Square pulse, Duty cycle and odd/even harmonics.

Discussion in 'General Electronics Chat' started by Antoniog2, May 1, 2010.

  1. Antoniog2

    Antoniog2 Thread Starter New Member

    Joined:
    May 1, 2010
    Messages:
    2
    Location:
    Laval-Quebec-Canada
    To KL7AJ or any one else who could explain…
    I was looking for information about relation between a pulse signal and production of odd/even harmonics, when I read this tread #16118 on 11-18-2008

    Mark / space ratio - does it matter?
    KL7AJ => “However, when you have UNSYMMETRICAL square waves, … … you start getting additive and subtractive components....products of your fundamental square wave frequency PLUS what you would have if your shortened pulses WERE symmetrical! For example, if you have a 1KHz square wave with a 25% duty cycle, you have not odd harmonics of 1KHz, but you have odd harmonics of 4 KHZ, PLUS all the sums and differences possible!”

    As I understand-it, a 25% duty cycle (250uS_On, 750uS_Off) will give with this pulse, a symmetrical signal of 250uS_On, 250uS_Off, or 2kHz and all its odds harmonics (2kHz, 6kHz, 10kHz, 14kHz…), “plus all the sums and differences possible”. (4kHz, 8kHz, 12kHz, 16kHz, …)
    All this is, in this case, the even harmonics of 1kHz, with no odd ones. Is it right?

    When I use a simulator as this little applet
    http://www.falstad.com/fourier/
    with a 25%_On duty cycle, (move arrow under the bottom of the square, in way to change Duty cycle) I find only harmonics of this order : 1, 2, 3,…,5 ,6 ,7 ,…,9, 10, 11, … with all 4x harmonics missing.

    Do you have a simple way to explain me what I’m missing?
    No argument intended!
    I want only to understand how to quickly find odd and/or even harmonics out of asymmetrical pulses, out of simple fractions of duty cycle, as 50/50 (easy, only odd), 33/66, 25/75, 20/80…

    Your way to consider a pulse as a part of a 1/2 symmetrical wave, giving only odd harmonics is very appealing, but not easy to follow for me now.

    Thank you.
    Antonio
  2. t_n_k

    t_n_k AAC Fanatic!

    Joined:
    Mar 6, 2009
    Messages:
    5,086
    If n is the notional harmonic and D is the duty then if nxD is an integer the nth
    harmonic value is zero.

    If D=0.25 then the 4th, 8th, 12th, etc... harmonics are zero.

    If D=0.1 then the 10th, 20th, 30th etc are zero.

    and so on.....
  3. Antoniog2

    Antoniog2 Thread Starter New Member

    Joined:
    May 1, 2010
    Messages:
    2
    Location:
    Laval-Quebec-Canada
    Thanks, tnk.
    This part of my question is clear now.
Similar Threads
Forum Title Date
General Electronics Chat removing overshoot and ringing from square pulse? Oct 1, 2014
General Electronics Chat PLC S7-1200 PTO square pulse distortion May 8, 2013
General Electronics Chat Adjusting Period of Square Wave Pulse Dec 27, 2012
General Electronics Chat Phase shift of square pulse using 555timer Aug 22, 2012
General Electronics Chat clock pulse oscillator and square wave osc. ? Aug 28, 2009

Share This Page