RMS jitter from eye diagram

Hi 3489,

I believe what you need to do is to compute the time interval error (TIE) as a function of zero-crossing and then determine its rms value. Please note, however, that "jitter" is defined over a specific bandwidth. Hence, the rms value you compute is limited by the sample size of the TIE you compute. For example, the minimum frequency of the time interval error is determined by the time span of the simulation you analyze and the maximum frequency is essentially 1/(2*your clock period). If you are trying to compute the rms jitter over a specific frequency band, you will need to set the simulation time and your analysis period accordingly. There is an calculator function called "abs_jitter" that will compute the time interval error on a periodic waveform. An example of its syntax for a clock waveform named hsclki_clipped with period 1/fin and switching threshold level vth is as follows:

hsclki_tie = abs_jitter( hsclki_clipped "rising" vth ?yUnit "s" ?Tnom 1.0/fin )

I hope this helps!

Shawn 

Unfortunately abs_jitter will only work with a periodic signal, not with a random bit stream. The Eye Diagram assistant (in IC615 onwards) gives the option of computing various metrics such as the standard deviation of the edge, but if you need more general calculations, you can use the SKILL code in solution 11395772.

 HI Andrew,

I beleve34892 is looking to compute the time interval error of a periodic signal as he states: "I am trying to find RMS jitter of the clock coming out of my ADC". This is why I suggested he try the abs_jitter() function. Your added information, however, is quite useful!

Shawn

HI Shawn/Andrew

So, I used the abs_jitter function to find the RMS jitter. I got a reasonable value, but I am wondering, what if I use period_jitter function in the calculator. I know there is a difference in so called 'period jitter' and 'TIE' jitter measurement, but I am not sure which one to use when and which result is more accurate.

FYI: I did use period_jitter function, and the RMS jitter value was 217fs (vs. that from abs_jitter was 249fs). They are close, but still different.

I haven't used the eyeDiagram method Andrew suggested, but will check that as well.

Thanks

-34892

Hi 34892,

Period jitter has a different definition that time interval error. Time interval error data contains the time differences between each waveform threshold crossing and the threshold crossings of a clock with an ideal period T. Period jitter data contains only the difference between the clock period (i.e. the time between the threshold crossings of subsequent rising or falling edges) and the ideal period. Period jitter can be viewed as a first "difference" function of the jitter since it relies on the difference in location of two consecutive threshold crossings. Time interval error provides an absolute difference. As such, the period jitter creates a weighted version of the jitter as some jitter amplitudes over frequency will be increased and some will be decreased. (In fact, period data may not be between adjacent periods. The JEDEC standard for measuring period jitter indicates that after measuring one period, one waits a random period of time before measuring the next period.) Period jitter can be derived from time interval error data. I hope this make sense to you.

 Hence, it is not surprising that the measurement results for rms TIE and rms period error are different. 

 The choice of which one to use (rms of time interval error or rms of period error) is determined by your application and its requirement. If only the time between adjacent rising or falling edges is of utmost importance, perhaps the rms period jitter is of greatest value. Without more knowledge about your specific application, I'm not able to suggest which metric is best - sorry!

Shawn 

Thanks a lot Shawn. It was helpful. 

 Great 3489! Thank you for letting us know. Say hi to Anthony if you happen to work at the company your location suggests!