From: "Klaus von der Heide" <v.d.heide@on-line.de>
To: rsgb_lf_group@blacksheep.org
Date: Tue, 20 Mar 2001 20:41:12 +0100
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Subject: Re: LF: Re: RE PICs and DDS etc.
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Hello Alberto, hello all,

> I thought again and, yes, the high order bit of the phase accumulator is not
> the squared up version of the sine output, but, after the phase accumulator,
> a DDS has a sine lookup ROM. What I really meant was the high order
> bit of the output of this ROM. Using this bit, I feel confident to state again
> that you don't need a DAC to generate a square wave of the wanted frequency.
> 
> With the AD9850 you have to use the comparator, as you do not have external
> access to the output of the sine ROM.
> You are right saying that a PIC probably doesn't have enough horse power
> for this task. Maybe some chips  of the Scenic family ???

The problem is the discrete time the DDS uses. The high order bit 
only changes at the clock edges. The result is a square wave with 
heavy phase jitter.  For example, draw a sine wave (3 waves), 
sample it at about 8/3 of its frequency in your sketch, and generate 
the square wave from these sample values (all on the paper). 
The one-periods and the zero-periods then are one or two clock cycles 
long. That's what Andy in other words already said.
If a DAC is used then time is continuous and a clean square wave can 
be obtained. 

73 de Klaus, DJ5HG