Platform forum

Transmission lines and impedance control PCB design

barakg , 09-18-2017, 09:54 AM
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 , 09-18-2017, 10:35 AM
Your "square wave", will contain harmonics of higher frequencies.
barakg , 09-18-2017, 12:04 PM
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.

Use our interactive Discord forum to reply or ask new questions.
Discord invite
Discord forum link (after invitation)

Didn't find what you were looking for?