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MRF89XA Folha de dados(PDF) 16 Page - Microchip Technology |
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MRF89XA Folha de dados(HTML) 16 Page - Microchip Technology |
16 / 140 page MRF89XA DS70000622D-page 16 Preliminary 2010-2017 Microchip Technology Inc. 2.4.4 IF GAIN AND SECOND I/Q MIXER Following the LNA and first down-conversion, there is an IF amplifier whose gain can be programmed from 13.5-0 dB in 4.5 dB steps, through the register DMODREG. For more information, refer to Section 2.14.2, Data And Modulation Configuration Register Details. The default setting corresponds to 0 dB gain, but lower values can be used to increase the RSSI dynamic range. 2.4.5 CHANNEL FILTERS The second mixer stages are followed by the channel select filters. The channel select filters have a strong influence on the noise bandwidth and selectivity of the receiver, and therefore, its sensitivity. Each channel select filter features a passive second-order RC filter, with a programmable bandwidth, and the “fine” channel selection is performed by an active, third-order, Butterworth filter, which acts as a low-pass filter for the zero-IF configuration (FSK), or a complex polyphase filter for the low-IF (OOK) configuration. For more information on configuring passive and active filters see Section 3.4.4, Channel Filters. 2.5 Frequency Synthesizer Block The frequency synthesizer of the MRF89XA is a fully integrated integer-N type PLL. The crystal oscillator provides the reference frequency for the PLL. The PLL circuit requires only a minimum of five external components for the PLL loop filter and the VCO tank circuit. Figure 2-4 illustrates a block schematic of the MRF89XA PLL. The crystal reference frequency and the software controlled dividers R, P, and S blocks determine the output frequency of the PLL. The VCO tank inductors are connected to an external differential input. Similarly, the loop filter is also located externally. FIGURE 2-4: FREQUENCY SYNTHESIZER BLOCK DIAGRAM 2.5.1 REFERENCE OSCILLATOR PINS (OSC1/OSC2) The MRF89XA has an internal, integrated oscillator circuit, and the OSC1 and OSC2 pins are used to connect to an external crystal resonator. The crystal oscillator provides the reference frequency for the PLL. The crystal oscillator circuit, with the required loading capacitors, provides a 12.8-MHz reference signal for the PLL. The PLL then generates the local oscillator frequency. It is possible to “pull” the crystal to the accurate frequency by changing the load capacitor value. The crystal oscillator load capacitance is typically 15 pF, which allows the crystal oscillator circuit to accept a wide range of crystals. Choosing a higher tolerance crystal results in a lower TX to RX frequency offset and the ability to select a smaller deviation in baseband bandwidth. Therefore, the recommended crystal accuracy should be 40 ppm. The guidelines for selecting the appropriate crystal with specifications are explained in Section 4.7, Crystal Specification and Selection Guidelines. PFD XO (Ri + 1) VCORS OSC1 OSC2 PLLP PLLN VCOTN VCOTP LO Vtune FCOMP 75 * (Pi + 1) + Si MRF89XA Note: Crystal frequency error directly translates to carrier frequency (frf), bit rate, and frequency deviation error. |
Nº de peça semelhante - MRF89XA |
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Descrição semelhante - MRF89XA |
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