Daewoo DDT-21H9ZZF, Service Manual, Page: 39 / 83

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Service Manual CP-099F
With the present system a high performance Hi-Fi stereo sound processing can be achieved. For a simplified application without a
Sound IF SAW filter the single reference QSS mixer can be switched to the intercarrier mode by I
2 C-bus. This single reference QSS
mixer generates the 2nd FM TV sound intercarrier signal. It is realized by a linear multiplier Which multiplies the SIF amplifier
output signal and the VIF VCO signal (90 degrees output), which is locked to the picture carrier. By this way the QSS mixer operates
as quadrature mixer in case of intercarrier mode and provides suppression of the low frequency video signals.
The QSS mixer output signal is fed via a high/low-pass combination internally to the FM demodulator as well as via an operational
amplifier to the intercarrier output pin 12.
5-2-11 Single reference QSS mixer
The amplitude modulated SIF amplifier output signal is fed both to a two-stage limiting amplifier for removing the AM and to a linear
multiplier. The result of multiplication of the SIF signal with the limiter output signal is AM demodulation (passive synchronous
demodulator). The demodulator output signal is fed via a low-pass filter for attenuation of the carrier harmonics and via the input
amplifier of the SIF-AGC detector to the audio amplifier.
The intercarrier signal from the intercarrier mixer is fed to the input of an AC coupled gain controlled amplifier with two stages.
The gain controlled output signal is fed to the phase detector of the narrow-band FM-PLL (FM demodulator). For good selectivity
and robustness against disturbance caused by the video signal, high linearity of the gain controlled FM amplifier and of the phase
detector as well as constant signal level are required. Gain control is done by means of an 'in phase' demodulator for the FM carrier
(from the output of the FM amplifier). The demodulation output is fed into a comparator for charging/discharging the integrated AGC
capacitor. This leads to a mean value AGC loop to control the gain of FM amplifier.
Narrow-band PLL
The FM demodulator is realized as a narrow-band PLL with external loop filter, which provides the necessary selectivity (bandwidth
about 100 kHz). To achieve good selectivity, a linear phase detector and constant input level are required. The gain controlled intercarrier
signal from the FM amplifer is fed to the phase Detector (PD). The PD controls via the loop filter the integated low radiation relaxation
oscillator. The designed frequency range is from 4 to 7 MHz.
The VCO within the FM-PLL is phase-locked with the incoming 2nd SIF. This 2nd SIF is frequency modulated (FM). On to this fact,
the VCO control voltage is superimposed by the AF voltage. Therefore the VCO tracks with the FM of the 2nd SIF. So the AF voltage
is present at the loop filter (about 5 mV RMS for 27 kHz FM deviation). This AF signal is fed via a buffer to the audio amplifier.
The correct locking of the PLL is supported by the digital acquisition help circuit (see Section "Digital acquisition help and AFC").
5-2-12 AM demodulator
Gain controlled FM amplifier and detector
5-2-13 FM demodulator and acquisition help
The audio amplifier and mute time constant
5-2-14 Audio amplifier and mute time constant
1. The AF preamplifier used for FM sound is an operational amplifier with internal feedback, high gain and high common mode
rejection. The AF voltage from the PLL demodulator (5 mV RMS for frequency deviation of 27 kHz) is amplified by 30 dB.
By use or a DC operating point control circuit (with external capacitor C
AF
), the AF preamplifier is decoupled from the PLL DC
voltage. The low-pass characteristic of the amplifier reduces the harmonics of the sound intercarrier signal at the AF output terminal.
For FM sound a switchable de-emphasis network (with external capacitor) is implemented between the peramplifier and the output
amplifier.
2. The AF output amplifier provides the required AF output level by a rail-to-rail output stage. A preceding stage makes use of an
input selector for switching between FM sound. AM sound and mute state. The gain can be switched betweem 10 dB (normal)
and 4 dB (reduced).
Switching to mute state is controlled automatically dependent on the digital acquisition help for the case the VCO of the FM-PLL
is not in the required frequency window. This is done by a time constant (fast for switching to mute state and slow (about 40 ms)
for switching to no-mute state)).
All switching functions are controlled via the I
2
C-bus :
AM sound, FM sound, forced mute
Auto mute enable, auto mute disable
De-emphasis off, de-emphasis on, 50 s , 75 s
Audio gain normal, audio gain reduced.