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[2] SINGLE CHIP IC FOR PIF, SIF and AFT
e@ PIF amplifier and P-detector circuit
The IF signal from the tuner is applied via the preampli-
fier (Q201) to the saw-wave filter (CF201) where it is
given the specific IF signal selectivity. The signal is then
fed into IC201 via pins () and ©) and is processed at the
three-stage IF amplifiers and AGC circuitry.
Then the signal is sent to the synchronous detector cir-
cuit in which a video signal is detected to enter the
white/black spot inverters, and it goes out of pin
of
1C201.
Since the video signal contains the audio carrier signal
(5.5 MHz), it has to be attenuated with CF202 traps
before it is supplied to the chroma and video circuits via
the 1st video amplifier (Q203). Use the following dia-
grams to know how the circuits work.
Fig. 16 Block diagram
Fig.
11 Waveforms on P-detector circuit
Fig. 12 AGC voltage characteristic
31
41
51
61
69
CHANNEL
CHANNEL
Figure 10.
Turning Voltage (VT) CURVES
At colour bar reception
At no signal reception
75%
WHITE
AGC (Automatic Gain Control)
The video signal coming from the video detector circuit
is applied to the white spot inverter, AGC noise inverter,
and then to the IF AGC detector. From here the result-
ing signal is applied to three-stage PIF amplifiers to cont-
rol the gain of these amplifiers. On the other hand, the
voltage from the voltage regulator RF AGC (R209) is
sent to pin (@) of IC201 to provide the delayed RF AGC.
The RF AGC voltage that appeared at pin
of IC201
is then applied to the RF AGC terminal to provide the
optimum gain of the RF amplifiers. Thus, the constant
signal amplitude
from the video detector is available
even with the variable antenna input signals.
See Fig. 12 for the relation between the AGC voltage and
antenna inputs.
AFT (Automatic Fine Tuning)
The carrier signal from the sync detector carrier coil
(T204) is sent to the AFT detector coil (f203) via preset
capacitors. The signal which differs in phase depending
on frequencies is then fed into [C201 via pins
and
@5)
, where the processor supplies the carrier to the
AFT detector, detects the phase difference, and provides
the AFT detector output at pin @4 .
The AFT detector signal is then applied via the AFT
switch (S1002) to AFT terminal of the tuner, thereby to
provide optimum
video
patterns with the controlled
local frequencies.
See Fig. 13 for the AFT detector voltages.
10
—~ 7 —
hn
ev «|
er
| Ste :
Pain
iy
3.5V
eee
OV
el
eee
VIDEO DET OUTPUT IC201 pin ® ( © )
Figure 11.
Waveforms on P-Detector Circuit
NOISE
Pin @ : Output voltage
from
AGC
de-
tector (pin (2)
of 1201)
Pin@® : RF
AGC
vol-
tage
of
the
tuner
10
Note:
Use the RF AGC cut-in
point
(about 60 dB of
— antenna
input)
and
R209
(RF
AGC
cont-
rol) for
"A"
level ad-
justment.
8
6
4
2
0
AGC
:
Voltage (V)
Antenna input (dB)
Figure 12.
AGC Voltage and Antenna
Inputs
-200kHz
+200kHz
The
figure
shows
the
11.0V
relation
between
the
o
tuner AFT
voltage with
the
activated
AFT
($1002
is on) and the
frequency
of P-carrier.
6.5V
It indicates
about 6 V
:
DC at the 38.9 MHz
(f,) P-carrier.
1.0V
OV
fo
oe
(38.9 MHz)
PI F-P-CARRIER-FREQ.
(MHz)
Figure 13.
AFT Detector Voltage
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