Philips Semiconductors
Product specification
DVB selective AGC amplifier
TDA9888TS; TDA9889TS
Reference generation
Tracking low-pass filter with reference control
The 4 MHz crystal is the reference for the downconversion
synthesizer and the filter synthesizer.
The low-pass filter is controlled by a reference signal
[generated by a frequency synthesizer and can be
switched by the standard selection (S0 and S1)].
The downconverted DVB frequency and the frequency
adjustment of the integrated filters are dependent on the
precision of the reference signal at pin REF. An operation
as crystal oscillator is possible as well as connecting this
input via a serial capacitor to another low-ohmic reference
frequency source.
The tracking low-pass filter is designed as a
Tschebyscheff filter to guarantee sufficient suppression for
the neighbouring picture carrier. An all-pass filter corrects
the characteristic to obtain a flat (equivalent ripple) group
delay behaviour.
The integrated divider-by-8 generates the internally
needed 500 kHz reference frequency for the DVB
synthesizer and the reference filter.
Low IF differential output stage
The output amplifier consists of two identical differential
operational amplifiers with a high common mode (DC)
rejection ratio, to provide a constant DC voltage at the
output stage. The amplified Low IF (LIF) signal is available
as a differential signal with an amplitude of 2 V (p-p) and
DC level of 2 V (typ.) between the output terminals of both
amplifiers.
Synthesizer for downconversion
The PLL synthesizer for downconversion consists of a
Voltage Controlled Oscillator (VCO), a divider with a
standard dependent divider factor, a frequency phase
detector (constructed as a digital 3-state comparator) and
a charge pump.
Logic control
The VCO operates as an integrated low radiation
relaxation oscillator at double the conversion frequency.
For downconversion the VCO frequency is divided-by-2 to
provide two differential square wave signals with exactly
90 degrees phase difference, independent of the VCO
frequency.
The logic control provides an easy selection of the most
common standards for Europe, USA and Japan with the
two switches S0 and S1 (frequency selection different for
TDA9888TS and TDA9889TS) and controls all internal
standard dependent stages.
The five available tuner AGC modes can be set via the
3-state input pin S2 and RTOP connection (at pin TADJ).
The frequency phase detector compares the down-divided
oscillator signal with the 500 kHz reference frequency
signal and controls, via the charge pump, the control
voltage at the external loop filter connected to pin LFS,
which is required to tune the VCO exactly to the wanted
frequency.
Internal voltage stabilizer
The band gap circuit 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.
The divider, the frequency phase detector and the
reference frequency divider are constructed in a low
radiation technique to optimize the synthesizer spurious
suppression in the downconverted output signal.
Downconversion and complex filtering
The gain controlled IF signal from the IF amplifier is fed to
two identical linear mixers, operating in the ‘in phase’ and
‘quadrature’ mode in accordance to the 0 and 90 degree
VCO signal from the synthesizer. With this, the
downconverted DVB low IF signal is available as an
I and Q signal in the frequency band from 1 to 9 MHz.
These signals are fed to the complex filter section. As a
result of adding both signals, the unwanted mirror signal
will be attenuated (approximately 34 dB). The unwanted
frequency components below 1 MHz are attenuated in a
high-pass filter. This signal is then fed through a
group-delay equalizer circuit.
2004 Nov 02
6
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