TDA8740 Datasheet, PDF (16/24 Pages) NXP Semiconductors – Satellite sound circuit with noise reduction

Philips Semiconductors
Satellite sound circuit with noise reduction
Product specification
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.
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.
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 V_P 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 V_P 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
R_S1 (R_S2) 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.
October 1994
16