Skyworks Si5338 - Configuring the Frequency Increment;Decrement; Step Size Resolution of 1 Ppm; Step Size as Small as.931 Ppb

Si5338-RM
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6. Configuring the Frequency Increment/Decrement
The Si5338 has a glitchless frequency increment/decrement (Finc/Fdec) feature that allows each output MultiSynth
frequency to be independently stepped up or down in predefined steps. Finc/Fdec is not provided on the feedback
MultiSynth. When using the Finc/Fdec feature, care must be taken to ensure that the resulting MultiSynth0,1,2,3
output frequency (Fout) stays in the range of 5 MHz to Fvco/8. The divider values, a, b, and c, in "3.2. Calculating
MultiSynth Values" on page 8 are used to calculate the Finc/Fdec parameters. Note that the Fout term in Equations
2 and 3 is a constant as defined by the equations in "3. Configuring the Si5338" on page 6. In other words, if a step
size (Fstep) of 10 kHz is programmed, the step size will stay 10 kHz regardless of the number of increments or
decrements that have occurred. The control of Finc/Fdec can be via external pins or internal register bits. See
Registers 52, 63, 74, and 85 for more information.
6.1. Step Size Resolution of 1 ppm
Under all conditions, a step size resolution as small as 1 ppm can be achieved. The actual step size would then be
an integer multiple of 1 ppm. Equation 2 shows how to configure the Si5338 with a 1 ppm step size resolution.
Equation 2. Frequency Increment/Decrement for 1 ppm Resolution
6.2. Step Size as Small as .931 ppb
The divider parameter c (= MSx_P3) can be up to 30 bits wide as needed to define the fractional part of the
MultiSynth output divider value. When the divider parameter c is limited to < 22 bits the Si5338 can achieve
increment/decrement step size as small as .931 ppb (2
–30
). Limiting the c parameter to < 22 bits has the effect of
limiting the precision of the MultiSynth output divider. However in practice it is extremely rare that more than 22 bits
are needed for the divider parameter c, hence in most cases this limitation of 22 bits for the c parameter will not be
an issue.
The following three conditions must be met for step sizes down to .931 ppb.
1. c < 2
22
; c is from "3.2. Calculating MultiSynth Values" on page 8.
2. Fout/Fstep <
2
30
; Fout is the frequency out of the MultiSynth.
3. Fout/Fstep is an integer.
The MSx_FIDPx parameters can be calculated as follows:
Equation 3. Frequency Increment/Decrement for Step Size >0.931 ppb
MSx_FIDP1 10
6
c
MSx_FIDP2 10
6
c
F
step
F
out
-------------


MSx_FIDP3 10
6
ac b+=

Where Fstep/Fout must be an integer multiple of 1 ppm.
=
=
MSx_FIDP1 c
F
out
F
step
-------------


MSx_FIDP2 c=

MSx_FIDP3 a c b+
F
out
F
step
-------------



=
=