TechGuru Technolic
ECT Project
Unit V
Q.1.Define frequency modulation. Discuss how FM waves results.
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Definition: The encoding of a carrier wave by variation of its frequency in accordance
with an input signal is called frequency modulation
How FM waves results:
FM signals can be generated using either direct or indirect frequency modulation:
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Direct FM modulation can be achieved by directly feeding the message into the input of a
VCO (voltage controlled oscillator). The VCO often consists of an oscillator with a
variable capacitance or inductance that allows the resonant frequency to vary with the
applied voltage.
Indirect FM generation first produces a narrow-band (relatively low
bandwidth/frequency). Then, it multiplies and mixes this wave's frequency properties to
achieve the desired frequency deviation and carrier frequency.
Direct FM generation has poor frequency stability, so it requires feedback to stabilize
this, which complicates the system. Indirect FM generation has good frequency stability,
but is noisier since the noise is multiplied during the frequency multiplication.
Diagram:-
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Q.2.Define Phase modulation. Define modulation index for FM.
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Phase Modulation: Phase modulation is a method of transmitting signals in which the
value of the signal is proportional to the phase angle of a carrier wave. It is often
combined with amplitude modulation to transmit digital information via modems.
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Modulation Index for FM: Modulation index is the ratio of the maximum deviation
frequency to the frequency of modulation. In other words, it is the ratio of the spread in
frequency spectrum to the frequency that used to modulate the carrier.
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Where,
mf = modulation index for FM.
fm = frequency of the modulation.
Let us take an example:
Let us take a FM broadcasting radio station, 98.3MHz with a power of 10 KW. The
bandwidth of the modulation signal is from 30 Hz to 15 kHz which is excellent for highfidelity broadcast. The maximum deviation set by the FCC, (δ), is 75 kHz. The range of
the modulation index is ;
mf (min) = δ/fm (max)
= 75 kHz/15 kHz
= 5 (for fm= 15 kHz)
and for
mf (max) = δ/fm (min)
= 75 kHz/30 Hz
= 2500! (for fm= 30 Hz)
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Q.3. Differentiate between AM & FM Systems
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The process of changing amplitude or frequency of a carrier wave in accordance with the intensity of the signal
is called Modulation.
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Accordingly there are two basic types of Modulations, namely; Amplitude Modulation (AM) and Frequency
Modulation (FM). These two systems can be differentiated as given below:
Amplitude Modulation (AM)
Frequency Modulation (FM)
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When the amplitude of high frequency
carrier wave is changed in accordance
with the intensity of the signal, it is
called Amplitude Modulation (AM).
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When the frequency of high frequency
carrier wave is changed in accordance
with the intensity of the signal, it is
called Frequency Modulation (FM).
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The frequency of carrier wave remains
constant with amplitude modulation.
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The amplitude of carrier wane remains
constant with frequency modulation.
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The carrier amplitude changes according
to the strength of the modulating signal.
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The carrier frequency changes
according to the strength of the
modulating signal.
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Amplitude modulated signal has poorer
sound quality due to smaller bandwidth.
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Frequency modulated signal has a
better sound quality due to higher
bandwidth.
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AM is more susceptible to noise because
noise affects amplitude, where
information is stored in an AM signal.
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FM is less susceptible to noise because
information in an FM signal is
transmitted through varying the
frequency, and not the amplitude.
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Q.4.What do you mean by sampling? State and explain sampling theorem.
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In signal processing, sampling is the reduction of a continuous signal to a discrete signal. A common example
is the conversion of a sound wave (a continuous signal) to a sequence of samples (a discrete-time signal).
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It is also known as the process of converting a signal into a numeric sequence.
Sampling Theorem:
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The Sampling theorem or Nyquist theorem states that a band limited signal can be reconstructed exactly if it is
sampled at a rate at least twice the maximum frequency component in it.
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For analog-to-digital conversion (ADC) to result in a faithful reproduction of the signal, slices, called samples,
of the analog waveform must be taken frequently. The number of samples per second is called the sampling rate
or sampling frequency.
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Any analog signal consists of components at various frequencies. The simplest case is the sine wave, in which
all the signal energy is concentrated at one frequency. In practice, analog signals usually have complex
waveforms, with components at many frequencies. The highest frequency component in an analog signal
determines the bandwidth of that signal. The higher the frequency, the greater the bandwidth, if all other factors
are held constant.
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Suppose the highest frequency component, in hertz, for a given analog signal is fmax. According to the Nyquist
Theorem, the sampling rate must be at least 2fmax, or twice the highest analog frequency component.
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The sampling in an analog-to-digital converter is actuated by a pulse generator (clock). If the sampling rate is
less than 2fmax, some of the highest frequency components in the analog input signal will not be correctly
represented in the digitized output.
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When such a digital signal is converted back to analog form by a digital-to-analog converter, false frequency
components appear that were not in the original analog signal. This undesirable condition is a form of distortion
called aliasing.
Signal sampling representation. The continuous signal is
represented with a green colored line while the discrete
samples are indicated by the blue vertical lines.
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Q.5. Write a note on Pulse Amplitude Modulation (PAM)
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In the analog pulse modulation, the amplitude width or position of the carrier pulse is changed in
accordance with modulating signal. This will result in Pulse Amplitude Modulation (PAM).
The amplitude of a constant width, constant position is varied in proportion with the instantaneous
amplitude of the modulating signal.
In the PAM system the amplitude of the pulse carrier is changed in proportions with the instantaneous
amplitude of the modulating signal x (t).
So the information is in the amplitude variation of PAM signal.
The carrier is in the form of narrow pulses
If u composed the PAM function with the sampling process you will find that these two process are
identical.
The PAM signal is then send by either wire or cable or it is used to modulate a carrier.
Diagram:-
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Q.6. Write note on Pulse Width Modulation (PWM).
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In this system, the width of carrier pulse is varied in accordance with the instantaneous amplitude of
modulating signal. The amplitude of pulse is fixed.
 From the diagram it seem that the width of pulse goes on
increasing as the modulating signal goes on increasing and
decreasing from decreasing modulation signal.
Advantages of PWM:
 Less effect of noise i.e. very good noise immunity.
 Synchronization between the transmitter and the receiver is
not essential.
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Pulse Width Modulation (PWM)
Disadvantages of PWM:
Due to variable pulse width, the pulse have variable power constant so the transmitter must be powerful
enough to handle maximum width pulse.
Requires large bandwidth.
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Q.7.Write a note on Pulse Position Modulation (PPM).
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When the time of occurrences of pulses in the train of pulses or digital carrier is changed in
accordance with the input analog waveforms, the modulation is called Pulse Position
Modulation(PPM)
In PPM, the amplitude & width of the sampled pulses is maintained as constant.
The position of each pulse w.r.t. the position of a recurrent reference pulse varies as a function of the
instantaneous sampled amplitude of the modulating signal.
PPM is achieved by shifting each pulse form its unmodulated position by an amount proportional to
the amplitude of the input analog waveforms.
The amplitude of the input waveforms decides the time of occurrences of the pulse in the resultant
waveform.
A pulse position modulated signal can be generated from a pulse width modulated signal.
PPM is often used in optical communication, such as fiber optics, in which there is little multipathway interference.
PPM has advantage over pulse amplitude modulation (PAM) in that it has a higher noise immunity.
One of the disadvantage that PPM has that it is highly sensitive to multi-pathway interference.
PPM can be used for a number of purposes, especially in RF (Radio Frequency) communications.
Diagram:-
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Q.8. Write a note on Pulse Code Modulation (PCM).
Pulse Code Modulation (PCM)
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Pulse Code Modulation (PCM) is a method by which standard audio signals or analog signal are
converted into digital audio signal or digital signal with little, or no, compression.
Information in an analog form cannot be processed by digital computers, so it is necessary to convert
them into digital form.
In PCM, the train of pulses is not transmitted. Instead, the coded values are transmitted over the
communication channel.
The input analog waveform which need to be transmitted, is first sampled using Pulse Amplitude
Modulation (PAM) methods.
This gives the discrete sample values at the time interval corresponding to the digital carrier.
These values are quantifies &are then converted into the digital values using A/D converter.
Under PCM, a binary number represents each pulse group, & this pulse group represent by the binary
number, is transmitted over a communication channel.
At the receiving end, the signal is decoded & it generated the input analog waveform using D/A
converter & filter.
The advantage of PCM is that, it permits the use of pulse regeneration.
The disadvantage of PCM is that, it requires a large bandwidth as compared to the others system.