Service Manual CP-099F
5-2-11 Single reference QSS mixer
With the present system a high performance Hi-Fi stereo sound processing can be achieved. aFor a simplified application without
2
Sound IF SAW filter the single reference QSS mixer can be switched C-bus. to the This intercarrier single reference mode by QSSI
mixer generates the 2nd FM TV sound intercarrier signal. It is realized by a linear ermultiplier Which multiplies the SIF amplifi
output signal and the VIF VCO signal (90 degrees output), which is locked to the picture ratescarrier. By this way the QSS mixer ope
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 aldemodulator as well as via an operation
amplifier to the intercarrier output pin 12.
5-2-12 AM demodulator
The amplitude modulated SIF amplifier output signal is fed both to a two-stage limiting linearamplifier for removing the AM and to a
multiplier. The result of multiplication of the SIF signal with the limiter output signal usis AM demodulation (passive synchrono
demodulator). The demodulator output signal is fed via a low-pass filter for attenuation nput of the carrier harmonics and via the i
amplifier of the SIF-AGC detector to the audio amplifier.
5-2-13 FM demodulator and acquisition help
Gain controlled FM amplifier and detector
The intercarrier signal from the intercarrier mixer is fed to the input of an AC coupled ges. gain controlled amplifier with two sta
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 asegain controlled FM amplifier and of the ph
detector as well as constant signal level are required. Gain control is done by means arrierof an 'in phase' demodulator for the FM c
(from the output of the FM amplifier). The demodulation output is fed into a comparator AGCfor charging/discharging the integrated
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 dththe necessary selectivity (bandwi
about 100 kHz). To achieve good selectivity, a linear phase detector and constant input intercarrierlevel are required. The gain controlled
signal from the FM amplifer is fed to the phase Detector (PD). The PD controls via trelaxationhe loop filter the integated low radiation
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 gethe FM of the 2nd SIF. So the AF volta
is present at the loop filter (about 5 mV RMS for 27 kHz FM deviation). This AF signal er.is fed via a buffer to the audio amplifi
The correct locking of the PLL is supported by the digital acquisition help circuit AFC").(see Section "Digital acquisition help and
5-2-14 Audio amplifier and mute time constant
The 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.
Buse y or a DC operating point control circuit (with AF), external the AF preamplifier capacitor C is decoupled from the PLL DC
voltage. The low-pass characteristic of the amplifier reduces the harmonics of the sound erminal.intercarrier signal at the AF output t
For sound FM a switchable de-emphasis network (with external capacitor) is implemented between the peramplifier and the output
aplifier.m
2. The AF output amplifier provides the required AF output level by a rail-to-rail output an stage. A preceding stage makes use of
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 mutcontrolled e state is automatically dependent on the digital acquisition help for the case the VCO of the FM-PLL
inot s 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)).
2
All switching functions are controlled C-bus :via the I
Asound, M FM sound, forced mute
Auto mute enable, auto mute disable
De-emphasis off, de-emphasis s ,on, 7550 s
Audio gain normal, audio gain reduced.
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ACEC
