[time-nuts] oscillator choice question

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun May 2 08:51:51 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?
A DDS can generate some close in spurs that are very close to the 
desired frequency and thus are difficult to filter even with a narrow 
band PLL as the spur offset and amplitude varies with the DDS output 
frequency in a very complex way.
A very narrow PLL requires a VCO with good short (for averaging times up 
to the inverse PLL bandwidth) term stability.
However if the offset is 500Hz its relatively easy to filter out the 
unwanted sum (or difference) frequency with a PLL using a VCO with good 
short term stability for averaging times of a few tens of millisec. The 
rejection can be improved by using an SSB mixer.

N.B. the author of the paper (and his web page) that you found has 
vanished without trace.
It turns out that the Diophantine frequency synthesis technique was 
patented (US Patent 5267182) some 17 years ago.
His literature search for previous papers/patents cant have been very 
Fortunately I managed to download all of his papers before they vanished 
along with the web page.
>> 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?

(thanks to Bob Camp for finding this gem)

There's nothing magic about an FPGA, its merely a convenient way of 
implementing the improved algorithm.
There's no way to implement it with a traditional DDS chip as the 
digital section needs to be extensively modified.

DDS chips do not use it because it has only recently been devised.


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