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

[Bob kb8tq]

HI

> On Feb 21, 2020, at 5:26 PM, Taka Kamiya via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> I'm sorry, I messed up.  I jumped on more advance topic than I intended.  I'm sure there were answers in the replies but they must have gone way over my head because some of original questions still remain.  I bulletized (is that a word?) the original question with my NEW understanding.  Would someone please respond for me, point-to-point?


> 
> 1)  A frequency counter that measures DUT basically puts out a reading every second during the measurement.  When TimeLab is well into 1000s or so, it is still reading every second; it does not change the gate time to say, 1000s.  I understand now, Adev is about phase, not the frequency.  But assuming DUT is sine wave, if there is enough phase change, frequency do change.  I think of phase change as frequency change that is less than full cycle.  So how does counters that outputs every 1 second end up in tau of 1000s?  It will entirely miss phase change that spans more than 1 cycle.

It does not come up with a 1000 s tau ADEV by doing that. The gate time would need to be open for 1000 seconds.

> 
> 2)  I recall reading on TICC manual, in time interval mode, anything that's reasonably good is good enough, because it has time stamp and the count reading.  Clock is used to chunk the data.  Is this still true?  Through this discussion, I ended up with conclusion that there is no inherent advantage over TI measurement when compared to frequency measurement.  Am I understanding this correctly?

If you are measuring the difference between two one 1 pps signals, and they are < 0.1 seconds apart, and the TICC has a resolution of 0.1 ns ….

Then the reference clock for the TICC only needs to be good to 0.1 ns / 0.1 s = 1x10^-9

> 
> 3)  I understand even the BEST counter is only good for Adev nE-12 measurement. Then, with my collection of counters, HP53132A (which averages tons of short period measurement), 5335A (not enough resolution), HP5370A (interval reading is no better than frequency), TICC by TAPR,   Do I even have a chance of doing any meaningful work?  (say work with GPSDO and Rb which some of it does reach E-13)  Yes, I know now, it is NOT possible to do 1 sec Adev but say over 100 seconds?  Right now, I don't have any standard that has adev that good at 1 sec anyway.

A counter that will do 1x10^-12 at 1 second would need errors below 1 ps. I do not know of any such counter. As you go to other tau’s (like 1,000 seconds)
a counter that will keep a gate open for 1000 seconds might do the trick. 

> 
> 4)  Would one person who has infinite patience and experience guide me through getting one reading done correctly with what I already have?  That may include email and phone call.  (I speak English and Japanese)  I don't want to lower S/N of this mailing list by doing this here.  

We have been doing that off list ….

> 
> 5)  One time, it was mentioned many of Adev graphs posted are basically a graph of instruments noise graph.  How do I tell when a given reading/graph is exceeding the limit of a setup?  I did do base line where same signal goes to counter's reference input and signal input.  I always have that on my chart so traces does not go below.  Is that enough?

Look at the counter’s stated accuracy information and do the math. If it’s a 100 ps counter, then that is 100 x 10^-12 at 1 second. If there are no other errors, and
the reference is perfect, you will hit 10X better each time the gate increases by 10X. 

Bob

> I appreciate everyone's input.  I am learning a lot but just not digesting well enough.  I'd like to do DMTD after I understand the basics.
> --------------------------------------- 
> (Mr.) Taka Kamiya
> KB4EMF / ex JF2DKG
> 
> 
>    On Thursday, February 20, 2020, 1:41:06 PM EST, Taka Kamiya via time-nuts <time-nuts at lists.febo.com> wrote:  
> 
> I have a question concerning frequency standard and their Allen deviation.  (to measure Allen Dev in frequency mode using TimeLab)
> 
> It is commonly said that for shorter tau measurement, I'd need OCXO because it's short tau jitter is superior to just about anything else.  Also, it is said that for longer tau measurement, I'd need something like Rb or Cs which has superior stability over longer term.
> Here's the question part.  A frequency counter that measures DUT basically puts out a reading every second during the measurement.  When TimeLab is well into 1000s or so, it is still reading every second; it does not change the gate time to say, 1000s.
> That being the case, why this consensus of what time source to use for what tau?
> I recall reading on TICC, in time interval mode, anything that's reasonably good is good enough.  I'm aware TI mode and Freq mode is entirely different, but it is the same in fact that measurement is made for very short time span AT A TIME.
> I'm still trying to wrap my small head around this.  
> 
> --------------------------------------- 
> (Mr.) Taka Kamiya
> KB4EMF / ex JF2DKG
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