Day Tv Operation

8/8/2019 Day Tv Operation

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The RF amplifiers in the tuner commonly aredual gate N-channel depletion-type MOSFETs.

The transistor amplify the weak RF signal toimprove sensitivity of the receiver.

The RF signal is input to one of the gates, and an

automatic gain control (AGC) voltage is input to theother gate.

RF amplifier FETs operate very much likevacuum tubes.

0.5 5 mV RF signal strength is applied to theinput of the RF amplifiers tarnsistor gate.

RF AMPLIFIER


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LOCAL OSCILLATOR Is part of a voltage-controlled frequency

generator. To adjust the frequency of the LC

Oscillator, the capacitance of the LC circuit isvaried with the varactor diode.

A reversed-bias voltage applied to thevaractor sets the LC circuit frequency45.75MHz above the desired channels videocarrier.

The voltage applied to the varactordiodes tunes the LC circuit through a range ofrequencies.


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MIXER In the tuners mixer stage, a carrier from the

oscillator and the television RF signal are beat orheterodyned together. This converts the television channel down toan intermediate frequency (I.F.) containing all the

channels signal information. for i.e. Channel 2 (55.25MHz) is beat with anoscillator frequency of 101 MHz. The differencebetween this frequencies is 45.75MHz, the IF video

carrier frequency. The I.F. circuits pass the channels 6MHz bandof signals between the audio carrier at 41.25MHzand the video carrier at 45.75MHz.


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VIDEO IF AMPLIFIER

Amplify and processes weakIntermediate Frequency (IF) signals.


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VIDEO DETECTOR

The output of this stage produce three signals:

(1)composite video signalcomprise the syncpulses, blanking pulses and video information

through video section.

(2) 3.58 Mhz Chroma signalfor colorinformation through Chroma section.

(3)4.5 Mhz FM sound signalthrough Sound IFsection.


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SOUND IF AMPLIFIER

Amplify and processes the 4.5 MHz

Sound Intermediate Frequency (IF) signals.


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SOUND DETECTOR

Detects the 4.5 MHz IntermediateFrequency Sound signals.

Separate and recover the information

from the 4.5 MHz sound carrier.


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AUDIO AMPLIFIER

The stereo audio is fed to theright and left audio power amplifiers

and then drive onto the speakers.


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VIDEO AMPLIFIER

provide sufficient gain to drive a CRT.

have the response characteristics to cover the requiredbandwidth.

have tie points to customer and service control.

either maintain or restore a dc reference voltage

related to the original transmission.

ensure that the proper phase video signal reaches theCRT.

accepts horizontal and vertical blanking waveforms

that cut off the CRT during blanking intervals.

Most importantly, though the video amplifier increasesthe magnitude of the detector output voltage withoutchanging any frequencies.


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SYNC SEPARATOR

Eliminates the video and blanking signalswhile amplifying only the horizontal sync,vertical sync and equalizing pulses.

Sync separation also involves the removalof 60Hz vertical sync pulses from 15,750Hzhorizontal sync pulses.


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VERTICAL OSCILLATOR

The oscillator have the characteristic to allow easy

synchronization by the sync pulse.The oscillator must quickly change its starting time and

match the new sync signals.

Use blocking oscillators and multi-vibrators requires for

rapid changes. Immune to triggering by noise pulses.

Vertical Hold Control adjusts the free-running frequencyof an oscillator that causes of rolling picture.

Most modern TV do not rely on a manual adjustmentbecause of the advances in solid-state technologies.


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VERTICAL DRIVE

Represents wave-shaping to provide a ramp orsaw-tooth waveform that has gradual and linearrise time.

The wave-shaping circuit will go into cut-off

during the retrace interval by adding a pulse tothe saw-tooth as a results evolves intotrapezoidal waveform.

Includes frequency determining components ofeither an RC or an LC network.

The vertical size, or height , and linearitycontrols interface with the waveshaping


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VERTICAL OUTPUT

Vertical deflection systems always includesome type of large-signal power amplifier inthe output stage.

Provides the large amount of current neededto drive the vertical scanning coils in thedeflection yoke.


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HORIZONTAL AFCThe horizontal AFC stage compares the frequency

and phase of feedback pulse taken from thehorizontal output stage with horizontal sync pulsearriving from the differentiator.

Through the comparison of those signals, the AFCcircuits maintain the correct 15,750Hz frequency of

the horizontal oscillator. If the oscillator frequency deviates from the correct

setting, a mismatch occurs between the signalsarriving at input of the AFC circuit.

Either an increase or a decrease in frequency willcause the AFC circuit to generate a positive ornegative dc control voltage.


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HORIZONTAL OSCILLATOR

Stabilize the Horizontal sweep generators to resist triggering bynoise pulses.

It completes 15,750Hz/60 or 262.5 cycles during the time that thevertical oscillator requires to complete one cycle.

Can take the time of almost 200 cycles to change phase with novisible changes occurring the picture.

Produces either rectangular wave or a pulse output. Horizontal Hold Control adjusts the free-running frequency of an

oscillator that causes of bending picture.

Most modern TV do not rely on a manual adjustment because of thadvances in solid-state technologies.


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HORIZONTAL DRIVE

Reshapes the rectangular pulse taken from the horizontal

oscillator output to provide the waveshape needed toproduce a horizontal output signal.

The driver stage uses a semiconductor device as a switch.

Conduction time determines the length of time that the dcsupply voltage connects to the horizontal deflection yoke for

each horizontal scan.The pulse width is an important factor when checking the

performance of the driver stage.


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HORIZONTAL OUTPUT

The raster will not have any brightness without functions

provided by the horizontal output stage.

Because the output pulse is applied to the flybacktransformer it produces high voltage for the CRT.

This stage requires a damper diode to minimize the

possibility of oscillation in the horizontal scanning current.The horizontal output transistor (HOT) supplies the

horizontal scanning current directly to the yoke coils.


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FLYBACK TRANSFORMER

Provide the approximately 25kV-30kV dchigh voltage required for the second of theCRT.

Provide sufficient current for the threeelectron beams generated within a colorCRT.

Regulate the high voltage.


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SYSTEM CONTROL

also called microprocessor control system dedicated microprocessor and system

memory section to issue and store operatingcommands

A frequency synthesis section designed forexact channel selection and continuousfrequency control

An electronic bandswitching system


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SWITCHED-MODE POWER

SUPPLIES (SMPS)

Offer advantage such as reduced size, weight, and cost.

The high frequency operation of an SMPS allows the useof smaller and lighter component than those seen inlinear power supplies.

Offers greater efficiency than a linear power supply.

Operates either fully on or fully off.

Type of power supply loses little power and has anefficiency of approximately 85%.


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AUTOMATIC GAIN CONTROL

The AGC circuit controls strong signals so that thegain produced by the first and second IFamplifiers will not overload the stages that follow.

Increases IF amplifier gain under weak signal. Any signal increase above the reference level

causes the AGC circuit to apply a dc correctionvoltage to the amplifier stages and restore the

proper sync tip amplitude.


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CHROMA SECTION

Remove and amplify the color burst signal.

Reinsert the suppressed color subcarrier.

Recover the original color difference signals.

Control the hue and saturation of thereproduced colors.

Disable the color signal during a monochrome

broadcast.

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