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

Magnus Danielson magnus at rubidium.dyndns.org
Wed Dec 9 17:51:37 EST 2015


DDSes used to be very crude in terms of spurs. However, things have 
improved significantly and some of the chips you can get now is pretty 
impressive. It has even inspired some significantly different radio 
designs in the ham world.

As for filters, yes, they can vary delay with temperature, but the first 
rule of thumb is to keep filters low-Q (that is, both resonances/poles 
and zeros) because delay is proportional to Q. Then, well, temperature 
sensitive components, so choose those which is stable. Dielectras with 
low relative dielectric constants tend to be more stable, just as 
low-permability magnetics.

73 de SA0MAD Magnus

On 12/09/2015 10:44 PM, Don Latham wrote:
> A friend and I have been messing with a DDS replacement for the VFO in older
> radios. The odds runs between 5 and 5.5 MHz. There are some mixers that
> generate the final LO frequency. We found many many birdies (caused by spurs
> for the non-hams) over the tuning ranges.  We had to put in a lo-pass filter,
> 7stage commercial type to get rid of the birdies. But as I recall, there have
> been several cautions on this list about filters causing temperature
> dependence.  I haven't read the whole of this thread, so it may have already
> been mentioned.
> Merry Christmas,
> Don
> Magnus Danielson
>> God kväll,
>> On 12/09/2015 11:47 AM, Attila Kinali wrote:
>>> God eftermiddag,
>>> On Tue, 8 Dec 2015 23:45:52 +0100
>>> Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
>>>> If you would setup essentially a micro-stepper design, such as those
>>>> being used for cesium and hydrogen masers, but maybe adapted to a
>>>> hobbyist needs and with straight-forward way of building and tune-up,
>>>> then we could alter the design pattern. The phase-noise and long term
>>>> stability issues is clear.
>>> It doesn't look too difficult to crank something out within a rainy
>>> weekend or two. But I am most likely underestimating the amount of work :-)
>> Indeed. As any engineering time estimate, you need to multiply with pi.
>> At work, we engineers divide our estimates with pi before giving it to
>> the project managers, as they will multiply with pi before putting it
>> into their time-plan. :)
>>>> Doing control loop using a phase-stepper is a little bit different, and
>>>> has a few minor design-challenges, but once mastered is essentially the
>>>> same. EFC or C-field control then becomes more an initial setup.
>>> What makes the control loop different (beside that you control phase
>>> and not frequency, and thus have to integrate)?
>> Well, that is a little bit different right there. Depending on your
>> setup, you might have to consider how phase-wrapping and similar
>> saturations that happens over a long time. If you think about it, it's
>> manageable.
>> One useful trick is to let the phase-wrapping be that of the numeric
>> wrapping, and then handle that case for time-stamps, so that the
>> numerical extension becomes trivial. If you don't, you can get some very
>> interesting problems.
>>>> An alternative approach divider wise is to use re-generative dividers.
>>>> For Rick's approach there would be a number of these at the same
>>>> frequency (nominally), so the same design-pattern would apply. However,
>>>> that would only be meaningful if you need really need to keep the noise
>>>> down.
>>> Yes, I thought about that as well, the problem here is that the low
>>> noise mixers designs use transformers, which make everything bulky
>>> and expensive (the usual suspects cost 2USD/piece and use about 1cm^2).
>>> The one design that comes to mind that doesn't need transformers is
>>> the tripple Gilbert-Cell design, but that might be higher in noise.
>>> (Heck, i should just sit down and do some noise calculations)
>>> Additionally, there is a need for relative steep filters for 667kHz
>> Indeed. For most uses, re-generative dividers will not be needed.
>> I should do more experiments on that stuff.
>> Cheers,
>> Magnus
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