SKY72302-21 Datasheet, PDF (6/23 Pages) Skyworks Solutions Inc. – SKY72302-21: Spur-Free, 6.1 GHz Dual Fractional-N Frequency Synthesizer

DATA SHEET • SKY72302-21 FREQUENCY SYNTHESIZER
Case 1: To achieve a desired Fvco_main frequency of 5900.4530 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
4 x Fdiv_ref
= 5900.4530
80
= 73.755663
The value to be programmed in the Main Divider Register is:
Nreg = Round[Nfractional] – 32
= Round[73.755663] – 32
= 74 – 32
= 42 (decimal)
= 000101010 (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 × (73.755663 – 42 – 32)]
= Round[262144 × (–0.2443375)]
= Round[–64051.6096]
= –64052 (decimal)
= 110000010111001100 (binary)
where 11 0000 0101 is loaded in the MSB of the Main Dividend Register and 1100 1100 is loaded in the LSB of the
Main Dividend Register.
Summary:
· Main Divider Register = 0 0010 1010
· Main Dividend Register, LSB = 1100 1100
· Main Dividend Register, MSB = 11 0000 0101
· The resulting main VCO frequency is 5900.4529 MHz
Note: The frequency step size for this case is 4 x 20 MHz divided by 2¹⁸, giving 305.2 Hz.
C1449
Figure 4. Fractional-N Applications: Sample Calculation (1 of 2)
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