Experiment 4: AM Demodulation

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Experiment 4: AM Demodulation
1. Turn on the NI ELVIS power switch at the back then turn on its Prototyping Board Power switch
at the front.
2. Launch the NI ELVIS software.
3. Launch the DATEx soft front-panel (SFP). From Step-4 to Step-10 it is aimed to setting 1V DC
voltage and 1Vp-p 10kHz sine signal.
4. Launch the Variable Power Supplies (VPS) VI. Adjust the VPS negative output about -6V.
Minimize VPS, but dont’t close it.
5. Locate the Adder module on the DATEx SFP and turn its soft G and g controls fully anticlockwise.
6. Connect the set-up shown in Figure 1 below.
Figure 1
7. Launch the NI ELVIS DMM VI. Set up the DMM VI for measuring DC voltages. Connect the
Adder module’s output to the DMM’s HI input and adjust the module’s soft g control to obtain
a 1V DC output. Close the DMM VI.
8. Launch the NI ELVIS Oscilloscope VI. Set up the scope the procedure with the following
changes:
 Trigger Source control- Immediate
 Channel 0 Coupling control - DC
 Channel 0 Scale control - the 500mV/div position
 Timebase control - the 50µs/div position
9. Adjust the Adder module’s soft G control to obtain a 1Vp-p sinewave. Set the scope’s Trigger
Source control to CH 0 and set its Trigger Level control to 1V. Activate the scope’s Channel 1
input to view both the message and the modulated carrier. Set Channel 1 Coupling control to
DC and Channel 1 Scale control to 1V/div position.
The scope should now display an AM signal with envelopes that are the same shape and size
as the message.
10. Modify the set-up as shown in Figure 2 below.
Figure 2
11. Set up the scope the procedure with the following changes:
 Trigger Source control- Immediate
 Channel 0 and Channel 1 Coupling control - DC
 Channel 0 Scale control - the 500mV/div position
 Channel 1 Scale control- the 1V/div pozition
 Timebase control - the 200µs/div position
12. Draw the two waveforms to scale in the space provided below leaving room to draw a third
waveform. Tip: Draw the message signal in the upper third of the graph and the rectified AM
signal in the middle third.
(25p)
13. Disconnect the scope’s Channel B input from the Rectifier’s output and connect it to the RC
LPF’s output instead. Draw the demodulated AM signal to scale in the space that you left on
the graph paper.
Question 1: What is the relationship between the original message signal and the recovered
message? (10p)
14. Vary the message signal’s amplitude up and down a little (by turning the Adder module’s soft
G control left and right a little) while watching the demodulated signal.
Question 2: Which demodulation method has been used in this part of experiment?(10)
15. Slowly increase the message signal’s amplitude to maximum while watching the demodulated
signal.
Question 3: What do you think causes the heavy distortion of the demodulated signal? Tip: If you’re
not sure, connect the scope’s Channel A input to the AM modulator’s output.(10p)
Question 4: Why does over-modulation cause the distortion?Explain in detail.(10p)
16. Modify the set-up as shown in Figure 3 below.
Figure 3
17. Set the message signal’s amplitude to 0.5Vp-p (using the Adder module’s soft G control).
Compare the Multiplier module’s output with the Rectifier’s output that you drew earlier.
Question 5: Given the AM signal is being multiplied by a 100kHz sinewave
(10p)
A) How many sinewaves are present in the Multiplier module’s output?
B) What are their frequencies?
18. Disconnect the scope’s Channel B input from the Multiplier module’s output and connect it to
the RC LPF’s output instead . Compare the RC LPF’s output with the message and the output
RC LPF’s that you drew earlier.
Question 6: If AM modulated signal is over modulated, is it possible to recover the mesage signal?
Explain.(15p)
19. Slowly increase the message signal’s amplitude to produce a near 100% modulated AM signal
by adjusting the Adder module’s soft G control. Slowly increase the message signal’s amplitude
to produce an AM signal that is modulated by more than 100% while paying close attention to
the demodulated message signal.
Question 7: Why the commercial implementation of AM modulation commonly involves a Class C
amplifier?(10p)
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