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Clock accuracy in ppm
Crystal Clock accuracy is defined in terms of ppm or parts per million and it gives a convenient way of comparing accuracies of different crystal specifications.
.
The following headings give practical calculations showing the typical errors you will encounter when using a clock of a specific type with a specific accuracy.
How good is a 1% accurate clock ?
If you look at a day's worth of timekeeping then you have 24 x 60 x 60 = 86400 seconds in a day. So the maximum error after a day has passed is 1% of 86400 = 864 seconds = 14 minutes!
How good is a typical crystal ?
A typical crystal has an error of 100ppm (ish) this translates as 100/10e6 or 0.001% (its the same number expressed as a percentage). So the total error on a day is 86400 x 0.01%= 8.64 seconds per day so in a month you would loose 30x8.64 = 4.32 minutes per month.
How good is a watch crystal ?
A watch crystal has an error of 20ppm (ish), but you have to design the board layout well, this translates as 20/10e6 (0.002%) which gives an error over a day of 86400 * 0.002% = 1.73 seconds per day so in a month it looses 30x1.72 = 51 seconds or 1 minute a month (approx).One of the other factors in a wrist watch is that you wear it on your wrist - and the human body is at a constant temperature. Crystals have a temperature coefficient graph meaning that another source of error is temperature (This is why you can buy an OCXO or Oven Controlled Crystal Oscillator - that generates heat and keeps a constant temperature). The watch crystal will be better because you keep it at a constant temperature!
How good is an oven controller crystal oscillator OCXO ?
A typical spec might be ±1 x 10-9 so the error after a day would be 84.6us and after a month 2.6ms (2.6e-3 seconds or 2.6 thousandths of a second!). They are not quoted in ppm as it becomes inconvenient to write e.g. this OCXO has a ppm value of 0.001 ppmNote: there are many types designed for many different applications and
all costing different amounts!
How good is a rubidium oscillator ?
This is also known as an atomic clock.A rubidium clock has an accuracy of about ±1 x 10-12 so the error after a day would be 84.6ns (84e-9 seconds 84 billionths of a second!) so the error after a month would be 2.6us. Again using ppm is also inconvenient for writing : 0.000001ppm
How good is a cesium oscillator ?
This is also known as an atomic clock.Cesium beam atomic clocks are stable to 1 x 10-13 (8.64ns/day 8 billionths of a second!) or 259ns (259e-9 seconds) a month (ppm is 0.0000001ppm !).
Note: A Cesium fountain is stable to 1 x 10-15.
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