tda9880t NXP Semiconductors, tda9880t Datasheet - Page 8

• Manufacturer Part Number: tda9880t
• Description: Alignment-free Multistandard Vision And Fm Sound If-pll Demodulator
• Manufacturer: NXP Semiconductors

Philips Semiconductors
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. The video signal of
1.1 V (p-p) for nominal vision IF modulation is fed
internally to the integrated sound carrier trap as well as to
the VIF-AGC detector. The second stage of the video
amplifier converts and amplifies the differential output
signal from the sound carrier trap to the single-ended
CVBS output signal at pin 13 with a 2 V (p-p) amplitude.
Noise clipping is provided. Furthermore the trap can be
bypassed by the implemented input switch of the second
amplifier stage, forced by connecting pin 12 to ground.
Sound carrier trap
The sound carrier trap consists of a reference filter, a
phase detector and the sound trap itself.
A sound carrier reference signal is fed into the reference
low-pass filter and is shifted by a nominal 90 degrees.
The phase detector compares the original reference signal
with the signal shifted by the reference filter and produces,
at the external capacitor C
discharging the capacitor with a current proportional to the
phase difference between both signals, respectively to the
frequency error of the integrated filters. The DC voltage is
converted to currents which control the frequency position
of the reference filter and the sound trap.
The sound trap itself is constructed of three separate traps
to realize sufficient suppression of the first and second
sound carrier. The right frequency position of the different
standards is set by the sound carrier reference signal.
Intercarrier mixer
The intercarrier mixer is realized by a multiplier, operating
in quadrature mode for suppression of low frequency video
signals. The VIF amplifier output signal is fed to the
intercarrier mixer and converted to an intercarrier
frequency by the regenerated 90 degree picture carrier
from the VCO. The mixer output signal is fed via a
band-pass filter and amplifier for attenuation of the high
frequency video signal components and carrier harmonics
to the output pin 11. The intercarrier signal is fed also to
the integrated FM demodulator.
2000 Nov 22
Alignment-free multistandard vision and
FM sound IF-PLL demodulator
TR
, a DC voltage by charging or
8
FM demodulator and acquisition help
The FM demodulator is realized as a narrow-band PLL
with external loop filter, which provides the necessary
selectivity. To achieve good selectivity, a linear phase
detector and constant input level are required. The
intercarrier signal from the intercarrier mixer is fed via a
gain controlled amplifier to the phase detector and it’s
output signal controls (via the loop filter) the integrated
relaxation oscillator. The possible frequency range is from
4 to 7 MHz. As a result of locking the oscillator frequency
tracks with the FM modulation of the input signal;
therefore, the oscillator control voltage is superimposed by
the AF voltage. In this way the FM-PLL operates as an
FM demodulator. The AF voltage is present at the loop
filter and is fed via a buffer with 0 dB gain to the audio
amplifier.
The digital acquisition help operates in the same way as
described in Section “Digital acquisition help and AFC”.
Audio amplifier
The audio amplifier consists of two parts:
1. The AF preamplifier is an operational amplifier with
2. The AF output amplifier (10 dB) provides the required
Internal voltage stabilizer
The band gap circuit internally generates a voltage of
approximately 2.4 V, independent of the supply voltage
and the temperature. A voltage regulator circuit, controlled
by this voltage, produces a constant voltage of 3.55 V
which is used as an internal reference voltage.
internal feedback, high gain and high common mode
rejection. The AF voltage from the PLL demodulator,
by principle a small output signal, is amplified by
30 dB. A DC operating point control circuit (pin 6)
decouples the AF amplifier from the DC voltage of the
PLL. The low-pass characteristic of the amplifier
reduces the harmonics of the intercarrier signal at the
sound output terminal. If required, a de-emphasis
network can be realized by the amplifier output
resistance and an external capacitor.
output level by a rail-to-rail output stage. This amplifier
makes use of an input selector for switching to mute
state, automatically controlled by the mute switching
voltage from the digital acquisition help in order to
avoid lock-in noise. During normal operation the
automatic audio mute function is not active.
Application of a 2.2 k resistor between the
intercarrier output (pin 11) and ground will activate the
automatic audio mute function.
Product specification
TDA9880T