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

Attila Kinali attila at kinali.ch
Tue Dec 8 19:00:27 EST 2015

On Tue, 8 Dec 2015 10:09:44 -0800
Jim Lux <jimlux at earthlink.net> wrote:

> > 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.

That might be an explanation.

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

Well, Rick's paper is already 20 years old. There has been lots of
discussion of DDS in the radio community and I think that community
has a lot of knowledge of the idiosyncracies of DDS. I'm not sure
how it works for the  time and frequency control community, though.

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

Compared to the 1-5W steady state of an OCXO?
But yes, the circuit would probably need something in the range of 2-3W.
> 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 might be naive, but I would have used an small FPGA with EEPROM cells
(like the Lattice ICE or the Altera MAX) and build an DDS by hand that
is tailored for exactly that purpose (to get minimal spurs), add an
10bit DAC, combine everything with an 60-100MHz VCXO that is PLL-locked to
the 10MHz input. Well, actually the VCXO might not even be necessary,
if the FPGA internal PLL is not too noisy, there is some heavy "damping"
of the noise through the divider stages anyways.

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

Unless you are a hobbyist.

			Attila Kinali

Reading can seriously damage your ignorance.
		-- unknown

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