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©2017 Integrated Device Technology, Inc.
October 20, 2017
9FGV1006 Register Descriptions and Programming Guide
Figure 4. Spread Step and Period
The SS modulation period is defined as the amount of time steps it takes for the triangle to move from its lowest to its highest point. The
period is essentially half of the modulation cycle or modulation rate. One time step is defined as one cycle of the output frequency FOUT.
The period register setting needs to be half of the period decimal value, so essentially ¼ of the modulation cycle.
Period (decimal) = FPFD / (2×FSS) (6)
Period [11..0] = DEC2HEX(ROUND2INT(Period(decimal) / 2)) (7)
Given the required spread percentage and the period value, the step size is calculated as:
Step (decimal) = (SS% / 100) × P / Period (8)
Step [15..0] = DEC2HEX(ROUND2INT(224 × Step(decimal))) (9)
Example 1 with down spread (= default configuration)
:
FVCO = 2500MHz, FCLOCK = 100MHz with -0.5% down spread and 31.5kHz modulation rate.
The crystal is 25MHz and the doubler is enabled so FPFD = 25MHz × 2 = 50MHz.
FFD value P = (FVCO / (2 × FPFD)) = (2500 / (2×50)) = 25.
FOD integer [7..0] = DEC2HEX(25) = 19-hex.
FOD fraction [15..0] = DEC2HEX(ROUND2INT(216×0)) = DEC2HEX(ROUND2INT(0)) = 00 00 hex.
Period (decimal) = FPFD / (2×FSS) = 50 / (2×0.0315) = 793.6508.
Period [11:0] = DEC2HEX(ROUND2INT(Period(decimal) / 2)) = DEC2HEX(397) = 1 8D hex.
Step (decimal) = (SS% / 100) × P / Period = (0.5 / 100) × 25 / 793.6508 = 1.575 × 10-4.
Step [15..0] = DEC2HEX(ROUND2INT(224 × Step(decimal))) = DEC2HEX(2642) = 0A 42 hex.
Example 2 with center spread
:
FVCO = 2430MHz, FOUT = 27MHz with ±1.0% center spread and 31.5kHz modulation rate.
The crystal is 25MHz and the doubler is enabled so FPFD = 25MHz × 2 = 50MHz.
FFD value P = (1 - SS% / 200) × (FVCO / (2 × FPFD)) = (1 + 2.0/200) × (2430 / (2×50)) = 1.01 × 24.3 = 24.543.
FFD integer [7..0] = DEC2HEX(24) = 18-hex.
FFD fraction [15..0] = DEC2HEX(ROUND2INT(216×0.543)) = DEC2HEX(ROUND2INT(35586.05)) = 8B 02 hex.
Period (decimal) = FOUT / (2×FSS) = 50 / (2×0.0315) = 793.6508.
Period [11:0] = DEC2HEX(ROUND2INT(Period(decimal) / 2)) = DEC2HEX(397) = 1 8D hex.
Step (decimal) = (SS% / 100) × P / Period = (2.0 / 100) × 24.543 / 793.6508 = 6.184836 × 10-4.
Step [15..0] = DEC2HEX(ROUND2INT(224 × Step(decimal))) = DEC2HEX(10376) = 28 88 hex.