TDA8722 Datasheet, PDF (19/28 Pages) NXP Semiconductors – I2C-bus programmable modulator for negative video modulation and FM sound

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