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MONOCHROME TELEVISION RECEIVER

Monochrome Receiver

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Page 1: Monochrome Receiver

MONOCHROME TELEVISION RECEIVER

Page 2: Monochrome Receiver

SECTIONS

• TUNER(RF SECTION)• I.F. SUBSYSTEM• VIDEO SECTION• AUDIO SECTION• DEFLECTION CIRCUITS• POWER SUPPLY

Page 3: Monochrome Receiver

INPUT FROM ANTENNA• ANTENNA INTERCEPTS OR PICKS UP

ELECTROMAGNETIC WAVES WHICH ARE RADIATED BY TRANSMITTERS.

• THE RECEIVED SIGNAL CAN VARY FROM 50 µV TO 1mV.

• ANTENNA CAN BE RESONANT OR NON-RESONANT.

• YAGI-UDA AND PARABOLIC REFLECTOR ARE MOST COMMON IN USE.

Page 4: Monochrome Receiver

BALUN

• THE IMPEDENCE SEEN BY THE ANTENNA AT THE RECEIVER I/P IS ABOUT 300.

• THE OPEN CIRCUIT IMPEDENCE OF THE ANTENNA IS ABOUT 75.

• THE BALUN PROVIDES THE IMPEDNECE MATCH BETWEEN ANTENNA AND THE RECEIVER.

Page 5: Monochrome Receiver

TUNER

• TUNER CONSISTS OF (1)RF AMPLIFIER(2)MIXER(3)LOCAL OSCILATOR• THEIR FUNCTIONS ARE SAME AS THEY DO IN A

CONVENTIONAL RADIO RECEIVER.

Page 6: Monochrome Receiver

RF AMPLIFIER• RF AMPLIFIER RECEIVES SIGNAL FROM

ANTENNA .• IT IS A LOW NOISE AMPLIFIER.• IT PROVIDES INITIAL GAIN AND SELECTIVITY

TO THE RECEIVED SIGNAL.

A

fPc-1.25 Pc Sc

IDEAL RF RESPONSE

IDEAL RF RESPONSE

Page 7: Monochrome Receiver

CONVERSION TO I.F.

• THE TUNER I/P IS A MODULATED COMPOSITE VIDEO SIGNAL(WITH SOUND).

• THE O/P IS AGAIN A COMPOSITE VIDEO SIGNAL BUT WITH A DIFFERENT CARRIER FREQ.,THE INTER MEDIATE FREQ.

• THIS CONVERSION INTO I.F. IS DONE BY HETERODYNING I/P SIGNAL WITH LOCAL OSCI. SIGNAL TO PRODUCE BEAT FREQ.

• PICTURE I.F.=38.9 MHz & SOUND I.F.=33.4 MHz

• AFTER CONVERSION THE SOUND FREQ. IS LOWER THAN PICTURE FREQ.

Page 8: Monochrome Receiver

CHOICE OF I.F.

• I.F. SHOULD NOT LIE IN VHF OR UHF BAND,ASSIGNED FOR T.V. TRANSMISSION

• IMAGE REJECTION RATIO.• LOCAL OSCI. RADIATION.• IMAGE FREQ. SHOULD LIE OUSIDE FM(88-

108 MHz) BAND.• NO INTERFERENCE FROM THE BAND

ASSIGNED TO OTHER SERVICES.• GAIN.

Page 9: Monochrome Receiver

I.F. SUBSYSTEM

• CONSISTS OF..1) I.F. SECTION2) AGC CIRCUIT3) VIDEO DETECTOR• THIS SECTION RECEIVES ITS I/P FROM MIXER

O/P AND ITS O/P IS BASEBAND COMPOSITE VIDEO SIGNAL

Page 10: Monochrome Receiver

I.F. SECTION

• THIS SUBSYSTEM PROVIDES MOST OF THE GAIN AND SELECTIVITY IN THE RECEIVER.

• IT IS A SERIES OF I.F. AMPLIFIERS WHICH CAN PROVIDE A GAIN OF AS MUCH AS 10,000.

• IT ALSO HAS A SAW FILTER TO PROVIDE VESTIGIAL SIDE BAND CORRECTION AND ADJACENT CHANNEL REJECTION.

Page 11: Monochrome Receiver

V.S.B. CORRECTION

• IN V.S.B. TRANSMISSION THE UPPER SIDE BAND IS FULLY TRANSMITTED AND LOWER SIDE BAND IS PARTIALLY TRANSMITTED.

• THIS PROVIDES BOOSTING OF LOW FREQ. COMPONENTS,UNNECESSARILY.

• SO, THEY ARE ATTENUATED BY AN APPROPRIATE FACTOR.

• THIS IS CALLED V.S.B. CORRECTION.

Page 12: Monochrome Receiver

THE OVER ALL I.F. RESPONSE0

-6

dB

Sif=33.4 Pif=38.9

31.9 33.15 40.15 40.4 f(MHz)

CH.3(WANTED): (54-61 MHz)

PC3=55.25 SC3=60.75 L.O.=94.15

CH.2(LOWER ADJACENT CHANNEL):(47-54MHz)

PC2=48.25 SC2=53.75

CH.4(UPPER ADJACENT CHANNEL): (61-68 MHz)

PC4=62.25 SC4=67.75

THE SOUND CARRIER OF CH.2 CAN PRODUCE A BEAT FREQ. OF 94.15-53.75=40.4MHz. THIS IS VERY CLOSE TO 38.9 AND MAY INTERFERE WITH IT.

THE PICTURE CARRIRER OF CH.4 CAN ALSO PRODUCE AN INTEREFERNCE BY PRODUCING A BEAT FREQ. OF 31.9 MHz WHICH IS CLOSE TO 33.4MHz.

Page 13: Monochrome Receiver

SAW FILTER

• SUFACE ACOUSTIC WAVE FILTER IS A BAND PASS FILTER WITH SHARP CUT-OFF.

• ITS RESPONSE CAN BE DESIGNED TO ACHIEVE DESIRED SELECTIVITY AND ROLL-OFF FACTOR.

• THIS FILTER PROVIDES BOTH V.S.B. CORRECTION AND ADJACENT CHANNEL REJECTION.

Page 14: Monochrome Receiver

A.G.C.

• IT IS USED TO CONTROL GAINS OF R.F. AND I.F. AMPLIFIERS.

• IT USES TIP OF THE SYNC PULSE AS A REFERENCE TO MEASURE THE SIGNAL STRENGTH.

• DELAYED/KEYED A.G.C. IS USED IN TV.• THIS DELAYS THE SIGNAL TILL IT AQUIRES A LELVEL

GREATER THAN THE BACKGROUND NOISE LEVEL.• IF THE RECEIVED SIGNAL IS WEAKER THAN NOISE LEVEL ,

NO A.G.C. IS PROVIDED.

Page 15: Monochrome Receiver

VIDEO DETECTOR

• IT IS BASICALLY A PEAK DETECTOR.• IT USES L-C COMBINATION,INSTEAD OF R-C ,

AS IN CONVENTIONAL RADIO RECEIVER, TO PREVENT UNDUE ATTENUATION OF VIDEO SIGNAL.

• THE DETECTOR I/P IS I.F. SIGNAL AND O/P IS BASE BAND COMPOSITE VIDEO SIGNAL.

Page 16: Monochrome Receiver

REQUIREMENTS

• THE DETECOR O/P MUST MAINTAIN DC LEVEL OF THE SIGNAL TO PRESERVE BRIGHTNESS.

• THE POLARITY OF O/P OF THE VIDEO DETECTOR SHOULD BE NEGATIVE.

• SINCE CATHODE DRIVE IS COMMON PRACTICE ,WHICH REQUIRES POSITIVE POLARITY OF THE VIDEO SIGNAL.

• WHILE O/P OF THE VIDEO AMPLIFIER IS INVERTED AS IT IS AN INVERTING AMPLIFIER.

• SO, I/P TO VIDEO AMPLIFIER IS NEGATIVE.

Page 17: Monochrome Receiver

FROM LAST IF AMPL.

BUFFERBUFFER

VIDEO AMPLI.

VIDEO AMPLI.

TO CATHODE OF PICTURE TUBE

Page 18: Monochrome Receiver

VIDEO SECTION• IT CONSISTS OF STAGES OF VIDEO AMPLIFIER.• ITS JOB IS TO PROVIDE SUFFICIENT GAIN TO DETECTED

SIGNAL SO THAT IT CAN DRIVE TUBE FROM CUT-OFF TO PEAK WHITE LEVEL.

• THE PEAK TO PEAK VOLTAGES AT PICTURE TUBE VARIES FROM 30V TO 150V.

• THE DC BIAS TO VIDEO AMPLI. MUST BE LARGE ENOUGH TO AMPLI. LINEARLY OVER THE ENTIRE RANGE OF VIDEO SIGNAL.

• NO AMPLITUDE OR PHASE DISTORTION IS TOLERABLE IN THE O/P OF VIDEO AMPLI.

• THE VIDEO AMPLI. SHOULD PROVIDE CONSTANT GAIN OVER 0Hz TO 5 MHz.

Page 19: Monochrome Receiver

SOUND SECTION

• THE SOUND SIGNAL IS ALSO AMPLIFIED ALONG WITH PICTURE SIGNAL IN I.F. STAGES.

• ITS JOB IS TO DETECT SOUND SIGNAL FROM DETECTED VIDEO SIGNAL.

• A TRAP CKT.(B.P.F.) IS USED TO SEPARATE SOUND SIGNAL FROM THE BASEBAND COMPOSITE VIDEO SIGNAL AT THE O/P OF VIDEO DETECTOR.

Page 20: Monochrome Receiver

SOUND DETECTION

FROM VIDEO DETECTOR

FROM VIDEO DETECTOR

SOUND TRAP 5.50.75MHz

SOUND TRAP 5.50.75MHz

AMP.+LIMITER 5.50.75MHz

AMP.+LIMITER 5.50.75MHz

FM DETECTOR

FM DETECTOR

AUDIO AMPL.

AUDIO AMPL.

Page 21: Monochrome Receiver

SYNC SEPARATION

TWO LEVEL CLIPPER

TWO LEVEL CLIPPER

INTEGRATOR LOWPASS RC

INTEGRATOR LOWPASS RC DIFFEREN-

TIATOR HIGH PASS RC

DIFFEREN-TIATOR HIGH PASS RC

HIGH PASS RC CAN ALSO ALLOW HIGH FREQ. NOISE TO PASS.THIS MAY CAUSE WRONG HORZ. TRIGGERING.AN AFC CIRCUIT IS USED TO PROTECT THIS.

Page 22: Monochrome Receiver

DEFLECTION CIRCUITS

• THEY ARE USED TO DEFLECT THE BEAM HORIZONTALLY AND VERTICALLY.

• FOT THIS, THE CIRCUITS USED ARE: HORIZONTAL OSCI.,HORZ. AMP.,VERTICAL OSCI. AND VERTICAL AMPL..

• THE OSCI. PROVIDE SYNCHRONIZATION FOR STILL PICTURE WHILE AMPLIFIERS PRODUCE SCANNING CURRENT.

Page 23: Monochrome Receiver

• THE OSCI. ARE BOTH FREE RUNNUNG OSCI.• THE SYNC I/P CONTROLS THEIR FREQ. OF

OSCI.• THE SYNC PULSES COULD BE DIRECTLY

GIVEN TO THE AMPLIFIERS.• BUT IN THIS SITUATION, WHEN THERE IS

NO VIDEO SIGNAL, THE RASTER IS EMPTY.• THIS IS TEASING TO THE VIEWER.• SO, OSCI. ARE USED AND ARE KEPT FREE

RUNNUNG.

Page 24: Monochrome Receiver

VERTICAL DEFLECTION

VERTICAL OSCI.

VERTICAL OSCI.

VERTICAL SYNC PULSE

CURRENT AMPLI.

CURRENT AMPLI.

VERTICALAMPLI.

VERTICALAMPLI.

VERTICAL DEFLEC-TION COIL

V-HOLD

Page 25: Monochrome Receiver

HORIZONTAL DEFLECTION

TIMING COMPARATOR

TIMING COMPARATOR

HORZ. SYNC PULSE

RC FILTER.

RC FILTER.

V.C.O.V.C.O.

HORZ. DEFLEC-TION COIL

Page 26: Monochrome Receiver

YOKE

• IT IS COMBINATION OF HORZ. AND VERTI. DEFL. COILS.

• THE AMPLIFIERS MUST PRODUCE SUFFUCIENT CURRENTS TO SCAN ENTIRE RASTER.

• THE VERT. AMP. PRODUCES HEIGHT AND HORZ. AMP. PRODUCES WIDTH.

• CURRENTS IN BOTH THE COILS ARE SAWTOOTH.

Page 27: Monochrome Receiver

VERTICAL SCANNING CURRENT

• AT 50Hz RESI. OF COIL IS MUCH GREATER THAN INDUCTANCE.

• SO, IT BEHAVES AS A RESI.• SO, TO PRODUCE SAWTOOTH CURRENT, WE

NEED SAWTOOTH VOLTAGE ACROSS VERTI. COIL.

• THIS MEANS O/P OF VERTI. AMPLI. MUST BE SAWTOOTH.

Page 28: Monochrome Receiver

HORIZONTAL SCANNING CURRENT

• AT 15625Hz RESI. OF COIL IS MUCH LESS THAN INDUCTANCE.

• SO, IT BEHAVES AS AN INDUCTOR.• SO, TO PRODUCE SAWTOOTH CURRENT, WE

NEED CONSTANT VOLTAGE ACROSS HORZ. COIL.

• THIS MEANS O/P OF HORZ. AMPLI. MUST BE RECTANGULAR.

Page 29: Monochrome Receiver

POWER SUPPLY• ALL ACTIVE DEVICES ICs AND PICTURE TUBE NEED A DC

SOURCE.• IT IS SUPPLIED BY RECTIFICATION OF AC MAINS VOLTAGE

AND FILTERS ARE PROVIDED TO REMOVE RIPPLES FROM DC O/P.THE RANGE OF VOLTAGES REQUIRED BY VARIOUS DEVICES CAN BE DIVIDED IN CONVINIENT GROUPS..

• LOW VOLTAGE:12-35 V FOR ICs’ AND SMALL SIGNAL AMPLIFIERS’ BIASING.

• MEDIUM VOLTAGE:150 V-500V FOR SCREEN AND FOCUS GREEDS AND VIDEO AMPLIFIERS.

• HIGH VOLTAGE:15- 25 kV FOR FINAL ANODE.