Beijer Electronics (formerly QSI Corporation)
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Clock of QTERM-G75
http://www.qsiforums.com/viewtopic.php?f=6&t=3944
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Author:  Jay [ Sun May 23, 2010 10:54 pm ]
Post subject:  Clock of QTERM-G75

Hi,
I am using QTERM-G75. I wanted to know the accuracy / drift of the in-built clock present in the same.
My application requires time synchronization for periods upto 6 months. Is it reliable to use in-built clock of QTERM-G75 for such applications? In such a case, by what amount can this clock drift?

Author:  Jeremy [ Mon May 24, 2010 6:26 am ]
Post subject:  Re: Clock of QTERM-G75

I do not know that anyone has formally studied that particular issue. I did ask one of the engineers who helped with the clock circuitry if he had any insight and will let you know his response.

I just grabbed a Qlarity terminal that has been on the shelf for a while (where while is somewhere between 6 and 18 months) and the clock is off by about 6 minutes. I realize that this is probably way too informal for any real use by you.

If your G75 will have network access over those 6 months, it could synchronize its clock via SNTP. We have several G75s installed here a QSI that use SNTP once a day or so and their clocks have remained within a second of each other for several years now.

Author:  mkelwood [ Thu May 27, 2010 12:39 pm ]
Post subject:  Re: Clock of QTERM-G75

The drift is primarily a function of the crystal frequency tolerance. For the crystal used on G75 this is 20 ppm. Assuming 30 day month (2592000 seconds) you would get a maximum 51.8 seconds drift in that time.

This neglects the effects of temperature (which can be very significant; the additional drift can be as much as 50 ppm at -10 C or 60 C) and crystal aging. For first order approximation, a minute (maximum) per month is a good figure assuming the unit is sitting close to room temperature for that period of time.

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