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IC advice to create a regolable oscillator from 0 to 500kHz

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ogden:

--- Quote from: imo on April 02, 2019, 05:00:27 pm ---The 9833's "MSB" is with 28bit resolution. Therefore the square wave too.
It is the same situation as with the 9851 and its comparator.

--- End quote ---

Yes you are so right. My bad  :P
It is output jitter up-to 1/FCLK for 9833 that differs between two.

stockvu:

--- Quote ---Square wave output of this DDS is just MSB of the DAC - far from DDS resolution. AD9851 have built-in comparator for proper square wave output, yet it is huge overkill for 0..500KHz application.
--- End quote ---

Hi. Well, I don't think the OP specified things like jitter or settling time -- but I may have missed it. I don't have a Jitter Test Set or a Spectrum Analyzer to evaluate these 9833 circuits. On my low cost scope, the outputs seem stable and agree well with freq counter.

I run one DDS 100x higher in frequency and then pipe it into the 2560 Timer-5 input. There its divided by 100 and T5's Output-Compare generates selectable duty cycle from 5-95% (no comparator needed). An added benefit is 0.1 Hz resolution now becomes .001 Hz. 

So I admit its overkill -- but I like overkill when its cheap. :)

ogden:

--- Quote from: stockvu on April 03, 2019, 03:28:14 am ---Hi. Well, I don't think the OP specified things like jitter or settling time -- but I may have missed it.

--- End quote ---

He did not. Knowledge may be useful for others who read this.


--- Quote ---I don't have a Jitter Test Set or a Spectrum Analyzer to evaluate these 9833 circuits. On my low cost scope, the outputs seem stable and agree well with freq counter.

--- End quote ---

It is hard to notice on low output frequencies. FCLK/4 have no jitter but FCLK/2.5 is worst case. Look at > 4 periods on scope and you will see. BTW you don't even have to put DDS into MSB mode - just look at DDS DAC output w/o LP filter at FCLK/2.5. It will be eye-opening on itself. Then you start to realize how critical is output low pass filter especially for high freq output of DDS, why sometimes it is designed to be steep 7-th order or even better, filter. Disclaimer: all noninteger (relating to FCLK) frequencies have this behavior, FCLK/2.5 was just simple worst case pick.

[edit] Ups. I talk about non-integer divisors, yet use round number, 3  :palm:  Fixed to FCLK/2.5

stockvu:

--- Quote ---It is hard to notice on low output frequencies. FCLK/4 have no jitter but FCLK/3 is worst case. Look at > 4 periods on scope and you will see.
--- Quote ---
--- End quote ---
BTW you don't even have to put DDS into MSB mode - just look at DDS DAC output w/o LP filter at FCLK/3. It will be eye-opening on itself.
--- End quote ---

I take output off the DAC MSB. I don't need the triangle or sine outputs. My use of this circuitry never sees an RF channel, it doesn't get low-passed since it feeds digital chips downstream.

Thanks for the tip on looking >4 periods, that's good to know. If I do have some jitter, its not a problem for my application.

I just threw in the DDS idea as another option for the OP to consider.


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