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High Speed PCB Design

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Johnny101

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Hello everyone I am looking for considerations to keep in mind while designing a pcb(Only double sided available) with high speed signals (upto 100MHz).
What I know up till know:
1)Using small traces to get low inductance and capacitance.
2)Using a complete layer of the two sided pcb as ground plane to act as a decoupling capacitor.
3)Avoid removing any part from the ground layer so that the return current is exactly under the signal current.

Questions:

4) Would using smds be a better option than through hole components and why?
5) If I use smds, I would have to use vias to provide ground is using vias for ground and also other connections a good option?
6)What other techniques can be used to minimize the stray capacitances and inductances at high frequency.
7)What is with the transmission line termination and control impedance?
8)Microstrip:what is it ? In case of double sided pcb with one ground layer and the other etched part is this microstrip?
9)What should I do with the power should it be provided on the etched side?
10)Removing the emi.

Please correct me where wrong Please try to answer all it would really help me.
 

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Johnny101 said:
1)Using small traces to get low inductance and capacitance.
Given that you are using a two layer board using small traces (does this mean narrow traces or just short runs?) i.e. narrow trace width the impedance won't be even close to 50 ohms.
http://www.eeweb.com/toolbox/microstrip-impedance

Johnny101 said:
2)Using a complete layer of the two sided pcb as ground plane to act as a decoupling capacitor.
Nope that isn't going to function as a decoupling cap. PCBs that are built with a GND/PWR pair embedded in the board that are separated by by 4 mils or so will act as a high frequency decoupling. A single ground plane won't act like a cap. You better make sure you actually have decoupling caps on the board.

Johnny101 said:
3)Avoid removing any part from the ground layer so that the return current is exactly under the signal current.
Avoid slotting the ground if you don't want a lot of ground loops and potentially weird unexplained behavior.

Johnny101 said:
4) Would using smds be a better option than through hole components and why?
5) If I use smds, I would have to use vias to provide ground is using vias for ground and also other connections a good option?
As I've never built a homemade board I can't give any recommendations. I've always had multi-layer boards built.

Johnny101 said:
6)What other techniques can be used to minimize the stray capacitances and inductances at high frequency.
7)What is with the transmission line termination and control impedance?
You really need to worry more about 7 as that is the one that will cause the design to fail. If you don't control the impedance and treat every signal (your high speed ones i.e. the edge rate of transitions) as a transmission line you'll end up with a board that has reflections on every signal, which really messes up your signal integrity.

Johnny101 said:
8)Microstrip:what is it ? In case of double sided pcb with one ground layer and the other etched part is this microstrip?
see my link the pictures of the various topologies are shown.

Johnny101 said:
9)What should I do with the power should it be provided on the etched side?
Hello, where else is it going to come from? You may get stuck routing some of it on the GND side of the board too.

Johnny101 said:
10)Removing the emi.
Use a multi-laayer board with grounds on both top and bottom and buried vias. Or put a can on the top side of the board that is connected to your ground plane. Otherwise it's not likely to be all that EMI friendly unless you use differential signals on the board.

Good luck I think your going to need it.

Regards
 

marce

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Without the schematic it is hard to give an educated guess, but I second the comments above....
Myself I would consider a 4 layer board the absolute minimum for this sort of design, you need a contiguous ground plane, anything less is to much of a compromise and not worth doing....
 

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