TDA8722 Philips Semiconductors, TDA8722 Datasheet - Page 18

• Manufacturer Part Number: TDA8722
• Description: I2C-bus programmable modulator for negative video modulation and FM sound
• Manufacturer: Philips Semiconductors

Philips Semiconductors
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).
1998 Jun 23
I
negative video modulation and FM sound
handbook, halfpage
modulation
Fig.10 Typical modulation depth as a function of
2
C-bus programmable modulator for
depth
(%)
100
90
80
70
60
the value of R17.
10
10
2
R17 (k )
MBE398
10
3
18
Table 7 Value of resistor for several Q factor ranges
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.
COIL QUALITY
80 to >100
FACTOR
30 to 40
40 to 50
50 to 60
60 to 80
PROPOSED VALUE FOR R3
82 to 33
33 to 27
27 to 22
22 to 18
18 to 15
Product specification
(k )
TDA8722