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TDA9830

TV sound AM-demodulator and audio source switch

Philips Electronics 

Philips Semiconductors

TV sound AM-demodulator and audio source switch

Product specification

TDA9830

PINNING

SYMBOL

IFIN

n.c.

CAGC

CREF

n.c.

AMOUT

AMIN

AFOUT

EXTIN

SWITCH

Vp2

MUTE

GND

Vp1

n.c.

IFIN

PIN

DESCRIPTION

1

sound IF differential input signal

2

not connected

3

AGC capacitor

4

REF voltage filtering capacitor

5

not connected

6

AM demodulator output

7

input signal (from AM) to audio switch

8

output signal from audio switch

9

input signal (from external) to audio switch

10 switch input select control

11 supply voltage +12 V (alternative)

12 mute control

13 ground (0 V)

14 supply voltage +5 to +8 V

15 not connected

16 sound IF differential input signal

Fig.2 Pin configuration.

FUNCTIONAL DESCRIPTION

Sound IF input

The sound IF amplifier consists of

three AC-coupled differential

amplifier stages each with

approximately 20 dB gain. At the

output of each stage is a multiplier for

gain controlling (→ current

distribution gain control). The overall

control range is approximately −6 to

+60 dB and the frequency response

(−3 dB) of the IF amplifier is

approximately 6 to 70 MHz. The

steepness of gain control is

approximately 10 mV/dB.

IF AGC

The automatic gain control voltage to

maintain the AM demodulator output

signal at a constant level is generated

by a mean level detector. This

AGC-detector charges and

discharges the capacitor at pin 3

controlled by the output signal of the

AM-demodulator compared to an

internal reference voltage. The

maximum charge/discharge current is

approximately 5 µA. This value in

combination with the value of the

AGC capacitor and the AGC

steepness determines the lower

cut-off audio frequency and the

THD-figure at low modulation

frequency of the whole

AM-demodulator. Therefore a large

time constant has to be chosen which

leads to slow AGC reaction at IF level

change. To speed up the AGC in case

of IF signal jump from low to high

level, there is an additional

comparator built in, which can provide

additional discharge current from the

AGC capacitor up to 5 mA in a case of

overloading the AM demodulator by

the internal IF signal.

AM-demodulator

The IF amplifier output signal is fed to

a limiting amplifier (two stages) and to

a multiplier circuit. However the limiter

output signal (which is not any more

AM modulated) is also fed to the

multiplier, which provides AM

demodulation (in phase

demodulation). After lowpass filtering

(fg ≈ 400 kHz) for carrier rejection and

buffering, the demodulator output

signal is present at pin 6. The AM

demodulator operates over a wide

frequency range, so that in

combination with the frequency

response of the IF amplifier

applications in a frequency range

from approximately 6 MHz up to

70 MHz are possible.

Audio switch

This circuit is an operational amplifier

with three input stages and internal

feedback network determining gain

(0 dB) and frequency response

(fg ≈ 700 kHz). Two of the input

stages are connected to pin 7 and pin

9, the third input stage to an internal

reference voltage. Controlled by the

switching pins 10 and 12, one of the

three input stages can be activated

and a choice made between two

different AF signals or mute state.

The selected signal is present at

June 1994

4

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