[SI-LIST] Re: Rx Eye Mask width, BER, and Jitter

  • From: Conrad Herse <herse@xxxxxxxxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Mon, 21 Jun 2010 15:39:08 -0500

Marty,

Thanks for the comments. My comments below:

Marty Bewildered wrote:
> My first comment is Rj is not a function of BER, Tj is ... or more directly 
> Jrms ... is a function of BER

Isn't Jrms basically 1 sigma (std dev), and Q is some number of sigmas 
to get to a target BER (or am I missing something)? Rj is derived from 
Jrms and the target BER Q, which then is included in Tj.

> Second comment ... if your margin at 1e-12 is 0.2UI, you're in big trouble at 
> 1e-15.

The 0.2 UI was a fabricated number for this example, it could be 
anything. However, more importantly the 0.2 UI is a *receiver eye width* 
derived from an Rx jitter *tolerance*, not a margin. I would expect the 
required Rx eye mask width to *increase* and my receiver jitter 
tolerance to *decrease* for a higher BER. Jitter will increase at a 
higher BER, but won't the receiver require a wider eye mask at a higher 
BER too?

Thanks,

Conrad

> Basically your Tj is getting bigger than 1UI ... so all the numbers don't 
> make sense anymore.

Conrad Herse

> 
> 
> 
> ----- Original Message ----
> From: Conrad Herse <herse@xxxxxxxxxxxxxxxxxx>
> To: si-list@xxxxxxxxxxxxx
> Cc: herse@xxxxxxxxxxxxxxxxxx
> Sent: Mon, June 21, 2010 12:40:13 PM
> Subject: [SI-LIST] Rx Eye Mask width, BER, and Jitter
> 
> Hello experts,
> 
> I've been working on trying to scale receiver eye mask widths to 
> different bit error rates. There is something which is puzzling me which 
> I'm hoping someone can clear up for me.
> 
> I've been studying the dual-Dirac jitter model given by the formula:
> 
> Tj = Dj + 2Q * Jrms
> 
> where Q is a constant from the Complimentary Error function for a given 
> BER (2Q*Jrms = Rj at a specific BER). So if I have a receiver with the 
> following jitter tolerance spec:
> 
> Tj = 0.8 UI
> Dj = 0.3 UI
> Rj = 0.5 UI
> BER = 1e-12
> 
> then, given 2Q = 14 for BER = 1e-12:
> 
> Jrms = 0.5 / 14 = 0.036 UI
> 
> The Rx eye mask width would be:
> 
> 1 - 0.8 = 0.2 UI
> 
> If I want to scale the Rx eye mask width to BER=1e-15 I would expect I 
> need to *grow* the eye mask width by Jrms.
> 
> Given that 2Q = 15.883 at BER = 1e-15, then my new eye mask width would be:
> 
> 0.2 + (15.883 - 14) * 0.036 = 0.268 UI
> 
> So far so good, assuming I did this correctly. Here's what puzzles me, 
> if I adjust my Rx jitter tolerance to accommodate the new Rx eye mask:
> 
> Tj = 1.0 - 0.268 = 0.732 UI
> Dj = 0.3 UI
> Rj = 0.732 - 0.3 = 0.432 UI
> BER = 1e-15
> 
> and recalculate Jrms:
> 
> Jrms = 0.432 / 15.883 = 0.027 UI
> 
> The Jrms number has changed, I wouldn't expect this to happen simply 
> because I'm extrapolating to a different BER. Can someone please 
> straighten me out?
> 
> Thanks!
> 
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