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group delay of a backplane channel (Read 6257 times)
ywguo
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group delay of a backplane channel
Dec 10th, 2012, 9:57pm
 
Hi Guys,

I study the group delay of channel. The ac analysis gives the following the  amplitude loss and group delay. The group delay has sharp increase at ~500kHz and sharp decrease near 700MHz.

What causes the group delay looks like that shape? What is the effect on jitter/eye if the datarate is below 600Mbps and above 1Gbps respectively?

Any comments are appreciated.
Yawei
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GroupDelay_18inch_backplane.png
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raja.cedt
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Re: group delay of a backplane channel
Reply #1 - Dec 11th, 2012, 12:31am
 
hello Yawei,
-ve group delay is little bit new to me, i know for some filters it will be fine, but i haven't seen any real channels with this behavior. Please double check once. Double check with s parameters.

Regarding your group delay impact on link performance, normally if ripple in the group delay is less than 10% of bit period then there is not much impact (according to sakinger book), in reality if you limit less than 6% it will be really good and you don't see any data dependent jitter. In your plot ripple is around 1ps, which is acceptable even at 5Gbps (here tb=200ps, so 10% ripple 20ps ).

Thanks,
Raj.
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loose-electron
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Re: group delay of a backplane channel
Reply #2 - Jan 16th, 2013, 8:37pm
 
Delay in a backplane will not model nicely as a linear model, with a simple set of parameters.
To get something close to reality you need to use a distributed parameter model.

You care to show us the model that you are using?
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andyseu
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Re: group delay of a backplane channel
Reply #3 - Sep 25th, 2013, 1:28am
 
hi raja

i have a question , you said " In your plot ripple is around 1ps". but,according to the plot, the ripple is about 1ns. why you said the ripple is 1ps. why have 10e-3 difference. i have saw some paper, all have 10e-3 difference. i can't understand , or can you offer me some document which i can get the answer from . thank you


raja.cedt wrote on Dec 11th, 2012, 12:31am:
hello Yawei,
-ve group delay is little bit new to me, i know for some filters it will be fine, but i haven't seen any real channels with this behavior. Please double check once. Double check with s parameters.

Regarding your group delay impact on link performance, normally if ripple in the group delay is less than 10% of bit period then there is not much impact (according to sakinger book), in reality if you limit less than 6% it will be really good and you don't see any data dependent jitter. In your plot ripple is around 1ps, which is acceptable even at 5Gbps (here tb=200ps, so 10% ripple 20ps ).

Thanks,
Raj.

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raja.cedt
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Re: group delay of a backplane channel
Reply #4 - Sep 27th, 2013, 6:03am
 
hello,
it's a typo, sorry for that. How cum some other documents are doing the same mistake? Please forward me those docs. Regarding -ve group delay, group delay will be the physical delay only when you when you have constant magnitude around that frequency spectrum.

Thanks,
Raj.
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andyseu
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Re: group delay of a backplane channel
Reply #5 - Sep 28th, 2013, 6:23pm
 
dear
"The group delay variation  for the 100cm V cable is 435 ps (which corresponds to 230 GHz when using the 10% rule) up to 50GHz hence there is no need for phase equalization at data rates below 50 GHz", why 435ps corresponts to 230GHz, i think 435ps shoud correspont to 0.23GHz(1/435ps*0.1). it have 10e-3 difference. the document is in the wabside"http://www.eecg.utoronto.ca/~sorinv/theses/andreea-balteanu_MASc_thesis.pdf", page 9 .
thank you




raja.cedt wrote on Sep 27th, 2013, 6:03am:
hello,
it's a typo, sorry for that. How cum some other documents are doing the same mistake? Please forward me those docs. Regarding -ve group delay, group delay will be the physical delay only when you when you have constant magnitude around that frequency spectrum.

Thanks,
Raj.

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