Post on 17-Jan-2016
description
F1 x F2
Sum and Mixing of Frequencies
fUSB = fc + fm and fLSB = fc − fm
eam=EcSin(Wct)+mEc/2Cos(Wc-Wm)t-mEc/2Cos(Wc+Wm)t
Carrier LSB USB
Sidebands and the Frequency Domain
Figure 3-8: The relationship between the time and frequency domains.
Ea =Emax − Emin
2m = Ea / Ec
Ec = Emax - Ea
Calculatiom of modulation index by envelope
BW = fUSB−fLSB=2fm
Ec
mEc/2mEc/2
Fc Fc+FmFc-Fm
Fc Fc+Fm-Fc -Fc+Fm-Fc-Fm Fc-Fm
Two sided spectrum
mEc/4mEc/4mEc/4mEc/4
Ec/2 Ec/2
Property of Active Device
Block Diagram of a Simple AM Transmitter
R
E
R
Ec
R
VrmsP cc 2
)2/( 222
4
2c
usblsb
PmPP
PT = (IT)2R where IT is measured RF current and R is antenna impedance
PT=PC+PUSB+PLSB
222
8
22/
R
Ecm
R
mEcPP usblsb
PcPmPm
Pt cc 44
22
Pcm
Pt ]2
1[2
21
2m
Pc
Pt 2
1
12
Pc
Ptm
Power relations in AM
PT = (IT)2R PT=PC+PUSB+PLSB
2
2
2
CC I
It
R
R
I
It
Pc
Pt
Power relations in AM in terms of current
21
22m
Ic
It
21
2m
Pc
Pt
2
1
2
21
m
IIt C
Pc = (Ic)2R
But
2
12
12
Ic
Itm
Transmission Efficiency
C
CC
T
USBLSB
Pm
Pm
Pm
P
PP
21
442
22
2
2
2 m
m
%100*2 2
2
m
m
Percent efficiency
Useful Power /Total power
Modulation by several sinewaves
Two modulating signals are given by
X1(t)=Em1CosWm1t X2(t)=Em2CosWm2t
ec=Ec CosWct Carrier wave
eam=A CosWct where A=Ec+X1(t)+X2(t)
eam= (Ec+Em1CosWm1t + Em2CosWm2t) CosWct
eam= Ec(1+Em1/Ec CosWm1t + Em2/ Ec CosWm2t) CosWct
eam= Ec(1+m1 CosWm1t + m2 CosWm2t) CosWct
eam= Ec CosWct+m1Ec/2 Cos(Wc+Wm1)t + m1Ec/2 Cos(Wc-Wm1)t+m2Ec/2 Cos(Wc+Wm2)t+m2Ec/2 Cos(Wc-Wm2)t
Fc Fc+fm1 Fc+fm2Fc-fm1Fc-fm2
Ecm1Ec/2 m2Ec/2m1Ec/2
m2Ec/2 BW=2fm2
Total power in AM Wave=
Pt=PUSB1+PUSB2+PLSB1+PLSB2
4
2
4
1
4
2
4
1 2222cccc PmPmPmPm
PcPt
2
2
2
11
22 mmPcPt
Modulation Index
222 21 mmmt 2
122 21 mmmt
R
E
R
Ec
R
VrmsP cc 2
)2/( 222
4
2c
usblsb
PmPP
PT = (IT)2R where IT is measured RF current and R is antenna impedance
PT=PC+PUSB+PLSB
222
8
22/
R
Ecm
R
mEcPP usblsb
PcPmPm
Pt cc 44
22
Pcm
Pt ]2
1[2
21
2m
Pc
Pt 2
1
12
Pc
Ptm
Power relations in AM
PT = (IT)2R PT=PC+PUSB+PLSB
2
2
2
CC I
It
R
R
I
It
Pc
Pt
Power relations in AM in terms of current
21
22m
Ic
It
21
2m
Pc
Pt
2
1
2
21
m
IIt C
Pc = (Ic)2R
But
2
12
12
Ic
Itm
Transmission Efficiency
C
CC
T
USBLSB
Pm
Pm
Pm
P
PP
21
442
22
2
2
2 m
m
%100*2 2
2
m
m
Percent efficiency
Useful Power /Total power
Modulation by several sinewaves
Two modulating signals are given by
X1(t)=Em1CosWm1t X2(t)=Em2CosWm2t
ec=Ec CosWct Carrier wave
eam=A CosWct where A=Ec+X1(t)+X2(t)
eam= (Ec+Em1CosWm1t + Em2CosWm2t) CosWct
eam= Ec(1+Em1/Ec CosWm1t + Em2/ Ec CosWm2t) CosWct
eam= Ec(1+m1 CosWm1t + m2 CosWm2t) CosWct
eam= Ec CosWct+m1Ec/2 Cos(Wc+Wm1)t + m1Ec/2 Cos(Wc-Wm1)t+m2Ec/2 Cos(Wc+Wm2)t+m2Ec/2 Cos(Wc-Wm2)t
Fc Fc+fm1 Fc+fm2Fc-fm1Fc-fm2
Ecm1Ec/2 m2Ec/2m1Ec/2
m2Ec/2 BW=2fm2
Total power in AM Wave=
Pt=PUSB1+PUSB2+PLSB1+PLSB2
4
2
4
1
4
2
4
1 2222cccc PmPmPmPm
PcPt
2
2
2
11
22 mmPcPt
Modulation Index
222 21 mmmt 2
122 21 mmmt
Amplitude Modulators
• There are two types of amplitude modulators. They are low-level and high-level modulators.
• Low-level modulators generate AM with small signals and must be amplified before transmission.
• High-level modulators produce AM at high power levels, usually in the
final amplifier stage of a transmitter.• Modulators are class C amplifiers and at output tank circuit.
Low level AM Transmitter
540KHz to 1640KHz
modulator
540KHz to 1640KHz
Amplifier Classes
Class A - bias point is set so that the amplifier conducts through a complete cycle (360 deg) of the input waveform. This class has low efficiency (~35%) but high linearity.Class AB - bias point is set so that the amplifier conducts through at least 180 deg but less than 360deg of the input waveform. This class has better efficiency (~55%) but lower linearity.Class B - bias point is set so that the amplifier conducts through a half cycle (180 deg) of the input waveform. This class has higher efficiency (~60%),but poor linearity.Class C - bias point is set so that the amplifier conducts through less than 180 deg of the input waveform. This class has higher efficiency (~70%), but even poorer linearity
Use of Tank circuit
Low level Class C Grid Modulator
High level Plate Modulator
Low level Transistor Modulator
Low level and High level AM Transmitter
modulator
540KHz to 1640KHz
Advantages of DSBFC =
1.Transmitters are less complex
2.Receivers are simple, detection is easy.
3.Cost efficient.
Disadvantages =
1.Power wastage - carrier doesn’t carry any information and USB & LSB contains same information.
2. Needs larger Bandwidth
3. Gets affected by noise.
Pcm
PcPwastage4
2
Types of AM=
1. DSBFC
2. DSBSC
3. SSB
4. ISB
5. VSB
Balanced ModulatorModulating signal
carrier
DSBSC
180 phase shift
DSB-SC Generation Methods
1. Ring Balanced Modulator2. Lattice Balanced Modulator3. Push pull Balanced modulator
eam=mEc/2Cos(Wc-Wm)t-mEc/2Cos(Wc+Wm)t
180 phase shift
Ec
mEc/2mEc/2
Fc Fc+FmFc-Fm
BW=2fm
Balanced Modulator
1.Ring modulator
2.Lattice-type balanced modulator.
Lattice Modulator
+ - - +
Push Pull Balanced Modulator
inet = aem + 2becem
Drain Current
Modulating signal
Two side bands
AM Waveforms
SSB Generation Methods
1. Filter Method2. Phase shift method3. Third method (Weaver method)
mEc/2mEc/2
Fc Fc+FmFc-Fm
mEc/2mEc/2
Fc Fc+FmFc-Fm
BW=fm
SSB Circuits
Figure 4-31 An SSB transmitter using the filter method.
This technique can be used at relatively low carrier frequencies. At high frequencies, the Q of the filter becomes unacceptably high. The required Q necessary to filter off one of the sidebands can be approximated by:
SSB Circuits
Figure 4-33 An SSB generator using the phasing method.
SSB phase shift
Important points=
1. Sharp cutoff Filters are not required
2. Freq Up conversion is not required
3. Easy to switch between sidebands. Simply change the oscillator position.
4. Designing a phase shift network for AF range is dificult.
Weaver Method or Third method
LSB
Independent side band transmitter
10MHz to 30MHz
VESTIGIAL SIDEBAND MODULATION
• VSB is used in TV transmission to transmit Video signal.
• In VSB full USB is transmitted with some part of LSB.
• As filter response is not sharp at the edges it may attenuate part of transmitted sideband if only SSB is used to transmit.
• Part of the LSB is called as Vestige.
• BW required is less than the DSBFC and DSBSC.
• No of channels can be increased.
VSB AM Technique-
USBLSB
0 5 MHz 5.75MHz1.25MHz
0.5
Picture career Sound career
AM Receivers
1. Tuned Radio Frequency (TRF)
2. Superheterodyne Receiver
fo - fs = fIF
+
-
+
-
Receiver CharacteristicsSensitivity- it must provide amplification to recover the original modulating signal from a very weak received signal.
Sensitivity refers to the weakest signal that can be received and still produce an acceptable out.
Sensitivity can be specified as a minimum voltage (microV) or as a power level (dBm).
Gain of RF and IF amp. decides sensitivity.
Selectivity =it must be able to select the desired signal from the thousands of other signals in the spectrum.
It is the ability to select the desired signal and reject all other.
Fidelity= Is A Measure Of The Ability Of A Communications System To Produce At The Output Of The Receiver, An Exact Replica Of The Original Source Information
540KHz to 1640KHz
Ganged tuning
Problems of Tuned Radio Frequency (TRF) Receiver
1. Instability- Overall gain of RF amplifiers is very very high so a very small f/b from o/p to i/p with correct phase can initiate oscillations . Due to stray capacitance at high freq.
2. Variation in BW- For 535KHz – 1640KHz Range BW=10KHz for fc=535 Q=fr/BW=535/10=53.5for fc=1640 Q=164
But max value of Q is 120 so BW=fr/Q=1640/120=13.7KSo receiver picks adjacent channels.
3. Insufficient Selectivity- Due to variable BW selectivity of TRF receiver is poor.
Superheterodyne Receivers
• Superheterodyne receivers convert all incoming signals to a lower frequency, known as the intermediate frequency (IF), at which a single set of amplifiers is used to provide a fixed level of sensitivity and selectivity.
• Gain and selectivity are obtained in the IF amplifiers.
• The key circuit is the mixer, which acts like a simple amplitude modulator to produce sum and difference frequencies.
• The incoming signal is mixed with a local oscillator signal.