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pcb trace as transmission line

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alok_ky

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when does a pcb trace acts as a transmission line ?

I know that there is some rule that the trace will act as a tx line when:

Rise time < 2* loaded flight time along the trace.


BUT, MY QUESTION IS : HOW THIS FORMULA CAME ???

I want to know the subjective reasoning behind that.
 

ramani

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pcb trace acts as a transmission line when frequency of operation increases so pcb traces can be modelled as a waveguide. and signal passing thro it bahaves as a guided signal in a waveguide.
normally when a rise time increases bandwidth also increases.
 

William Cheung

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Any book about microtrip can tell you about this. Basically, when the wavelength is comparable to the length of the microstrip, the microstrip behaves like an antenna. We know that in antenna theories, higher the frequency, shorter is the length of the antenna required. That is why we need a longer antenna for our cordless phone at home, but a short one for our mobile phone. This is also why when the clock frequency of our CUP get higher and higher, we have to pay more attention to the radiation from it which may become interference.
 

v_c

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You can calculate the parameters using this
http://www.emclab.umr.edu/pcbtlc/

Just choose a structure and enter in your parameters.

If you want to know more about this topic you should take a look at the books by Howard Johnson "High-Speed Digital Design" and "High-Speed Signal Propagation".
 

jallem

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see section 1.3 on High Speed Digital Design by
Johnson/Graham
 

alok_ky

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i mean to ask, that

how this formula is derived from signal integrity issues ?

What will happen to the travelling wave, if this rule is not met ?
 

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