SKY72302-21 Datasheet, PDF (7/23 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 5217.1776 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
4 x Fdiv_ref
= 5217.1776
4 x 19.2
= 67.9320
The value to be programmed in the Main Divider Register is:
Nreg = Round[Nfractional] – 32
= Round[67.9320] – 32
= 68 – 32
= 36 (decimal)
= 000100100 (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[1024 × (67.9320 – 36 – 32)]
= Round[1024 × (–0.068)]
= Round[–69.632]
= –70 (decimal)
= 1110111010 (binary)
where 11 1011 1010 is loaded in the MSB of the Main Dividend Register.
Summary:
· Main Divider Register = 0 0010 0100
· Main Dividend Register, MSB = 11 1011 1010
· The resulting main VCO frequency is 5217.15 MHz
Note: The frequency step size for this case is 4 x 19.2 MHz divided by 2¹⁰, giving 75 kHz.
C1450
Figure 4. Fractional-N Applications: Sample Calculation (2 of 2)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
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