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Posts: 3
Registered: ‎01-05-2017

Si5324 Jitter tolerance

I have inherited a design with the following Frequency Plan but without any commentary or explanation:

CKIN1 = 27MHz (Generated as hi 6.37ns, lo 33.67ns, hi 6.73ns, lo 26.93ns, repeat. Thus there is regular jitter of 6.74ns p-p at 13.5MHz.)

N31 = 14

N2_HS = 11
N2_LS = 254

N1_HS = 6

NC1_LS = 6

BW= 8 (BWSEL_REG = 10)


Of the thirty or so production items that have been built, about ten show reluctance for the Si5324 to lock to the 27MHz input. Mostly this is a problem only at cold (-20C), but some do not lock even at room temperature.

I took one production item that had never locked, even at room temperature, and fed it a jitter-free 27MHz. It locked every time. Thus it seems that it does not like the jitter.


My questions are:

Is the low value of N31 a problem in that it presumably passes a 6.74ns p-p jitter at 964kHz into the DSPLL?

Would a higher value of N31 be of benefit?

Would the associated higher value of N2 then be a problem?

Posts: 3
Registered: ‎01-05-2017

Re: Si5324 Jitter tolerance

After a bit more thinking, I have another question:

Does the even value (14) of N31 mean that for the CKIN1 described there should be zero regular jitter at the input to the DSPLL?

 

Posts: 70
Registered: ‎03-03-2014

Re: Si5324 Jitter tolerance

The biggest lock issue with Si5324 has been the LOCKT setting is not optimized, a 0x4 setting is usually best. What is the current LOCKT setting?

 

Thanks

Fran

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Posts: 3
Registered: ‎01-05-2017

Re: Si5324 Jitter tolerance

LOCKT is 0x1. Maybe the recommendation of para 6.2.1.1 of the Reference Manual was being followed.