# Transmission line impedance

1. ## Transmission line impedance

Dear Sirs,

I read LVDS spec a few day ago. I found these words, transmission line impedance. I don't know how to measure this value on printed circuit board. And how can I reach output impedance match? Wish your help. Thanks.

Best Regards.

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2. ## Re: Transmission line impedance

LVDS goes up to 200MHz Right?
Well as you get higher in Freq you circuit traces take on a L-C characteristic.
So at a high freq a traces that's .200" will look like a (Let's Say )200 Ohm
line, and if your Freq goes up by say 80% that same line might look like a
90 Ohm trace.

The best way to measure it is to make a trace 1 Wavelenght long and measure it
with a Network analyzer. But chances are you can not do this. So you might try
calculating the value. You can try HP's Appcad, Rogers web site has a calculator,
and I think Microwave Office has T-Lin for free. But even if you get all the parameters
for the T-line impedance what you calculate will most likely be off, that’s just the
way it is. One other rule of thumb is to try and keep similar signals on similar lines (Lengths and Widths)

Good Luck

3. ## Re: Transmission line impedance

Impedance on a PCB is measured using a Time Domain Reflectometer - the length of the line has no bearing on the AC impedance, it determines whether or not you line is "transmissive" - in the case of LVDS, (can go a lot higher than 200 Mhz), you need to calculate differential impedance, there are many for sale calculators and a couple of free ones, available - a new one I just found is at hxxp://www.4thdpcb.com/ -

SiGiNT

Articles can be found at hxxp://www.ultracad.com, along with a free single-ended Impedance calculator.

SiGiNT

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4. ## Transmission line impedance

when we say that track length is transmission line ?
my question may be silly!

Binu G

5. ## Re: Transmission line impedance

Some textbook say that when track length is >λ/10 you
should consider it a transmission line.

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6. ## Transmission line impedance

That's a reasonable rule of thumb. I'd say λ/20.

THe fast rising and falling edges in digital circuits contain high frequency energy eg a 10MHz square wave
contains significant energy at multiples of 10MHz.
The harmonic energy needs to be considered when looking at transmission line effects.

7. ## Re: Transmission line impedance

Check this discussion

https://www.edaboard.com/viewtopic.p...913&highlight=

if you do a little of math you will find the answer.

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