Transcript t - CSE-C, SRM vdp.

 Bandwidth Efficiency
 Single Side Band
 Vestigial Side Band
 Power Efficiency
 Reduced Carrier
 Suppressed Carrier
 Complication
 Circuits: generation, tuning, detection
 Filter, tone variation, carrier recovery
Double Side Band Suppressed Carrier (DSBSC)
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It is a technique where the sidebands are transmitted
without the carrier (carrier is being suppressed/cut)
Characteristics:
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Power content less
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Same bandwidth
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Disadvantages - receiver is complex and expensive.
Frequency spectrum of DSB-SSC
Phasor diagram of DSB-SC
 We know that, the total power transmitted in AM is
 If the carrier is suppressed, then the total power
transmitted in DSB-SC-AM is P t = PLSB+PUSB
Single Side Band (SSB)
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Improved DSBSC and standard AM, waste power and occupy
large bandwidth.
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SSB is a process of transmitting one of the sidebands of the
standard AM by suppressing the carrier and one of the sidebands
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Advantages:
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Saving power
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Reduce BW by 50%
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Increase efficiency, increase SNR
Disadvantages
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Complex circuits for frequency stability
Frequency spectrum of SSB
Vestigial Side Band (VSB)
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VSB is mainly used in TV broadcasting for their
video transmissions.
TV signal consists of
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Audio signal – transmitted by FM
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Video signal – transmitted by VSB
A video signal consists a range of frequency and
fmax = 4.5 MHz.
If it transmitted using conventional AM, the
required BW is 9 MHz (BW=2fm). But according to
the standard, TV signal is limited to 7 MHz only
So, to reduce the BW, a part of the LSB of picture
signal is not fully transmitted.
The frequency spectrum for the TV
signal / VSB:
Generation of Amplitude Modulation
 It is broadly divided into two types namely
(i) Linear modulator or large signal modulator.
(ii) Non Linear modulator or small signal
modulator
Linear modulator or large signal modulator:
 In this type of modulators the devices are operated in
the linear region of its transfer characteristics thus the
relation between the amplitude of the modulating signal
and the resulting depth of modulation is linear. It can be
divided into two types
(i) Transistor modulator
(ii) Switching modulator
In this linear modulator the pair of non-linear
elements having (as near as possible) the same
characteristics in a balanced circuit cancel out the
unwanted components. `
Non-linear Modulator or Small signal
modulator
 These modulators makes use of non-linear V-I
characteristics of the devices and are, in general
suited for use at low voltages.
 The important type is: Square law modulator
 In simple words it can be defined as the devices
used in these modulator are operated in non linear
region of its characteristics.
Square law modulator
 A square law modulator requires to add up the carrier
and modulating signal to obtain AM with carrier.
 Thus a square law modulator has three features.
1. Summer-To add carrier and modulating signal
2. A non-linear (active) element.
3. Bandpass filter for extracting desired
modulating products.
Basic Square Law modulator
Sqr. Law modr. using Diode
A. Modulating Signal
B. Carrier
C. Sum of carrier and
modulating signal
D. Diode current
E. AM output across
tuned circuit
Square law modr. using FET
 The FET is biased in a non-linear region of its
transfer characteristics, to obtain the desired
output.
 The output tank circuit RLC is tuned to the carrier
frequency to select the desired modulating
components.
Mathematical Analysis
 The current of a non-linear element suitably
biased in its non-linear region is given by the
square law equation
i.e., io = a1V1+a2V12 +...........
Where V1 = input voltage applied to the FET
V1 = Am sin ωmt +Ac sin ωct
Substituting above equation in prev. eqn.
Neglecting the second order terms,
 When the bandpass filter is tuned to the carrier
frequency it allows only wc , (ωc − ωm) and (ωc + ωm)
terms and it eliminates all other terms. Hence we
obtain,
Product Modulators
 Used for generation of DSB-SC signals.
 Here, a DSB-SC signal is obtained by simply
multiplying the modulating signal Vm(t) with carrier
signal cos ωct.
 Let the modulating signal be Vm (t) = Am sin ωmt
and the carrier signal C(t) = Ac sin ωct
 When multiplying both the carrier and message
signal, the resulting signal is the DSB-SC-AM signal.
S(t)DSBSC =Vm (t)C(t)
 Therefore S(t)DSBSC = Am sin ωmt Ac sin ωct
Balanced Modulators (practical)
for DSB-SC generation:
 There are two ways of generating DSB-SC
(i) Balanced modulator.
(ii) Ring modulator.
Balanced modulator:
 a pair of symetric non-linear elements are used.
 They cancel out the unwanted components
Diode balanced Modulator:
 The carrier voltage is applied in parallel to diode D1
and D2
 the modulating voltage in push-pull to the diodes.
 A bandpass filter is that type of filter which allows to
passband of frequencies.
 Since the bandpass filter is centred around ωc it will
pass a narrow band of frequencies
 BPF is centred at ωc with a small bandwidth of 2ωm to
preserve the sidebands.
 Therefore, the output of BPF centered around ωc is
given by
 Which is the expression for a DSB-SC signal.
Switching or Ring or Chopper modulator.
 A ring modulator uses four diodes in ring form.
 diodes are controlled by a square wave carrier signal
applied through a center-tapped transformer.
 If the carrier is large enough to cause the diodes to
switch states, then the circuit acts like a diode
switching modulator
 The modulating signal is inverted at the carrier rate.
This is essentially multiplication by ±1.
 When the carrier is +, outer diodes will conduct.
 When the carrier is - , inner diodes will conduct.
 Thus carrier switches the diodes on / off.
 Bandpass filters are used to extract the frequency of
interest.
Diode conduction for + and –
carrier:
Generation of SSB-SC AM
 SSB-SC waves can be generated in two ways:
(i) Frequency discrimination or filter method
(ii) Phase discrimination method
 Phase discrimination method itself can be divided into
two types.
(a) Phase shift method
(b) Modified phase shift-weavers method.
Filter method of SSB generation:
 first a DSB-SC signal is generated simply by using an
ordinary product modulator or a balanced modulator.
 After this from the DSB-SC signal one of the two
sidebands is filtered out by a suitable Band pass filter
(BPF).
 The design of band pass filter is quite critical and puts
some limitations on the modulating or baseband and
carrier frequencies.
Limitations of frequency discrimination method.
 is useful only if the baseband signal is restricted at its
lower edge due to which the upper and lower
sidebands are non-overlapping.
 the base band signal must be appropriately related to
the carrier frequency (design of the BPF becomes
difficult if ωc >> ωm.
Phase shift method of SSB generation :
 The phase shift method avoids filter.
 makes use of two balanced modulators and two phase
shifting networks.
 One modulator receives the carrier with the phase shift
of 90◦ and the modulating signals directly.
 The other modulator receives modulating signal with
the phase shift of 90◦ and the carrier signal directly.
 For BM 1
Vm (t) = Am sin (ωmt)
Vc (t) = Ac sin (ωct +90◦)
 For BM 2
Vm (t) = Am sin (ωmt +90◦)
Vc (t) = Ac sin ωct
Output of balanced modulator M1 will contain sum
and difference frequencies
Output of balanced modulator M1 will contain sum
and difference frequencies