SL1714CKG 查看數據表（PDF） - Mitel Networks

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SL1714CKG

Quadrature Downconverter

Mitel Networks 

SL1714

APPLICATION NOTES

These application notes should be read in conjunction with

circuit diagrams contained in Fig. 7 and 10, and a

recommended front end tuner solution contained in Fig. 12.

These boards have been designed to demonstrate

performance and to allow for initial evaluation of the SL1714.

Varator Tuned Oscillator

Refer to Fig. 7 circuit diagram and Figs. 8 and 9PCB layout.

This application uses a synthesised VCO with a tuning

range of 460 MHz to 500 MHz. The surface mount inductor L1

is 12 nH. The VCO frequency is controlled by the SP5611

synthesiser which is programmed via an I2C bus. The RF input

to the synthesiser is from the SL1714 prescaler outputs

coupled via RF inductors L3 and L4.

For functional checking the VCO can be tuned by

physically shorting the base of transistor T3 to ground and

then adjusting the +30 volt supply to tune the VCO. Under

these conditions, due to the unlocked state of the LO, the

board WILL NOT BE representative of locked gain and phase

match or phase noise performance.

In real applications the VCO control voltage will be

provided by the QPSK demodulator circuit, such as the

VP305. This circuit provides a line voltage to align the

reference LO in the SL1714 in both frequency and phase to the

centre of the modulation bandwidth, normally 402.75 or 479.5

MHz.

As in all feedback loops the bandwidth of the varactor line

must be optimised for the symbol rate of the received

modulation.

It is recommended for optimum performance that the VCO

application is implemented symmetrically, in presented drive

and impedance to the VCO ports, as demonstrated in the

evaluation schematic and PCB.

In the recommended application the varactor diodes are

referenced to the VCO port DC bias voltages. This limits the

minimum tuning voltage on the varactor line to 3V. If lower

tuning voltage is required the tank can be AC coupled to the

VCO ports by 390pF capacitor and a DC reference voltage for

the varactor diodes applied by centre tapping the tank

inductors. NB the varactor diodes require a minimum of 1V

reverse bias for correct operation.

In real applications the maximum tuning range required for

the VCO will be determined by the required lock range of the

tuner and the manufacturing tolerance of the tank, assuming

the quadrature downconverter section will be alignment free.

This tuning range will typically be much smaller than the

demonstration board, which will consequently improve the

VCO phase noise performance.

This application can be ported direct to real system

implementations. Normal good RF practice must be applied to

the layout implementation.

Prescaler Outputs (varactor tuned VCO)

The VCO frequency divided by 32 is available at the

differential prescaler outputs, pins 10 and 11.

These enable the VCO frequency to be synthesised by a PLL

frequency synthesiser; on the demo board an SP5611 is used

for this function however in a real application this function will

be provided by the QPSK demodulator function contained in

for example the VP305.

It is recommended that the prescaler outputs are loaded

symmetrically to balance radiation effects.

Saw Resonator Oscillator

Refer to Fig. 10 circuit diagram and Figs 11 and 12 PCB

layout.

In the standard application the oscillator uses a varactor

diode tuned tank circuit which allows fine tuning of the

oscillator frequency via a voltage control line. This control

voltage is usually derived from the QPSK/FEC decoder

VP305/VP306.

Certain applications do not require this fine tune facility so

a fixed frequency application using a SAW resonator has been

developed. In this application the frequency of the oscillator is

determined by the SAW resonator. The SAW is AC coupled

into the VCO pins of the device pins 13 and 14 via 100pF

coupling capacitors.

The SAW resonator used in this application is a ;

Murata Part No SAR479.45MB10X200

Prescaler Outputs (SAWR tuned VCO)

The VCO frequency divided by 32 is available at the

differential prescaler outputs, pins 10 and 11. Normally these

outputs will not be required since the derotation and fine tuning

required will be processed by the QPSK demodulator.

However these frequencies could be used if required for other

system reference frequencies or clocks.

If used it is recommended that the prescaler outputs are

loaded symmetrically to balance radiation effects.

VCO Disable

The on-chip oscillator can be disabled by connecting

VCODIS, pin 15, to ground and enabled by connecting to Vcc

via a 4K7 pull up resistor.

AGC

The AGC facility can be used to control the conversion gain

of the SL1714.

On the demonstration boards the conversion gain is

adjusted by means of a potentiometer, which is set to 2.5V so

giving a conversion gain of 40 dB. The voltage adjustment

range for the AGC is approximately 0.5 to 4.5 V.

It is important that the AGC voltage minimum does not give

a conversion gain of greater than 44dBs otherwise the

channel amplitude match may be degraded. In real

applications the AGC can be either set at a fixed control

voltage or controlled by means of the AGC control signal from

the QPSK demodulator dependant on the overall dynamic

range requirement of the tuner and it’s gain distribution.

11

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