[time-nuts] Adapting my GPSDO to the FE-5680A

Nick Sayer nsayer at kfu.com
Wed Apr 20 18:31:04 EDT 2016


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 <attila at kinali.ch> wrote:
> 
> On Wed, 20 Apr 2016 07:17:58 -0700
> Nick Sayer via time-nuts <time-nuts at febo.com> 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
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