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LMH0384 equalizer

Lajos Andras
Prodigy 10 points

Other Parts Discussed in Thread: LMH0384, LMH0394

In some of our projects we use LMH0384 equalizer IC. We found that if we connect a 140m long BELDEN 1694A cable (specified maximum length for 3G-SDI in LMH0384 datasheet) to the input with 3G-SDI signal, there is no image at output with any input launch amplitude setting. If we set input launch amplitude with 6dB-22% à 6dB+40% sequence after connecting the cable we get picture at output.

 

It is remarkable that the jitter measured directly at the output of the LMH0384 is more than one and a half times less regardless to the input cable length, input signal datarate (see details on a table below).

It seems that input launch amplitude setting resets something in the IC.

Could you give details on that, please? Why input launch amplitude have to be set than set back to get better signal quality?

 

Input launch amplitude after cable connection

Cable Length [m]

Signal type

Jitter [UI]

0dB (constant)

23

SD-SDI

0,01

23

3G-SDI

0,48

140

SD-SDI

0,03

140

3G-SDI

0,81

6dB-22% à 6dB+40%

23

SD-SDI

0,02

23

3G-SDI

0,31

140

SD-SDI

0,02

140

3G-SDI

0,37

 

  • 0
    TI__Expert 4400 points

    The LMH0384 is an adaptive equalizer.  It automatically adjusts to reverse the effects of the cable loss.

    The LMH0384 Launch Amplitude settings (SPI register 0x02 bits 7:3) allow additional manual adjustments to the equalizer gain to compensate for attenuation prior to the equalizer.  This can be used to compensate for a non-optimal launch amplitude (i.e. the amplitude of the signal prior to the cable attenuation is not 800 mVp-p +/-10%), or attenuation due to a passive splitter or non-ideal input termination network on the PCB prior to the equalizer.

    Sequencing through the Launch Amplitude settings (as you have done) will at some point boost the equalizer gain temporarily.  If you are operating at a rate/cable length just beyond the LMH0384's equalization capabilities (e.g. 140+ meters at 3G), then boosting the gain temporarily could allow the equalizer to adapt and equalize a signal that is otherwise out of its range.  This could have the effect of tricking the equalizer into equalizing longer lengths than normal at 3G.

    If you are on the edge for 3G cable reach, then you could achieve a similar effect and allow operation at longer cable lengths by enabling the Extended 3G Reach Mode (SPI register 0x00 bit 2).

    The LMH0384 maximum cable reach could be limited by poor quality connectors or cable couplers, or imperfections in the PCB layout that introduce impedance discontinuities.  If you require longer reach in your applications, the LMH0394 equalizer has significantly more gain than the LMH0384 and can equalize up to 200m Belden 1694A cable at 3G rates.