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

[Azelio Boriani]

When used as a time interval counter, the PM6680 (as well as other
counters) counts the reference oscillator cycles + a residual got from an
interpolator. Suppose you have to measure 234nS: with a 100MHz oscillator
you count 23 cycles (the 100MHz is 10nS cycle) and then you are left with
an additional 4nS. To account for the 4nS, various digital and analog
techniques are available. Your 100MHz has to be stable at least only for
the time you make the measurement, 234nS in this example. For this short
time interval almost every oscillator will do and the final interpolator
doesn't take any advantage from a stable oscillator: must be well designed
and stable by itself. In short: if used as a frequency counter, yes, you
have to feed a good reference at the EXT REF, when used as a time interval
counter the good EXT REF is not so important, it is mandatory to use the
good reference at one of the two counting inputs. Don't forget that with
the PM6680 you can directly measure fractional frequency by using the MATH
menu and the K=, L= and M= keys and smooth the measure by the STAT key.
Later on you will discover that more resolution is necessary and you will
be interested in counters such as the HP5370B (20pS) or the SR620 (25pS).
Take a look at the PM6681 (50pS) too.

On Tue, Jul 3, 2012 at 7:38 PM, cfo <xnews3 at luna.dyndns.dk> wrote:

> On Tue, 03 Jul 2012 19:08:56 +0200, Azelio Boriani wrote:
>
> > 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.
>
> Azelio , first ... Thanx for walking me through this.
>
> I'll try the above later on.
> But what you're saying is that when used as a TIC A->B , i don't need the
> TBolt on ext-ref ? , or do i need the TBolt on both Ext-Ref & on Chan-A ?
>
> If i don't use the TBolt on Ext-Ref , won't i get hit by having a Std-Osc
> installed ?
>
> I have a GPS Antenna-Splitter , another TBolt (just need to assemble the
> PSU) , and also a X72 Rubi & a few FEI-5680's.
>
> So i could try those when i'm a bit more familiar with the Counter &
> program.
>
>
> >
> >The default sampling time is 200mS
> >
> Ahhh ... Thanx now i know why my 1hr Trace Duration was so fast.
> I assumed that Timelab would set the sampling interval written in Timelab
> via GPIB. But i'll just select 1sec manually.
>
>
>
> > 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).
> >
>
> I will try this now , and i assume i should then select Frequency in the
> Data Type - In TimeLab , as i don't use A->B measurements .. Or ?
>
>
> I'll also dig into - How to calculate measurement errors , as you
> describe above.
>
>
> Regards
> CFO - Tnut-Beginner
>
>
>
>
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