Philips TDA8303A - Manual (Page 5)

July 1992 5
Philips Semiconductors Preliminary specification
Small signal combination IC for black/white
TV TDA8303
TDA8303A
PINNING
AGC circuit
The AGC circuit of the TDA8303/TDA8303A is a top-sync
detector. The video signal coming from the video amplifier
passes a 2nd order low-pass filter before it is compared
with an internal reference level. The comparator stage is
gated when the horizontal oscillator is synchronized with
the video signal, such that interference pulses outside the
gating time have no influence on the gain control.
PIN DESCRIPTION
1 AGC take-over
2 vertical ramp generator
3 vertical drive
4 vertical feedback
5 tuner AGC
6 ground
7 supply voltage input
8 video IF input
9 video IF input
10 IF AGC
11 volume control/start horizontal oscillator
12 audio output
13 sound demodulator
14 sound IF decoupling
15 sound IF input
16 ground (for some critical parts)
17 video amplifier output
18 AFC output
19 AFC S/H, AFC switch
20 video demodulator tuned circuit
21 video demodulator tuned circuit
22 coincidence detector
23 horizontal oscillator
24 phase 1 detector
25 sync separator input
26 horizontal drive output
27 horizontal flyback input
28 phase 2 detector
Sound circuit
The sound quality of the TDA8303/TDA8303A compared
with the predecessors has been improved at weak signal
conditions. The improvement has been achieved by the
new IF amplifier which is less sensitive for radiation from
the sound IF amplifier and by change of the ground and
supply connections in the IC. When out-of-sync condition
is detected by the coincidence detector the sound output
is muted. When no mute is required the minimum voltage
level on pin 22 should be clamped to a high level of 5 V.
At this level the gating of the AGC is switched off and the
phase 1 detector has a high output current for reliable
catching of a new transmitter.
Vertical synchronization
The TDA8303/TDA8303A embodies a synchronized
divider system for generating the vertical sawtooth at pin 2
having several advantages and features such as:
•
The vertical frequency is alignment free. The divider
automatically adapts to a vertical frequency of 50 Hz or
60 Hz including automatic amplitude correction and its
operating modes offer maximum
interference/disturbance protection.
•
A discriminator-window checks the accuracy of the
vertical trigger pulse. Internally clock pulses are
generated by doubling the line frequency. The divider
operates in the 60 Hz mode when the trigger pulse
appears before count 576, otherwise the 50 Hz mode
will be active.
•
The divider system operates with two different reset
windows for maximum interference/disturbance
protection. The windows are activated via an up/down
counter. The counter increases its counter-value by 1 for
each time the separated vertical sync pulse appears
within the selected window, otherwise the counter value
is decreased by 1.
Modes of operation
Large search window: divider ratio between 488 and 576.
This mode is valid for the following conditions:
•
Divider is looking for a new transmitter
•
Divider ratio found does not comply with the narrow
window specification limits
•
Up/down counter value of the divider system, operating
in the narrow window mode, drops below count 10