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smarty

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return loss measurement
Apr 25^th, 2007, 9:44pm

Hi,
I have just started working on high speed SerDes design and I am starting on the receiver supporting PCI-express 2.0 standards(5Gbps).
I am starting to understand the different spec and trying out simulations on them to improve my understanding.
There is a spec on receiver that it should meed a specified CMRL(common-mode return loss) and DRL(differential return loss).

I am simulating this in spectre to measure both. From my understanding, relating to return loss it is measured using the S11 parameter ( please correct me if I am wrong).

Can anybody suggest me as to how this can be done?

I provide a method I am trying out. Please provide your feedback on this.

1. I have my receiver end terminated to 50ohms.
2. I use the n-port from analog Lib and including the corresponding s-param file to model the package model.
3. I uses a PORT0 from analog Lib to provide my ideal input side voltage source with series resitance as 50ohms( source side resistance).
4. To measure differential RL I use to connect one line to one end of PORT and the other line to the another end of the PORT.

I then do an sp analysis and measure the RL as db(s11).

Kindly provide your valuble feedbacks, whether this is a correct setup/method to measure return loss.

Thanks and Regards,
SBR

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Ken Kundert Global Moderator Offline Posts: 2386 Silicon Valley	Re: return loss measurement Reply #1 - Apr 25^th, 2007, 11:27pm Take a look at the papers on differential measurements in the Analysis section of this site. -Ken
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smarty Community Member Offline Posts: 46	Re: return loss measurement Reply #2 - Apr 26^th, 2007, 1:21am Hi Ken, Thanks for the information. I will look at the papers. Regards, SBR
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smarty

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Re: return loss measurement
Reply #3 - May 14^th, 2007, 11:07pm

Hi Ken,
Could you pls clear me few things regarding the diferential circit testbanch pdf, it would be of much help.

1. I need to measure the common-mode return loss and Differential return loss for PCI-e reciever from dc till 1.25Gz. In normal operation the
input varies from +/-600mv from ground on both D+ and D-.

I would like to know to have a better understanding, using this setup how can that be measured. I am dotting my understanding. correct me if you find my way different.

I would like to use the setup in fig4.

To measure, the input to be given are vdm=1.2;vcm=0;Rid=100ohms;Rcm=very high(10K).

One more is could we use the PORT cell from analogLib, and define input as a sine wave(freq:2.5GHz, amp:600mv, dc=0) and connect the port to a se2diff cell and use the output of that cell to the reciever input.

I am new in this field, and from what I have seen Return loss is measured by s11 parameter(20dB ofs11), by running a sp analysis in spectre.
If this is the case, how would I measure return loss for both these setup?. In your experience which would provide, a more realistic method.

Kindly help this out for me to have better understanding.

Thanks and Regards,
SBR

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Ken Kundert Global Moderator Offline Posts: 2386 Silicon Valley	Re: return loss measurement Reply #4 - May 15^th, 2007, 9:29am It seems like you are on the right track, though it is not clear why you would want to measure the common-mode return loss when the common-mode impedance is not controlled. Also, I don't know what se2diff is, so I cannot say whether its use is appropriate. -Ken
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smarty Community Member Offline Posts: 46	Re: return loss measurement Reply #5 - May 17^th, 2007, 5:33am Ken, Thanks for your inputs. Common-mode return loss is measured as a part of the PCIe specs. Best Regards, SBR
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