Parts per million, or ppm, is a way to describe very, very small concentrations of something. It tells you how many parts of one substance are present for every million parts of the total mixture. In electronics, the term “ppm to” is often used when talking about the stability and accuracy of oscillators and other devices.
PPM and Frequency Variation
PPM describes how much the frequency of a signal varies from its center frequency. A certain PPM value will translate to a specific frequency deviation, measured in Hertz (Hz).
Here’s the formula for converting PPM to Hz:
Frequency Variation (Hz) = PPM Center Frequency (MHz) / 1,000,000
For example, 100 PPM of 100 MHz is a 10 kHz variation. Conversely, a frequency varying from 100 MHz to 100.01 MHz (10 kHz) is a 100 PPM variation.
Why does PPM matter?
PPM is a common way to describe the stability of oscillators. A lower PPM means the oscillator’s frequency is more stable.
Even small PPM variations can affect timing. For example, if you have a 100 MHz clock with a 100 PPM variation, it can result in a 2 picosecond difference in the period.
In Summary
PPM is an important spec for frequency control, because it has a direct effect on accuracy. Knowing how to convert between ppm and Hz is essential when you’re working with oscillators and similar circuits.