written 6.7 years ago by | • modified 2.8 years ago |
Subject : Principle of Communication Engineering
Topic : Pulse-Modulation and demodulation
DIfficulty: Medium
written 6.7 years ago by | • modified 2.8 years ago |
Subject : Principle of Communication Engineering
Topic : Pulse-Modulation and demodulation
DIfficulty: Medium
written 6.6 years ago by | • modified 6.6 years ago |
Sampling theorem
fs ≥ 2fm
Where,
fs is the sampling frequency and
fm is the maximum modulating signal frequency
Flat top sampling
In flat top sampling, the top of the samples remains constant and equal to the instantaneous value of the modulating signal at the start of the sampling.
Thus the amplitude of the pulse after sampling is kept constant and the top of the sampled pulse do not follow the contour of the modulating signal unlike Natural sampling.
The duration of each sample is τ and the sampling rate is fs=1/Ts. Ts=1/fs.
Sample and hold circuit is used for the generation of the sampled signal to attain flat top sampling, which is shown in the Fig.1 below.
Fig.1 Sample and hold circuit to generate flat top samples
Fig.2 (a) Modulating signal (b) sampling signal and (c) Flat top sampling spectrum
The starting edge of the pulse corresponds to the instantaneous value of the modulating signal x(t).
Flat top sampling can be mathematically considered as convolution of the sampled signal and the pulse signal.
Flat top sampling is mostly used in digital transmission
Aperture effect
The amplitude of the flat top signal must be constant, but sometimes it is not constant due to the high frequency roll off of the sampling signal.
This results in the attenuation in the high frequency part of the message spectrum.
Thus the sampled signal in the flat top sampling consists of attenuated high frequency components and this effect is known as Aperture effect.
Aperture effect can be improved by selecting value of pulse width τ to be very small and by using equalizer circuit.