frequency/clock stability

[maTheMatic]

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?

 

[PeteS]

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?

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 ( << 1ppm) accuracy and stability from
such devices.

Also keep in mind that an oscillator is more than just the crystal;
it's the crystal plus amplifier plus control loop circuitry (for really
accurate devices amongst other things) and when specifying an
oscillator, the entire circuit must be taken into account.

Cesium (atomic) clocks use the natural resonance of cesium (which
starts as liquid and is heated in an oven to it's gaseous state) to
lock in a crystal oscillator. By using cesium as the reference feedback
element, accuracies of 0.00000002ppm (about 2 E-14) have been achieved
for reference units (such as the one used at NIST).

This one, incidentally, is known as a Primary Standard or a Laboratory
standard.

Secondary standards are more commonly used.

You can read all about cesium clocks :
http://tycho.usno.navy.mil/cesium.html

Cheers

PeteS

 

[Stanislaw Flatto]

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?

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

Stanislaw
Slack user from Ulladulla.

 

[[email protected]]

Ive been at the freq stability game for a long time.
Stability is simple .
For long term performance like wrist watches and receiver on tune add
the initial setting tolerance to the effects of temperature over the
band of interest.
say -10 +60 10ppm + 3ppm per year to give 13 ppm at the end of the
first year to be corrected annually.
For regenerators in digital systems and close in noise in carriers look
at the stability not in years or weeks but second to second or shorter
time. Picosecond jitter translates to ssb noise.
..

 

[Stanislaw Flatto]

Ive been at the freq stability game for a long time.
Stability is simple .
For long term performance like wrist watches and receiver on tune add
the initial setting tolerance to the effects of temperature over the
band of interest.
say -10 +60 10ppm + 3ppm per year to give 13 ppm at the end of the
first year to be corrected annually.
For regenerators in digital systems and close in noise in carriers look
at the stability not in years or weeks but second to second or shorter
time. Picosecond jitter translates to ssb noise.
.

It is one thing to accept the existing trend in design, completely
different thing to convince the scientific community that your design
answers the precision that you promissed.
Time/freq did this trick during my lifetime.

Have fun

Stanislaw
Slack user from Ulladulla.