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Bug #5672

open

sfp-experimenter / voltage swing of LVDS driver insufficient for SFP spec

Added by laforge 3 months ago.

Status:
New
Priority:
Normal
Assignee:
-
Target version:
-
Start date:
09/02/2022
Due date:
% Done:

0%

Spec Reference:

Description

I'm surprised this never caused any problems, but

  • the SFP MSA speifies _The inputs will accept differential swings of 500 – 2400 mV (250 – 1200 mV single-ended), though it is recommended that values between 500 and 1200 mV differential (250 – 600 mV single-ended) be used for best EMI performance.
  • the LVDS driver we use states The outputs comply with the TIA/EIA-644 standard and provide a minimum differential output voltage magnitude of 247 mV into a 100-Ω load at signaling rates up to 630 Mbps...

So the differential magnitude of the driver is much lower t han the SFP MSA requires it to be.

I guess real-world SFP transceivers appear to be much more sensitive and accept lower voltage swings - and hence we never saw that problem so far.

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