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DS25BR120: Pre-emphasis response

Part Number: DS25BR120
Other Parts Discussed in Thread: DS25BR100EVK, DS25CP102,

Hi,

I am currently using DS25BR100EVK evaluation board in order to evaluate pre-emphasis and egalisation performances.

In the pre-emphasis paragraph of your application note AN-1957 (LVDS Signal conditioners Reduce Data-Dependent Jitter), we can see the waveform of a 2.5Gbps PRBS-7 signal measured at the output of the DS25CP102 with pre-emphasis disabled and with pre-emphasis enabled:

I can see the 6dB gain the DS25CP102 provides when the pre-emphasis is enabled (and also the 0dB gain at low frequency).

I assume that DS25BR120 has the same "type of preemphasis" (high pass filter with a 3dB, 6dB or 9dB gain at 1.56Ghz).

At first, I tested the component without transmission loss in order to see the pre-emphasis effect on the output.

I used a 2.304Gbps PRBS7 data signal input and measured this on the oscillosope:

First picture shows the output of DS25BR120 without pre-emphasis and the second one shows the output of DS25BR120 with a medium pre-emphasis (6dB).

I don't see the 6dB gain. I am not working at the optimal frequency (1.56GHz) but I am assuming that i should see a gain very close to 6dB when working at 1.15GHz ?
On top of that, I can see a static gain which I am not supposed to see.

Please help me out to understand why I obtain this measurement ?

Do you have the pre-emphasis frequency response ?

Thank you,

Gladys

I use a 1.15GHz PRBS7 data input signal an

  • Hi Gladys,

    Agreed these waveform do not show 6dB de-emphasis. I am assuming you have checked your PE0/PE1 settings and these indeed enables 6dB de-emphasis. Additionally it would be a good idea to run your BERT at 3.125Gbps. 

    Regards ,, Nasser

  • Hi,

    I indeed checked my PE0/PE1 settings and i did another measurement at 3.125Gbps but it didn't work as expected.
    Here are my results:

    PRE OFF                                                                    PRE LOW
                                                                                      Gain PP : 3.29dB / Static Gain : 1.92dB

    PRE MED                                                                PRE HIGH
    Gain PP : 3.57dB / Static Gain : 2.46dB                  Gain PP : 3.86dB / Static Gain : 2.91dB

    As you can see, even at the right frequency (3.125Gbps) I don't see the 3dB/6dB/9dB gain and also i have a static gain which i am not supposed to see. Moreover there is no many differences between the 3 preamphasis settings.

    Please help me out to understand why I obtain this measurement ?

    Do you have the pre-emphasis frequency response ?

    Thanks,

    Gladys

  • Hi Gladys,

    1). Can you please use PRBS7 type of pattern at 3.125 with scope waveform only - no need for eye diagram.

    2). It would be good if you can generate 8 ones and a zero at at 3.12Gbps . This could make it easier to see the pre-emphasis levels.

    Regards ,, nasser

  • Hi,

    Here are the waveforms of the PRBS7 pattern at 3.125Gbps:

    PRE OFF

    PRE LOW

    PRE MED

    PRE HIGH

    At the moment I don't have access to the board so I can't give you the 8 ones and a zero pattern.

    Gladys

  • Hi Glays,

    As you had noted earlier, it seems PE setting change does not have any influence on PE settings. Can you please send me a snap shot of your schematic to see how you set PE0 & PE1.

    Regards ,, nasser