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LMH0387 - Mechanism of Cable Length Detection

Kawai
Guru 19785 points
Other Parts Discussed in Thread: LMH0387

Hi, I have a question from the customer, how LMH0387 recognizes the cable length.

Could you please tell us the mechanism how the device detect cable length.

 

I think the device calculates the cable loss by comparing launch amplitude information and received signal amplitude.

Then, LMH0387 uses the Belden1694A cable loss curve table information to estimate cable length from loss information.

Am I correct ? 

  • 0
    Guru 22270 points

    I would think it would be something a little simpler like measuring the resistance by applying a current to the line.

  • 0
    TI__Genius 13475 points

    There is a feedback loop which adjusts the amount of equalization being applied to the input, and it adjusts the equalization to make the signal coming from the equalization filters to be as close as possible to the original signal - which is known by the SMPTE specs (known rise/fall and amplitude).   By looking at how much equalization has to be applied, we are able to estimate the cable length.

  • 0 in reply to Mark Sauerwald88474
    Guru 22270 points

    Hello Mark,

    Now does this equalization take place as a result of injecting more current into the line or is simply through an increase in voltage?  Basically, how is this equalization derived?

    Thanks,

  • 0 in reply to Greenja
    TI__Genius 13475 points

    Simplified explanation:

    The attenuation that a signal sees when going through coax cable is approximately proportional to the sqrt(f).   The equalizer has a filter in it which has poles positioned so that it approximates a sqrt(f) transfer function.   The gain of this filter is adjusted to compensate for the length of the cable connected to the input.