U4468B Datasheet, PDF (2/11 Pages) ATMEL Corporation – Quasi-split Sound Circuit and AM Demodulator IC

Circuit Description
IF Amplifier and AGC
Standard Switch
Internal Voltage
Stabilizer
The symmetrical IF input signal from the external SAW filter is fed to the pins 1 and 2. In
case of FM/NICAM applications, a SAW filter with double band-pass characteristic is
required for the vision and sound carrier. In case of the L-standard, a band-pass filter
with the center frequency at the L-sound carrier is used.
The amplifier consists of three AC-coupled wide-band IF stages. Each differential ampli-
fier stage is gain controlled by the AGC (Automatic Gain Control) voltage, available at
pin 3. The output signal of the IF amplifier is then applied to the FPLL carrier generation,
to the mixer for FM/NICAM signals and to the AM demodulator.
The IF-AGC is derived from the average level of the AM carrier (L-standard) or from the
peak value of the FM carrier (e.g., B/G standard). It controls the three-stage wide-band
amplifier to provide a constant SIF signal to the AM demodulator or to the QSS (quasi-
split sound) mixer.
If the standard switch (pin 7) is in position “low” (for AM sound), the AGC is operating as
an average level detector. In position “open” or “high” (for FM/NICAM sound), the AGC
acts as a peak value detector.
The internal band gap reference voltage (4.25 V at pin 4) ensures constant perfor-
mance, independent of supply voltage and temperature.
FPLL, VCO
The FPLL (frequency-phase-locked loop) circuit consists of a frequency detector and a
phase detector to generate a control voltage for the VCO tuning. In locked mode, the
VCO is controlled by the phase detector while in unlocked mode, the frequency detector
is superimposed. The VCO operates with an external resonance circuit (L and C paral-
lel) and is controlled by integrated varicaps.
A practicable VCO alignment of the external coil is the adjustment of the loop filter volt-
age (pin 5) to 2.3 V. At this value, the capture and hold range are centered.
AM Demodulator
The alignment-free AM demodulator is realized by a synchronous detector. The modu-
lated IF signal from the wide-band IF amplifier output is multiplied in phase with the
limited SIF signal to remove the AM. Then, the resulting AF signal of the demodulator
output is fed to the output amplifier and to the AGC stage.
Quasi-Split-Sound (QSS)
Mixer
The QSS mixer is realized by a multiplier circuit. The IF signal (FM/NICAM) is converted
to the intercarrier frequency by means of a quadrature signal from the generated picture
carrier provided by the PLL. The intercarrier signal is fed via an output amplifier to
pin 12.
Mute Switch
The AM sound output (pin 6) can be muted by switching the pulling mute switch (pin 10)
to “low”.
2 U4468B
4777A–TVVCR–01/04