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When reading about high speed serial interfaces like RapidIO or RocketIO, I find data rates of 3.125 Gigabits/second and more.

My question is: 3.125 Gb/s = ?? GHz

What's the relation between Data rates (in bits per second) and Frequencies (in Hz) ?

I want to route these high speed signals and want to know its actual frequency on trace so that I can take care of signal integrity.

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RapidIO employs one or more LVDS (low voltage differential signal) lanes in parallel.

The Unit Interval (simply the duration of one bit) of each single lane is 320 ps (equals 3.125 GHz) when transmitting at 3.125 Gbaud.

It is square-waved, with defined rise and fall times of the signal.

Read more at chapter 8 of http://www.rapidio.org/data/specs/SerialSpec_v_1pt2.pdf

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Firstly, Gigabit links are generally much closer to sine waves than the more square low-speed signals, so you can treat them as such in a first approximation (you won't be far off).

Frequency wise, you have to be a little careful. Every transition in a Gigabit link translates to the next piece of data, so a full 1-0-1 cycle would represent a full 'clock' cycle.

The net result of this is the effective operating frequency of the link is half the line rate (i.e. 1.5GHz for a 3Gb/s link). In other words, you get 2 bits of data for each 1-0-1 transition.

With respect to routing the traces, try to avoid more than 2 vias for each trace of the differential pair when getting from the source to the destination. At higher frequencies it's also worth sandwiching the traces between a pair of ground planes, and using blind or microvias - at 10Gb/s even a normal through-via can create problems. We normally use a 2-core construction with a blind via through the top half and sandwich the serial links between grounds on layers 2 & 4. You may also want to consider PCB materials other than FR4, depending on the trace distance, especially when going > 5 Gb/s. For lower speeds and short routes this is probably overkill, but you should always have a continuous reference plane next to the traces in any case.

If you're really worried, do a proper signal integrity model using something like HyperLynx or a 3D waveform solver, but you probably don't need to worry about that unless (like us) you have > 144 x 10 Gb/s links in a dense board.

Just to summarise some rules of thumb:

  1. Keep traces short (< 10 inches or shorter, depending on frequency).
  2. Antipads on unconnected layers around vias (reduces excess capacitance).
  3. Minimise via transitions.
  4. Keep reference plane continuous.
  5. Microstrip or stripline.
  6. Match skew on differential traces before your vias.
  7. Space from other traces by at least 5x width of traces in differential pair (preferably more).
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Serial RapidIO links are no longer using LVDS I/Os. This was true for 8-bit Parallel RapidIO that ran up to 1 Ghz.

With Serial RapidIO, links can operate at 1.25, 2.5 or 3.125 Gbaud for version 1.3. On the link, we use 8b/10b encoding on the differential pairs. This ensures that the proper number of transitions happen to keep the links trained.

3.125 Gbaud or 3.125 Gbps mean the link is runnign at the 3.125 GHz clock rate.

The effective data rate of a 3.125 Gbaud link running 8b/10b encoding is 2.5 Gbps of useful data being transfered as 20% of the bandwidth is added to ensure enough transistions between 0 and 1 occur on the link.

The initial concern was regarding routing Serial RapidIO links at 3.125Gbps. Please consider accessing the IDT website and the Serial RapidIO support section for a list of suggested board routing rules that will lead your board layout to a successful experience.

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In general 1 Gbit = 1 GHz...

BUT...

This would be a sine wave and not a square wave. A good square wave has at least 5 harmonics, so you might be looking at close to 5 GHz.

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  • \$\begingroup\$ More accurately, as Jxj answered, 1 Gb/s requires minimally about 0.5 GHz to transmit. But as you say, more harmonics are desirable. But 5 harmonics would bring you up to 2.5 GHz rather than 5 GHz. Realistically, a lot of Gigabit designs get by with much less than 5 harmonics. \$\endgroup\$ – The Photon Feb 19 '12 at 3:08

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