TDA8745 datasheet(8/40 Pages) PHILIPS

8 / 40 page

1996 Mar 11

Philips Semiconductors

Preliminary speciﬁcation

Satellite sound receiver with I2C-bus control

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 2n+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’).

TDA8745 датащи(PDF) 8 Page - NXP Semiconductors

TDA8745 датащи(HTML) 8 Page - NXP Semiconductors

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

датащи скачать

ссылки URL