# What is the ppm in the crystal oscillator?

I'm a student, and I'm working on a low power communication project, I am trying to design a PCB using the TI CC2540 sample design. There is a MC-306(32.768kHz, 12.5pf, and 20/50ppm) I don't know what is the 20/50ppm. For me very important the size, so I decided to replace with the following FX135A but there the ppm is -20/+20. Will it cause a problem if I us this one?

What is the ppm in the crystal oscillator?

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## 2 Answers

Like Olin said, ppm stands for parts per million, and it indicates how much your crystal's frequency may deviate from the nominal value. The MC-306 exists in a 20 ppm and a 50 ppm version. For the 20 ppm version this means that the frequency will be between 32.7673 kHz (32.768 - 20 ppm, or x 0.999980) and 32.7687 kHz (32.768 + 20 ppm, or x 1.000020). These numbers may give you a comfortable feeling, but remember that a month is 2.6 million seconds, so if you want to use a 20 ppm crystal to build an alarm clock, it may have an error of 1 minute per month.
Crystals are available is different precisions, +/-20 ppm is more or less standard, for 10 ppm you'll pay more.
Also, this is basic precision. This frequency may deviate depending on environment factors, mainly temperature.

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PPM stands for "parts per million". It's like percent which is really parts per hundred, but based on million (10^6) instead of hundred (10^2). Therefore, 1% = 10000 PPM.

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