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MC33696FCAE Datasheet(PDF) 9 Page - Freescale Semiconductor, Inc |
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MC33696FCAE Datasheet(HTML) 9 Page - Freescale Semiconductor, Inc |
9 / 64 page Frequency Planning MC33696 Data Sheet, Rev. 9 Freescale Semiconductor 9 10 Frequency Planning 10.1 Clock Generator All clocks running in the circuit are derived from the reference frequency provided by the crystal oscillator (frequency fref, period tref). The crystal frequency is chosen in relation to the band in which the MC33696 has to operate. Table 4 shows the value of the CF bits. 10.2 Intermediate Frequency The IF filter is controlled by the crystal oscillator to guarantee the frequency over temperature and voltage range. The IF filter center frequency, FIF, can be computed using the crystal frequency fref and the value of the CF bits: • If CF[0] = 0 : FIF = fref/9*1.5/2 • If CF[0] = 1 : FIF = fref/12*1.5/2 The cut-off frequency given in the parametric section can be computed by scaling to the FIF. Example 1. Cut-off Frequency Computation Compute the low cut-off frequency of the IF filter for a 16.9683 MHz crystal oscillator. For this reference frequency, FIF = 1.414 MHz. So, the 1.3751 MHz low cut-off frequency specified for a 1.5 MHz IF frequency becomes 1.3751*1.414/1.5 = 1.296 MHz 10.3 Frequency Synthesizer Description The frequency synthesizer consists of a local oscillator (LO) driven by a fractional N phase locked loop (PLL). The LO is an integrated LC voltage controlled oscillator (VCO) operating at twice the RF frequency (for the 868 MHz frequency band) or four times the RF frequency (for the 434 MHz and 315 MHz frequency bands). This allows the I/Q signals driving the mixer to be generated by division. Table 4. Crystal Frequency and CF Values Versus Frequency Band RF Frequency (MHz) CF1 CF0 LOF1 LOF0 FREF (Crystal Frequency) (MHz) FIF (IF Frequency) (MHz) Dataclk Divider Fdataclk (kHz) Digclk Divider Fdigclk (kHz) Tdigclk (µs) 304 0 0 0 0 16.96745 1.414 60 282.791 30 565.582 1.77 315 0 0 1 0 17.58140 1.465 60 293.023 30 586.047 1.71 426 0 1 0 1 23.74913 1.484 80 296.864 40 593.728 1.68 433.92 0 1 0 1 24.19066 1.512 80 302.383 40 604.767 1.65 868.3 1 1 0 1 24.16139 1.510 80 302.017 40 604.035 1.66 916.5 1 1 1 1 25.50261 1.594 80 318.783 40 637.565 1.57 1. Refer to parameter 3.3 found in Section 19.1, “General Parameters.” |
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