[time-nuts] 53230A TIC and TimeLab

[Magnus Danielson]

Hi,

On 10/4/18 8:56 AM, Ole Petter Ronningen wrote:
> Somewhat longer answer; I assume you have set up a frequency measurement
> (as opposed to time interval)
> 
> 1. Timelab uses SCPI-command "READ" to fetch readings from the 53230a (and
> most other counters *). The 53230a does not return gap free frequency
> measurements in this case. There will be deadtime which will be ignored, as
> you say, but the measurements will also be biased. In addition, the
> CONTinous gap free mode is broken when it comes to ADEV, as has been
> documented elsewhere. From my experience, using the 53230a in a time
> interval measurement takes care of these issues, at the expense of a
> slighly more cumbersome setup.

Regardless, the 53230A does a filtered frequency estimation, which is
great for achieving high precision frequency measures quickly, but not
good for ADEV measures, as it introduces a bias due to the lower
bandwidth of the filtering as compared to the sampling interval.
Dead-time gaps biases also comes in.

For any software (TimeLab or Stable32) to process sufficiently unbiased
values, only phase values should be used to avoid both the filtering and
dead-time gap issues.

> 2. You may know this, but from your text it could be a misunderstanding:
> timelab does not configure the instrument; whichever sample interval you
> select in timelab should match the sample interval you have set up the
> 53230a to use.

TimeLab actually assumes that the user really know what they are doing,
but attempts to assist in the overall task. Adding a instrument
configuration tool into TimeLab would be possible, but for most part
consider the configuration you do for a measurement the opportunity to
input to TimeLab under which conditions the measurement was done, how
the instrument was setup etc. From there it just consumes measurements,
scales them according to the configuration, unwrapp phase etc. prior to
record the measurement record that is then used for analysis.

> 3. Regarding the ADEV number discrepancy; are the measurements made *at the
> same time*? For "long enough"? (The numbers jump all over the place before
> they settle down, thats normal from my experience.)

One of the great learning experiences you can get from TimeLab is how
the real-time update of ADEV swings in the highest tau range of the
plot, but as more samples comes in, the same tau range becomes less
uncertain, the confidence bounds (chi-square based) goes down and
TimeLab illustrates this very nicely. Thus, one wants to have enough
samples before judging too much of a value. It's experience one has to
built.

> Of course, theres the question of what reference you are measuring against.

Always. It is always good to look at specs and other measurements to see
if the measurement is reasonably in compliance with what one can expect
from other measurements. Even with very good instruments that does a lot
to solve the measurement problem, you always needs to validate it and
check it. One need to understand how the measurement instrument and
processing will impact the value and understand what part of this is
needed to get "proper" value and what is various forms of impairments.

Cheers,
Magnus

> Ole
> 
> * at least this was the case the last  time I checked, but I have not
> looked at the source for 1.31 to verify.
> 
> On Thu, Oct 4, 2018 at 7:42 AM AC0XU (Jim) <James.Schatzman at ac0xu.com>
> wrote:
> 
>> I have recently acquited a 53230A counter and have tried to measure ADEV
>> of a GPSDO with it. I am suspicious of the ADEV numbers reported by TimeLab
>> V1.31 because
>>
>> 1) Timelab disagrees significantly with the ADEV numbers reported by the
>> 53230A itself.  In this case, with 1 sec sample intervals, the 53230A
>> reports an ADEV of about 370 micro Hz (or 3.7e-11). Time lab reports
>> 1.5e-11 @0.01 sec sample period, 2e-11 @0.1 sec sample period, 6e-11 @1sec
>> sample period - all at tau=1sec.
>>
>> 2) When I use TimeLab to select different sample intervals, the whole ADEV
>> curve shifts left (with smaller sample intervals) or right (with larger
>> sample intervals).
>>
>> Its as if TimeLab does not correctly account for the sample interval, but
>> am I just not using the Timelab/counter setup correctly?
>>
>> Thanks!
>>
>>
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