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.

http://en.wikibooks.org/w/index.php?title=File:Trapazoidal_display.gif&filetimestamp=20080809025035

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.