Poul-Henning,
On 12/09/2015 09:24 AM, Poul-Henning Kamp wrote:
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In message <[email protected]>, Magnus Danielson writes:
So, what did I miss? Why do people use DAC-EFC control instead of
the DDS scheme?
The main reason I would say is habbits, people have habbits and stick to
them.
And I think right after that comes the problem of spurs.
Controlling DDS spurs is mostly a solved problem, when you get to choose
your input and output frequencies for that purpose.
But as "afterburner" for an OCXO, there is no restriction on the two
frequencies, and they are almost guaranteed to be close-in multiples,
so coping with the spurs becomes non-trivial.
That basically means that you need 50% higher DDS and DAC resolution
than everybody else in the market an suddenly it isn't cheap any more.
Well, consider that using a circuit similar to the one Rick has in his
article, then frequency deviation and spurs will be significantly scaled
down. For a good crystal, you will maybe have a handfull of ppms, let's
say +/- 10 ppm range. A scale-down of 1000-100000 would be worth
considering. That helps with DDS spurs.
Cheers,
Magnus
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