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FS6011-02 bảng dữ liệu(PDF) 5 Page - List of Unclassifed Manufacturers |
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FS6011-02 bảng dữ liệu(HTML) 5 Page - List of Unclassifed Manufacturers |
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5 / 15 page ![]() XT July 1998 5 7.20.98 )6 'LJLWDO $XGLR9LGHR &ORFN *HQHUDWRU ,& ,62 4.5 Reference Frequencies (CLK27) The CLK27 output frequency is controlled by register bit D[8] that selects either the VCXO reference frequency or the UCLK frequency. Table 7: CLK27 Frequency Select D[8] CLK27 Output 0 VCXO Frequency 1 UCLK Frequency 4.6 VCXO Coarse Tuning and Enable The VCXO may be coarse tuned by a programmable ad- justment of the crystal load capacitance via D[12:9]. The actual amount of frequency warping caused by the tuning capacitance will depend on the crystal used. The VCXO tuning capacitance includes an external 6pF load ca- pacitance (12pF from the XIN pin to ground and 12pF from the XOUT pin to ground). The fine tuning capability of the VCXO can be enabled by setting D[13] to a logic-one or disabled by clearing the bit to a logic-zero. Table 8: VCXO Tuning Capacitance D[12] D[11] D[10] D[9] VCXO TUNING CAPACITANCE (pF) 0 0 0 0 10.00 0 0 0 1 10.84 0 0 1 0 11.69 0 0 1 1 12.53 0 1 0 0 13.38 0 1 0 1 14.22 0 1 1 0 15.06 0 1 1 1 15.91 1 0 0 0 16.75 1 0 0 1 17.59 1 0 1 0 18.43 1 0 1 1 19.28 1 1 0 0 20.13 1 1 0 1 20.97 1 1 1 0 21.81 1 1 1 1 22.66 Figure 4 shows the typical effect of the coarse and fine tuning mechanisms. The difference in VCXO frequency in parts-per-million (ppm) is shown as the fine tuning volt- age on the XTUNE pin varies from 0V to 5V. The coarse tune range as shown is about 350ppm. As the crystal load capacitance is increased (with increasing Coarse Tune setting) the frequency is pulled somewhat less with each coarse step and the fine tuning range decreases. The fine tuning range always overlaps a few coarse tun- ing ranges, eliminating the possibility of holes in the VCXO response. Note that different crystal warping char- acteristics will change the scaling on the Y-axis, but not the overall characteristic of the curves. Figure 4: VCXO Coarse and Fine Tuning VCXO Range (ppm) vs. XTUNE Voltage (V) -200 -150 -100 -50 0 50 100 150 200 0123 45 6789 10 11 12 13 14 15 Coarse Tune Setting D[11:14] XTUNE Volt age = 0.0V XTUNE Volt age = 5.0V |
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