SKY72300-21 Datasheet, PDF (5/23 Pages) Skyworks Solutions Inc. – SKY72300-21: Spur-Free, 2.1 GHz Dual Fractional-N Frequency Synthesizer

DATA SHEET • SKY72300-21 FREQUENCY SYNTHESIZER
Fractional-N Applications. The desired division ratio for the
main and auxiliary synthesizers is given by:
N _fractional
F_VCO
F_{div _ ref}
where N_fractional must be between 37.5 and 537.5.
The value to be programmed by the Main or Auxiliary Divider
Register is given by:
N_reg Round( N _fractional ) 32
NOTE: The Round function rounds the number to the nearest
integer.
When in fractional mode, allowed values for N_reg are from 6 to 505
inclusive.
The value to be programmed by either of the MSB/LSB Dividend
Registers or the Auxiliary Dividend Register is given by:
dividend Round [ divideru( N _fractional  N_reg  32 )]
where the divider is either 1024 in 10-bit mode or 262144 in
18-bit mode. Therefore, the dividend is a signed binary value
either 10 or 18 bits long.
NOTE: Because of the high fractionality of the SKY72300-21,
there is no practical need for any integer relationship
between the reference frequency and the channel spacing
or desired VCO frequencies.
Sample calculations for two fractional-N applications are provided
in Figure 4.
Integer-N Applications. The desired division ratio for the main or
auxiliary synthesizer is given by:
N _{int eger}
F_{vco _ main}
F_{div _ ref}
where N_integer is an integer number from 32 to 543.
The value to be programmed by the Main or Auxiliary Divider
Register is given by:
N N  32 _reg
int eger
When in integer mode, allowed values for N_reg are from 0 to 511
for both the main and auxiliary synthesizers.
NOTE: As with all integer-N synthesizers, the minimum step size
is related to the crystal frequency and reference frequency
division ratio.
A sample calculation for an integer-N application is provided in
Figure 5.
Register Loading Order. In applications where the main
synthesizer is in 18-bit mode, the Main Dividend MSB Register
holds the 10 MSBs of the dividend and the Main Dividend LSB
Register holds the 8 LSBs of the dividend. The registers that
control the main synthesizer’s divide ratio are to be loaded in the
following order:
x Main Divider Register
x Main Dividend LSB Register
x Main Dividend MSB Register (at which point the new divide ratio
takes effect)
In applications where the main synthesizer is in 10-bit mode, the
Main Dividend MSB Register holds the 10 bits of the dividend. The
registers that control the main synthesizer’s divide ratio are to be
loaded in the following order:
x Main Divider Register
x Main Dividend MSB Register (at which point the new divide ratio
takes effect)
For the auxiliary synthesizer, the Auxiliary Dividend Register holds
the 10 bits of the dividend. The registers that control the auxiliary
synthesizer’s divide ratio are to be loaded in the following order:
x Auxiliary Divider Register
x Auxiliary Dividend Register (at which point the new divide ratio
takes effect)
NOTE: When in integer mode, the new divide ratios take effect as
soon as the Main or Auxiliary Divider Register is loaded.
Direct Digital Modulation
The high fractionality and small step size of the SKY72300-21
allow the user to tune to practically any frequency in the VCO’s
operating range. This allows direct digital modulation by
programming the different desired frequencies at precise instants.
Typically, the channel frequency is programmed by the Main
Divider and MSB/LSB Dividend Registers, and the instantaneous
frequency offset from the carrier is programmed by the
Modulation Data Register.
The Modulation Data Register can be accessed in three ways as
defined in the following subsections.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
101217N • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • September 11, 2009
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