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AD9577BCPZ-R7 数据表(PDF) 36 Page - Analog Devices |
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AD9577BCPZ-R7 数据表(HTML) 36 Page - Analog Devices |
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36 / 44 page  AD9577 Data Sheet Rev. 0 | Page 36 of 44 Table 27. CkDiv and FracStep Values Used in Worked Example CkDiv Ideal FracStep Rounded FracStep Table 26 shows the relevant register names and programmable ranges. Table 26. Registers Used to Program SSCG Operation Parameter FracStep Error 2 −1.5675 −2 21.6% Register Name Range NumSteps 3 −2.35125 −2 17.6% BS2[7:0], BS3[7] +1 to +511 FracStep 4 −3.135 −3 4.5% BS1[7:0] −128 to 0 5 −3.91875 −4 2.0% CkDiv BS3[6:0] +2 to +127 6 −4.7025 −5 6.0% 7 −5.48625 −5 9.7% Because the register values need to be expressed as integers, there are no guaranteed exact solutions; therefore, some approximations and trade-offs must be made. The fact that neither FracRange nor fMOD needs to be exact is exploited. Note that the SSCG pin must be toggled every time the SSCG parameters are adjusted for the changes to take effect. 8 −6.27 −6 4.5% 9 −7.05375 −7 0.77% 10 −7.8375 −8 2.0% Both CkDiv and NumSteps must be integers. To minimize error, CkDiv = 9 and FracStep = −7 was chosen. With a target for FracRange = −313.5, Equation 12 is used to find the ideal value of NumSteps = 44.79, which is rounded to 45. From Equation 12, the actual used value for FracRange is Worked Example: Programming for fMOD = 31.25 kHz, Downspread = −0.5%, fPFD = 25 MHz Assume Nb = 100, MOD = 625, and FRAC = 198. In addition, a large number of frequency steps are desired to cover −0.5%. The objective is to find values for FracStep, NumSteps, and CkDiv that result in the required frequency modulation profile. FracRange = −7 × 45 = −315 The accuracy of this solution needs to be verified. Putting the derived values into Equation 13 gives The total feedback divider ratio is kHz 30.86 9 45 2 MHz 25 2 = × × = × × = CkDiv NumSteps f f PFD MOD NTOT = Nb + MOD FRAC = 100 + 198/625 = 62,698/625 FracRange is set to −0.5% of 62,698, which results in an ideal value of −313.5. By rearranging Equation 12 and Equation 13, it results in ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ × × × = PFD MOD f f FracRange CkDiv FracStep 2 (14) In addition, the percentage frequency deviation is obtained as % 502 . 0 625 62698 625 315 100 100 − = × − × = × × = TOT N MOD FracRange eviation FrequencyD The fMOD and the percentage frequency deviation are very close to the target values. The register settings required for this example are detailed in Table 29. Putting in the values for FracRange, fMOD, and fPFD from the previous information, the following results: FracStep = CkDiv × (−0.78375) (15) SSCG Register Summary An approximate solution must be found to Equation 15 that produces an integer value for CkDiv, which gives a value that is very close to an integer for FracStep. In this case, considering CkDiv values in the range of 2 to 10 gives the FracStep values shown in Table 27. Table 28 summarizes the programmable registers required to set up SSCG. Table 28. Register Values for SSCG Parameter Register Names Range NumSteps BS2[7:0], BS3[7] +1 to +511 FracStep BS1[7:0] −128 to 0 CkDiv BS3[6:0] +2 to +127 FRAC BF0[7:0], BF1[7:4] 0 to +4094 MOD BF1[3:0], BF2[7:0] 0 to +4095 Nb BF3[5:0] 0 to +51 |
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