[time-nuts] Looking for RTC or some combination of MPU/crystal for0.1ppm accuracy.

WB6BNQ wb6bnq at cox.net
Sat Sep 12 23:52:37 UTC 2009


I agree with Hal in another post that you are not giving enough information for a
properly reasoned response.  So here are some questions:

1. What is the environment these sensors will be experiencing [temperature -
humidity - NOISE etc.] ?

2. What are the sensors measuring ?

3. What is the purpose for the timed measurements [that is what is expected to be
achieved with the time stamp] ?

4. How did you determine your need for a time stamp resolution ?

5. What is the duration of the measurement process [minutes/days etc.] ?

6. What is the expected time frame between being able to "check/verify" each
sensor's clock accuracy [that is how often would the sensor be brought back to a
shop/lab to re-adjust/re-align/calibrate (whatever) the sensor's various
adjustments if any] ?

7. What do you mean by "time beacon ?"

8. What is the purpose of the "time beacon ?"

9. What is the form of this "time beacon ?"

10. How is the "time beacon" transmitted [i.e., by wire / wireless etc.] ?

11. How is the sensor expected to align with this transmitted "time beacon ?"

12. How is the sensor sending its information "out" ?

13. With multiple sensors, how are they to avoid colliding with each other when
sending their data [is the response to be asynchronous or synchronous] ?

As you can see, a proper answer is not so simple.  The Dallas DS32B/C35 stability
is for a temperature range from 0c to 40c and may perform much better if the
temperature of the device were held {more or less} constant.  However, that would
have to be measured by you as Dallas does not directly make such offerings.

They do have a rather curious spec called "Frequency Sensitivity per LSB" that
shows the rate of error over temperature.  One could infer that for a temperature
between +25c and +70c that the worse case error would be +/- 0.4 ppm.  However,
you would need to include other error terms like voltage stability, etc.

Another point to consider is the fact that the DS32B/C35 only records time at
"one" second intervals.  The way I see it this determines the true granularity of
your time stamps with respect to other asynchronous sensors.  That is to say, you
can only determine the true time difference between sensors to the "one" second
level.  I could be all wet on this point, perhaps Bruce will comment on this ?


Rick Harold wrote:

> Hi,
> I need to find a solution for a RTC which provides 0.1 ppm accuracy in the
> form of a RTC/MPU or some other combination, oscillator etc..
> I'm creating some sensors which all need to do timed measurements and need
> to be at the same time.  They can get a time beacon
> every 10 seconds but then need to accurately perform independently 20x /
> sec.  They cannot communicate with each other and other than the time beacon
> are unconnected.  After a period of time they will send their time-stamped
> data out.
> I haven't seen anything other than RTC DS32B35/DS32C35 which perform at
> ~2ppm.  I need an order of magnitude better.  An atomic clock
> would be great, but the cost/size has to be low as these are sensors and it
> would be overkill.
> Any ideas?
> Rick
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