TDA8722M/C3 데이터 시트보기 (PDF) - Philips Electronics

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TDA8722M/C3

I2C-bus programmable modulator for negative video modulation and FM sound

Philips Electronics 

Philips Semiconductors

I2C-bus programmable modulator for

negative video modulation and FM sound

Product specification

TDA8722

handbook, h8alfpage

P/S

(dB)

12

MBE399

16

(1)

(2)

(3)

(4)

20

0

10

20

30

40

C17 (pF)

R17 = 120 kΩ.

(1) 4.5 MHz.

(2) 5.5 MHz.

(3) 6.0 MHz.

(4) 6.5 MHz.

Fig.11 Typical picture-to-sound ratio as a function

of the value of C17.

14

handbook, halfpage

RF

harmonics

(dBc)

18

third harmonic

MBE400

22

26

30

450

second harmonic

550

650

750

850

RF (MHz)

Fig.12 Typical level of RF harmonics for an

asymmetrical application.

VHF operation

This IC can operate on frequencies as low as 200 MHz

(and especially for VHF 3 band) provided the impedance

of the tuned circuit between pins 4 and 6 is >1 kΩ.

NICAM and stereo

Because of the fact that the ADJUST pin (pin 17) is an

access point to the RF mixer, it is possible to use this pin

to inject an external modulated subcarrier into the IC.

This is especially interesting when it is necessary to

transmit a second frequency modulated audio subcarrier

for stereo sound (f = 5.72 MHz) or a NICAM QPSK

modulated carrier for digital audio transmission

(f = 5.85 or 6.552 MHz).

The incoming signal must be externally modulated either

in FM with the desired signal corresponding to PAL B/G

specification for stereo sound transmission, or in QPSK in

accordance with the NICAM transmission system.

The input impedance on pin 17 is approximately 3500 Ω,

the incoming signal must be capacitive coupled, the

resistor R17 between pin 17 and ground must remain to

adjust the modulation depth, the capacitor C17 between

pin 17 and ground may be changed depending on the

capacitance brought on by the incoming network. If this

capacitance is large, it is possible to remove C17.

Figure 13 shows a possible application for injecting such

kind of signal into the modulator IC.

Following this application, to get a picture-to-second

sound carrier ratio of −20 dB, it is necessary to apply a

level of approximately 800 mV (peak-to-peak value) at

the second carrier input, when the picture-to-first sound

carrier ratio is approximately −13 dB.

In addition, the internal FM sound modulator can be

switched off by short-circuiting pins 2 and 3.

1998 Jun 23

19

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