[time-nuts] Another "atomic" clock question

Magnus Danielson magnus at rubidium.dyndns.org
Fri Mar 7 23:47:40 EST 2014

On 07/03/14 19:28, Lars Walenius wrote:
> Chris, about using one Arduino for two GPSDO controllers:
> Even if a microcontroller has lots of capacity I would recommend to use separate controllers for each oscillator. One of the reasons is what Tom van Baak said about using only one interrupt to avoid jitter and even if you trigger both channels from the same PPS and have just one interrupt you will have a problem that you can´t read two ADC´s at the same time.
> Even the HC390 I wouldn´t use for two different oscillators to prevent crosstalk. Both the processor and HC390 is so cheap it isn´t worth the risk IMO.

Cross-talk typically happens though ground-bounce. Just using separate 
chips reduces the effect. May not be much of an issue at ns level, but 

> Actually I would also recommend to put them in separate boxes even if it is more work  (and I´m lazy ) to get best performance.
> Having two GPSDO´s that you can compare is very nice as long as you understand how they correlate , if that is not what you want to test. Of course you can also set one or both in hold mode to test them freerunning.

Some telecom rubidiums have fairly noisy output. Steering an OCXO for 
clean-up might actually provide the best of both worlds. Holdover of the 
rubidiums and phase-noise of the OCXO. In that case, keeping them in the 
same box makes sense. The arduino could contribute long-term integrator 
memory and possibly do temperature compensation of the OCXO as a 
feed-forward approach.

> I have thought of connecting the M12 to the Arduino and if someone can help with code to get the sawtooth correction value into the Arduino and decoded I would be glad to have it.

I've proposed to some of my local friends here, and we will probably do 
something with LPROs. We need to look at what GPS modules there is.

I think sawtooth correction should be added. It's not that hard. One 
really wants two serial ports, one for the GPS and one for monitoring.


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