[time-nuts] oscillator choice question

jimlux jimlux at earthlink.net
Sun May 2 16:27:00 UTC 2010

Hal Murray wrote:
> bruce.griffiths at xtra.co.nz said:
>> If there is no electronic tuning available one can use a DDS based
>> synthesiser to produce a corrected output frequency. However close in spurs
>> will be problematic unless one use a couple of  simple mix and divide stages
>> or resorts to a Diophantine synthesiser  using phase noise truncation spur
>> free output frequencies from the DDS  chip(s). 
> I think I understand the classic spurs from a DDS.
> I wasn't familiar with Diophantine techniques.  Google found this
>   http://www.hindawi.com/journals/ijno/2008/416958.html
> which is readable at my level.
> But I don't think I understand the big picture.  The example numbers they 
> give involve mixing 500 Hz with 10 MHz.  Assuming I want the sum, how do I 
> get rid of the difference?  It's going to be a good strong signal, as strong 
> as the one I want.  I think anything that leaks through the filter into the 
> next mixer is likely to make mirror sidebands that are right where we don't 
> want them.
> Why is that going to be easier to get rid of than traditional spurs?
>> Alternatively if one implements the DDS in an FPGA its possible to
>> virtually eliminate such spurs using a modified algorithm. However this
>> requires an external DAC to produce the required output. 
> Got a URL?  What's magic about a FPGA?  Why don't traditional DDS chips use 
> that modified algorithm?

Commercial DDSs are sold in large quantities for generalized 
applications, so they tend not to use exotic techniques for spur 
reduction over small ranges. You can also burn gates in exchange for 
performance, a decision that would be tough to make for a manufacturer 
concerned about power dissipation, etc.

  It's easy, for instance, in a FPGA, to implement several different 
length cosine lookup tables, so that all the frequencies you want to 
generate exactly match the table length.  You can also do things like 
error filtering, various spur cancellation techniques, etc.

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