TDA8741 Datasheet, PDF (15/24 Pages) NXP Semiconductors – Satellite sound circuit with noise reduction

Philips Semiconductors
Satellite sound circuit with noise reduction
Product specification
TDA8741; TDA8741H
APPLICATION INFORMATION
General
The satellite baseband signal is routed to a mixer to
transfer the various secondary channels to fixed
intermediate frequencies of 10.7 and 10.52 MHz. Tuning
to the required carrier frequencies can be achieved by
varying the oscillator frequency e.g. tuning to 7.02 and
7.20 MHz carrier frequencies requires an oscillator
frequency of 17.72 MHz and tuning to 7.38 and 7.56 MHz
requires an oscillator frequency of 18.08 MHz. The IF
signals are routed to the inputs pin 8 and 16 (3 and 11) via
10.7 and 10.52 MHz ceramic bandpass filters. For the
secondary channels the bandpass filter terminating
resistor is integrated on-chip (330 Ω typical) thus no
external resistors are required.
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. For the main channel a ceramic bandpass filter
(e.g. 6.5 MHz) is used to filter out the main sound carrier
directly from the baseband signal. Simultaneous
demodulation of one main channel and various secondary
channels is now possible with a minimum of external
components. As no bandpass filter terminating resistor for
the main channel pin 18 (14) is integrated on-chip, an
external resistor is required.
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 of the main channel is
also transferred to a fixed intermediate frequency. In this
event the main audio output level should be adjusted to
−6 dBV by inserting a 470 Ω resistor in series with the
electrolytic capacitor on pin 41 (37).
If simultaneous demodulation of main- and secondary
sound carriers is not required, a low cost solution as given
in Fig.6 can be considered. One mixer and frequency
synthesizer are used to transfer all sound carriers to
intermediate frequencies of 10.7 and 10.52 MHz. When
the bandwidth of the main channel is larger than the
secondary channel bandwidth, a separate (e.g. 10.7 MHz)
ceramic filter plus resistors are required for the main
channel.
The secondary sound carriers can be demodulated as
described above. If e.g. a 6.65 MHz main sound 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 to 17.20 MHz a 6.5 MHz main sound
carrier can be demodulated. When main sound is to be
demodulated and this IF signal is also available on the
secondary Channel 1 input the PLLs of the secondary
channels can be disabled by making SCD pin 11 (6) HIGH.
The output selector should be in position 4 (main sound on
all outputs) if output pins 25 and 24 (21 and 20) are used.
For high-end applications the input level of the NR can be
adjusted to give optimum 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: 1 dB error in 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 an 50 Hz modulated
signal.
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.
October 1994
15