# FEDEVELPlatform forum

## Transmission lines - velocity of signal

pixelx , 11-07-2022, 11:11 AM
Hello,

In this video was calculated length of track and I don't understand how many is ,,v" and this is velocity of the signal. This parameter is necessary to calculate the maximum lenght of trace.

3:19
What's the difference between a transmission line and a trace? Here's a hint: all traces and their ground planes are transmission lines! In this video, Tech ...

qdrives , 11-07-2022, 01:57 PM
v=C / sqrt(Er)
C=speed of light (300Mm/s or 300mm/ns)
Er=dielectric constant, FR4 is about 4...4.5
So very simplistic v=150mm/ns
pixelx , 11-07-2022, 02:02 PM
I understand, thank you!.
I have the next question. Where can I find time tr in datasheet elements?
I think it is more complex in my opinion tr is dependent on frequency and how can I calculate it?​
qdrives , 11-08-2022, 12:59 PM
The rise time is rarely in the datasheet.
For theoretical values you could use the IBIS model (digital components).
Otherwise there is no other way then to measure it.
robertferanec , 11-12-2022, 02:58 AM
there may be multiple ways where to get tr,
- as @qdrives mentioned, ibis model can help (usually you can set three different variants, slow, normal and fast)
- interface specification may give you some values
- good datasheets will have them, usually in the AC Electrical characteristics, look for rows with units in ns or ps to scan the tables quickly. Sometimes this may depend on register settings, this can give you an idea: https://youtu.be/gOS8LyExKDk?t=152