tda9817t NXP Semiconductors, tda9817t Datasheet - Page 7

• Manufacturer Part Number: tda9817t
• Description: Single/multistandard Vif/sif-pll And Fm-pll/am Demodulators
• Manufacturer: NXP Semiconductors

Philips Semiconductors
FUNCTIONAL DESCRIPTION
The integrated circuit comprises the functional blocks as
shown in Fig.1:
Vision IF amplifier and VIF AGC detector
The vision IF amplifier consists of three AC-coupled
differential amplifier stages. Each differential stage
comprises a feedback network controlled by emitter
degeneration.
The AGC detector generates the required VIF gain control
voltage for constant video output by charging/discharging
the AGC capacitor. Therefore for negative video
modulation the sync level and for positive video
modulation the peak white level of the video signal is
detected. In order to reduce the reaction time for positive
modulation, where a very large time constant is needed,
an additional level detector increases the discharging
current of the AGC capacitor (fast mode) in the event of a
decreasing VIF amplitude step. The additional level
information is given by the black-level detector voltage.
Tuner AGC
The AGC capacitor voltage is converted to an internal
IF control signal, and is fed to the tuner AGC to generate
the tuner AGC output current at pin TAGC (open-collector
output). The tuner AGC takeover point can be adjusted at
pin TADJ. This allows to match the tuner to the SAW filter
in order to achieve the optimum IF input level.
Frequency Phase Locked Loop detector (FPLL)
The VIF-amplifier output signal is fed into a frequency
detector and into a phase detector via a limiting amplifier.
During acquisition the frequency detector produces a
DC current proportional to the frequency difference
between the input and the VCO signal. After frequency
lock-in the phase detector produces a DC current
2004 Jun 29
Vision IF amplifier and VIF AGC detector
Tuner AGC
Frequency Phase Locked Loop detector (FPLL)
VCO, Travelling Wave Divider (TWD) and AFC
Video demodulator and amplifier
Sound IF amplifier and SIF AGC
Single reference QSS mixer
AM demodulator
FM-PLL demodulator
Audio Frequency (AF) signal processing
Internal voltage stabilizer.
Single/multistandard VIF/SIF-PLL and
FM-PLL/AM demodulators
7
proportional to the phase difference between the VCO and
the input signal. The DC current of either frequency
detector or phase detector is converted into a DC voltage
via the loop filter, which controls the VCO frequency. In the
event of positive modulated signals the phase detector is
gated by composite sync in order to avoid signal distortion
for overmodulated VIF signals.
VCO, Travelling Wave Divider (TWD) and AFC
The VCO operates with a resonance circuit (with L and C
in parallel) at double the picture carrier frequency. The
VCO is controlled by two integrated variable capacitors.
The control voltage required to tune the VCO from its
free-running frequency to actually double the picture
carrier frequency is generated by the frequency-phase
detector (FPLL) and fed via the loop filter to the first
variable capacitor. This control voltage is amplified and
additionally converted into a current which represents the
AFC output signal. At centre frequency the AFC output
current is equal to zero.
For TDA9818: the VCO centre frequency can be
decreased (required for L accent standard) by activating
an additional internal capacitor. This is achieved by using
the L accent switch. In this event the second variable
capacitor can be controlled by a variable resistor at the
L accent switch for setting the VCO centre frequency to the
required L accent value.
The oscillator signal is divided by 2 with a TWD which
generates two differential output signals with a 90 degree
phase difference independent of the frequency.
Video demodulator and amplifier
The video demodulator is realized by a multiplier which is
designed for low distortion and large bandwidth. The
vision IF input signal is multiplied with the ‘in phase’ signal
of the travelling wave divider output. In the demodulator
stage the video signal polarity can be switched in
accordance with the TV standard.
The demodulator output signal is fed via an integrated
low-pass filter for attenuation of the carrier harmonics to
the video amplifier. The video amplifier is realized by an
operational amplifier with internal feedback and high
bandwidth. A low-pass filter is integrated to achieve an
attenuation of the carrier harmonics for B/G and
L standard. The standard dependent level shift in this
stage delivers the same sync level for positive and
negative modulation. The video output signal at pin CVBS
is 1.1 V (p-p) for nominal vision IF modulation, in order to
achieve 1 V (p-p) at sound trap output.
TDA9817; TDA9818
Product specification