What is 'Solid Ground Plane'?

[rainman1]

And what is the difference between solid ground plane and non-solid ground plane?

Thanks.

 

[Mikebits]

Solid ground plane means there is no break in the copper, just a solid plane. Non-solid would then imply ground plane mixed with signal traces.

 

[rainman1]

Is ground made of copper?

 

[Mikebits]

Is ground made of copper?

That is usually the case. I think gold plate is used in some cases, silver sometimes. I have a few bare PWB's that are gold plated. The ground plane principle remains the same, whatever the conductor material is.

 

[rainman1]

Thanks a lot man.
I love the chinies girl who hangs out in here!

 

[rainman1]

I have a question please,
It is said that solind ground is absolutely necessary for designs with large amounts of high speed devices (edge rates < 5ns).

Why is that?
(what does it mean edge rates < 5ns)

 

[Mikebits]

Edge rates means rise time of a signal some signals are slow rising like / others are fast |. The fast edges implies higher frequency components within the waveform. With high frequencies, groundplane becomes very important.

 

[rainman1]

Edge rates means rise time of a signal some signals are slow rising like / others are fast |. The fast edges implies higher frequency components within the waveform. With high frequencies, groundplane becomes very important.

Thanks!

That was just my question (my other one ), why is solid ground plane specially important for high frequency currents?

 

[Ubergeek63]

unless you have proper decoupling or a 4 layer board, a good ground plane can be rendered useless.

even at relatively low frequencies you need both to pass EMI.

Dan

 

[rainman1]

Thats good to know, thank you.

But why is it specially indicated that a solid ground is needed for high-frequency currents?
What is the different consequenes of high and low frequency currents?

 

[crutschow]

But why is it specially indicated that a solid ground is needed for high-frequency currents?
What is the different consequenes of high and low frequency currents?

A high-frequency or fast rise-time pulse requires a ground plane in close proximity to carry the return signal. else the signal will radiate and the high frequency signal or signal edge becomes distorted. Even a small gap in the ground plane which causes the return signal to take a different path can cause significant problems with the signal. For very high frequencies, microstrip traces are used where the trace width, and distance to the ground plane are controlled to maintain a specific impedance and minimize signal reflections.

A low frequency return signal can take a longer or different path without serious radiation or distortion so a ground plane is not as essential. The frequency at which a ground plane is required is not exact. Generally to be safe, a ground plane is used for most circuit boards operating much above audio frequencies including most digital boards.

 

[rainman1]

A high-frequency or fast rise-time pulse requires a ground plane in close proximity to carry the return signal. else the signal will radiate and the high frequency signal or signal edge becomes distorted. Even a small gap in the ground plane which causes the return signal to take a different path can cause significant problems with the signal. For very high frequencies, microstrip traces are used where the trace width, and distance to the ground plane are controlled to maintain a specific impedance and minimize signal reflections.

A low frequency return signal can take a longer or different path without serious radiation or distortion so a ground plane is not as essential. The frequency at which a ground plane is required is not exact. Generally to be safe, a ground plane is used for most circuit boards operating much above audio frequencies including most digital boards.

What does it mean when a signall radiates?

I read that in high-frequencies, if a return current will go thru a big loop, The wire will become a relatively large inductor, and due to high frequency, its impedance will be very large, and therefore (here is my conclusion) the 0V of the circuit wont be 0V but lets say x volt, is it right? (what is x then? more than a few mV?)

I'd love to hear about the problems that it makes, like the radiation problem you arouse which i didnt quite understand.


Thank you!

 

[Sceadwian]

Radiation means the energy pops off the wire into free space and radiates outwards, IE your circuit becomes a radio. At high frequencies this is very easy to do, even over short distances if the PCB is designed poorly. It depends entirly on the frequencies you're working with.

 

[rainman1]

Radiation means the energy pops off the wire into free space and radiates outwards, IE your circuit becomes a radio. At high frequencies this is very easy to do, even over short distances if the PCB is designed poorly. It depends entirly on the frequencies you're working with.

Thanks.
Is it what called antena loop?
I really wanted to learn about it in next few days and now i'm wondering if that is the case you're talking about.

EDIT:

What causes this radiation is that that if no solid ground plan would be exist then the current will go in loop?

 

[dknguyen]

Current always travels in a loop. WIthout good decoupling and a ground plane, the current will travel in a larger loop (making a bigger antenna and radiating more noise).

 

[Sceadwian]

Not sure, RF on PC boards is a bit of black magic for me. I barely understand it in free air with widely spaced antennas.

 

[Ubergeek63]

Current always travels in a loop. WIthout good decoupling and a ground plane, the current will travel in a larger loop (making a bigger antenna and radiating more noise).

LOL

we had an engineer here that just could not grasp the concept. 3" loops to the "decoupling" capacitors and multiple 1-2" long traces stitched through the PCB between portions of the "ground plane". Nothing like a good multiphase resonant antenna, eh?

Dan

 

[crutschow]

Another way to look at a high speed digital signal, is that as the pulse propagates down the trace, a mirror-image current pulse is propagating underneath the trace in the ground plane. You don't want that ground plane pulse to take a different path then the signal pulse. If the ground has a break which causes the plane pulse to take a longer path, then it will spread out and more readily affect other circuits. The ground pulse will also arrive at the destination at a different time then the trace pulse will will cause a distortion of the signal at the destination.