[time-nuts] oscillator choice question

Hal Murray hmurray at megapathdsl.net
Sun May 2 07:17:58 UTC 2010

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
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?

These are my opinions, not necessarily my employer's.  I hate spam.

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