[time-nuts] My HP 5370B reads 6 nS out!

[Frank Stellmach]

Pete,

you do not specify, whether you use FREQ or T.I. when you use the 
averaging function.

First of all, its OCXO can be adjusted to a few parts in 1e-9 only, as 
the trimmer is too unprecise.
If the OCXO is running for several weeks already (idle state), its drift 
may be as low as a few parts in 1e-10 or better.

If you put the instrument to FREQ mode, you may measure and the 10MHz of 
the GPSDO standard to about 2e-11 resolution, if you use 1sec time base 
on the 5370B.
That should work also, if you directly measure 1pps, but you have to 
properly adjust the trigger level.
Important: Don't use the 10k statistics, set the 5370A also to 1sec time 
base!
Due to this low frequency, jitter should be higher, see specifications.

You better do statistics by means of a PC, over GPIB.
That will show the 30ps jitter of the 5370B, and the jitter of the 
GPSDO, on the order of 1e-10.

You may also calibrate the OCXO of the 5370B this way, instead of that 
oscilloscope method.

I strongly recommend Timelab from John Miles to do these measurements 
properly.
http://www.ke5fx.com/timelab/readme.htm


If you use the internal 10k statistics 10k, pay attention!!

In this instance, the 5370B will do the frequency measurement in a 
different manner..
Not 100% sure, it will be a sort of a T.I. measurement, calculated to 
frequency.
And that may produce a constant offset, if the internal T.I. calibration 
is not done properly.
Look into the specs, its absolute T.I. uncertainty is 1ns only, although 
it resolves 20ps.


You may check that behaviour, if you apply its own 10Mhz OCXO ouput to 
the FREQ input, and measure this frequency first on FREQ, 1sec.
That should give nearly exactly 10MHz, < 1e-10 jitter or deviation.
Mine reads 9.999 999 999 85 MHz, for example.

If you now switch to AVERAGE, SAMPLE SIZE 1, 100, 1k, 10K, you will see, 
that you will get big deviations as big as 0.1%, although it should 
measure its own OCXO to precisely 10.00000MHZ.
Mine reads 9,989 294 5 MHz, for example.

That's due to the different measurement method, and should explain 
6..7ns deviation on the 1pps signal also.

This averaging should only be used with T.I.!

Frank