoming RF, the LO signal, the sum
of the RF and LO signals, and the difference between the LO and the RF signal (the
IF signal). The following BPF is centered at the IF frequency, it is used to reject all
but the IF signal. Following that will be one or more tuned IF amplifiers to increase
the IF signal power to drive the diode in the envelope detector into conduction. After
this comes the envelope detector followed by a baseband amplifier and onto the
speaker in the case of audio signals.
The superheterodyne receiver used in this experiment is prebuilt on a board. It
is designed to receive commercial AM broadcast. The IF frequency is 455kHz, while
the tuning range is from 1Mhz to 2.06Mhz.The purpose of the experiment is to
observe the main signals and operations carried on them in a system approach. Figure
3.3 shows the complete schematic diagram of the experimental AM receiver.
Automatic Gain Control (AGC)
To prevent overloading of the IF amplifiers when a strong signal is received
some form of gain control is necessary. By passing the received signal through a long
time constant LPF (~1s) an average of the recived signal power can be formed. This
control voltage is applied to the base of the amplifier transistors in the IF stages and
possibly the RF stage thereby changing their gain to suit the received signal power.
2.3 Equipment
AM/DSB Transmitter Insertion Panel ( SIP350A )
AM Radio Receiver Panel ( SIP327B )
Power Supply Base ( S300PSB )
Dual-Trace Oscilloscope
Digital Multimeter
Function Generator
2.4 Procedure
Part A- AM Transmitter
1. Make sure +15V knobs on the power supply panel base are reduced to
minimum.
2. Switch on the power supply panel and adjust the voltage knobs to +15V dc.
3. Connect ch1 of the oscilloscope to TP2 on the transmitter board, observe and
record the message signal. If no signal is present, increase the modulation
level control sli ghtly until the signal is visible.
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Amplitude (V Pk-Pk )
Frequency (kHz)
4. Connect ch2 to TP4, observe and record the carrier signal. What are the
amplitude and frequency of the carrier
Amplitude (V Pk-Pk )
Frequency (kHz)
5. Turn the carrier suppress knob fully clockwise ( AM setting ).
6. Connect ch2 to TP8 to observe the o/p of the LM1496. Use the message signal
on ch1 (TP2) as the trigger source. Make sure there is no overmodulation.
Record both signals on the same graph.
7. Calculate the modulation index.
8.
Modulation Index (m)
Switch the oscilloscope to XY mode. Record the trapezoidal pattern. Find the
modulation index from the trapezoidal pattern, and compare it with step 7. Is
there distortion in the modulation process
Modulation Index (m)
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9. Use the trapezoidal pattern and change the modulation level to get 100%
modulation.
10. Remove the oscilloscope XY mode and record the resulting AM waveform.
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11. Change the message amplitude by varying the modulation level, and find the
modulation index for the message amplitudes (0, 0.5, 1.0, 1.5V PP , and 100%
modulation). Plot a graph of message amplitude vs. modulation index.
V Message Pk-Pk 0 0.5 1.0 1 .5
Mod. Index 100%
12. What does the graph shape mean
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Part C- AM Receiver
1. Connect CH2 of the oscilloscope to TP22 on the AM receiver board.
2. Switch on the speaker in the power supply base, then tune the receiver to the
best signal reception (largest amplitude on CH2 and clearest sound).
3. Record the receiver’s local oscillator (LO) signal at TP5.
Amplitude (V Pk-Pk )
Frequency (kHz)
4. Calculate the intermediate frequency f IF = f LO - f c .
f IF (kHz)
5. Observe the signal at TP6, what does this signal represent
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6. Record the 1 st IF stage o/p at TP9 by expanding the horizontal display of the
CRO. Measure f IF .
7. Record the 2 nd IF stage o/p at TP12, find the gain of the second IF stage in dB.
Gain (dB)
8. Connect the oscilloscope to the envelope detector output at TP14. Record the
o/p of the envelope detector and compare with the original message on the
same graph.
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9. Measure f m and compare with the original message. Comment on the shape of
the received message.
f m (kHz)
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Part D- AGC
1. Connect the multimeter to TP 16 and set it to DC coupling.
2. Record the AGC voltage.
AGC (V)
3. Touch the antenna with your finger. Describe what happens to the shape of the
received signal.
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4. Record the AGC voltage while touching the antenna. And explain the change
in the AGC voltage.
AGC (V)
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