Motherboard bus speed physical maximum

[DwithQs]

I read somewhere (article, messageboard, i can't find it anymore) that motherboard bus speeds are limited because if the frequency gets too high then the signal will disappear. I guess I understood it to mean that the bus wire will become like and antenna if the frequency is too high.

They then went on to explain that it is the reason why processors use multipliers to run at higher speeds even though the bus isn't capable.

Is that true?

 

[Danger]

I don't know. Maybe that's why people are working so hard to get optronics up and running.

 

[phinds]

I doubt if they meant the signal will "disappear" but likely they meant it will degrade/weaken. Possibly due to "skin effect" if the motherboard "wires" are very small (which they are) and the frequency gets high enough for the skin effect to be significant.

 

[256bits]

The signals on the motherboard traces act as if on a transmission line, with impedance, capacitance and all that fun stuff to take into account. Since motherboards can reduced to a certain size to accept components, and copper traces can only but placed so many per inch, propagation delays result. Someone better versed in transmission line theory would be able to a better description than myself, but for starters you might want to consider this PDF.
Most likely you can make better sense of it than I can.
http://www.polarinstruments.com/support/cits/Critical_length.pdf

 

[Svein]

The transmission line concept is valid, but at the current speed of microprocessors the dimensions are extremely critical.

Example: On a typical motherboard the propagation speed of electrical waveforms is about 20cm/ns. If the clock frequency of your Core 5 is 2.5GHz, this corresponds to a wavelength of 8cm.This again means that the layout of your CPU should have no dimension of this order (otherwise the internal signals would be out of phase just due to differences in traveled distances. A good rule of thumb is to keep the high-speed circuitry within 10% of the wavelength - which means 8mm circuit size.

If we are talking about transmission lines, the situation is different. Here we know that the signal will take whatever time is necessary to reach the other end of the line. In this case we are concerned about waveform degradation, reflections and line termination. The critical parameter for a transmission line is not so much the clock speed of the signals as the waveform rise and fall times.