[time-nuts] Is Timelab with a Prologix-Eth and a PM6680 - working ?

Azelio Boriani azelio.boriani at screen.it
Tue Jul 3 17:08:56 UTC 2012


Yes, you can ask here about these things.
Anyway, the best is to use the TBolt as a reference (not necessarily on the
external ref of the counter) and test another oscillator or GPSDO. For
example: feed the TBolt PPS to A, the GPSDO_under_test PPS to B and measure
the stability of the time interval using TIME A-B. Ideally it should be a
constant value (even if not exactly 0, but constant). The phase data (the
time interval errors) can be collected and an Allan deviation computed. You
can feed the TBolt PPS to A, a delayed replica to B (using 50 meters of
coaxial cable) and test the PM6680 (and the thermal stability of the
coaxial cable delay).
Now, beware of the PM6680 uncertainty: read page 11-14 of the manual and
see that the frequency uncertainty is LeastSignificantDigit=
(500pS*frequency)/sampling_time. The default sampling time is 200mS so, for
10MHz you have 0.025Hz + or -. The time interval uncertainty is minimum
100pS.
So, to try your setup (TBolt on EXT REF and A): first inform the TimeLab
that you have 1 second frequency samples, then set the sampling time to 1
second (use FUNCTION button on the PM6680) and retry collecting data.In
this case the predicted uncertainty is 0.005 (5E-10) and sholud average out
to 5E-13 in 1000 seconds taking into consideratin that the full 12 digit
resolution is available at the PM6680 GPIB (that is, the GPIB has more
digits than the display can show, see page 11-18 of the manual).

On Tue, Jul 3, 2012 at 6:37 PM, Azelio Boriani <azelio.boriani at screen.it>wrote:

> Well, let's see: usually the Allan deviation is computed by collecting
> phase (time error) samples. You have to select time interval in TimeLab and
> on the PM6680 TIME A-B. If you want to use frequency samples (this is
> strange for me, never used frequency samples) then you must select
> frequency in TimeLab and FREQ A on the PM6680.
> Selecting TIME A-B requires that you use both the A and B inputs (start
> and stop) to time your signals but since you are trying to measure the
> TBolt with itself you must delay the signal going to A before it reaches B.
> Use a long coaxial cable as a delay line (50 meters should work). Measuring
> a signal with a delayed replica of itself can be useful to see how good is
> the counter.
>
> On Tue, Jul 3, 2012 at 6:26 PM, cfo <xnews3 at luna.dyndns.dk> wrote:
>
>> On Tue, 03 Jul 2012 18:11:26 +0200, Azelio Boriani wrote:
>>
>> > When you start TimeLab, which mode does the PM6680 show? FREQ A or TIME
>> > A-B?
>> >
>> It shows Data Type = time interval , i could also select frequency in the
>> drop down box (This is the latest TimeLab - NonBeta).
>>
>> Now have i goofed ?
>> Is time interval A->B inputs (blush)
>>
>> CFO
>>
>>
>>
>>
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>
>


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