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

Don Latham djl at montana.com
Wed Dec 9 16:44:40 EST 2015


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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>


-- 
Felix qui potuit rerum cognoscere causas.
Lucky is he who has been able to understand the causes of things.
Virgil
-------------------------------
"Noli sinere nothos te opprimere"

Dr. Don Latham, AJ7LL
Six Mile Systems LLC, 17850 Six Mile Road
Huson, MT, 59846
mailing address:  POBox 404
Frenchtown MT 59834-0404

VOX 406-626-4304
CEL 406-241-5093
Skype: buffler2
www.lightningforensics.com
www.sixmilesystems.com





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