INSTRUMENTATION & CONTROL TELEMETRY SYSTEM

1. UNIT – 4 PROCESS SIGNAL MODULATION AND DEMODULATION
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MODULATION:
 Modulation is the process of combining the low frequency signal with a very high
frequency radio wave called carrier wave.
 The resultant wave is called modulated carrier wave. Low frequency signal is called
modulating wave.
MODULATING SIGNAL:
 Some characteristics of a carrier signal is varied in accordance with the
instantaneous amplitude of another signal called modulating signal which has low
frequency and carry information.
CARRIER SIGNAL:
 The high frequency signal whose characteristics varies in accordance with
instantaneous amplitude of modulation signal is known as carrier signal.
MODULATED WAVE:
 The signal resulting from the process of modulation is called modulated wave.
NEED OF MODULATION:
 Modulation increases operating range.
 It reduces the size of transmitting and receiving antennas.
 It permits transmission without wire.
 Improves quality of reception.
 Avoids mixing of signals.

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TYPES OF MODULATION:
1) Analog modulation
a) Amplitude modulation
b) Angle modulation
I. Frequency modulation
II. Phase modulation
2) Pulse digital modulation
a. Digital modulation
i. Pulse code modulation
b.analog modulation
I.Pulse amplitude modulation (PAM)
ii.Pulse width modulation (PWM)
iii.Pulse position modulation (PPM)
 ANALOG MODULATION:
Amplitude Modulation(AM):
 The process by which the amplitude of a carrier wave is varied in accordance with
the modulating signal is called amplitude modulation.
 In amplitude modulation method, the carrier is a sine wave with frequency fc.
 The carrier frequency is much higher than the frequency of modulating signal (fm).
 The process of amplitude modulation is shown in fig.
 The amplitude modulated (AM) signal is transmitted by a transmitter.
 The information is contained in its amplitude variation.
 The frequency of the carrier remains constant.
 AM is used in radio and TV broadcasting applications.
 Carrier signal equations
 Looking at the theory, it is possible to describe the carrier in terms of a sine wave as follows:
C (t) = C sin (ωc + φ)

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Where:
carrier frequency in Hertz is equal to ωc / 2 π
C is the carrier amplitude
φ is the phase of the signal at the start of the reference time
 Both C and φ can be omitted to simplify the equation by changing C to "1" and φ to "0".
2. Modulating signal equations
The modulating waveform can either be a single tone. This can be represented by a cosine
waveform, or the modulating waveform could be a wide variety of frequencies - these can
be represented by a series of cosine waveforms added together in a linear fashion.
For the initial look at how the signal is formed, it is easiest to look at the equation for a
simple single tone waveform and then expand the concept to cover the more normal case.
Take a single tone waveform:
m (t) = M sin (ωm + φ)
Where:
modulating signal frequency in Hertz is equal to ωm / 2 π
M is the carrier amplitude
φ is the phase of the signal at the start of the reference time
Both C and φ can be omitted to simplify the equation by changing C to "1" and φ to "0".
It is worth noting that normally the modulating signal frequency is well below that of the carrier
frequency.
3. Overall modulated signal for a single tone
The equation for the overall modulated signal is obtained by multiplying the carrier and the
modulating signal together.
y (t) = [ A + m (t) ] . c (t)
The constant A is required as it represents the amplitude of the waveform.
Substituting in the individual relationships for the carrier and modulating signal, the overall
signal becomes:
y (t) = [ A + M cos (ωm t + φ ] . sin(ωc t)
The trigonometry can then be expanded out to give an equation that includes the components
of the signal:
y (t) = [ A + M cos (ωm t + φ ] . sin(ωc t)
This can be expanded out using the standard trigonometric rules:
y (t) = A . sin (ωc t) + M/2 [ sin ((ωc + ωm) t + φ) + M/2 [ sin ((ωc - ωm) t - φ)

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In this theory, three terms can be seen which represent the carrier, and upper and lower
sidebands:
Carrier: A . sin (ωc t)
Upper sideband: M/2 [ sin ((ωc + ωm) t + φ)
Lower sideband: M/2 [ sin ((ωc - ωm) t - φ)
Note also that the sidebands are separated from the carrier by a frequency equal to that of the
tone.
Sidebands on an amplitude modulated carrier
when modulated with a single tone
It can be seen that for a case where there is 100% modulation, i.e. M = 1, and where the carrier
is not suppressed, i.e. A = 1, then the sidebands have half the value of the carrier, i.e. a quarter
of the power each.

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FIG. – AMPLITUDE MODULATION
Amplitudemodulation blockdiaggram:
Advantages:
Coveragearea of AM is wider

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Itis cheaper not complex
Disadvantages:
Signal of AM is not stronger
Noise increase with increase in power
AM bandwidth is limited
ANGLE MODULATION:
 Angle modulation is divided into 3 types.
1) Frequency modulation

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2) Phase modulation
Frequency Modulation(FM):
 The process by which the frequency of a carrier wave is varied in accordance with
the modulating signal is called frequency modulation.
 In the frequency modulation method, the carrier is a sine wave with frequency fc.
 The carrier frequency is much higher than the frequency of modulating signal (fm).
FIG. – FREQUENCY MODULATION
 The process of frequency modulation is shown in fig.
 Here, frequency compression and expansion will take place.
 The FM signal is transmitted by the transmitter and it carries the information in the
frequency variation.
 The amplitude of carrier remains constant.

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FM is used for TV and radio broadcasting and police wireless transmission. requency
modulation uses the information signal, Vm(t) to vary the carrier frequency within
some small range about its original value. Here are the three signals in mathematical
form:
 Information: Vm(t)
 Carrier: Vc(t) = Vco sin ( 2  fc t + 
 FM: VFM (t) = Vco sin (2 fc + (f/Vmo) Vm (t)t + 
We have replaced the carrier frequency term, with a time-varying frequency. We have
also introduced a new term: f, the peak frequency deviation. In this form, you should
be able to see that the carrier frequency term: fc + (f/Vmo) Vm (t) now varies between
the extremes of fc - f and fc + f. The interpretation of f becomes clear: it is the
farthest away from the original frequency that the FM signal can be. Sometimes it is
referred to as the "swing" in the frequency.
We can also define a modulation index for FM, analogous to AM:
 = f/fm , where fm is the maximum modulating frequency used.
The simplest interpretation of the modulation index, is as a measure of the peak
frequency deviation, f. In other words, represents a way to express the peak
deviation frequency as a multiple of the maximum modulating frequency, fm, i.e. f
=  fm.
Example: suppose in FM radio that the audio signal to be transmitted ranges from 20
to 15,000 Hz (it does). If the FM system used a maximum modulating index, , of 5.0,
then the frequency would "swing" by a maximum of 5 x 15 kHz = 75 kHz above and

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below the carrier frequency
Here, the carrier is at 30 Hz, and the modulating frequency is 5 Hz. The modulation
index is about 3, making the peak frequency deviation about 15 Hz. That means the
frequency will vary somewhere between 15 and 45 Hz. How fast the cycle is
completed is a function of the modulating frequency
Frequency modulation block diagram:

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Advantages:
 Reduction to noise
 As the signal varies audio amplitude not vary
 Does not require linear amp
 Greater efficiency
Disadvantages:
 More complicated demodulator
 Sidebands extends to infinity for their filters are used
Phase Modulation (PM):
 The process by which the phase shift of a carrier signal is varied in accordance with
the modulation signal is called phase modulation.
 In phase modulation method, the carrier is a sine wave with frequency fc.
 The carrier frequency is much higher than the frequency of modulating signal (fm).
 The process of phase modulation is shown in fig.

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FIG. – PHASE MODULATION
 The amplitude of carrier remains constant.
 Thus, the information is carried in the phase variations of the modulated signal.
 Phase modulation is very similar to frequency modulation. The only difference is that
the phase of the carrier varies instead of varying the frequency.
Advantages:
Modulation is easy
Disadvantages:
We need frequency multiplier to increase phase modulation index

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PULSE MODULATION:
 Pulse modulation is divided into 3 types.
3) Pulse amplitude modulation
4) Pulse width modulation
5) Pulse position modulation
1) Pulse Amplitude Modulation (PAM):
 In PAM, the amplitude of the pulsed carrier is varied in accordance with the
amplitude of the modulating signal.

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FIG. – PULSE AMPLITUDE MODULATION
 The process of PAM may be accomplished by sampling the analog signal at constant
intervals.
 The samples are pulses whose amplitude is that of the original analog signal at the
sampling instant.
 Thus, the information is carried in the amplitude variations of the modulated signal.
 The carrier is in the form of narrow pulses having frequency fs.
 The sampling rate must be at least twice the highest analog signal frequency.
 PAM is used in time division multiplexing.

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Advantage:
 Simplicity of generation and detection.
Disadvantages:
 The effect of noise is maximum.
 Large bandwidth.

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2) Pulse WidthModulation(PWM):
 In PWM, the width of the pulsed carrier is varied in accordance with the amplitude
of the modulating signal.
FIG. – PULSE WIDTH MODULATION
 As shown in fig., the amplitude and frequency of the PWM wave remains constant
only the width changes.
 Thus, the information is carried in the width variations of the modulated signal.
 As the noise changes the amplitude of the PWM signal, at the receiver, it is possible
to remove these unwanted amplitude variations very easily.
 As the information is contained in the width variations, it is unaffected by the
amplitude variations introduced by noise.
 Thus, the PWM system is noise immune than the PAM signal.

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
Advantages:
 Less effect of noise.
 Synchronization between the transmitter and receiver is not needed.
Disadvantage:
 Large bandwidth.
3) Pulse PositionModulation(PPM):
 In PPM, the position of the pulsed carrier is varied in accordance with the amplitude
of the modulating signal.
FIG. – PULSE POSITION MODULATION

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 As shown in fig., the amplitude, frequency of the PPM wave remains constant only
the position changes.
 Thus, the information is carried in the position variations of the modulated signal.
 The PPM pulses can be derived from the PWM pulses as shown in fig.
 Note that with increase in the modulating voltage, the PPM pulses marked 1, 2 and 3
go away from their respective reference lines. This corresponds to increase in
modulating signal amplitude.
 Then as modulating voltage decreases, the PPM pulses 4,5,6,7 come closer to their
respective reference lines.

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Advantage:
 The noise can not affect.
Disadvantages:
 Synchronization between the transmitter and receiver is necessay.
 Large bandwidth.
 DIGITAL MODULATION:
Pulse Code or Digital Modulation:
 PCM is a digital pulse modulation system. Its output is in the coded digital form.
 In PCM, the available range of signal voltages is divided into samples and each
sample is represented by a binary number.
FIG. – PCM TRANSMITTER

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 PCM consists of 3 steps to digitize an analog signal:
1. Sampling
2. Quantization
3. Binary encoding
FIG. – SAMPLING, QUANTIZATION AND ENCODING
 PCM transmitter is shown in fig. in fig., the message signal x(t) [fig. (a)] passes
through low pass filter to limit the maximum frequency of the signal and is given to
sample and hold circuit and then to quantizer.
 Quantizer reduces the effect of noise and generates quantized PAM [fig. (d)] and it
will be the input of encoder (A to D converter).

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 The encoder generates the output in digital form and is converted into a stream of
pulses by the parallel to serial converter.
 Thus, by PCM we get the output in form of train of digital pulses.
Advantages:
 Very high noise immunity.
 It is possible to store the PCM signal (because it is digital in nature).
Disadvantages:
 Large bandwidth.
 Complex circuitry.
Comparison of PAM, PWM and PPM:
PARAMETER PAM PWM PPM
Type of carrier Train of pulses Train of pulses Train of pulses
Variable
characteristics of
the pulsed carrier
amplitude width position
Bandwidth required Low high high
Noise immunity Low high high
Information is
contained in
Amplitude
variations
Width variations Position variations
Transmitted power Varies with
amplitude of pulses
Varies with width of
pulses
Varies with position
of pulses
Need to transmit
synchronizing pulses
Not needed Not needed necessary
Complexity of
generation and
detection
Complex easy complex
Similarity with other
modulation systems
Similar to AM Similar to FM Similar to PM
DEMODULATION:

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 At the receiver, the original information signal is separated from the carrier. This
process is known as demodulation or detection.
 Detection is exactly the opposite process of modulation.