[time-nuts] Frequency standards for different tau in Allen Dev measurement

Sat Feb 22 15:51:40 UTC 2020

Hi Taka,

On 2020-02-22 16:08, Taka Kamiya via time-nuts wrote:
> As I understand, 3 cornered hat is nothing more than system of equations where you have 3 variables and 3 equations, that allows for solving for each.  I'm going to hold off on that one.  I forgot about thermal and equipment's internal noise.  It all makes sense.  I hope to be doing some measuring this weekend if all works as planned (hope, hope, hope....)
Good. As you get measures, more and more of this will sink in as you see
them.
> I mentioned earlier, I have various standards.  I also have TAPR's board to create 1 pps from any standards I have.  To start and follow your last example on how-to, what would you recommend for DUT and the standard?  I'd like to do one that I can show you, and immediately say if it's what you'd expect or not.
> I have newly repaired HP105B, PRS10, Efratom FRK-c?, LPRO-101, and various GPSDO including T-bolt proper.  For the counter, I think I'll use the old HP interval counter, rather than 50132A.  The latter is all menu driven.  It's so easy to mis-set or otherwise screw it up and not notice it.  HP5370A is all switches and knobs.  How long of measurement?  In my experience, 2 days will show the inflection point.  

I would start doing a few 1 hour measurements. Only when the measurement
makes sense you extend it.

The HP53132A requires you to set trigger to manual and then adjust
trigger level manually. A little painful in the beginning, but you learn
to do it. In the beginning it can be good to use an oscilloscope to
learn where a good trigger point is.

HP5370 will work. Use PPS from one of your GPSDOs for start. Use any
oscillator of choice as DUT. Once the measurement on one makes sense,
keep that and go to the next.

Mistakes in setup causes huge face-jumps, which is why wrapped phase
(w), phase (p) and frequency (f) is useful as one learns. After learning
to set things up, one end up not using wrapped phase anymore, one sees
the error and just fixes it. Use the 10 MHz output of a counter with no
external clock attached as a clock with worse perfromance than rubidiums.

Cheers,
Magnus

>
> --------------------------------------- 
> (Mr.) Taka Kamiya
> KB4EMF / ex JF2DKG
>  
>
>     On Saturday, February 22, 2020, 3:03:37 AM EST, Magnus Danielson via time-nuts <time-nuts at lists.febo.com> wrote:  
>  
>  Hi Taka,
>
> On 2020-02-22 04:55, Taka Kamiya via time-nuts wrote:
>> It's not like drinking from a fire hydrant.  It's like drowning in hoover dam, get sucked into an inlet, pulverized by turbine blade, and getting spit out into a stream.  
> Hmm, not exactly what I had hoped for as your experience... whatever,
> hope it is worth it.
>> One question :  
>>
>> You said this: 
>> "The resolution of your counter tells you about where your 1/tau curve
>> will cut tau = 1 s, and it goes from there. There is a slight scaling
>> factor, but if we assume it is 1 for now, it is pretty simple. Your
>> 5335A has 1 ns single-shot resolution, this gives 1E-9 at 1 s, but 1E-10
>> at 10 s, 1E-11 at 100 s and 1E-12 at 1000 s. You see very clearly when
>> the linear slope ends and "lands" in the noise, at which time the noise
>> becomes dominant and is giving you the interesting reading."
>> By using the same logic, I can keep going up and up on longer gate time and tau keeps getting better and better.  I know at one point, inflection happens and that indicates noise taking over.  But what kind of noise (phase?) and how does that happen?
> OK. Good question.
>
> The instrument noise in itself can be while phase noise and flicker
> phase noise. The resolution limit will be a systematic disturbance. All
> these three has the same 1/tau slope so from these three, nothing
> happens as tau increases. At some point naturally will thermal
> instability come in, but that will be another systematic, but it will be
> hard to cancel. So that is the limit of the instrument.
>
> However, the noise of the reference and DUT will dominate for larger
> taus, and thus it will be good enough to measure that noise. Besides,
> length of measurement becomes a limit.
>
>>   Only thing that changes in this equation is the gate time.  Everything is constant.  You mean gate time is no longer accurate enough to support the minute shift in phase?
>> I'm still confused about the precision (not accuracy) of the time base.  Am I still ultimately constrained by this?
> There are ways to get around it.
>>   Without DMTD, or some kind of pre-scaling of DUT, if I measure Rb with time base using another Rb, they are both rubber-bands, correct?
> Now, what you can do is to do a three-cornered hat measurement.
> Essentially you measure three sources at once as three pair-wise
> measurements. You calculate your ADEV of each pair. Now, as you have the
> noises n1, n2 and n3 of each source, measurement m1 = n1+n2, m2 = n1+n3
> and m3 = n2+n3. This equation problem can be solved to bring out n1, n2
> and n3 separate. The trouble is, this does not always work out as
> precisely as one would like, because the measurements isn't precise and
> well converged. Therefore it is a bit of sensitive process, but if one
> gets it working, you can achieve the separation.
>> I'm infinitely curious by nature.  I need to know everything, even to a minute detail, to be satisfied.  I hope you don't get tired of this.    
> So far, you ask relevant questions that can be given an answer. Being a
> curious person myself, I've digged down.
>
> Cheers,
> Magnus
>
>
>
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