[time-nuts] Best counter setting for ADEV?

Bob Camp lists at rtty.us
Thu Oct 4 19:19:33 EDT 2012


Hi

Doing the repetition may give you a more fine grained idea of what the clocks are doing. It does not impact the resolution of the single measurement. 

A single ns in 100,000 seconds is indeed 1.0x10^-14…

Bob

On Oct 4, 2012, at 6:11 PM, Azelio Boriani <azelio.boriani at screen.it> wrote:

> OK, because you think: (Y-X)/100K is the drift of the two clocks for each
> second over 100K seconds and for each second I have a 1nS/100K resolution.
> Now think about this: repeat the measure over and over and after each 100K
> seconds you have your table. This table (your counter is 1nS) can have
> figures like, say, 52nS, 53nS, 58nS, that is with a 1nS step. Your drift
> maybe not exactly 52: it maybe 52.1 52.3 52.4 but you only see 52. So you
> are loosing an entire nS between 52nS and 53nS. If you have a 100pS counter
> you can see 52.0, 52.1 and so on.
> 
> On Thu, Oct 4, 2012 at 10:13 PM, Bob Camp <lists at rtty.us> wrote:
> 
>> Hi
>> 
>> If I have two clocks:
>> 
>> 1) I know they are X ns apart at time = 0
>> 2) I know they are Y ns apart at time = 100,000 seconds
>> 3) The resolution of the X and Y measurements is 1 ns
>> 
>> I know the relative time between the two clocks to a lot better than
>> 5.0x10^10...
>> 
>> Bob
>> 
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
>> Behalf Of Azelio Boriani
>> Sent: Thursday, October 04, 2012 3:43 PM
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] Best counter setting for ADEV?
>> 
>> If you take 2 samples out of a 1nS counter than you can estimate your
>> measure with a 500pS resolution (5E-10) with an uncertainty (noise) of
>> 7E-10.
>> 
>> On Thu, Oct 4, 2012 at 8:58 PM, Bob Camp <lists at rtty.us> wrote:
>> 
>>> Hi
>>> 
>>> What if I only take two single measurements:
>>> 
>>> One at time = 0, the other at time = 100,000 seconds.
>>> 
>>> No averaging, no signal processing just two measurements. I look at the
>>> time
>>> difference  between the two signals at time = 0, and then again a bit
>> more
>>> than a day later.
>>> 
>>> Bob
>>> 
>>> -----Original Message-----
>>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
>>> Behalf Of Azelio Boriani
>>> Sent: Thursday, October 04, 2012 2:48 PM
>>> To: Discussion of precise time and frequency measurement
>>> Subject: Re: [time-nuts] Best counter setting for ADEV?
>>> 
>>> You're right: it is better to put it down correctly:
>>> for a theoretical resolution of 10E-14 at 100K seconds, start with a 1nS
>>> counter that takes 1 second samples for 100K seconds and average those
>> 100K
>>> samples. You have your resolution and a noise (an error bar) of 3E-12. If
>>> you use a 100pS counter and do the same run, you will end up with a
>> 10E-15
>>> reolution and 3E-13 noise, and so on. It turns out that to have a real
>>> 10E-14 measure (@100K seconds), not blurred with noise, you must start
>> with
>>> a 10pS counter.
>>> 
>>> On Thu, Oct 4, 2012 at 6:41 PM, Bob Camp <lists at rtty.us> wrote:
>>> 
>>>> Hi
>>>> 
>>>> Maybe we're talking about two different things here.
>>>> 
>>>> Simplistically, resolution is simply what the counter puts out as a
>>> usable
>>>> LSB. There are a lot of examples out there that will do a one
>> nanosecond
>>>> single shot measurement. That measurement includes the normal trigger
>>> noise
>>>> and "stuff" in the counter. There are a few assumptions about slew rate
>>> of
>>>> the input.
>>>> 
>>>> If I take two one shot measurements spaced 100,000 seconds apart, my
>>>> resolution over that period is 1.0x10^-14. The measurement is
>>>> representative
>>>> of what the sources have done to that level. Weather the sources are
>>> stable
>>>> to that level is independent of the resolution of the measurement.
>>>> 
>>>> Bob
>>>> 
>>>> -----Original Message-----
>>>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]
>> On
>>>> Behalf Of Azelio Boriani
>>>> Sent: Thursday, October 04, 2012 12:27 PM
>>>> To: Discussion of precise time and frequency measurement
>>>> Subject: Re: [time-nuts] Best counter setting for ADEV?
>>>> 
>>>> Yes, for a theoretical resolution of 10E-14 at 100K sec start with a 1nS
>>> and
>>>> average for 100K seconds but ending up with a noise (an error bar) of
>>>> 3E-12.
>>>> 
>>>> On Thu, Oct 4, 2012 at 4:25 PM, Bob Camp <lists at rtty.us> wrote:
>>>> 
>>>>> Hi
>>>>> 
>>>>> For a *resolution* of 1.0 x 10^-14 at 1.0 x 10^5 seconds I only need
>> a
>>>> 1.0
>>>>> x
>>>>> 10^-9 second reading out of the counter. Indeed, 5 or 10X more than
>>> that
>>>>> would be better if I was after a 1 x 10^-14 accuracy.
>>>>> 
>>>>> Bob
>>>>> 
>>>>> -----Original Message-----
>>>>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]
>>> On
>>>>> Behalf Of Azelio Boriani
>>>>> Sent: Thursday, October 04, 2012 7:58 AM
>>>>> To: Discussion of precise time and frequency measurement
>>>>> Subject: Re: [time-nuts] Best counter setting for ADEV?
>>>>> 
>>>>> Taking 10 samples from a 1nS counter leads you to a 100pS resolution
>>> but
>>>>> the noise at best (if it is really random) is reduced by SQRT(10).
>> So,
>>> as
>>>>> already pointed out here, there is no real substitute for lower
>> noise,
>>>>> higher resolution measurements.
>>>>> 
>>>>> On Thu, Oct 4, 2012 at 3:01 AM, Bob Camp <lists at rtty.us> wrote:
>>>>> 
>>>>>> Hi
>>>>>> 
>>>>>> A 1 ns resolution TI counter will do the measuring part just fine.
>>>>> Hitting
>>>>>> the number, is where it gets a bit crazy. A *good* GPSDO might get
>>> near
>>>>>> that.
>>>>>> 
>>>>>> Bob
>>>>>> 
>>>>>> On Oct 3, 2012, at 8:56 PM, Kevin Rosenberg <kevin at rosenberg.net>
>>>> wrote:
>>>>>> 
>>>>>>> On Oct 3, 2012, at 6:52 PM, Kevin Rosenberg <kevin at rosenberg.net
>>> 
>>>>> wrote:
>>>>>>>> Nice plot! Yes, I'd have trouble measuring 10E-14 at 10E5
>> seconds.
>>>>>>> 
>>>>>>> Sorry, 1E-14 at 1E5.
>>>>>>> 
>>>>>>> 
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