Transmission lines and impedance control PCB design

[barakg]

Hello everyone,

I have wondered about the rule of thumb about transmission lines that says - if trace length is over 10% of wavelength of the signal then transmission line effects should be taken into account.

Let's say that I route a USB 2.0 signals with 480 Mbps the frequency is equivalent to ~200MHz square wave (according to google )
So if I make the signals length bellow 150mm(10% of the wavelength) can I route the signals without impedance control, length matching or termination resistors?

Is my theory right? or am I missing something? Please tell me.

If my theory is correct, why does engineers consider impedance control ,length matching or termination resistors when the signals length are bellow 10% of wavelength?

Thanks alot!

[robertferanec]

Your "square wave", will contain harmonics of higher frequencies.

[barakg]

Thank you very much Robert for the quick response.

Ok, I got it.

I have another example. lets say that I have routed DDR2 signals and the signal's length from CPU to RAM is 25mm.
in this case it would mean that frequency should be roughly above 1 GHz .RAM clock frequency is only 133 MHz and even the first few harmonics are under the 1 GHz.
is it should work fine? or I need to consider impedance control, termination resistors and length matching.