If you use a DDS then it pays to use the Analog Devices design tool.
It is at this URL
[1]http://designtools.analog.com/dtDDSWeb/dtDDSMain.aspx
You can open multiple instances in different browser windows. Also,
there is an area under the graphs called DISPLAY which allows for some
selections. One of them is for a filter. If you click on the blue
configure to the right of the filter selection it allows you to set the
parameters for the filter.
It is interesting to note just how imprecise these DDS devices really
are ! For instance, the tendency to use the house standard (i.e., 10
MHz) as the DDS source clock produces the following values for 32 KHz
clock.
1. AD9850 DDS {32 bit tuning/10 bit output} with 10 MHz clock
produces: 32768.00038293 Hz
2. AD9956 DDS {48 bit tuning/14 bit output} with 10 MHz clock
produces: 32767.999999983 Hz
You would think scaling the clock by [sub] multiples would make no
difference. However, this is not true. You would think using a clock
that was a binary multiple, just like the 32 KHz number, would make it
come out precisely. As well, this was not the case !
Experiment with the better 48 bit DDS and varying the clock from 10 MHz
down with the following results:
* CLOCK FREQUENCY
*
* 0.01 MHz 32768.0000000000 Hz perfect but no output ! See Mr.
Nyquest.
*
* 0.1 MHz 32768.0000000001 Hz
*
* 0.2 MHz 32768.0000000001 Hz
*
* 0.3 MHz 32768.0000000004 Hz
*
* 0.4 MHz 32768.0000000001 Hz
*
* 0.5 MHz 32768.0000000008 Hz
*
* 0.6 MHz 32767.9999999994 Hz
*
* 0.7 MHz 32767.9999999990 Hz
*
* 0.8 MHz 32767.9999999987 Hz
*
* 0.9 MHz 32768.0000000004 Hz
*
* 1 MHz 32768.0000000008 Hz
*
* 2 MHz 32767.9999999972 Hz
*
* 3 MHz 32767.9999999972 Hz
*
* 4 MHz 32768.0000000044 Hz
*
* 5 MHz 32768.0000000008 Hz
*
* 6 MHz 32767.9999999972 Hz
*
* 7 MHz 32768.0000000115 Hz
*
* 8 MHz 32768.0000000044 Hz
*
* 9 MHz 32767.9999999972 Hz
*
* 10 MHz 32767.9999999830 Hz
What is interesting is there were few repetitive values. Also
surprising is the lower the clock the closer to the correct output.
BUT BIG SURPRISE ! Dont forget Mr. Nyquest. To get a reasonable wave
form you really need to have the clock 3 times the highest output. So
that leaves us with 100 KHz as the minimum practical clock frequency.
The error at 100 KHz is quite small and essentially beyond the range of
the DDSs 48 bit tuning word.
Of course one should be mindful of the spurious responses out of the
DDS. The 200 KHz selection { an easy sub of 10 MHz} would be better
based on the Analog Devices program after setting the filter factors.
I certainly am no expert but the Elliptical filter, which I have some
small familiarity with, seemed to be the best choice for the frequency
range.
Bill....WB6BNQ
References
1. http://designtools.analog.com/dtDDSWeb/dtDDSMain.aspx
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