SKY72300-21 Datasheet, PDF (6/23 Pages) Skyworks Solutions Inc. – SKY72300-21: Spur-Free, 2.1 GHz Dual Fractional-N Frequency Synthesizer

DATA SHEET • SKY72300-21 FREQUENCY SYNTHESIZER
Case 1: To achieve a desired Fvco_main frequency of 902.4530 MHz using a crystal frequency of 40 MHz with operation
of the synthesizer in 18-bit mode. Since the maximum internal reference frequency (Fdiv_ref) is 25 MHz, the crystal
frequency is divided by 2 to obtain a Fdiv_ref of 20 MHz. Therefore:
Nfractional = Fvco_main
Fdiv_ref
= 902.4530
20
= 45.12265
The value to be programmed in the Main Divider Register is:
Nreg = Round[Nfractional] – 32
= Round[45.12265] – 32
= 45 – 32
= 13 (decimal)
= 000001101 (binary)
With the modulator in 18-bit mode, the value to be programmed in the Main Dividend Registers is:
dividend = Round[divider × (Nfractional – Nreg – 32)]
= Round[262144 × (45.12265 – 13 – 32)]
= Round[262144 × (0.12265)]
= Round[32151.9616]
= 32152 (decimal)
= 000111110110011000 (binary)
where 00 0111 1101 is loaded in the MSB of the Main Dividend Register and 1001 1000 is loaded in the LSB of the
Main Dividend Register.
Summary:
· Main Divider Register = 0 0000 1101
· Main Dividend Register, LSB = 1001 1000
· Main Dividend Register, MSB = 00 0111 1101
· The resulting main VCO frequency is 902.453 MHz
· Step size is 76.3 Hz
Note: The frequency step size for this case is 20 MHz divided by 2¹⁸, giving 76.3 Hz.
C1414
Figure 4. Fractional-N Applications: Sample Calculation (1 of 2)
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September 11, 2009 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • 101217N