INTRODUCTION TO COMMUNICATION SYSTEMS -...

INTRODUCTION TO COMMUNICATION SYSTEMSWhat is Communication ?• Communication is the process of conveying information from one• point to another• What is the purpose of Communication system?• The purpose of communication systems is to communicate

information bearing signals from a source located at one point in spaceto a user destination located at another point.

• The three most common sources of information are: speech (or sound), video and data.

• Regardless of the source, the information that is transmitted andreceived in a communication system consists of a signal, encoding theinformation in some appropriate fashion

MODEL OF COMMUNICATION SYSTEMS

MODULATIONWhat is modulation?

• Modulation is performed at the transmitting end of the communication system.

• At the receiving end of the system we usually require the original baseband signal to be restored, this is usually accomplished by using a process known as demodulation

• which is the reverse process of the modulation

• In basic signal processing terms, we thus find that the transmitter of an analog communication system consists of a modulator and the receiver consists of a demodulator.

What are the reasons for modulation?

If modulation is not employed however, the system designer

could confront the following problems:

• Antenna Height

• Narrow Banding

• Poor radiation and penetration

• Diffraction angle

• Multiplexing

• To overcome equipment limitations

• To reduce noise and interferences

What are the Different ModulationMethods?

Amplitude modulation

ANNOUNCERTRANSMITTINGANTENNA

Frequency modulation

Phase modulation.

HILBERT TRANSFORM

Pre-envelope

Complex Envelope

Double sideband suppressed carrier( DSB-SC)

AM MODULATOR CIRCUIT

Depending on location of modulator in transmitter

Low level modulator

Modulator placed before the output element of final stage of transmitter

Medium power modulator

modulation take place in the final stage , directly connected to antenna.

Low level AM modulator

• Carrier signalbase of amplifier

• Modulating signalemitter

• No modulating signal operate class C amp

• Depend on gain variations carrier amplified.

• Advantage:

less modulation signal power required & lower power design.

• Disadvantage:

Amplifiers following modulator stage must be linear.

High power in efficient

Medium power amplifierCollector modulator

• Transistor biased well beyond the cut off regionsothat operate in class C mode high efficiency.

• RF drive– carrier signal drive the transistorapplied to base of transistor.

• Modulating signal power amp low freqtransformer vm(t)

• AM produced across the LC circuit.

• Vm(t) changes net supply voltage change RF carrier Voltage also varied.

• Adv: no constraints of linear operation & good efficiency.

• Dis Adv: require high power for achive % of modulation.

SQUARE LAW MODULATORGenerate AM wave.Consists of 1. summer: add carrier and modulating signal.2. Non linear element: implement square law modulator.(Diode ,Transistors)3. Band pass filter: extract desired term.(Single or double Tuned Filter)

Ka= 2a2/a1 amplite sensitivity of AM wave.Un wanted terms removed by filter with the following specifications.

AM TRANSMITTER• HIGH LEVEL TRANSMITTER

• Crystal osc generate carrier.

• Buffer amp & driver amp amplify power level of carrier.

• Audio amp amplify modulating signal further amplify by power amp.

• Class C modulator amplifiermodulate carrier according to i/p modulating signal.

• Antenna mismatching Networktuned LC circuit in collector circuit.

• AM modulator operate at high power deliver to antenna.

LOW LEVEL TRANSMITTER• Linear Class B amplifier is used after

Class C amplifier.

• Class B amp : perform major power amplification & feed AM signal to antenna.

• Modulator : perform modulation at low power levels.

AM broadcast transmitter using partially modulator driver stages

• Crystal oscillator : generate carrier.

• Buffer amp: raise power level of carrier.

• Class C driver: partially modulated then modulated at high power level by class C power amp.

• Audio modulating signal amplified and fed to modulator driver and couple to class C driver.

• Linear detector : demodulate part of transmitted signal.

• Negative feedback linearizes the modulation characteristics.

• Advantages:

One or more tubes failed - driver stage still provide modulated output.

Total power distributed in main and power amplifier.

Single side band modulation• DSB FC: Unmodulated carrier and USB ,LSB

• Unmodulated carrier: doesn’t convey any information & consume 2/3 of total power.

• USB & LSB : carry same information.

• So only one carrier capable of same information in DSBFC.

Advantages:

• Bandwidth = fm only.

• Reduce power requirement, narrow band noise, fading.

Disadvantages:

• Costly, complex design, unsuitable for video transmission.

Mathematical representation of SSB SC

Suppression of carrier

• Balance modulator - suppress the carrier in AM.

Balanced modulator or ring modulator• Called Lattice type BM.

• Modulating signal T1

• Carrier center taps of T1 & T2.

• DSB o/p collected at secondary of T2.

• Let us consider Modulating i/p =0

• Magnetic field in upper & lower part of winding are equal and opposite ,so cancel each other.

• No o/p produced on T2.

During positive half cycle

• D1 & D2 forward bias

• D3 & D4 reverse bias

During negative half cycle

• D1 & D2 reverse bias

• D3 & D4 forward bias

Balanced Modulator Using FET

• Modulating signal applied to transformer T1.

• Carrier signal center taps input transformer T1 & output transformer T3 through T2 further applied to 2 gates of FET .

• Modulating voltage appears 180 deg out of phase at gates.

• No modulating signal FET current due to carrier are equal in amplitude and opposite in direction in primary of transformer and cancel each other.

• Modulating signal applied id1, id2 flow in the primary of T3.

• FET currents due to modulating signal are equal but not opposite do not cancel.

Suppression of Unwanted Sidebands

• We have to suppress the carrier as well as one of the sidebands.

Filter method

• Balanced Modulator produce DSB

• Sideband suppression filter remove unwanted bands.(filter flat pass band & extremely high attenuation)

• Balanced mixer & crystal osc boost up transmitter frequency.

• BM1Modulating signal directly applied and carrier 90 deg phase shift.

• BM2Modulating signal applied 90deg phase shift and carrier signal directly applied.

• BM1 USB,LSB each shifted by 90.

• M1=cos(Wct-Wmt+90)-cos(Wct+Wmt-90)

• BM2 USB+90 &LSB –90.

• M1=cos(Wct-Wmt-90)-cos(Wct+Wmt+90)

• Summer USB =90+90 ->double amplitude, LSB +90-90 cancel.

Phase shift method to generate SSB

Weavers Method

Vestigial Sideband Transmission(VSB)

• One of the sideband partially suppressed and vestige (portion) of other side band is transmitted.

• This vestige (portion) compensates the suppression of the sideband.

Demodulation of VSB

Spectrum of VSB

Tuned Radio Frequency Receiver

Superheterodyne Receiver

Fidelity

Diode or Coherent detector

Synchronous or coherent detector

Automatic Gain Control

COMPARISON OF SSB ,DSB, VSB

Limitations of AM

• AM depends upon depth of modulation, hence power not constant.

• Amplitude not constant, superimpose noise consider as signal.

• Modulation index maximum value 1. above 1 signal lost.

• AM uses MF and HF . Noise interference is increased.

• Carrier consumes significant power.

• AM suffer due to adjacent channel interference.