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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!
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.
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.
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