Faculty of Engineering Sciences
Electronics and Communications Department
Fall 2012
ELE 353
Data Communication Systems
Sheet(2)
Amplitude modulation (Envelop detector)
Dr. Amr Abouel Magd Ahmed
Eng : Ahmed Al Wakeel
2012 – 2013
Sheet ( 2 )
AM:
(1)
The figure bellow shows a scheme for coherent demodulation. Show that this scheme can demodulate
the AM signal [A + m(t)] cos wct regardless of the value of A
Fig. 1
(2)
Sketch the AM signal [A +m(t)] cos wct for the periodic triangle signal m(t) shown in the figure
corresponding to the modulation index: (a) μ= 0.5; (b) μ= 1; (e) μ= 2; (d) μ= .
How do you interpret the case μ= ?
Fig. 2
(3)
For the AM signal in Prob. 2 with μ= 0.8:
(a) Find the amplitude and power of the carrier.
(b) Find the sideband power and the power efficiency η
(4)
(a) Sketch the DSB-SC signal corresponding to m(t) = cos 2πt
b) This DSB-SC signal m(t) cos wct is applied at the input of an envelope detector. Show that output of
the envelope detector is not m(t), but |m(t)|. Show that, in general, if an AM Signal .[A + m(t)] cos wct
is envelope-detected, the output is |A + m(t)| I. Hence, show that the condition for recovering m(t) from
the envelope detector is A + m(t) > 0 for all t
(5)
Show that any scheme that can be used to generate DSB-SC can also generate AM. Is the converse true?
Explain
(6)
Show that any scheme 'that can be used to demodulate DSB-SC can also demodulate AM. Is the
converse true? Explain
(7)
In the textbook, mentioned in the syllabus, the power efficiency of AM for a sinusoidal m(t) was
derived. Carry out a similar analysis when m(t) is a random binary signal as shown in the figure and μ=
1. Sketch the AM signal with μ= 1 Find the sideband's power and the total power (power of the AM
signal) as well as their ratio (the power efficiency η).
Fig. 3