Excuse this late reply, but it seems that the maximum email size that is
accepted is 40Kb, and I didn't know that my email was more like 400Kb with
attached screenshots!


On 2/11/06, Dave Dilatush <davedilatush@comcast.net> wrote:
>
> Sean Schouten wrote...
>
> >I have a designg for a digitally controlable sign wave generator, using
> one
> >of Analog Devices AD9834 DDS's and am looking for a way to turn it's
> output
> >signal into a sine-wave that looks near-perfect on an old analog scope.
>
> Given that you intend to clock the AD9834 at 10MHz and are going
> to be generating sine waves only up to 25kHz, that should be very
> easy.
>
> >I had the idea to use a Sallen-Key filter with a butterworth
> configuration
> >(2nd order), to filter my DDS's output, but after numerous simulation it
> >seems that the filter starts deforming the DDS's output because of it's
> >fase-shift  characteristic of the filter.
>
> You don't say exactly how the sine wave is being "deformed", but
> whatever the nature of this distortion it is unlikely that it has
> anything to do with the filter's phase shift.  When passing a
> sine wave, the phase shift of a filter will only affect the delay
> between input and output-- not the shape of the wave.


Glad to hear that. I am still tempted not to believe it though.


I would guess that any distortion you are seeing in your
> simulation is the result of some fluke in the simulation device
> models, or a consequence of the way your particular circuit is
> designed; without seeing the circuit, it's hard to tell.


I've just been running some more simulations and I still have a problem with
deformations starting in the higher frequencies where the phase shift goes >
X - degrees, just before the cutoff frequency. I have attached some
screenshots so you can see.

3k.gif = perfect sine @ 3Khz, http://seantech.offside.fromadia.com/3k.gif
30k.gif = perfect sine @ 30Khz http://seantech.offside.fromadia.com/30k.gif
300K.gif = more like a triangle @ 300Khz.
http://seantech.offside.fromadia.com/300k.gif

filter_and_response.gif = my current *test* design, and it's simulated
responce characteristic.
http://seantech.offside.fromadia.com/filter_and_response.gif

Why on earth does my output signal deform at 300Khz? I am overlooking
something.

Sean S.
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