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TDA8722M

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

Modulation depth

With 500 mV (peak-to-peak value) video input signal, the

wanted modulation depth must be set by the value of R17

(resistor between pin 17 and ground) as shown Fig.10. For

a good accuracy, it is recommended to use a 1% type

resistor.

It is also possible to use an adjustable resistor, see Fig.8.

Depending on the layout of the PCB, it may be necessary

to slightly change the value of R17 from the one given in

Fig.10 to get the wanted modulation depth.

Sound oscillator design

The frequency of the sound subcarrier is fixed by the tank

circuit connected between pins 2 and 3. This frequency

can be adjusted between 4.5 and 6.5 MHz covering all

existing standards in the world.

The damping resistor R3 between pins 2 and 3 is

necessary to decrease the quality factor of the tank circuit

allowing the frequency to be modulated by the audio

signal. The value of this resistor is calculated for several

Q factor ranges of the coil for a sound frequency of

5.5 MHz (see Table 7).

100

handbook, halfpage

modulation

depth

(%)

90

80

70

MBE398

Table 7 Value of resistor for several Q factor ranges

COIL QUALITY

FACTOR

30 to 40

40 to 50

50 to 60

60 to 80

80 to >100

PROPOSED VALUE FOR R3

(kΩ)

82 to 33

33 to 27

27 to 22

22 to 18

18 to 15

The use of a coil with a quality factor <30 may result in a

non operating oscillator. For safety, it is recommended to

use a coil with a quality factor ≥50.

Picture-to-sound ratio

The picture-to-sound ratio can be adjusted in the

application by changing the value of C17 (capacitor

between pin 17 and ground); see Fig.11.

Figure 11 shows us that the picture-to-sound ratio will

change for a constant value of C17 when the sound

subcarrier frequency will change.

RF harmonics

This IC has been designed to have the lowest level of

unwanted RF harmonics at the frequencies where these

are the hardest to be filtered out, especially for the second

harmonic of the RF carrier at the lowest frequencies of the

UHF band.

The level of the second and third RF harmonic is shown in

Fig.12 for an asymmetrical application. This chart gives a

typical value while the level of these harmonics can vary

depending on the design of the application.

It is possible to reduce the level of the second harmonic by

using a wide band transformer at the output of the IC and

create a symmetrical application (see Fig.9).

To reduce the out-of-band harmonics and especially the

third one, it is necessary to use a low-pass filter at the

output of the IC.

60

10

102

R17 (kΩ)

10 3

Fig.10 Typical modulation depth as a function of

the value of R17.

1998 Jun 23

18

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