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.
