# FEDEVELPlatform forum

## Signals Travel In Different Speed

tariq1996 , 05-20-2020, 05:34 AM
Hi Robert,
You said " The signals in top/bottom layers travels in difference speed than signals in internal layers", the question is WHY?
Why is the signals travels in different speed if the layers where different even though they have rated speed of (300 Mbit/s) for example.

Thank you!
robertferanec , 05-21-2020, 11:03 AM
Because environment around the tracks is different. Also here I speak about it a little bit, maybe it can help: https://youtu.be/rdlEm2xjCsc?t=919
xaviondono , 05-22-2020, 08:36 AM
Originally posted by tariq1996
Why is the signals travels in different speed if the layers where different even though they have rated speed of (300 Mbit/s) for example.
Thank you!
Careful with mixing signal speed with bitrate. If I send you 10x 12TB hard disk with 24h shipment, the bitrate is over 1GB/s, but it will still take a day to get there 😉

robertferanec , 05-22-2020, 08:51 AM
Careful with mixing signal speed with bitrate. If I send you 10x 12TB hard disk with 24h shipment, the bitrate is over 1GB/s, but it will still take a day to get there 😉
- I need to remember this example
chitransh92 , 05-23-2020, 04:14 PM
Hello,

The signal travel in form of electromagnetic (EM) waves.
Theoretically these travel at the speed of light that is c=3x10^8 m/s.
Practically this depends on the medium in which the signal is travelling and it can be calculated by a factor "sqrt(Er)" which is square root of Dielectric constant of the medium and math turns out to be "v = c/ sqrt(Er)" where v = velocity of signal.

When the signal is in the internal layers it is surrounded on both sides by the dielectric medium.
For Top and bottom layer one side is air which has a dielectric constant of 1 and other side is FR4 (Could be any other material) hence "The signals in top/bottom layers travels in difference speed than signals in internal layers".

Thanks.
tariq1996 , 05-24-2020, 04:37 AM
Thank you all.
robertferanec , 05-25-2020, 01:17 AM
@tariq1996, this video may also help: Understanding PCB Layout Essential - Change the way how you look at the tracks https://youtu.be/EUAJbNnqhLs
beamray , 05-29-2020, 10:51 AM
I may add something theoretical here