[time-nuts] DDS'ery

[Mike Feher]

Actually it is (n*Fs +/-Fo), however, they do go down in amplitude as a
SinX/X. Theoretically, they would also be there after the Sin ROM, and the
only one really of concern in the real world is the Fs - Fo, hence the LPF.
When I used to design DDSs in the early 70's, we typically tried to use an
Fs that was about 2.5 times the max desired Fo, to make filtering easier. We
also used discrete parts then :). Now, with the higher speed devices
available a real high Fs can be selected to meet a desired Fmax. Of course
the limit is always being pushed, and, the devices are never fast enough.
Regards - Mike 

Mike B. Feher, EOZ Inc.
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960 office
908-902-3831 cell

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Graham / KE9H
Sent: Monday, June 20, 2011 11:12 AM
To: time-nuts at febo.com
Subject: Re: [time-nuts] DDS'ery

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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