2. Contents
RF Tuner
IF Subsystem
Video amplifier
Sound section
Sync separation and processing
Deflection circuits
Scanning Currents in the yoke
DC power supplies
Electronic tuners
IF Subsystem
3. Contents
Y Signal channel
Chroma decoder
Separation of U and V colour phasors
Synchronous demodulators
Sub carrier generation and control
Matrixing for drive circuits
Receiver Servicing
Video pattern generator
Sweep & Marker generator
Colour TV Pattern Generator
Vectroscope
5. RF tuner
This section consists of RF amplifier, mixer and local
oscillator
The purpose of the tuner unit is to amplify both sound
and picture signals picked up by the antenna and to
convert the carrier frequencies and their associated
bands into the intermediate frequencies and their
sidebands
The receiver uses superhetrodyne principle as used in
radio receivers
The setting of the local oscillator frequency enables
selection of desired station
6. RF tuner
The standard intermediate frequencies for the 625-B
system are-Picture IF = 38.9 MHz, Sound IF = 33.4
MHz
10. Choice of selecting IF
The undesired signal which gets received is spaced at a
gap of twice the IF frequency, and is known as ‘Image
Signal’
The image rejection ratio is defined as the output due
to desired station divided by output due to image
signal
With RF amplifier the output due to image signal can
be very much reduced or completely eliminated
Here as IF will be greater, the image frequency will be
greater and there are more chances to eliminate it
through well designed filter
11. Choice of selecting IF
2) Pick-up Due to Local Oscillator Radiation from TV
Receivers
If the output from the local oscillator of a TV receiver
gets coupled to the antenna, it will get radiated and
may cause interference in another receiver
Here again advantage lies with higher IF frequency,
because with higher IF there is a greater separation
between the resonant circuits of local oscillator and RF
amplifier circuits
13. Choice of selecting IF
3) Ease of Separation of Modulating Signal from IF
Carrier at the Demodulator
For ease of filtering out the IF carrier freuency, it is
desirable to have a much higher IF frequency as
compared to the highest modulating frequency
In radio receivers the IF frequency is 455 KHz and the
highest audio frequency is only 5 KHz
In TV receivers, with the highest modulating frequency
of 5 MHz, an IF frequency of atleast 40 MHz is desirable
14. Choice of selecting IF
4) Image Frequencies Should Not Lie in the FM Band
The FM band is from 88 MHz to 110 MHz
With IF frequency chosen close to 40 MHz, the image
frequencies of the lower VHF band fall between 121 to
148 MHz and thus cannot cause any interference in the
FM band
Higher TV channels are much above the FM band
15. Choice of selecting IF
5) Interference or Direct Pick-Up from Bands Assigned
for other Service
Amateur and industrial applications frequency band
lies between 21 to 27 MHz
If the IF frequency is chosen above 40 MHz, even the
second harmonics of this band will not cause any
serious direct pick-up problems
16. Choice of selecting IF
6) Gain
The television receiver should produce enough gain at
high IF frequencies
This is achievable through today’s highly accurate
transistors
18. Video Amplifier
The video amplifier is dc coupled from the video detector
to the picture tube, in order to preserve the dc component
for correct brightness
However, in some video amplifier designs, on account of
complexities of a direct coupled amplifier, ac coupling is
instead used
The dc component of the video signal is restored by a diode
clamper before feeding it to cathode or grid of the picture
tube
In transistor amplifier designs, a suitable configuration of
two transistors and a driver often becomes necessary to
obtain the same gain
19. Video Amplifier
Besides gain, response of the amplifier should ideally
be flat from dc (zero) to 5 MHz to include all essential
video components
This needs rigorous design considerations because the
band of frequencies to be covered extends from dc
through audio range to radio frequencies
A loss in gain of high frequency components in the
video signal would reduce sharpness of the picture
whereas a poor low frequency response will result in
loss of boundary details of letters etc
It is also essential that phase distortion in the amplifer
is kept to a minimum
20. Sound section
The relatively weak FM sound
signal is given at least one
stage of amplification before
feeding it to the FM detector
The FM detector is normally a
ratio detector or a
discriminator preceded by a
limiter
The characteristics of a typical
FM detector are shown in Fig
21. Sound section
A tuned amplifier, with enough bandwidth to pass the FM
sound signal is used to boost the FM signal
The volume and tone controls form part of the audio
amplifiers
The power amplifier is either a single ended or push-pull
configuration employing transistors
Special ICs have been developed which contain FM
demodulator and most parts of the audio amplifier
22. Sync separation and processing
The horizontal and vertical sync pulses that form part
of the composite video signal are separated in the sync
separator
A sync separator is a clipper that is suitably biased to
produce output, only during sync pulse amplitude of
the video signal
The pulse train as obtained from the sync separator is
fed simultaneously to a differentiating and an
integrating circuit
23. Sync separation and processing
The differentiator, being a high-pass filter, develops
output in response to noise pulses in addition to the
spiked horizontal sync pulses
24. Sync separation and processing
This results in occasional wrong triggering of the
horizontal oscillator which results in diagonal tearing
of the reproduced picture
To overcome this difficulty, a special circuit known as
automatic frequency control (AFC) circuit is employed
The AFC circuit employs a discriminator arrangement
which compares the incoming horizontal sync pulses
and the voltage that develops across the output of the
horizontal deflection amplifier
25. Deflection circuits
The necessary sawtooth voltage for vertical and
horizontal is developed by charging and discharging a
capacitor with different time constants
For vertical deflection, the frequency of the oscillator
is controlled by varying the resistance of the RC
coupling network and is locked in synchronism by the
vertical sync pulses
A part of the coupling network resistance is a
potentiometer that is located on the front panel of the
receiver
This is known as ‘Vertical Hold Control’
26. Deflection circuits
The frequency of horizontal oscillator is controlled by
dc control voltage developed by the AFC circuit
Since the noise pulses in the control voltage are
completely suppressed, most receivers do not provide
any horizontal frequency (hold) control
Since the deflection coils need about one amp of
current to sweep the entire raster, the output of the
oscillator is given one stage of power amplification (as
for vertical deflection) and then fed to the horizontal
deflection coils
27. Scanning Currents in the yoke
Vertical scanning current: The vertical oscillator uses
an RC network to develop a sawtooth waveform
With this as an input to the vertical amplifier, a
sawtooth current flows through the vertical deflection
coil to cause vertical scanning
The vertical output stage is a power amplifier which
acts as a current source to produce a linear rise of
magnetic field in the deflection coil
The stage is cut off for brief period of retrace only
28. Scanning Currents in the yoke
Horizontal scanning current: The horizontal coil is
given a current changes of several amperes in 52us
Due this large change a self induced voltage is
generated across the coil which creates the horizontal
trace
The horizontal amplifier is continuously switched ON
and switched OFF
The horizontal output stage consumes more than 75
percent of the total power used by the receiver
29. DC power supplies
Various DC sources needed in a typical television
receiver are as under
Low voltage: about 12 to 35 volts for IC and small signal
amplifiers
Medium voltages: about 150 v for horizontal output
stage, 300 to 400v for the screen and focus grid of
picture tube and about 175 v for the video amplifier
High voltage: 15 to 18KV for final anode of picture tube
31. Electronic tuners for colour
television
Varactor is a special silicon diode, the junction
capacitance of which is used for tuning
This capacitance varies inversely with the amount of
reverse bias applied across the diode
The resonant frequency of the tuned circuits in which
they are connected, is controlled merely by changing
the reverse bias across the varactor
Figure shows a basic circuit for varactor diode tuning
33. IF Subsystem for colour television
IF system for colour television consists of
band shaping filter circuit
IF amplifiers(AGC controlled)
AGC(Automatic gain control)
AFT(Automatic frequency tuning)
Intercarrier sound IF detector
Video detector
Buffer video amplifier
35. IF Subsystem
After passing through IF subsystem the colour signal is
given in to 4 blocks
Luminance or Y channel
Chroma decoder
AGC circuit
Sync-separator and Raster circuit
37. Y signal Channel
Y signal represents the brightness of the picture signal
The colour signals get added on Y signal to reproduce
coloured scene on raster
To recover colour information from Y signal, we need
to use comb filter
It selects frequencies that need to be passed and
rejects the other frequencies in Y signal band
To achieve this video signal v is applied to a delay line
of 64 us and inverter
Output of delay line and inverter are added and
chroma signal frequencies are recovered
41. Chroma decoder
The main function of chroma decoder is to recover U
and V colour difference signals which are combined
with Y to obtain R,G and B video signal
For this the decoder has to perform following
functions
Chroma signal seperation and amplification
Separation of U and V
Demodulation of U and V
Generation of subcarriers for two demodulators
To develop ‘ident’ signal
47. Sub carrier generation and control
To extract the colour burst signal from composite
video signal, a burst phase discriminator is used
It determines the phase difference of input colour
burst and sets the reference oscillator accordingly to
use it as a reference subcarrier for U and V
demodulators
Also a colour killer block and burst phase
identification block is used to turn ON the second
stage amplifier only when a colour signal is received
In absence of colour burst, the amplifier will be OFF
and the signal is considered as monochrome signal
49. Matrixing for drive circuits
The value of R1, R2 and R3 are so chosen that the G-Y
amplifier block receives input as per following
equation
(G-Y) = -0.51(R-Y) – 0.186(B-Y)
Once all colour difference signals are extracted, they
are applied to RGB matrix for cathode drive
Luminance signal Y is added to these difference signals
to obtain the Vr, Vb and Vg colour voltages