CDR when eye open with only 0.5UI

[raymond_luo2003]

Dear all,

I really wonder the eye diagram definition in most serial link standard such as SATA, 1394 and USB2.0. I understand the eye closure is decided by deterministic jitter and random jitter which are above jitter corner frequency. Also I understand those high frequency jitter is not tracked by CDR at receiver side if CDR have low pass filter with same jitter corner frequency.

Usually jitter corner frequency is defined as fc/1667 or fc/2500 while real DJ frequency (from data dependant jitter, duty cycle jitter, supply noise jitter) are much high than jitter corner frequency.

In some of circumstance, the eye opening at receiver side is only 0.5UI. When we implement the clock data recovery circuitry to recover the data from such eye, usually we have two clocks, CLK_I (align to data transition edge) and CLK_Q (sample the data at middle point of data period) with 0.5UI distance.

Since the 0.5UI (DJ and RJ) is not recovered by CDR, I just wonder how can CLK_Q can sample the correct data with no margin [UI- 0.5UI (distance between CLK_I and CLK_Q) - 0.5UI (DJ+RJ) ==0) even the CDR have no internal generated jitter?

At some extreme case, the DJ+RJ=0.6UI, that means eye-opening is only 0.4UI left. Do we have to use over-sampling style CDR to recover the data?

Does this mean the DJ is partially traceable by CDR?? Anyone can help me to understand thisHuh

Many thanks!

 

[Fahmy]

I think that eye opening of 0.5UI doesn't mean that the input duty cycle is distorted to the extent that the spacing between the optimum sampling point and the data transition is close to 0.5UI /2 (0.25 UI).
All what happens is that the jitter makes the data transition point moves forward and backward closing the eye and the optimum sampling point is following it . In case of high frequency jitter, the CDR can not track the rapid movement in the data transition edge but if it locked to the optimum sampling point it will be able to sample the data in eye opening where a correct value is sampled.

 

[CDRCDR]

The CDR actually tracks some DJ(such as ISI jitter) partially because of high frequency jitter tolerance. For the RJ, the CDR can only samples at the average RJ position.

 

[adsunnk]

The DJ+RJ=0.5UI means the eye still open 1-0.5=0.5UI, so you Q clk still have a timing margin=0.5UI