I am reading about the 10Gb/s Ethernet PHY for Altera FPGAs (datasheet here). I was impressed to know that, at the hardware level, the 10Gb/s is done serially.

Naively, I would think to do 10Gb/s serially, one needs a 10GHz clock. However, 10GHz seems awefully high for a clock, and the datasheet does not specify a 10GHz clock anywhere.

How is 10Gb/s serial communication done? What clocks drive such transfers?

  • \$\begingroup\$ They now have 28Gbps working. Amazing 10GHz+ needs to be modulated with a suitable RLL code like 8B10B. I have not seen any specs but Gallium Arsenide over diamond substrate comes to mind at those frequencies with Laser over single and multi-mode fibre in single or quadrature modulation. \$\endgroup\$ Commented Dec 3, 2012 at 15:18
  • 2
    \$\begingroup\$ Note that you can have multi-bit symbols, so if instead of using binary levels you use 4-levels coding, you halve the required symbol frequency, at the expense of noise margin and circuit complexity. I'm not sure if it applies to Ethernet, though. \$\endgroup\$
    – clabacchio
    Commented Dec 3, 2012 at 15:25
  • \$\begingroup\$ actually 10+Gbps will use 64B/66B where 8B10B is for 1Gbps and many under. Yes they use ◾Very high speed ADC: 5-bit, 4.25GS/s 3-bit ADC's, 17 GS/s \$\endgroup\$ Commented Dec 3, 2012 at 15:25
  • \$\begingroup\$ Dont know much about it, but some R&D in this area uses CMOS 40nm and 28nm for the SERDES clock and data. \$\endgroup\$ Commented Dec 3, 2012 at 15:34

1 Answer 1


On the Altera parts you provide some reference base clock like 156.25Mhz. Then the transceiver section has a PLL that brings the frequency up to I think half the rate. So it'll be 5 Ghz for for the 10Gb/s link. Or it might be lower if instead of having one 10Gb/s link you break it into 4 lanes like we do for the XAUI interface. That clock and the parallel data get fed into the serializer and out comes 10Gb/s serial data. That's the gist of it anyway. You can read more about how the Altera transceivers work here.

Here's an cut out from their documentation.

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