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

Bob Camp lists at rtty.us
Tue Jul 3 19:42:37 UTC 2012


Hi

That works out fine *if* the edges are close to each other. 

Consider one edge arriving a half second before the other one. Then the reading is off by a nanosecond for each 2 ppb the reference is off frequency. If you have a TCXO as the reference for your counter, you likely will be further off than that. There are a *lot* of surplus counters that originally went into setups with external references. They have (at best) a loose TCXO for a standard when the reference is not present. 

Bob

On Jul 3, 2012, at 3:32 PM, Azelio Boriani wrote:

> 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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