SKY72310 Datasheet, PDF (6/19 Pages) List of Unclassifed Manufacturers – SKY72310: Spur-Free, 2.1 GHz Single Fractional-N Frequency Synthesizer

DATA SHEET • SKY72310 FREQUENCY SYNTHESIZER
Case 2: To achieve a desired Fvco_main frequency of 917.7786 MHz using a crystal frequency of 19.2 MHz with operation
of the synthesizer in 10-bit mode. Since the maximum internal reference frequency (Fdiv_ref) is 25 MHz, the crystal
frequency does not require the internal division to be greater than 1, which makes Fdiv_ref = 19.2 MHz. Therefore:
Nfractional = Fvco_main
Fdiv_ref
= 917.7786
19.2
= 47.80097
The value to be programmed in the Divider Register is:
Nreg = Round[Nfractional] – 32
= Round[47.80087] – 32
= 48 – 32
= 16 (decimal)
= 000010000 (binary)
With the modulator in 10-bit mode, the value to be programmed in the Dividend Registers is:
dividend = Round[divider × (Nfractional – Nreg – 32)]
= Round[1024 × (47.80087 – 16 – 32)]
= Round[1024× (–0.1990312)]
= Round[–203.808]
= 204 (decimal)
= 1100110100 (binary)
where 11 0011 0100 is loaded in the Dividend MSB Register.
Summary:
· Divider Register = 0 0001 0000
· Dividend MSB Register = 11 0011 0100
· The resulting VCO frequency is 917.775 MHz
· Step size is 18.75 kHz
Note: The frequency step size for this case is 19.2 MHz divided by 2¹⁰, giving 18.75 kHz.
S1060
Figure 4. Fractional-N Applications: Sample Calculation (2 of 2)
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July 30, 2008 • Skyworks Proprietary and Confidential information • Products and Product Information are Subject to Change Without Notice • 200705E