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COMMUNICATION SYSTEM EECB353 Chapter 3 Part I ANGLE MODULATION. Dept of Electrical Engineering Universiti Tenaga Nasional. Introduction. Amplitude Modulation (AM) – Amplitude of the carrier varies accordingly to the message signal.
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Dept of Electrical Engineering
Universiti Tenaga Nasional
where m(t) = angle modulated waveform
Vc = peak carrier amplitude
c = carrier radian frequency (=2fc)
(t) = instantaneous phase deviation (radians)
(t) = F [vm(t)] (2)
where vm(t) = Vmsin(mt)
Vm = peak modulating signal amplitude
m = modulating signal radian frequency (=2fm)
Figure: Angle modulation vs amplitude modulation
Figure: Angle modulated wave in frequency domain
Instantaneous phase deviation = = (3)
Instantaneous frequency deviation = = (4)
where Kpand Kf are constant (deviation sensitivity of phase and freq)
Figure : Phase and Frequency Modulation of sinusoidal carrier by a single-freq modulatingsignal.
The phase deviation of modulator o/p is prop. to m(t). The freq deviation is prop. to the derivative of the phase deviation, thus the i is max when the slope of m(t) is max and min when the slope of m(t) is min
Freq. deviation is prop. to m(t), thus the i is max when the m(t) is max and min when m(t) is min
Note that m(t) is not shown along with the modulator o/p. It would not be possible to distinguish the PM and FM o/p
Output of PM and FM modulators for a sinusoidal, Accos 2fct
(a) Message signal. (b) Unmodulated carrier. (c) Phase modulator output (kp = ½). (d) Frequency modulator output.
i =fc for t<t0 and t>t0
The phase of unmodulated carrier is advanced by kp=/2 rads for t>t0 giving rise to a signal that is discontinues at t=t0
i =fc for t<t0 and,
= fc+ fdfor t>t0
The modulator o/p phase is, however, continues at t=t0
Kf = deviation sensitivity for frequency
Kp = deviation sensitivity for phase
m = modulation index and peak phase deviation, ∆Ѳ rad
% modulation = (27)
4. For a direct FM modulation, an information signal, is used to modulate a carrier, . For a deviation sensitivity of 2.5rad/s/V, calculate