Hi Most of these telco Rb’s start out around 2x10^-11 at 1 second tau. Their ADEV then drops as square root of tau. At 100 seconds they hit 2x10^-12 and at 10,000 seconds they (might) hit 2x10^-13. A *good* OCXO will do 2x10^-12 at 100 seconds. An excellent part will hold sub 1x10^-12 out to 1,000 seconds.
Very roughly speaking the Rb will beat out the OCXO somewhere past 100 seconds and somewhere before 1,000 seconds. Bob > On Apr 20, 2016, at 6:31 PM, Nick Sayer via time-nuts <[email protected]> > wrote: > > Thanks. I’ve taken your suggestion for the sine-to-square converter. > > I believe there are two separate commands for tuning the 5680 - one is > “temporary” and one writes through to the EEPROM. I’ll be using the latter, > of course. > > http://www.ka7oei.com/10_MHz_Rubidium_FE-5680A.html > > I do agree that the short term stability of the 5680A isn’t as good as an > OCXO, but at tau ~2s or so, the tables are turned. I’m getting a good measure > of my undisciplined 5680A as we speak to get a good control, but it’s > difficult, as I’m testing it against a Thunderbolt, and I think I’m seeing > its “hump” (http://www.leapsecond.com/pages/gpsdo/) from 10s to 300s coming > through. In any event, I’m getting awfully close to the limits of my 53220a. > I may go down the road of trying to make a mash-up as you suggest, but I’m > going to start by seeing if I can give myself a choice between whether I want > short term stability (OCXO) or medium term (Rubidium) for my reference. > > >> On Apr 20, 2016, at 1:57 PM, Attila Kinali <[email protected]> wrote: >> >> On Wed, 20 Apr 2016 07:17:58 -0700 >> Nick Sayer via time-nuts <[email protected]> wrote: >> >>> I spent some time yesterday mashing together my FE-5680A "breakout" board >>> with my GPSDO to make a GPS discipline board for it. Before I send the >>> board >>> off to OSHPark, I'd like to open the design to criticism (and I mean that >>> in >>> its neutral sense) here first. >> >> Looking at your schematics, I would replace the input squarer (IC4) >> by something different than a schmitt-trigger with an input bias >> voltage. For one, schmitt-triggers are more noisy than normal buffers >> for an other, the bias voltage will result in a slightly skewed duty >> cycle. If you want to use a gate, then the canoncical way would be to >> use an inverter with an input capacitor like you did, but let it self-bias >> itself by using a 100k-1M resistor from its output to its input. >> Important: don't use a buffer, as this will only work with an inverter. >> But I'd rather use a different squaring circuit, if you want to use >> the input directly for the output. There are many discussions on >> squaring circuits in the archives. Probably the most simple, yet sofisticated >> is the one you can find in the TADD-2. >> >> >> But: >> As you can see on http://www.ke5fx.com/rb.htm the phase noise of the >> FE 5680's is horrible at best, hence I wouldn't use it as source for >> anything directly. Additionally, the tuning word you write through >> the RS-232 is stored in an EEPROM inside the FE-5860 (unless i mix >> it up with another Rb). Writing this tuning word often will wear out >> the EEPROM pretty quickly. Hence you should not do this too often. >> >> What I would do instead is, use your current GPSDO design, with OCXO >> and all, but add something with which you can measure the phase/frequency >> of an external 10MHz reference. One way would be to use a digital DMTD[1,2]. >> Another would be to sample the reference using an ADC and build DMTD in >> the digital domain. For this you wouldn't need a high sampling rate, a >> couple of kHz should be enough, as long as the analog bandwidth of the >> ADC is high enough (>10MHz, better >20MHz). What you need is some PLL >> though, as you need to create a frequency that is not an integer divisible >> of 10MHz, as the ADC clock is used to downmix the reference frequency. >> >> Eg: >> If you can generate a 10001Hz ADC sampling clock from the OCXO, >> you will get a 1kHz beat frequency. You can "lock" to this using a >> digital PLL combined with an NCO (numerically controlled oscillator). >> Then use the steering word for the NCO as an input for the control >> loop of the OCXO, toghether with the corrections calculated from the >> GPS PPS. >> >> The advantage of this is, that you get the low phase noise and good >> short term stability of the OCXO, but can use the Rb to get the nice >> mid-term stability (somewhere from 1 to 10s up) while getting the >> accuracy of a GPSDO, whithout ever the need of writing to the tuning >> word of the Rb. That keeps your Rb more stable (the internal conditions >> of the Rb do not change) and allows you to compensate for quite large >> frequency offsets for Rb refernces that are working outside the spec, >> but are otherwise fine. >> >> One thing that you have to take care of is spurs, though. Because >> the ADC does some heavy down-mixing, or rather sub-sampling, this >> approach is quite sensitive to spurs. In order to not introduce some >> weird oscillations in the control loop due to spurs in the reference >> signal, you should use some narrow 10MHz filter at the input (at most >> half the sampling frequency wide). One way to achieve that is using a >> ceramic resonator which are available at 10MHz. >> >> Attila Kinali >> >> PS: I'm pretty sure I am not the first one with this idea. But I have never >> seen anyone else mention it, much less implement it. Does anyone know why? >> >> >> [1] "Digital Dual Mixer Time Difference for Sub-Nanosecond Time >> Synchronization in Ethernet", by Moreira, Alvares, Serrano, Darwezeh and >> Wlostowski, 2010 >> >> [2] "Digital femtosecond time difference circuit for CERN's timing system", >> by Moreira, Darwazeh, 2011 >> http://www.ee.ucl.ac.uk/lcs/previous/LCS2011/LCS1136.pdf >> >> -- >> Reading can seriously damage your ignorance. >> -- unknown >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
