SI5325C-C-GM Datasheet, PDF(74/178 Page) Silicon Laboratories

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SI5325C-C-GM Datasheet, PDF (74/178 Pages) Silicon Laboratories – ANY-FREQUENCY PRECISION CLOCKS

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Si53xx-RM

6.5.3. Free Run Reference Frequency Constraints

XA/XB Frequency Min

109 MHz

37 MHz

XA/XB Frequency Max

125.5 MHz

41 MHz

Xtal

3rd overtone

Fundamental

C-----K-----I--N--

N31

X-----A-------X----B--

N32

C-----K-----O-----U----T--

XA-XB

ï¹

ege

ï® All crystals and external oscillators must lie within these two bands

ï¬ï Not every crystal will work; they should be tested

ï¬ï An external oscillator can be used at all four bands

ï® The frequency at the phase detector (f3) must be the same for both CKIN1 and XA/XB or else switching cannot

be hitless

ï® To avoid spurs, avoid outputs that are an integer (or near integer) of the XA/XB frequency.

6.5.4. Free Run Reference Frequency Constraints

ï® While in Free Run:

ï¬ï CKOUT frequency tracks the reference frequency.

ï¬ï For very low drift, a TCXO or OCXO reference is necessary.

ï® CKOUT Jitter:

ï¬ï XA/XB to CKOUT jitter transfer function is roughly one-to-one.

ï¬ï For very low jitter, either use a high quality crystal or external oscillator.

ï¬ï 3rd overtone crystals have lower close-in phase noise.

ï¬ï In general, higher XA/XB frequency > lower jitter.

ï® XA/XB frequency accuracy:

ï¬ï For hitless switching, to meet all published specifications, the XA/XB frequency divided by N32 should match the CLKIN

frequency divided by N31. If they do not match, the clock switch will still be well-behaved.

ï® Other than the above, the absolute accuracy of the XA/XB frequency is not important.

Rev. 1.2

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