TDA8745 Datasheet, PDF (8/40 Pages) NXP Semiconductors – Satellite sound receiver with I2C-bus control

Philips Semiconductors
Satellite sound receiver with I²C-bus control
Preliminary specification
TDA8745
FUNCTIONAL DESCRIPTION
Satellite sound
The baseband signal coming from a satellite tuner
comprises the demodulated video signal plus a number of
sound carriers in the event of reception of a PAL, NTSC or
SECAM satellite signal.
Nearest to the video signal is the main sound carrier which
carries the mono sound related to the video. This is an FM
modulated carrier with a fixed pre-emphasis. The carrier
frequency can be in the range of 5.8 to 6.8 MHz.
Additionally, a number of optional secondary sound
carriers may be present. These can be used for stereo or
multi-language sound related to the video signal, or for
unrelated radio sound. These carriers are also FM
modulated, and for better sound quality (improved
signal-to-noise performance) broadcast satellites
(e.g. ‘ASTRA’) use a noise reduction system (adaptive
pre-emphasis circuit, combined with a fixed
pre-emphasis). These secondary carrier frequencies can
be in the range of 6.30 to 8.28 MHz.
For accurate tuning to the many sound carriers, an internal
frequency synthesizer and mixer are used to transfer the
sound carriers to intermediate frequencies of
10.7 and 10.52 MHz.
The TDA8745 contains all circuitry for the processing of
the main channel and secondary channels, from baseband
signal to line (SCART) output drivers. By means of
external band-pass filters the desired frequencies coming
from the synthesizer/mixer are routed to the IF
limiter/demodulator inputs.
Band-pass filter and mixer
Before the incoming baseband signal is applied to the
mixer, the signal is filtered. Related to the sound carriers,
the level of the video part of the baseband signal can be
much higher, so to avoid overload it is desirable to
attenuate the latter, this is also to avoid interference
(additional unwanted mix of signals after mixing).
The internal band-pass filter (pass band from
approximately 4 to 10 MHz) is completed by a simple
external notch filter. The external filter provides substantial
attenuation of the video colour carrier. The notch filter is
chosen to be external because the required notch
frequency is TV standard dependent and also because an
accurate on-chip notch filter requires a tuning mechanism
(consuming additional chip area).
The mixer is a double-balanced mixer with degeneration,
this to accommodate the level of the filter output signal.
The mixer transfers the different sound carrier frequencies
to fixed intermediate frequencies of 10.7 and 10.52 MHz.
These frequencies are fed via an internal buffer stage to
external ceramic band-pass filters before they are routed
to the two demodulator inputs. The buffer stage can drive
up to three external ceramic band-pass filters (assuming
330 Ω filter terminations) but this can be increased to four
or more by adding an external buffer.
Synthesizer
The synthesizer consists of the following parts:
• Reference oscillator
• Reference divider
• A 10-bit programmable divider
• Phase detector
• Charge pump
• Voltage Controlled Oscillator (VCO)
• Divide-by-two circuit.
The reference frequency circuit consists of a 4 MHz crystal
oscillator and a divider (by 200). The resulting reference
frequency of 20 kHz is fed to the phase detector.
The programmable divider consists of a series of cells
(divide by 2 or 3) connected as a ripple counter.
The minimum division ratio is 2ⁿ and the maximum division
ratio is 2^{n + 1} − 1.
The programmable divider output signal is also fed to the
phase detector. The charge pump provides output current
pulses in accordance with the signals from the phase
detector. The final tuning voltage for the VCO is provided
by the loop filter and a buffer amplifier.
The oscillator frequency range is from
29.04 to 40.94 MHz, depending on the setting of the
programmable divider (by the TUN signal). The tuning
voltage is clipped to limit the VCO frequency range.
The frequency of the oscillator is divided-by-two before it
is applied to the mixer (to obtain the desired 10 kHz
resolution).
Left and right channel inputs
A maximum of six inputs are available (pins 9, 11, 13, 15,
7 and 8). External ceramic band-pass filters, which are
tuned to the desired intermediate frequencies, route the
signals to the inputs.
For stereo purposes the TDA8745 contains two identical
secondary sound processing channels (secondary
channel 1 will also be referred to as ‘left’ or ‘language 1’
and secondary channel 2 as ‘right’ or ‘language 2’).
1996 Mar 11
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