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

Magnus Danielson magnus at rubidium.dyndns.org
Tue Dec 8 17:58:30 EST 2015


Jim,

On 12/08/2015 07:09 PM, Jim Lux wrote:
> 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.

Depends on what you do. I think the first reaction would be "why make it 
complex?" as the DAC and EFC is relatively simple and "understood".
Second arguments would be component count and board space. The uncounted 
case would be in the herritage department, which would prohibit the 
thought to even be considered in the first place.

However, the modern DDS chips makes it more and more useful for many cases.

> 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)

Consider that their use here would mean that such effects would be mixed 
down considerably. Doing a good DDS steering algorithm with plenty of 
updates would help to keep things smooth.

Personally, I use a combination of the techniques, you can play 
interesting tricks with DDSes if you build them yourself. I then lock up 
oscillators when that is the best approach, for say phase-noise.

Cheers,
Magnus



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