[time-nuts] General questions about making measurements with time interval counter.

Bob Camp kb8tq at n1k.org
Thu Jan 12 20:38:30 UTC 2017


Hi

Keep in mind that when you do 1800 samples at 1 second, that data will only meet the 
100 sample requirement out to tau = 18 seconds. Past that you are in the “under 100 samples
region”.

Bob

> On Jan 12, 2017, at 2:32 PM, Bob Stewart <bob at evoria.net> wrote:
> 
> Hi Bob,
> OK, thanks for explaining.  When you and others use highly technical terms like "small number of samples" it's not always clear to me what you mean.  =)  Ten samples?  That's not enough for anything.  Normally I run at least 1800 samples; at least if I plan to share them with someone.
> 
> Bob -----------------------------------------------------------------
> AE6RV.com
> 
> GFS GPSDO list:
> groups.yahoo.com/neo/groups/GFS-GPSDOs/info
> 
>      From: Bob Camp <kb8tq at n1k.org>
> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
> Sent: Thursday, January 12, 2017 1:03 PM
> Subject: Re: [time-nuts] General questions about making measurements with time interval counter.
> 
> Hi
> 
> It varies from 5370 to 5370. You see a lot of plots that run out to 10 samples or less. Anything below 100 samples
> is risky in some senses.
> 
> Bob
> 
>> On Jan 12, 2017, at 12:25 PM, Bob Stewart <bob at evoria.net> wrote:
>> 
>> Hi Bob,
>> OK, what's a small number of data points?  Attached is a screencap of captures for 25, 50, 75, 100, and 150 seconds.  Yeah, at 25 seconds, the 1S tau is up at 4.56E-11, but it falls pretty quickly.  I will mention that this particular 5370 is much better than my other one.  So, maybe this one is an exceptional example?
>> 
>> Just for grins, I also included a screencap of the phase points.
>> 
>> Bob -----------------------------------------------------------------
>> AE6RV.com
>> 
>> GFS GPSDO list:
>> groups.yahoo.com/neo/groups/GFS-GPSDOs/info
>> 
>>       From: Bob Camp <kb8tq at n1k.org>
>> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>> Sent: Thursday, January 12, 2017 11:04 AM
>> Subject: Re: [time-nuts] General questions about making measurements with time interval counter.
>> 
>> Hi
>> 
>> There is a big difference between RMS and single shot. Single shot, the 5370 is a very different beast. 
>> That’s not a big deal when you have a few thousand readings and it all averages down. Unfortunately 
>> we all love to do runs with a very small number of points and then draw conclusions from them. As the 
>> sample size goes down, you no longer have a 2 to 4 x 10^-11 beast, it’s more like 5X that.
>> 
>> Bob
>> 
>> 
>>> On Jan 12, 2017, at 11:31 AM, Bob Stewart <bob at evoria.net> wrote:
>>> 
>>> Hi Bob,
>>> Normally I see somewhere between 2E-11 and 4E-11 at 1S tau on my 5370A, as in the blue trace on the attached plot.  Am I misunderstanding your meaning?  Granted, I am clocking the 5370A with a GPSDO, but I believe I see about the same thing with the HP10811.  This test was 1PPS vs 1PPS on two different units.
>>> The plot also has a test run by Tom, in orange, using his H Maser and a Timepod to show how poor the 5370 is compared to the Timepod below about 60S tau.  These are essentially apples vs apples tests.
>>> 
>>> Bob
>>> 
>>> 
>>> 
>>> 
>>>       From: Bob Camp <kb8tq at n1k.org>
>>> To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>>> Sent: Thursday, January 12, 2017 8:27 AM
>>> Subject: Re: [time-nuts] General questions about making measurements with time interval counter.
>>> 
>>> Hi
>>> 
>>> There are a number of ways to improve the resolution (and accuracy) of your data without spending 
>>> big piles of cash. They have been discussed here on the list many times over the last few years. 
>>> What I’m suggesting is that you dig into that ahead of taking data. You will dive into it eventually as you 
>>> look more and more at devices that are locked to some sort of stable reference internally. 
>>> 
>>> Ideally you would like a device with a floor 5X to 10X better than what you are measuring. For ADEV style
>>> data, the 5370 is a 1x10^-10 sort of device single shot (so 1x10^-9 is the limit at 10:1). With a lot of averaging 
>>> (which is not something you do with ADEV) you can get about 5X better than that as a floor. In either case, it is getting in the way of any
>>> readings that are much below 1x10^-9 at one second. A low cost XO can hit that level of performance.  
>>> 
>>> Bob
>>> 
>>> 
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>> 
>> 
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> 
> 
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