[time-nuts] GPSDO and oscillator steering - EFC vs DDS schemes?

Magnus Danielson magnus at rubidium.dyndns.org
Wed Dec 9 19:51:20 UTC 2015


Poul-Henning,

On 12/09/2015 09:24 AM, Poul-Henning Kamp wrote:
> --------
> In message <56675DA0.4050609 at rubidium.dyndns.org>, 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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