### Author Topic: PCB traces and transmission line  (Read 1721 times)

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#### joniengr081

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##### PCB traces and transmission line
« on: June 29, 2023, 08:18:14 am »
Hi,

I have heard that at some point when the signal clock frequency or bandwidth is increased beyond a certain value than the PCB traces turn into a transmission line. What does that mean ?

#### nctnico

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##### Re: PCB traces and transmission line
« Reply #1 on: June 29, 2023, 11:13:17 am »
Hi,

I have heard that at some point when the signal clock frequency or bandwidth is increased beyond a certain value than the PCB traces turn into a transmission line. What does that mean ?
That is the wrong question. When a conductor (any kind of conductor) is longer than 1/10th of the wavelength (as a rule of thumb), you should consider it a transmission line. So what you want to learn is transmission theory. Yes, 50Hz/60Hz mains grid distribution means dealing with transmission lines.
« Last Edit: June 29, 2023, 11:23:15 am by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.

#### joniengr081

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##### Re: PCB traces and transmission line
« Reply #2 on: June 29, 2023, 01:27:27 pm »
I use Saturn tool to calculate the maximum length of conductor in FR4. I just put clock frequency 1000 MHz and 1/10 wavelength divide factor, I get 1.18 inches. How much this value is realistic. How about if I need to route 2 inches a digital signal with clock frequency of 1 GHz in FR4.

#### ataradov

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##### Re: PCB traces and transmission line
« Reply #3 on: June 29, 2023, 03:31:50 pm »
It is realistic, but it does not mean it is impossible to have longer traces. All you need is impedance match your traces and follow the guidelines for transmission line routing (no stubs, avoid vias, have solid ground planes). And any devices that can generate or receive 1 GHz would specify the impedance values, your goal is to match them to avoid reflections.
Alex

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