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Design >> High-Speed I/O Design >> Jitter amplification
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Message started by raja.cedt on Dec 2^nd, 2014, 8:13am

Title: Jitter amplification
Post by raja.cedt on Dec 2^nd, 2014, 8:13am

dear all,
1. Can any one explain why jitter after passing through low pass system(let us say a PCB channel) will be amplified or simply from jitter perspective it is high pass filter? I have seen lot of math regarding this but can any one explain intuitively
2. Similar question but after passing through High pass filter..

Slide 14 in the following presentation.
http://www.ece.tamu.edu/~spalermo/ecen689/lecture13_ee720_fwd_clk_deskew.pdf

Thanks,
Raj.

Title: Re: Jitter amplification
Post by loose-electron on Dec 2^nd, 2014, 8:40pm

intuitive answer:

Consider a system with a square wave (up to 9th harmonic for sake of discussion)
and some additive noise. (call it white noise that terminates at the 6th harmonic)

Filter the signal and pass the 1,3,5 harmonic.

What happens?

1. The rise/fall edge of the square wave occurs over a longer period of time.
2. The in channel noise remains the same.

Now since the same noise is present on a signal where
the rise/fall of the edges is greater, the apparent jitter
of the transition edge spreads out more.

Intuitive enough?

(No equations were produced in this explanation.... )

:)

Title: Re: Jitter amplification
Post by raja.cedt on Dec 3^rd, 2014, 3:54am

Hi,
thanks for your explanation,
by the same way can we explain high pass filter impact on jitter like ac coupling? Since it is high pass filter it should improve rise and fall, correct me if I am wrong.

Thanks,
Raj!

Title: Re: Jitter amplification
Post by loose-electron on Dec 3^rd, 2014, 11:54am

How can it improve something that is not there?

Any filter takes away spectral content.

If HPF, which is commonly used to get rid of a DC bias, and you set the filter break point below the fundamental, you have not removed signal content.

However, if you start to take out the fundamental, your resulting signal is going to lose it's rise-fall edges as well.