U4488B Datasheet, PDF (2/9 Pages) TEMIC Semiconductors – Quasi-Split Sound Circuit and AM Demodulator

U4488B
Circuit Description
IF Amplifier and AGC
The symmetrical IF input signal from the external SAW
filter is fed to the Pins 1 and 2 resp. Pins 15 and 16. For
FM/NICAM application, a SAW filter with double
bandpass characteristic is required for the vision and
sound carrier. In case of L-standard, a bandpass filter with
the center frequency at the L-sound carrier is used.
The amplifier consists of three AC-coupled wideband IF
stages. Each differential amplifier 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
wideband amplifier to provide a constant SIF signal to the
AM demodulator or to the QSS (quasi-split sound) mixer.
IF Input Switch
If Pin 11 is open, the IF input 1 is switched on (Pins 1
and 2). If Pin 11 is connected to ground, the IF input 2 is
switched on (Pins 15 and 16).
Standard Switch
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 is a peak value detector.
Internal Voltage Stabilizer
The internal bandgap reference voltage (4.25 V at Pin 4)
ensures constant performance, 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 parallel) and is controlled by integrated
varicaps.
A practicable VCO alignment of the external coil is the
adjustment of the loop filter voltage (Pin 5) to 2.3 V. At
this value, capture and hold range are centered.
AM Demodulator
The alignment-free AM demodulator is realized by a
synchronous detector. The modulated IF signal from the
wideband 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 in 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
AM sound output (Pin 6) can be muted in pulling mute
switch (Pin 10) to ‘low’.
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TELEFUNKEN Semiconductors
Preliminary Information
Rev. A2, 15-Jan-98