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

Jim Lux jimlux at earthlink.net
Tue Dec 8 13:09:44 EST 2015


On 12/8/15 8:32 AM, Attila Kinali wrote:
> Moin,
>
> I've been digging through some stuff and stumbled (again) over Rick's
> paper on high resolution, low noise DDS generation[1] and got confused.
> The scheme is very simple and looks like to be quite easy and reliably
> to implement. If I understood it correctly, the critical points are the
> DDS, its sideband generation and the LO/RF feedthrough in the mixers.
> Nothing that is not known and nothing that is too difficult to handle
> (the 10.7MHz filter get rid of most of the feedthrough already and
> there has been a lot written on how to design DDS for specific applications).
>
> What puzzled me is, why this has not been used more often to correct
> the frequency of OCXOs instead of using some DAC-to-EFC scheme?
>

Heritage... if you have a design that works, and there's a lot of them 
in the field, and the idiosyncracies are well known and understood, then 
one tends to stay with the old design.

DDS are "brand new", at least in terms of generating low spurs, etc. 
The idiosyncracies are not as well understood.

I think also the power consumption might be an issue.  Most good DDS 
burn a lot of power, compared to a DAC.

There's also systems that depend on smooth sweeps without steps (yes, 
one can design a DDS with a digital ramp generator driving the increment 
in a phase accumulator to get arbitrarily smooth sweeps, but the "off 
the shelf" parts don't do this)

I don't think parts cost is a big driver.





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