Many types designed for many different applications andĪ rubidium clock has an accuracy of about 1 x 10 -12 so the errorĪfter a day would be 86.4ns (84e-9 seconds 84 billionths of a second!) so theĮrror after a month would be 2.6us. this OCXO has a ppm value of 0.001 ppm or 1ppb. They are not quoted in ppm as it becomes inconvenient to Sealed in a small chamber with a controlled heating element inside toĪ typical spec might be 1 x 10 -9 (1ppb) so the error after aĭay would be 86.4us and after a month 2.6ms (2.6e-3 seconds or 2.6 thousandths Watch crystal will be better because you keep it at a constantĪn OCXO is an Oven Controlled Crystal Oscillator. Oscillator - that generates heat and keeps a constant temperature). Temperature (This is why you can buy an OCXO or Oven Controlled Crystal Temperature coefficient graph meaning that another source of error is and the human body is at a constant temperature. One of the other factors in a wrist watch is that you wear it on your wrist Over a day of 86400 * 2e-5 = 1.73 seconds per day so in a month it loosesģ0x1.72 = 51 seconds or 1 minute a month (approx). How good is a watch crystal ?Ī watch crystal has an error of 20ppm (ish), but you have to design theīoard layout well, this translates as 20/1e6 (2e-5) which gives an error In a month you would loose 30x8.64 = 259 seconds or 4.32Įrror: 8.64 seconds per day. So the total error on a day is 86400 x 1e-4= 8.64 How good is a typical crystal ?Ī typical crystal has an error of 100ppm (ish) this translates So the maximum error after a day has passed is 1%Įrror: 14.4 minutes error per day. If you look at a day's worth of timekeeping then you have 24 x 60 x 60 =Ĩ6400 seconds in a day. Note: A typical microcontroller crystal has a 100ppm specification
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