SL1714
FUNCTIONAL DESCRIPTION
The SL1714 is a wideband quadrature downconverter,
optimised for application in both professional and consumer
digital satellite receiver systems and requiring a minimum
external component count. It contains all the elements
required for construction of a quadrature demodulator, with
the exception of tank circuit for the local oscillator.
A block diagram is shown in Fig. 2.
The SL1714 oscillator can be used with either a varactor
tuned tank circuit or with a SAW resonator. Both
configurations are described in the Application Notes section
of this Data Sheet.
A typical digital satellite tuner application from tuner input
to data transport stream is shown in Fig. 13.
In normal applications the second satellite IF frequency of
typically 402.75 or 479.5 MHz is fed from the tuner SAW filter
to the RF preamplifier, which is optimised for impedance
match and signal handling. The amplifier output signal is then
split into two balanced channels to drive the In-phase and
Quadrature mixers. The typical RF input impedance is shown
in Fig. 3.
In-phase and Quadrature LO signals for the mixers are
derived from the on board local oscillator, which uses an
external varactor tuned resonant network and is optimised for
low phase noise. The VCO also drives an on board divide by
32 prescaler whose outputs can be used for driving an external
PLL control loop for the VCO, where the PLL loop is contained
within the QPSK demodulator, for example the VP305. For
optimum performance in the varactor tuned application the
VCO should be fully symmetric. The VCO has a disable facility
by grounding pin 15, VCODIS; in normal application this pin is
pulled to Vcc via a 4K7 resistor.
The mixer outputs are fed to balanced baseband AGC
amplifier stages, which provide for a minimum of 12 dB of AGC
control. The typical AGC characteristic is shown in Fig. 4.
These amplifiers then feed a low output impedance true
differential to single-ended converter output stage. In normal
application the output can be either directly AC coupled to the
ADC converter such as the VP216, which will generally have
a high input impedance, or to drive an anti alias filter. In this
later case the maximum load presented to the SL1714 must
not exceed a parallel combination of 1KΩ and 15pF. The
typical baseband output impedance is contained in Fig. 5.
It is recommended that the device is operated with an
output amplitude of 760mV under lock conditions.
Under transient conditions the output should not exceed
the clipping voltage.
Input and output interface circuitry is contained in Fig. 6.
The typical key performance numbers at 480 MHz IF, 5V
Vcc, 1 KΩ load and 25 deg C ambient are contained in table
headed 'QUICK REFERENCE DATA'. With SAWR oscillator
application the gain and phase match performance will
typically exceed these numbers.
3
MITEL [ MITEL NETWORKS CORPORATION ]
