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SL1714C Arkusz danych(PDF) 11 Page - Mitel Networks Corporation |
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SL1714C Arkusz danych(HTML) 11 Page - Mitel Networks Corporation |
11 / 20 page 11 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. |
Podobny numer części - SL1714C |
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Podobny opis - SL1714C |
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