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Original question:

LMH1297: VIN and VDD_CDR Combined or Separate?

LMH1297: LMH1297 Long Reach Performance

Randy May
Prodigy 70 points
Part Number: LMH1297

Greetings,

This is a continuation of post "LMH1297: VIN and VDD_CDR Combined or Separate?"  I was able to improve performance on some of my channels by adding a shield can over the LMH1297 and 75ohm traces.  With a -40db loss cable, there is still a significant benefit from increasing the output amplitude beyond the 800mV+%10 spec.

Under section 9.2.1.2, comment 8 of the LMH1297 datasheet, “Configure the LMH1297 in CD Mode. Tune the SDI_VOD output amplitude control pin for optimal signal quality depending on the cable length attached at SDI_IO+ and SDI_OUT+. Use register control for more tuning options if necessary.”  Is adjusting the output amplitude a common procedure when setting up an SDI system, or is the expectation that most links will work with the default 800mV setting?

Section 9.2.1.3 of the LMH1297 uses PRBS10 for testing.  Is there a specific reason PRBS10 was used, vs a higher polynomial?

The LMH1297 Programming Guide has detailed instructions on programming the CTLE in AM0 mode, but this appears to only apply to the FR4 (IN0) side of the chip.  Is there a way to manually program the equalization values on the Cable side?

Lastly, I attempted to send a Nyquist/Clock pattern through some of my longer cables, but the Cable Equalizer/CDR was not able to lock.  Thoughts?  Shorter cables were able to lock.

Thanks

 

  • 0
    TI__Guru* 93400 points

    Hi Randy,

    Please note comments.

    1). It is expected that applications would work with 720mV to 880mV(800mV +/-10%) launch amplitude. Device has the capability to change output amplitude in 5mV per step - if needed. However, per SMPTE standard we are limited to 800mV+/-10% and we should be able to operate in this range. Values outside of this range may cause interoperability issue.

    2).  PRBS10 was chosen because this pattern resembles color bar pattern and also it is readily available.

    3). You are correct we can force the CTLE setting on FR4 side however on the coax side different cable length can be attached and this is not possible.

    4). Based on your experiment, at longer cable length we are bandwidth limited. Insertion loss of the cable is attenuating the incoming clock signal to the point it cannot be recovered. Perhaps if you have a lower loss cable or if you reduce the clock frequency this may enable longer cable length.

    Regards,, Nasser