[time-nuts] DAC performance [WAS: Papers on timing for lunar laser ranging]

Azelio Boriani azelio.boriani at gmail.com
Mon Jul 17 17:54:46 UTC 2017


This implies that in a Rb or Cs there is not a voltage reference source?

On Mon, Jul 17, 2017 at 3:49 AM, Bob kb8tq <kb8tq at n1k.org> wrote:
> HI
>
> This is a limitation on an OCXO based GPSDO. That’s really the bottom line here.
> It’s a limitation in an OCXO based part, but not in one based on an Rb or a Cs. If
> the added component costs far more than a Cs, it’s not an answer.
>
> Bob
>
>> On Jul 16, 2017, at 7:25 PM, Chris Albertson <albertson.chris at gmail.com> wrote:
>>
>> What about josephson standards?   After all, this is "Time Nuts" and we are
>> allowed to propose silly-complex solutions to simple problems if it
>> improves performance even a little.
>>
>> But seriously I thought the issue of making a perfect voltage standard was
>> solved because the Volt is defined to be whatever the Josephson array
>> produces.     Yes expensive because to runs at nearly absolute zero.
>>
>> On Sun, Jul 16, 2017 at 4:10 PM, Bob kb8tq <kb8tq at n1k.org> wrote:
>>
>>> Hi
>>>
>>>> On Jul 16, 2017, at 6:33 PM, Richard (Rick) Karlquist <
>>> richard at karlquist.com> wrote:
>>>>
>>>>
>>>>
>>>> On 7/16/2017 1:51 PM, Bob kb8tq wrote:
>>>>> Hi
>>>>> One gotcha with any ADC or DAC is going to be the reference. There, you
>>> are in the same
>>>>> “get what you pay for” dilemma. Stable and noisy, can do. Quiet and not
>>> very stable, can do.
>>>>> Both stable and quiet, not so easy if you want it cheap.
>>>>> Noise can also be the sigma delta ADC’s weak point. Even at slow rates,
>>> some of them need
>>>>> a lot of averages to quiet down.
>>>>
>>>> The reference initially used in the E1938A turned out to be too
>>> noisy/unstable.  It was non trivial to find an upgrade.  The
>>>> HP Smart Clocks of 20 years ago were limited in their performance
>>>> by the reference used.
>>>>
>>>> Has there been much improvement in references in the intervening
>>>> 20 years?
>>>
>>> They still don’t seem to have the hysteresis problem licked. Yes, you can
>>> do an oversized reference
>>> and take care of the issue. More or less that’s what you would have done
>>> 20 years ago.
>>>
>>> Bob
>>>
>>>
>>>>
>>>> Rick N6RK
>>>
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>>
>>
>>
>> --
>>
>> Chris Albertson
>> Redondo Beach, California
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>
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