[time-nuts] Holdover, RTC for Pi as NTP GPS source

Magnus Danielson magnus at rubidium.dyndns.org
Wed Nov 1 10:37:38 EDT 2017


This is also true for labs. One NMI lab I visited had issues with their 
5071As and H-maser clocks. It took some time to correlate it, but it 
turned out to correlate with the florescent lamps in the lab and people 
walking in and out. Changing these lamps to milder labs removed that 
disturbance, so people needing things like light can be a problem even 
for higher end grade labs.

The draft has been a re-occurring problem that we have seen, so it was 
really nice to show that simple blockage of air helped to significantly 
reduce that impact. Again, people walking up to the lab-bench was an issue.

A third example was a couple of students doing stability measures. They 
had a long-term measurement that all of a sudden became much flatter, 
and they asked me what it could be. They had started the measurement at 
15:00 and three hours late the semi-cyclic variation in phase stopped 
and things became much more quiet. I could then quickly conclude that it 
was the A/C for the building turned off, not to heat it during night. 
They didn't believe me, so I popped out the board and showed how the 
air-stream passed directly over the crystal metal housing and hence had 
a good thermal connection to the surrounding air temperature. I later 
had them put a few strands of foam-tape over the crystal, and they 
measured the same stable behavior over the full work-day. Silly people 
want a relative comfortable temperature and well, building A/C is 
typically bang/bang regulated so you get what you paid for.


On 11/01/2017 02:17 PM, Bob kb8tq wrote:
> Hi
>> On Nov 1, 2017, at 12:14 AM, MLewis <mlewis000 at rogers.com> wrote:
>> (I suspect this is drifting from the original thread too much, so new subject)
>> Temperature ranges from 65F to 78F, with the potential for drafts, but is more typically 76F.
> The gotcha in a real environment often involves people. They walk by (creating a draft). They
> turn on all the lights and equipment. They open or close the blinds to let in or block the sun.
> They tend to do this in an unpredictable / chaotic fashion. All of this makes a correction
> process based on “normal operation” a bit difficult. Something goes wrong, and the unit
> goes into holdover. People suddenly start dashing around and the temperature is not
> what it has been ….
> Bob
>> I read about the NTPsec runs with insulating a Pi and running a load generating program to better maintain a stable core temperature.
>> Just today I've put my GPS module inside a case for an RF shield that is also semi insulated. It's feeding LH on a PC while I do the next step.
>> The Pi 3 is going inside a large enough tea tin and that will be lined with insulation.
>> I'm wondering about insulating the RTC...
>> The low cost for a 'precision' RTC means it is cheap to test.
>> I'd completely discounted coasting with the system clock, as I have fixed in my head the variable loads on my production machine mean that Window's time lags variable amounts, as the CPU load is variable with variable burst loads every 1/8 of a second.
>> Michael
>> On 31/10/2017 11:45 PM, Hal Murray wrote:
>>>> I'm intending to add a "precision" (well, precision to the Pi world) RTC  to
>>>> my Pi 3 to use for a holdover source when it hasn't got PPS from the  GPS
>>>> module.
>>>> An RTC that +/- 3 PPM over 24 hours would be great for holdovers of one  to
>>>> 20 minutes.
>>> Run some experiments to collect some data and play with the numbers.
>>> How stable is the temperature in your environment?
>>> The key to keeping sane time on a PC or Raspberry PI is to calibrate the
>>> crystal.  Most CPUs have a register that counts at the CPU clock frequency -
>>> or something in that range.  Most systems smear the clock to keep the FCC
>>> happy...
>>> Most OSes keep time by watching that register and dividing by the clock rate.
>>>   The actual clock rate doesn't usually match the number printed on the
>>> crystal.  It's close, but ntpd can easily measure the error and tell the
>>> kernel so the kernel can use the right value.  If you turn on loopstats, ntpd
>>> will log it and you can graph it.
>>> If you are writing an embedded system, you will want that sort of logic too.
>>> My guess is that in the under 30 minute range, you will get better results by
>>> just coasting with the system clock rather that using a RTC.  It would be an
>>> interesting experiment.  Implement both clocking schemes and compare them.
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