high speed nets on outer layers.

[buenos]

hi

Intel says, put all high speed nets (DDR, IDE, PCI, USB) into inner layers, and make them to be striplines. I think its good, but is it necessary?

If i use a not intel, and not GHz processor (500MHz, AMD-GeodeLX embedded uP), but i use the same interfaces, (DDR, IDE, PCI, USB) then should i put them inner? If i do that, my layer count starts at 8 layers. The AMD says, we can develop 4-6 layer PCBs for that uP, it means, there are high speed buses on outer layers.

So, who has wright?
Both of them would pass the FCC?

Another aspect: there are Wlan and bluetooth modules connected to the motherboard.

 

[rjainv]

I think its abt SI. Is AMD targetting same speed for DDR as intel board ?
For other i/f, the speed is pretty much standard, so these difference in recommendation from two might be just overdesign from Intel, or may be the Intel device has poor driving/receiving charaterisitics/margins, and they are improviding margins by better board design as compared to AMD..

 

[lladnar23]

I think the recommendation to use striplines for the high speed interfaces is because you get better noise performance from stripline (vs. microstrip).
Since microstrip is considered an inhomogenous transmission line (dielectric below the line is not the same as the dielectric above -- air), you get a lot of far-end crosstalk. Furthermore, the far-end crosstalk gets worse with length of line and fast rise-time. So it's probably best to stick with Intel's recommendation, unless you are able to run some tests to show that you can get away with microstrip...

 

[Santa]

My 2 cents to this topic :

- Microstrip has lower loss due to the fact that air is less lossy than board dielectric -> better for high frequencies like PCIe
- Stripline needs thinner traces to achieve same impedance than microstrip -> better density
- Copper thickness on internal layers is usually more constant when they did not require plating for blind vias -> more constant impedance -> less reflections -> less loss for stripline due to this effect
- Stripline reduces EMI compared to microstrip which is exposed at the surface.