[time-nuts] Sub Pico Second Phase logger
Joe Gwinn
joegwinn at comcast.net
Fri Dec 19 17:57:35 UTC 2008
Bruce,
At 3:54 AM +0000 12/19/08, time-nuts-request at febo.com wrote:
>
>Message: 5
>Date: Fri, 19 Dec 2008 16:51:55 +1300
>From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
>Subject: Re: [time-nuts] Sub Pico Second Phase logger
>To: Discussion of precise time and frequency measurement
> <time-nuts at febo.com>
>
>Joe
>
>Joe Gwinn wrote:
>> At 11:48 PM +0000 12/18/08, time-nuts-request at febo.com wrote:
>>
>>> Message: 5
>>> Date: Fri, 19 Dec 2008 12:48:27 +1300
>>> From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
>>> Subject: Re: [time-nuts] Sub Pico Second Phase logger
>>> To: Discussion of precise time and frequency measurement
> >> <time-nuts at febo.com>
> >>
> >> [snip]
> >
>>> Using a second sound card to generate the test signal may overcome this
>>> problem at increased cost, and for some it may not even be an option.
> >>
>>
>> It may not work with PCI soundcards, as the card clock may be
>> synchronized to the PCI bus clock. Firewire/USB cards will have
>> their own independent clocks.
>>
>>
>Most recent design PCI sound cards have their own independent crystal
>oscillator.
It won't be a great crystal, but a crystal nonetheless.
>Some claim to be able to sync to an SPDIF input but the resultant jitter
>may be large.
Why large jitter? Bad implementation?
> >
>>> >> 10Hz resolution whilst avoiding phase truncation spurs is impractical
>>>
>>>>> with a DDS chip by itself.
>>>>> Depending on the DDS and its clock frequency, the frequency spacing of
>>>>> phase truncation spur free outputs may be as large as several kHz.
>>>>>
>>>>>
> >>> Is this true of concatenated DDS chips? I recall a patent to the
> >>> contrary.
> >> >
> >> Which patent?
> >
>> Hmm. It's at work. I'll look it up in January. As I recall, the
>> second DDS made a small integer conversion, and so had low spurs,
>> while the first DDS was set to whatever was needed.
>>
>>
>Do you mean US5598440?
Yes, that's it.
> >> If the zero crossings are time stamped and do not occur simultaneously
>>> in each channel then the phase noise of the offset oscillator will
> >> affect the measurement.
> >
>> I'm not following. Please expand. The zero crossings are never
>> aligned unless there is no phase delay.
>>
>>
>>
>Yes, thats the point, the offset generator phase noise contribution isnt
>the same for both zero crossings.
>Greenhall et al correct for this to some extent, but at least for short
>tau, one is then no longer measuring ADEV, MDEV etc.
>There is some advantage in having a higher beat frequency as the offset
>generator phase noise has less time to accumulate.
I recall reading an article on this by Greenhall, probably _the_
article. My reaction was that I would be fortunate to have a setup
where I could even detect such a problem.
> >> >> A few divide and mix stages will be required to achieve a spur free
>>>
>>>>> resolution of 10Hz.
>>>>>
>>>>>
>>>> That is a traditional approach. But are there alternate approaches that
>>>> have now become practical?
>>>>
>>>>
>>>>
>>> Diophantine frequency synthesis?
>>>
>>
>> From the sound of the name I think so, at least in the last DDS
>> stage, as done by that patent.
>>
>> But I was fishing.
>>
>>
>Conventional Diophantine synthesis uses number theory together with 2 or
>3 conventional synthesiser loops to achieve very fine resolution whilst
>maintaining a high PLL phase detector input frequency.
In a sense, the concatenated DDS approach is a divide-and-mix chain.
Perhaps there is a parallel here.
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