I agree with Marc.

I'm presently doing this using the delays in the XOR.  As a frequency
doubler and 4 XOR's.  Also, you can use the Dallas Semiconductor DS1010-400
and maybe the DS1012.

Another way that I've done it is by building up a "phase shifer".  This is
an analog approach using a center tapped transformer and a RC combination.
By varying R the phase can be adusted to anything one wants.

David



----- Original Message -----
From: Marc <marc@AARGH.FRANKEN.DE>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Monday, May 31, 1999 1:58 PM
Subject: Re: [OT] Phase delay of square wave


> > I'm looking for a means of phase shifting a square wave clock pulse by
> > 90 degrees.
> >
> >      ______      ______
> >     |      |    |      |
> >     |      |    |      |
> > ----        ----        ----
> >         ______      _____
> >        |      |    |     |
> >        |      |    |     |
> >    ----        ----       ----
>
> One possible way (not necessarily the easiest) is to:
>
> 1) differentiate the signal by XORing each port reading with the previous
>    one:
>
>
>       ||     ||   ||      ||
>       ||     ||   ||      ||
>   ---- ------  ---  -----   ----
>
> 2) low-pass filter this new signal.  If it is of unknown and variable
>    frequency, the easiest way is to:
>
>    a) count the number of samples between two 1s (= period)
>
>    b) output 1/2 that many samples of 1, and 1/2 that many as 0.
>
>    That effectively does generate a new waveform with the desired
property:
>
>        ___    __   ___     __
>       |   |  |  | |   |   |  |
>       |   |  |  | |   |   |  |
>   ----    ---   --    ----   ---
>
>   Wow, you have doubled the frequency!
>
> 3) XOR the output of 2) with the original input:
>            _____       ______
>           |     |     |      |
>           |     |     |      |
>   --------       -----        ----
>
>
> That's it!! You have shifted phase by 90 degree - on any single frequency
> input - no matter which frequency (as long as it is substantially lower
> than your sample rate).
>
> Your example waveform is a good example: it is not a pure "square sinus"
:)
> All frequency deviations were preserved during the processing, and the
> output is a 90 degree shift, as good as the ASCII art resolution can
> get.
>
>
> Make sure that you read the port only ONCE per sample, and use the
buffered
> values for processing. If you read it twice and the calculations base on
> two versions of it, the output might become flickery..
>