Hi Tom:
Think of it this way. Offset is a measure of how well someone has set an oscillator. Stability is a measure of how well the oscillator maintains it's frequency. Stability is the money spec. A lab grade crystal oscillator may have a stability of parts in E-10 per day. That means that if the offset is set to zero a day later the frequency may by off by parts in E-10.
The world in not a perfect place and any Cesium oscillator also has aging, it's not perfect, that's why there's a spec on the 5071A saying less than 1E-14 per day. NIST has a fountain Cesium where they say less than 1E-15 per day. That number is the stability NOT the offset. To see the stability you need to accumulate enough data to see the parabolic shape. It took me over a month after I got the wrinkles out of my test system.
Can you hookup a Cesium source and make a time interval measurement for a period of a month? Do you have software that will tell you the stability (not offset) of the oscillator being tested?
Having Fun,
Brooke
Tom Van Baak wrote:
The FTS4060 time interval is following the following equation (it takes about a month to get this equation):
y = -1.2594x2 + 236.37x - 10318
where Y is in ns and X is the Day Of the Year. The first term is the fractional frequency stability, i.e. drift rate and is
1.14E-14 per day which is pretty good.
Do be careful here. Excel will blindly report equations with 5 significant digits no matter what the data looks like.
Here's something to try: break up your data into three 10-day segments and see how well the x2 term of the equations agree.
Or convert phase to frequency and then plot 30 days of frequency. If you have real drift it should be clear from this plot.
DougH, JohnA, and I have Stable32 which makes this a snap if you want to send any of us the raw phase data.
The HP-Agilent 5071A is specified at <1E-14 per day.
Note that the fractional frequency stability of a good lab grade crystal standard is about 1E-10 per day, so Cesium is 10,000 times better, but still has drift.
All frequency standards have frequency instabilities. Hydrogen masers, Quartz, and Rubidium have drift, but Cesium standards are generally considered to have zero drift. That's one reason UTC is based on that technology.
And note that fractional frequency [in]stability is not the same thing as frequency drift. I can go into this in more detail if you wish.
This explains a lot about why setting the C field near 1E-14 is difficult, the frequency is changing all the time.
If you make frequency plots in addition to phase plots you will see dramatically why setting the C-field of your 4060 to 1e-14 is hopeless. Frequency plots will graphically show frequency instability (the width of the line) and frequency drift (the slope of the line).
/tvb
Now Having Fun,
Brooke Clarke, N6GCE
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