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Original question:

DS90UB954-Q1: cable length for FPD-Link III

DS90UB954-Q1: maximum cable length for Back Channel

Dirk Bender
Prodigy 90 points
Part Number: DS90UB954-Q1

Hallo,

for the forward Channel the maximum attenuation between the SerDes is 20 dB. This can i calculate If I use the typical Output Voltage (425mV eye height)) of the DS90UB953 and the minimum Input Voltage (40mV) of the DS90UB954.

Now I checked the maximum attenuation for the Back Channel. There is the typical Output Voltage of the DS90UB954 at 130 mV (eye hight) and the minimum Input Voltage of the DS90UB953 is at 120 mV. I get only 0,7 dB.

My System works good with 7,5 Meter coax cable with approx 20 dB (at 2.1 GHz) attenuation. But for the Back Channel the attenuation is by 2.5-3.5 dB (at 50 MHz).

I do not get any CRC or Parity Error with 7.5 Meter but the attenuation of the Back Channel is theoretically to high.

Where is my fault?

How can I calculate the Back Channel attenuation?

  • 0
    TI__Genius 11340 points

    Hi Dirk,

    Are you running this in synchronous mode or non-sync mode?

    Are you seeing errors in the system at all? is lock maintained? Is the CSI data able to transmit properly? 

    Best,

    Jiashow

  • 0 in reply to Jiashow Ho
    Prodigy 90 points

    Hello,

    I run in synchronous mode.

    There are no errors, as long as I stay below aprox. 20 dB attenuation. If I go above 20 dB, CRC Errors occure first in the back channel. Parity Erorrs are very rare and difficult to reproduce.

    The LOCK is activ.

    The CSI date works correct.

    I want to understand where the limit values are. Hence the question of the maximum attenuation of the back channel and the Input and Output Voltage.

  • 0 in reply to Dirk Bender
    TI__Genius 11340 points

    Hi Dirk,

    The back channel runs at a significantly lower speed at 50MHz vs. 2GHz for the forward channel. Unlike the forward channel, the back channel doesn't have an adaptive equalizer. That's why the back channel has it's own jitter requirements.

    Best,

    Jiashow

  • 0 in reply to Jiashow Ho
    Prodigy 90 points

    Hallo,

    but is it correct, that where are only 0,7 dB / 10 mV  (worst case) attenuattion for the Back Channel? Or am I reading the datasheets wrong.

    I think that should mean that the back channel would have to determine the maximum line length despite the lower frequency.

  • 0 in reply to Dirk Bender
    TI__Genius 11340 points

    Hi Dirk,

    For coax configuration, the 953 datasheet specified a minimum back channel input of 120mV, and the 954 datasheet has a minimum back channel single-ended output of 190mV. This translates to about 4dB of attenuation.

    The maximum cable length should be determined by your forward channel attenuation.

    Best,

    Jiashow