TDA8822 Datasheet, PDF (22/28 Pages) NXP Semiconductors – Universal I2C-bus programmable RF modulator

Philips Semiconductors
Universal I²C-bus programmable RF
modulator
Preliminary specification
TDA8822
TPSG input/output (pin 17)
As already mentioned, this pin can be used either as an
input or as an output.
• As an input, it allows to turn on the TPSG, without
changing anything to the word the TDA8822 is
programmed through the I²C-bus.
In this mode, it is simply necessary to connect a switch
between the pin TPSG and DGND (see Fig.8). If the
switch is open, then the TPSG is selected
corresponding to the I²C-bus programming; if the switch
is closed, then the TPSG is on.
• As an output, it allows to indicate e.g. with an LED that
the TPSG has been programmed on using the I²C-bus.
In this mode, the pin acts as an open-collector output
port, it is possible to connect a LED to the 5 V power
supply with a series resistor to limit the current to about
10 mA (see Fig.9).
XTAL pin (pin 12)
This pin is connected to a 4 MHz crystal in series with a
capacitor. The value of this capacitor has to be as close as
possible to the load capacitance of the crystal.
It is also possible to drive the IC with an external 4 MHz
signal from a voltage source. A level of 50 mV(RMS)
insures stable operation. A capacitor of about 18 pF and a
resistor of 680 Ω needs to be placed in series with the
voltage source.
ACP pin (pin 2)
This pin is the charge pump output for the sound subcarrier
PLL as well as the input of the sound subcarrier VCO.
It is necessary to connect the loop filter between this pin
and ground. The loop filter indicated in Fig.7 gives a cut-off
frequency lower than 20 Hz.
If a cutoff frequency slightly higher than 20 Hz can be
accepted, it is possible to reduce the value of the 2.2 µF
capacitor (C2) to 220 nF. In this case C26 needs to be
changed from 68 nf to 22 nF and R3 needs to be changed
from 4.7 kΩ to 33 kΩ.
RF outputs (pins 18 and 19)
For inexpensive applications, it is possible to use the IC
with an asymmetrical output.
In an asymmetrical application, the unused output pin
must be loaded with a load as close as possible to the load
connected to the used pin.
A good improvement in performance is obtained using a
symmetrical to asymmetrical transformer
(balun; balance-to-unbalance) connected between the two
outputs. In this event both outputs have their loads
matched. The level of the RF second harmonic, and the
spurious outside channel is decreasing. The parasitic
coupling between RF outputs and RF oscillator is also
reduced.
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.
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.
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.
1997 Jan 08
22