Hal Murray wrote:
[email protected] 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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