Analog Communication
TDM
• TDM: is more simple than FDM, used in
analog and digital communication
– For analog HVAC
– For digital TDM/PCM, PAM, ….
• In TDM, each message signal occupies the
channel (eg. A transmission line) for a short
period of time.
TDM Transceiver
SW1 Transmission line SW2
m1(t)
m1(t)
m2(t) TX RX m2(t)
m3(t) m3(t)
m4(t) m4(t)
Switches SW1 and SW2 are rotates in synchronism, and in effect samples
each message input in a sequence m1(t), m2(t), …..
Multiplexer signals
m1(t) m2(t) m3(t) m4(t)
t t t t
Output from Multiplexer (rotating switches time = 1 period
We must use buffer before input signals to the switch to prevent loss information
Demultiplexer signals
m1(t)
t
m2(t)
t
m3(t)
t
m4(t)
t
To remove this problem we must use buffer at the end of demultiplexer
Pulse Modulation
• It is a system in which continuous waveform are sampled
at regular interval.
• Information regarding the signal is transmitted only at the
sampling times.
• At the receiving end, the Fs=1/Ts
Fm=1/Tm
original waveform
may be reconstructed Fs >= 2Fm
from the information
regarding the samples,
if these are taken Ts
frequently enough.
Tm
Types of mudulation
• Continuous modulation
– Analog modulation
• AM, FM, PM
– Pulse modulation
• PAM, PWM, PPM
• Digital modulation
– PCM, FSK, ASK, PSK, QAM
Why pulse modulation
• We can use the space between pulses for
another signal
• Needs less energy to transmit a part of signal
than all
• Digital circuits are more economical than
analog circuits
Sampling principles
• Sampling: is the process of converting a
continuous signal into numeric sequence
(discrete signal). Under condition of no losing of
information when we reconstruct this signal.
• Sampling theorem: the minimum sampling
frequency of a pulse modulation system should
be equal to (or more than) twice the highest
information signal frequency.
• Fs >= 2Fm
Spectrum 1
M(f)
Let the information signal m(t) has the following
spectrum f
-fm fm
And the spectrum of carrier pulses sequence is like these
s(t) S(f)
t f
-3fs -2fs -fs fs 2fs 3fs
Spectrum 2
Xs(f)
Fs = fm f
fm fs 3fm 2fs 5fm 3fs
2fm 4fm 6fm
Xs(f)
Guard band
f
Fs > fm
fm fs-fm fs fs+fm 2fs-fm 2fs 2fs+fm
Xs(f) Aliasing Distortion
Fs < fm f
fm
fs-fm fs fs+fm
Sampling types-I
• There are three types of sampling:
1. Instantaneous or impulse sampling
fs
+
output
m(t)
-
Sampling types-II
2. Natural Sampling
multiplier
m(t)
X output
Pulse train
Sampling types-III
• Flat-Top Sampling
– Using Quantizer after sampling process to flat tops
of pulses. As shown below:
Quantization
Proof of sampling theorem-I
• The original signal can be recognized from the
sampled waveform
• The sampling theorem requires that we assume
that m(t) has no frequency components above
fm. (by using LPF before modulation process)
• And the carrier pulse train
M(f)
s(t) can be expanded in Fourier
Series, as an even function, by
using trigonometric series f
contain only cosine terms. -fm fm
Proof of sampling Theorem_II
Xs(f)
Guard band
ao a1 a2
f
-fs -fm fm fs-fm fs fs+fm 2fs-fm 2fs 2fs+fm
Proof of sampling theorem-III
• The shape center at the original is the
transform of ao x(t), and shifted versions
represent the transforms of the various
harmonics terms.
• The various terms can be separated from each
other using linear filter
• A LPF with cutoff frequency of fm can be used
to recover ao x(t) term.
Pulse Amplitude Modulation (PAM)
• The Amplitude of pulses of carrier pulse train
is varied according to the instantaneous value
of modulating signal.
• Type of PAM
– Dual polarity PAM
– Single polarity PAM
Dual polarity PAM
• It’s a normal type
Single polarity PAM
• In which a fixed DC
level is added to the
signal to ensure that
the pulses always are
positive.
Demodulation of PAM
• For Demodulation of PAM wave, a low pass
filter is used to remove high frequencies and
pass low frequencies. Like cover detection in
Amplitude Demodulation system.
Pulse Time Modulation PTM
