3

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 1K

Ω 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 K

Ω 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.