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LMH0397: Optimal setup for longer cables

Danijel Sudec
Prodigy 20 points
Part Number: LMH0397

Tool/software:

Hello,

We have a PCB with the LMH0397 chip. We want to test what the maximum length of the cable is to still receive the SDI signal.

On the first PCB, we use SDI IO as input and SDI out0 as output. We then connect our cable to the out0 and connect it to the other PCB on in0.

We then test the signal we get from the second PCB on SDI OUT.

Furthermore, we did a couple of tests, and if the cable is under 10 meters long, we get a signal on the output without any issues.

On longer cables, we start having issues. We get either no lock at all or the lock LED starts blinking instead of being constantly on, like with our successful tests.

We do have access to the SPI communication with the chip, and we tried changing a few settings. We changed out0 VOD and de-emphasis registers, which improved the signal a little bit.

We would like to know what would be the optimal register configuration for our setup that would allow us to get a signal on the output with the longest possible cable.

  • 0
    TI__Genius 9209 points

    Hi Danijel,

    To clarify, what cable length are you adjusting? Is the cable an input to the first chip SDI_IO or is the cable between the OUT0 and IN0? Our longest length of cable is driven by the 75 ohm outputs.

    Danijel Sudec said:
    We would like to know what would be the optimal register configuration for our setup that would allow us to get a signal on the output with the longest possible cable.

    Based on the fact that adjusting VOD and de-emphasis improves performance, the issue is most likely with your signal to noise ratio. The optimal setting depends on the total cable loss, and you can adjust the VOD and de-emphasis accordingly. Additionally the input equalization settings on the second chip can have a positive impact. Ensure that the equalization is enabled on the second chip. 

    Best,

    Nick

  • 0 in reply to Nick Peabody
    Prodigy 20 points

    We are using out0 and in0. We are using a standard Cat6 Ethernet cable for tests, but we plan to use it with a slightly different but similar cable in the future. We tried using a 10 m Ethernet cable, and we did manage to get the signal through. But when we tried to extend the 10 m cable with a coupler and another 2 m cable, we were unable to successfully transmit the signal. We are also aware that the coupler added some additional losses.
    We did try to use some different equalizer values, and it did seem to help a little bit, but we couldn't see a significant improvement.

    Note that we are using 1080p 30Hz video, which I believe is 1.5 GHz. If we assumed correct, in0 should be in manual mode for 1.5G by default, so we just changed the 0x3A register to some different values. 0xA0 seemed to have the best results.

    Is there perhaps something else we could possibly try? Or is using a better cable (for example, Cat7 standard Ethernet) and better coupling the best way to increase the cable length?

  • +1 in reply to Danijel Sudec
    TI__Genius 9209 points

    Hi Danijel,

    Thanks for the description of the case. That makes more sense it seems to me that you are using 1080p 30Hz 1.5 GHz SDI. Since you've tried both adjusting EQ on the second LMH and adjusting VOD and de-emph on the first device there is not much more that you can do to improve the cable reach from a device programming perspective. It may take a few iterations to find the best values for both devices, but regardless the best way to improve cable reach would be to reduce cable loss using Cat-7 and remove impedance discontinuities along the transmission line (on the PCB or adapters). Removing the coupler could be a way to remove loss as well. 

    Hope you can find a combination that works. Let me know if you have any more questions.

    Best regards,

    Nick