(only in English)
What is the main difference is between an analog board for 1GHz and a digital board for 1Gbit/s?
The simple – but incorrect – answer is that the digital board only will go to 500MHz since the clock is normally half the data rate.
The digital 1Gbit/s board is in fact much more difficult to design than a 1GHz analog board. If the specification for a board is ‘1GHz analog’ it means that the signal level at the specified frequency is -3dB or half the amplitude. For a digital board, the situation is much more complex. To convey a 1Gbit/s signal it’s true that the clock only needs to be 500MHz, but on the other hand at least the 3rd overtone needs to be present which is 1.5GHz. It is actually not the frequency that’s interesting in a digital board but the edge rates. A digital circuit capable of 1Gbit/s data rate may have signal edges in the range of ~200ps. The normal rule of thumb is that the bandwidth = 0.35/edge rate which in this case means 1.75GHz!
Boards carrying digital signals at 3.3V logic levels with edges of 200ps requires a lot of attention!
Crosstalk is a major concern and needs to be checked continuously. All traces must be designed as transmission lines and everything needs to be impedance controlled and terminated to reduce reflections and EMI. Power integrity must be maintained to ensure that the circuits are fed with low enough power impedance to be able to maintain the signal edges. The board stack-up must be defined to maintain trace impedance as well as power impedance. 20 layers for a high-speed digital board is nothing unusual.
Do you feel up to this challenge?
We have done many of these boards in the past and look forward to making many more!