DATA SHEET • SKY72302-21 FREQUENCY SYNTHESIZER

Fractional-N Applications. The desired division ratio for the

main synthesizer is given by:

F

VCO

_main

4F

div

_ref

---------------------------------

=

The desired division ratio for the auxiliary synthesizer is given by:

F

VCO

_aux

F

di v

_ref

-------------------------

=

synthesizer or between 37.5 and 537.5 for the auxiliary

synthesizer.

The value to be programmed by the Main or Auxiliary Divider

Register is given by:

32

)

N

(

Round

N

fractional

reg

NOTE: The Round function rounds the number to the nearest

integer.

inclusive.

The value to be programmed by either of the MSB/LSB Dividend

Registers or the Auxiliary Dividend Register is given by:

)]

32

N

N

(

divider

[

Round

dividend

reg

fractional

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 SKY72302-21,

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:

ref

_

div

main

_

vco

eger

int

F

F

N

The value to be programmed by the Main or Auxiliary Divider

Register is given by:

32

N

N

eger

int

reg

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:

Main Divider Register

Main Dividend LSB Register

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:

Main Divider Register

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:

Auxiliary Divider Register

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 SKY72302-21

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.

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101216L • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • September 9, 2009

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