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MRF89XA Datasheet(PDF) 61 Page - Microchip Technology |
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MRF89XA Datasheet(HTML) 61 Page - Microchip Technology |
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61 / 140 page  2010-2017 Microchip Technology Inc. Preliminary DS70000622D-page 61 MRF89XA 3.4 Receiver The MRF89XA is set to Receive mode when the CMOD<2:0> bits (GCONREG<7:5>) are set to ‘011’ (see Register 2-1). The receiver is based on the superheterodyne architecture. (In a superheterodyne architecture, you need to use a saw filter to give better image rejection). The front-end is composed of an LNA and a mixer whose gains are constant. The mixer down-converts the RF signal to an intermediate frequency, which is equal to one-eighth of the LO frequency, which in turn is equal to eight-ninths of the RF frequency. Behind this first mixer is a variable gain IF amplifier that can be programmed from a maximum gain of 13.5-0 dB in steps of 4.5 dB by altering the IFGAIN<1:0> bits (DMODREG<1:0>). After the variable gain IF amplifier, the signal is down- converted into two I and Q baseband signals by two quadrature mixers that are fed by reference signals at one-eighth the LO frequency. These I and Q signals are then filtered and amplified before demodulation. The first filter is a second-order passive R-C filter whose bandwidth can be programmed to 16 values with the PASFILV<3:0> bits (FILCREG<7:4>). The second filter can be configured as either a third-order Butterworth active filter, which acts as a low-pass filter for the zero-IF FSK configuration, or as a polyphase band-pass filter for the low-IF OOK configuration. To select the Butterworth low-pass filter operation, the POLFILEN bit (SYNCREG<7>) is set to ‘0’. The bandwidth of the Butterworth filter can be programmed to 16 values by configuring the BUTFILV<3:0> bits (FILCREG<3:0>). The low-IF configuration must be used for OOK modulation. This configuration is enabled when the POLFILEN bit (SYNCREG<7>) is set to ‘1’. The center frequency (fo) of the polyphase filter can be programmed to 16 values by setting the POLCFV<3:0> bits (PFCREG<7:4>). The bandwidth of the filter can be programmed by configuring the BUTFILV<3:0> bits (FILCREG<3:0>). In OOK mode, the value of the low-IF is equal to the deviation frequency defined in FDEVREG. In addition to the channel filtering, the function of the polyphase filter is to reject the image. Figure 3-5 illustrates the two configurations of the second IF filter. In the Butterworth configuration, FCBW is the 3 dB cut- off frequency. In the polyphase band-pass configuration, FOPP is the center frequency given by the POLCFV<3:0> bits (PFCREG<7:4>), and FCPP is the upper 3 dB bandwidth of the filter whose offset, referenced to FOPP, is given by BUTFILV<3:0> bits (FILCREG<3:0>). 3.4.1 MRF89XA SECOND IF FILTER DETAILS FIGURE 3-5: IF FILTERS IN FSK AND OOK MODES After filtering, the I and Q signals are each amplified by a chain of 11 amplifiers having 6 dB of gain each. The outputs of these amplifiers and their intermediate 3 dB nodes are used to evaluate the received signal strength (RSSI). Limiters are located behind the 11 amplifiers of the I and Q chains, and the signals at the output of these limiters are used by the FSK demodulator. The OOK demodulator uses the RSSI output. The global bandwidth of the entire baseband chain is given by the bandwidths of the passive filter, the Butterworth filter, the amplifier chain, and the limiter. The maximum, achievable global bandwidth when the bandwidths of the first three blocks are programmed at their upper limit is approximately 350 kHz. 3.4.2 LNA AND FIRST MIXER In Receive mode, the RFIO pin is connected to a fixed gain, common-gate, Low Noise Amplifier (LNA). The performance of this amplifier is such that the Noise Figure (NF) of the receiver is estimated to be approximately 7 dB. 3.4.3 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 IFGAIN<1:0> bits (DMODREG<1:0>). The default setting corresponds to 0 dB gain, but lower values can be used to increase the RSSI dynamic range. For more information, refer Section 3.4.7, received signal strength (RSSI). Butterworth Low-Pass Filter for FSK Polyphase Band-Pass Filter for OOK FCBW FCPP FOPP 2 * FOPP – FCPP |
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