[time-nuts] DDS'ery

Graham / KE9H timenut at austin.rr.com
Mon Jun 20 11:11:41 EDT 2011


Luis:

No, not the same.

The most significant bit out of the accumulator has the alias information
on it (Fs +/- Fo), so it still needs to be run through the low pass filter
to clean off the alias signals.  The alias signals manifest themselves 
as jitter,
so no amount of just clipping will remove them.

If your application is not sensitive to the alias frequencies, then OK to
drive out of the DDS directly.

If you are driving something like a mixer in a wide band radio, then you
still need to use the low pass filters.  They don't call them anti-alias 
filters
for no reason.

--- Graham / KE9H

==

On 6/20/2011 9:46 AM, Luis Cupido wrote:
> Folks, a quick one...
>
> A DDS, that is an accumulator with a DAC followed by a low pass filter 
> and comparator (zero crossing) to produce a square wave to drive a PLL 
> or a MIXER or else (at logic levels).
>
> Isn't it the very same thing as just using the most significant bit of 
> the accumulator.
>
> Or am I missing something here ?
>
> Comments appreciated.
> thanks.
>
> Luis Cupido.
> ct1dmk.
>
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