weiming.qian

Sythesized Tap values swapped in FaseEye Channel Analyzer?

Discussion created by weiming.qian on Jul 20, 2011
Latest reply on Jul 26, 2011 by weiming.qian

I am running FastEye Channel Analyer, the resulted channel pulse response is shown below. I denote two pre-cusors (X-2, X-1), main cursor (X0) and two post-cursors (X1, X2) on the curve. The value is the difference between the corresponding point and the baseline at -480mV.

FIR_coefficient.jpg

 

When I set Tx pre-emphasis Taps to 3, the synthesized optimal values is following:

FIR_coefficient1.PNG

 

When I set Tx pre-emphasis Taps to 5, the synthesized optimal values is following:

FIR_coefficient2.PNG

It seems to me that somehow the Tool got confused about Pre-Taps with Post-Taps and swapped them. My understanding is that, to first order, Pre-Taps are used to reduce/minimize the pre-cursor of the channel pulse response, and Post-Taps are used to reduce/minimize the post-cursor. The channel pulse response got a much bigger post-cusor than pre-cursor, so my naive thinking is the Post-Tap should have a much bigger absolute value than that of Pre-Tap.

 

In 3-Tap case, using ZFS (Zero Forcing Solution) method with following matrix,

 

[y(-1)]      [X0 X-1  X-2]   [C-1]

[ y(0)]  =  [X1  X0  X-1] * [C0]

[Y(1)]       [X2  X1  X0]    [C1]

 

Setting y(-1)=y(1)=0 and C0=1, we got C-1 = -0.11, C1=-0.4635. Compare these two values to those synthesized in 3-Tap case, they are close but swapped.

 

In 5-Tap case, the tool synthesized result gives 3 Pre-Taps and 1 Post_Tap, whereas the channel pulse response got a much longer post-cursor than pre-cursor. So this also suggests the Tool swapped Pre-Tap with Post-Tap.

 

To verify my suspicion, I set the Xilinx GTX Tx pre-emphasis settings (3-tap) according to the tool synthesized Tap values, the resulted eye is closed. If I set Xilinx GTX Tx pre-emphasis settings using swapped synthesized Tap values or my own calculations (C-1, C1 above), the resulted eye is open.

 

So my real questions are: a) Are those tool sysntesized Tap values just swapped or completely wrong? b) Am I wrong in above analysis?

 

 

My HyperLynx version is:

 

LineSim v8.1.1

[V8.1 build 510(510,0)]

Jan 20 2011

 

 

Regards,

Weiming

Outcomes