# How to caculate wavelength of a pulse wave?

Discussion in 'General Electronics Discussion' started by eefocus, Sep 11, 2012.

1. ### eefocus

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Sep 6, 2012
I am designing a ultrawide band transmitter which emits short pulses.To decide how long for a trace can be counted as transmission line,I need to know the wavelength of the pulses.
These are periodic Gaussian pulses of period 100ns.Pulse duration is 1ns.Two possible thoughts are:
1 Since the Fourier Transform of a single pulse have a central frequency of 4GHz and bandwidth of 2GHz,wavelength is light speed in medal(copper)/4Ghz.
2 Since wavelength is the distance of pulse travels in one period,it will be pulse duration*light speed in medal.
Obviously these two are different.

Thanks a lot!

2. ### Harald KappModeratorModerator

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Nov 17, 2011
Your error is within the first part of your question. A signal with a period of 100ns will be represented as a fourier series with a lower frequency of f=1/T=10MHz plus overtones. Not 4GHz (although 4GHZ will be part of the spectrum due to the small pulse). You cannot consider the pulse width alone for the frequency analysis. You have to take into account the full period of the signal.

The two equations
wavelength=c/f and
wavelength = c*T
where c=speed of light, f=frequency, T=period are otherwise identical since f=1/T.

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May 8, 2012

4. ### eefocus

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Sep 6, 2012
For a trace,if its length is over 1/20 of the wavelength,it will be regarded as transmission line and consider impedance match.Otherwise it won't.

5. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
This I can understand but a transmission line isn't a match unless terminated in its characteristic impedance.