[time-nuts] DDS'ery

Luis Cupido cupido at mail.ua.pt
Mon Jun 20 15:36:43 UTC 2011


Thanks for the comments...

Yes the key is obviously the low pass filtering
one has the other doesn't.

I had mainly in my mind a PLL following that ACC.
so actually driving a phase comparator of
a PLL (narrow enough loop bw) that problem would
not exist. Ok great.

Good point on the mixer we got to have nothing in
the alias region for it to be ok. (not my application
at the moment but will keep that in mind)

Thanks guys...

Luis Cupido.
ct1dmk.




On 6/20/2011 4:11 PM, Graham / KE9H wrote:
> 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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