# frequency/clock stability

• posted

Hi,guys I am a little confused about definition of clock/oscillator stability. it is x ppm over some temperature range in common spec and there usally another aging spec in addion to stability, so I assume the stability was not very related with the measuring duration. but recently I am read some material about atom clock which used in gps, and they specify the stability with a duration(short or long) and it seemed they assuming some unchanged temperature. so can any one help me clarify the concept or recommend some reference?

• posted

x-posted to and followups set to sci.electronics.basics

Oscillator specs and crystal specs (often used as the frequency setting element) vary a little.

Crystal specs usually state their stability across temperature, loading capacitance, cal tolerance (initial accuracy) and long term ageing.

Long term ageing occurs simply because a crystal is an electromechanical device. One I have in front of me specifies those things as:

Stability: 50ppm from -40C to +85C Initial accuracy: 30ppm Loading capacitance : 7pF Ageing : 3ppm max first year

There isn't any more on ageing, as this device is not expected to be used in a reference oscillator.

As you mentioned, there is a temperature issue.

For very accurate clocks, we run the crystal in a temperature stabilised oven with an AFC (automatic frequency control) loop, which maintains the temperature such that the oscillator maintains it's frequency very accurately. This is known as a TCXO (Temperature Compensated Crystal Oscillator), and we can get very accurate (

• posted

I understand your confussion, especially if you try to make sense from the literature from the last two generations (2x25 years). Time (t) measurement and frequency (f=1/t) went through VERY intensive development during those times and the precision of measurement and means of supplying those quantities developed exponentially. To such extent that even your measuring stick (meter; inch) are specified today as part of distance traveled by light in vacuum in epoch of one second. (Such is the precision of the 'second' measurement!) The ppm over temp means two identical watches one worn on your hand 24H a day with 36.6 deg. C supplied by your anatomical air conditioning system, the other left on your night table with the daily ~10-15 deg. changes, after some time you will see disagreement. In the gps all clocks are syncronised to a master(s) and get corrected when needed. The master is pampered by steady and contolled conditions to the best that can be supplied. References? anything that talks about oscillators even the start of HP company and their progress to manufacturing THE standard atomic frequency/time reference.

HTH