Xref: utzoo comp.dsp:1657 sci.electronics:20161
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From: mcphail@dataco.UUCP (Alex McPhail)
Newsgroups: comp.dsp,sci.electronics
Subject: Re: 180 deg phase shift
Message-ID: <625@fudd.dataco.UUCP>
Date: 13 May 91 19:03:54 GMT
References: <1991May5.233533.18783@ux1.cso.uiuc.edu> <5781@media-lab.media.mit.edu.MEDIA.MIT.EDU> <1991May8.222501.19572@syd.dms.CSIRO.AU> <1991May10.003817.5593@milton.u.washington.edu>
Reply-To: mcphail@taarna.UUCP (Alex McPhail,DC )
Organization: Canadian Marconi Company (Datacomm), Ottawa, Ontario
Lines: 87

>> Inverting is NOT the same as 180 deg phase shift. For a symetric waveform
>>(eg a sine wave) it looks the same, but with something assymetric what you
>>will see is the waveform upside-down, which is not the same as shifted 180
>>deg. Phase shifting moves a waveform along the time axis: it stays the same
>>way up. Try it on an oscilloscope.

In article <1991May10.003817.5593@milton.u.washington.edu> whit@milton.u.washington.edu (John Whitmore) writes:

>	That's wrong.  The inverter is a perfectly good 180 degree
>phase shifter, and if you test it at ANY frequency you will see
>180 degrees of phase shift; the phase shift versus frequency is
>EXACTLY what was asked for.
>	Your 'assymmetric waveform' has a lot of Fourier components,
>and time-shifting it as you seem to be describing is NOT a
>well-defined operation.  You have to find some particular frequency,
>derive a time delay from THAT ONE FREQUENCY COMPONENT, and apply
>that time delay to get the time-shift, and that is NOT the correct
>phase shift for any frequency component except the one you
>chose as 'most significant'.
>
>	John Whitmore


Sorry, John.  I usually don't butt into other people's business here, but
you really are dead wrong.  The phase shift described above is indeed 
correct.  Since any periodic function is always measured against time, 
shifting the function's phase is also a time-related operation.  

In specific response to your article, frequency and Fourier components
have nothing to do with phase shifting.  The shift of a phase, in degrees,
can be expressed by:

dt
-- x 360
L

where:  dt is the shift of the function
        L is the wavelength of the function

Diagramatically, the following example shows the difference 
of a sawtooth wave shifted by 180 degrees, and an inverted sawtooth wave.

      .      .      .      .
     /|     /|     /|     /|
    / |    / |    / |    / |
   /  |   /  |   /  |   /  |
  /   |  /   |  /   |  /   |
 /    | /    | /    | /    |
/     |/     |/     |/     |          Normal sawtooth wave


         .      .      .      .
        /|     /|     /|     /|
       / |    / |    / |    / |
      /  |   /  |   /  |   /  |
     /   |  /   |  /   |  /   |
    /    | /    | /    | /    |
   /     |/     |/     |/     |       Shifted sawtooth wave


\     |\     |\     |\     |
 \    | \    | \    | \    |
  \   |  \   |  \   |  \   |
   \  |   \  |   \  |   \  |
    \ |    \ |    \ |    \ |
     \|     \|     \|     \|
      `      `      `      `         Inverted sawtooth wave
============================================================================
________
|\     /  Alex McPhail
| \   /
|  \ /    mail to mcphail@dataco              
|---X     (uunet!mitel!melair!dataco!mcphail)
|  / \
| /   \   "Wwwwwhat is the air speed of an average swallow?"
|/_____\   
  Alex


 

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