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 <xne...@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 >> >> >> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.