Chroma signal amplifier circuit and Yagi-uda antenna

By AshtonPoojaPrathik

AVE Colour TV

• Circuit diagram of Chroma signal amplifier- 5 different stages

- Stage 1(Q1) and 2(Q2 and Q3), colour saturation, colour killer bias and burst

blanking pulse stage.

• Yagi – uda antenna and radiation pattern.

Pal – D colour receiver

• Video detector suppies composite video signal to colour decoder (pal) and y channel decoder.

• o/p pal d = V and U ( scaled down colour difference signals U=0.493(B-Y) and V=0.877(R-Y)

• o/p y channel = luminance (brightness)signal (y)

• Given to matrix ckt for RGB colour signal generation.

Why separation of chroma signal and colour burst signal?

• The chroma signal separation circuit used.• Encoding= process of combining the 2 colour difference

signals by translating them to a higher freq. domain.• Since the 2 carrier freq. are suppressed in the balanced

mod. of the encoder, it is necessary to regenerate them at the receiver for demod. of the chroma signal.

• So,8-10 cycles of colour sub-carrier oss. o/p are transm. with sync pulses in short bursts called colour burst.

Stage 1 (Q1)

• Input to Q1 via C1• Gain controlled by XL1• Magnitude of ACC

voltage fed to base Q1 via R1 R2

• C2 avoids undesirable feedback between Q1 and ACC.

• Ie of Q1 = 0.5Vpp

• o/p to stage 2 via C6• o/p to burst stage via C5

Function of ACC amp. = o/p Voltage used to control gain of 1st stage of chroma amp. for constant chroma signal amplitude.

Stage 2 (Q2 and Q3)

• Consists of transistors Q2 and Q3

• Q2 acts as emitter follower

• Q3 acts as phase splitting amplifier i.e. to provide anti-phase chroma signal for chroma delay line circuit that separates U and V signals.

Colour saturation control section

• SaturationDetermines the pureness

of the colour.Level of saturation

indicates amounts of other colours present in received light.

• Saturation control section - function is as a variable attenuator to change mag. of the chroma signal led to U and V demodulator.

• Consists of D1,D2 with R7,R9.

Burst pulse blanking

• D3,D4 and R18 prevent the colour burst pulses from getting through the second stage.

• Blanking level > to avoid visibility during fly back time.

Colour killer control

• Function – rectifies burst pulses to produce steady dc V. and keeps the 2nd stage amp operative allowing the chroma signal to reach the demodulators.

• In case B&W, no burst pulses, no dc V, and 2nd amp blocked, no colour.

Thank you !