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

Bob Camp kb8tq at n1k.org
Tue Jun 16 07:01:52 EDT 2015


Hi
> On Jun 15, 2015, at 9:51 PM, Peter Reilley <peter at reilley.com> wrote:
> 
> Frank;
> 
> Thanks for your long and detailed explanation.
> 
> I was able to get the internal OCXO to that precision but it was probably
> luck
> to get the trimmer that close.   I worked at it for a while.
> 
> I am using T.I. mode with the averaging mode.   I assumed that it took 10K
> readings
> and averaged the results.   Is that not correct?   Is something mode
> complicated
> going on?
> 
> I will have to set up the GPIB and give that a try.   I did get TimeLab.
> This will be new territory to me.
> 
> I did try measuring it's own 10 MHz frequency with 1 sec gate time.   It
> bounces 
> around by about +- 4 in the 10th decimal place.
> 
> Is my calibration with the rubidium oscillator valid?   Could it be
> that far off?

An “old style” analog tune Rb could tune a max of 1x10^-8. It’s rare
to see them more than 3x10^-9 off. Most are running under 5x10^-10 when
received from the salvage yard. 

The “new style” digital tuned Rb’s have a range limited by their VCXO (if
it’s limited at all). They *should* be < 3x10^-9 as received from salvage.
They could be much further off.

Since all of these are tunable devices. The only way to be sure of their
accuracy is to calibrate them against something good.

Bob

> 
> I will have to ponder this some more.
> 
> 
> Pete.
> 
> 
> -----Original Message-----
> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Frank
> Stellmach
> Sent: Monday, June 15, 2015 5:01 PM
> To: time-nuts at febo.com
> Subject: [time-nuts] My HP 5370B reads 6 nS out!
> 
> 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
> 
> 
> 
> 
> 
> 
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