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Monochrome Receiver

Nov 18, 2014

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MONOCHROME TELEVISION RECEIVER

SECTIONS

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

INPUT FROM ANTENNA ANTENNA INTERCEPTS OR PICKS UP ELECTROMAGNETIC WAVES WHICH ARE RADIATED BY TRANSMITTERS. THE RECEIVED SIGNAL CAN VARY FROM 50 V TO 1mV. ANTENNA CAN BE RESONANT OR NONRESONANT. YAGI-UDA AND PARABOLIC REFLECTOR ARE MOST COMMON IN USE.

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.

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

Pc-1.25 Pc

Sc

f

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.

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(88108 MHz) BAND. NO INTERFERENCE FROM THE BAND ASSIGNED TO OTHER SERVICES. GAIN.

I.F. SUBSYSTEM 1) 2) 3)

CONSISTS OF.. I.F. SECTION AGC CIRCUIT VIDEO DETECTOR THIS SECTION RECEIVES ITS I/P FROM MIXER O/P AND ITS O/P IS BASEBAND COMPOSITE VIDEO SIGNAL

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.

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.

THE OVER ALL I.F. RESPONSEdB 0 -6 CH.3(WANTED): (54-61 MHz) PC3=55.25 SC3=60.75 L.O.=94.15

CH.2(LOWER ADJACENT CHANNEL):(47-54MHz) Sif=33.4 Pif=38.9 PC2=48.25 SC2=53.75

CH.4(UPPER ADJACENT CHANNEL): (61-68 MHz) 31.9 33.15 PC4=62.25 40.15 40.4 f(MHz) 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.

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

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.

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.

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.

FROM LAST IF AMPL.

BUFFER

TO CATHODE OF PICTURE TUBE

VIDEO AMPLI.

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.

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.

SOUND DETECTIONFROM VIDEO DETECTOR SOUND TRAP 5.5s0.75MHz AMP.+LIMI TER 5.5s0.75MHz FM DETECTOR

AUDIO AMPL.

SYNC SEPARATIONTWO LEVEL CLIPPER

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.

INTEGRATOR LOWPASS RC

DIFFERENTIATOR HIGH PASS RC

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.

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.

VERTICAL DEFLECTIONVERTICAL SYNC VERTICAL PULSE OSCI. CURRENT AMPLI. VERTICAL AMPLI. VERTICAL DEFLECTION COIL

V-HOLD

HORIZONTAL DEFLECTIONHORZ. SYNC PULSE TIMING COMPARA TOR RC FILTER. V.C.O.

HORZ. DEFLECTION COIL

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.

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.

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.

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.

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