TDA8740 datasheet(16/24 Pages) PHILIPS

16 / 24 page

October 1994

Philips Semiconductors

Product speciﬁcation

Satellite sound circuit with noise reduction

TDA8740; TDA8740H

APPLICATION INFORMATION

MCS pin 7 (2) = logic 0; (see Fig.5)

The lock-in range of the main channel PLL is 5.5 to

7.5 MHz. The satellite baseband signal is routed to nine

bandpass filters, eight for the secondary channels and one

for the main channel. For the secondary channels the

bandpass filter terminating resistors are integrated on-chip

(330

Ω typically) so no external resistors are required. For

the main channel this is not the case therefore filters with

different characteristics can be used.

MCS pin 7 (2) = logic 1; (see Fig.6)

The lock-in range of the main channel PLL is now 10.0 to

11.5 MHz. The carrier frequency for the main channel is

transposed to 10.7 MHz by an external frequency

synthesizer and mixer; e.g., if a 6.65 MHz carrier is to be

demodulated this frequency is mixed to 10.7 MHz by

tuning the frequency synthesizer to 17.35 MHz. If the

synthesizer is tuned 17.20 MHz a 6.5 MHz carrier can be

demodulated. In this event the audio output level should

be adjusted to

−6 dBV by inserting a 470 Ω resistor in

series with the electrolytic capacitor to pin 41 (37).

MCS pin 7 (2) = MID position

The main channel PLL is off if the voltage is between 1.8

and 2.8 V on pin 7 (2). This situation can be obtained by a

floating MCS pin if VP is between 10.8 and 13.2 V or by

forcing this MCS pin to the desired voltage by an external

voltage source. In that event VP can be in the full range of

8 to 13.2 V. This feature can be used to slightly improve

the signal-to-noise ratio of the secondary channels if the

main channel is not used.

For the main channel the baseband signal is routed to the

mixer, and to eight ceramic bandpass filters for the

secondary channels. For high-end applications the input

level of the NR can be adjusted to give optimal

performance. 0 dB is the maximum input level which

corresponds to the maximum frequency deviation of the

incoming FM signal (50 kHz for secondary channels). If

the NR input signal is too low (HIGH) the NR will attenuate

(favour) the higher audio frequencies too much due to the

expansion character. In general a 1 dB error in the NR

input level will give a 1 dB difference between

LOW (50 Hz) and HIGH (15 kHz) audio frequencies. With

RS1 (RS2) the input level and so the frequency response of

the NR can be adjusted: at 0 dB input level (which

corresponds to a frequency deviation of 50 kHz) the output

level of a 15 kHz modulated signal should be 0.25 dB

lower than that of a 50 Hz modulated signal.

If the main channel only is to be demodulated the PLLs of

the secondary channels can be disabled by setting SCD

pin 11 (6) to logic 1.

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