What is ppm rating in crystals/Oscillators?

I was going through a processor datasheet where a system PLL clock output is driving USB controller. It was mentioned that the clock must be having a stability factor 480MHz ± 500ppm. This article is for those who are interested in knowing what this ppm means.

You must have seen the specifications of a oscillator in it's datasheet and it must have mentioned stability as ppm. We generally don't think much about this ppm and tend to select as per the requirements given. If one asks, ±20ppm we just select the oscillator with that stability factor. A crystal frequency may vary as per the environmental conditions and ageing. The amount of variations is indicated by the ppm value. Let us see few points about this ppm.

• ppm is nothing but parts per million
• A change in frequency (dF) as per ppm value is calculated using the formula, (crystal frequency * ppm)/(10^6). 
• This dF when added to crystal frequency gives the frequency variation.
• So, for one year period, the crystal vary by F±dF

There are other types of oscillators use instead of normal oscillators to get temperature stability. They are temperature controlled oscillators, oven controlled oscillators, atomic clocks. Oven controlled oscillators have a ppm in the range of ±1x(10^-9). Atomic clocks are much more stable than this.

What is the difference between oscillator and crystal?

• Crystal is the basic building block of oscillator.
• Oscillator can be constructed from either quartz crystal or ceramic resonator. Quartz crystal gives more stability.
• A crystal included with semiconductor elements like transistor, inverter and capacitors form a oscillator
• If you see a crystal, you need not give power but for oscillator power is required.
• the output of oscillator is buffered to drive higher loads.
• Oscillators are available in plastic or metla package.