# Calculating signal travelling time in FR4

Status
Not open for further replies.

#### joniengr

Hi, I am working with a PCB layout having 500 MHz signal and I need to calculate, not very precisely, the time required to travel 400 mil (1.016 cm) in typical FR4.

I guess the signal speed in typical FR4 is something like 15 cm / ns (0.015 cm / ps). Is this speed is frequency dependent ? If yes, then how much is the speed of 500 MHz signal in typical FR4 ?

Assuming typical speed in FR4 is 15 cm / ns (0.015 cm / ps), I am trying to calculate the time to travel 400 mil (1.016 cm). I am not sure if the following calculation is correct.

Speed = Distance x Time

Time = Speed / Distance

Time = 0.015 / 1.016 = 0.0148 ps

The two last lines are completely wrong, time = distance/speed

Propagation speed is light speed/sqrt(Er,eff). Your assumed 0.15 m/ns (0.5 light speed) is roughly correct for embedded microstrip on FR4, microstrip has typically higher speed (Er,eff < 4), stripline lower speed (Er,eff > 4). You can use Saturn Toolkit, tab Conductor Impedance to calculate propagation speed for your actual transmission line geometry.
--- Updated --- I need to calculate time in picosecond.

Speed = 15 cm / ns
Distance = 400 mil
Time = ?

My finding is 0.0148 ps, Can someone please check this ?

My finding is 0.0148 ps, Can someone please check this ?
I already checked it and told you, that it's completely wrong.

Time = distance/speed. Assuming 15 cm/ns (possibly not exactly correct for your PCB, but at at least in the ball park)

you get t = 1.016/15 = 0.0677 ns = 67.7 ps.

Points: 2