TDP1204: channel loss tool

Fred

Linear or limited mode is determined by the re-driver design. Some re-driver only supports linear mode (RX EQ only, no TX output control such as SWING or Pre-shoot/De-emphasis) and some re-driver supports limited mode (RX EQ and TX output control such as SWING or Pre-shoot/De-emphasis). 

So you will have more compensation in the limited mode (RX + TX compensation) than the linear mode (RX compensation), so you can have longer pre-channel trace length in the limited mode.

Thanks

David

HI David,

thanks your illustration.

but I thought EQ decide pre-channel and DE decide post-channel and they both are separate and independent.

since I try to change the DE but it doesn't change the suggest pre-channel length

my question will be if pre-channel already take TX DE into account, then why we still see suggested  post-channel length changing with DE value?

Fred

In limited mode, the pre-channel and post-channel are independent from each other so you can see when you change the TX MAX DE but keep the RX MAX EQ the same, the calculated pre-channel length does not change but post-channel length does change.

When you change the RX MAX EQ but keep TX MAX DE the same, the pre-channel length then changes but post-channel length stays the same.

Thanks
David

Hi David,

appreciate your explanation,

but I just can't get it that if they're both independent then why this tool still take DE compensation into account when estimate pre-channel length,

which makes the pre-channel length in limited is  shorter than linear.

Fred

Can you please share your calculation spreadsheet?

Thanks

David

0083.TX trace length budgeting revB.xlsx

like I said, pre-channel is strangely different.

Fred

With the same channel input, the pre-channel with high confidence for limited mode is 9.8in while for linear mode is 7.5in, so I am not sure why you are saying pre-channel length in limited is shorter than linear.

4604.TX trace length budgeting revB.xlsx

Thanks

David

Hi David,

sorry, the other way around,

rephrase it again, why longer than linear mode and why there's a difference with same EQ?

Fred

With linear re-driver, its equalization can apply toward compensating for the pre and/or post-channel, But with limited re-driver with its TX SWING and Pre-emphasis, the link is broken into two parts, pre-channel and post-channel. The pre-channel will be compensated by the RX EQ only while the post-channel will be compensated by the TX Pre-emphasis, so you calculate the pre-channel and post-channel loss separately. 

In this case, you will see the pre-channel in linear mode to be less than the pre-channel in the limited mode.

Thanks

David

Hi David,

thanks a lot , totally understand now,

one last question ,

 if in linear mode, I decide to compensate all for only post-channel or only pre-channel, where can I change this setting?

and  is this linear mode thing you said also valid on other interface redriver like USB or PCIe?

Hi David,

appreciate , this is a good stuff.

I'll take some time to read it, thanks

Hi David,

I read the materiel , have one question.

since it mentioned the gain stage of limited redriver is high gain-clipping unlike linear one,

does this mean when we change the mode of TDP1204 to limited one, the gain stage would also change to below high-gain behavior?

Fred

The TDP1204 has an internal gain stage, so in the limited mode, you can program it from -15% to 20%,

Thanks

David

Hi David,

I know linear keep the same always,

but does this  -15% means the swing will be 1000x(1-0.15)?

how about how steep  gain curve become ?

Fred

but does this  -15% means the swing will be 1000x(1-0.15)

*** Correct

how about how steep  gain curve become ?

I honestly don't know since we didn't characterize it.

Thanks

David 

got it ,

so you mean the gain curve will be the same as linear one but only VOD swing change right?

so seems like the only difference between limited and linear in TDP1204 is FFT( de-emphasis)

Fred

No, I am saying that since we did not characterize the TDP1204 TX gain curve when TDP1204 is in limited mode, I don't know how steep the curve will be.

Thanks

David