M491B Pin 12

TDA4433 Pin 2

M491 B Pin 13

TDA4433 Pin 6

Function

(referred to the tuning voltage)

H

L

L

H

L

H

L

H

Up

Down

Middle

No Operation

These pins work according to the truth table given below :

These inputs have two different functions depend-

ing on whether the system is in the search or in

normal operation (AFT control).

The inputs have internal pull-up resistors of 30k

Ω

typ.

A) Search mode : after depressing the Automatic

search or preset keys, the levels of the signals

coming from the TDA4433, applied to these

pins, control the search function and determine

when the search must stop, i.e. a TV stationhas

been recognized.

The circuit operates in the following sequence

(see Figure 10 for reference) :

1 - after pressing the search start key the

search occurs in the FAST UP mode.

2 - eventual transitions available on these

inputs are ignored during the first 15 search

steps if the system is in the UHF or CATV

bands.

If the system operates in VHF I and III bands,

the first 60 search steps are ignored. The

acceptance delay of 15 (60) search steps has

been introduced to prevent the system from

stopping at the previous station.

Aft er this time the FAST UP speed is

automatically reduced to half during each UP

signal (MEDIUM UP = FAST UP/2).

A DOWN signal preceded by at least an UP

signal will set the search to MEDIUM DOWN

mode (FAST UP/4).

3 - the next UP signal will switch the search to

SLOW UP speed (61Hz).

At this point the systems is in normal AFT

operation.

B) Digital AFT operation : when a station is

perfectly tuned, the input signals coming from

TDA4433 are at middle condition.

If the tuning moves lower than the threshold

below 38.9MHz, the Pin 12 is put H and Pin 13

is put L ; the 13 bit internal counter is moved

SLOW UP speed to increase the varicap

voltage.

When a detuning occurs in the opposite

direction the input 12 goes Low and 13 goes

High and the tuning voltage falls at VERY

SLOW DOWN speed (7.6Hz).

The increase or decrease of the tuning voltage

is stopped as soon as the input returns to

middle conditions.

Therefore during normal operation Pins 12 and

13 act as digital AFT control commands.

C) Recall from memory : when the digital AFT is

enabled and data is recalled from Memory, a

fixed value of 8 steps (

≈ 31.2mV) is subtracted

from the tuning voltage.

This corresponds to a detuning of 0.6MHz

(UHF) and of 0.3MHz in VHF III into that part of

the IF response curve which corresponds to the

fully transmitted sideband.

At this point the AFT operation takes over as

described in point B above and the exact tuning

is achieved in about 0.2 sec.

This feature increases the AFT capture range

and fullfills the stability requirements of the

tu ne r, vol tage refere nces and the D/A

converter.

memory content is read without any change.

M491B

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