Timebase Cal Problem on HP 53131A SW Cal

Started by Hawaii596, 07-11-2011 -- 09:55:55

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Hawaii596

07-11-2011 -- 09:55:55
I run into this every once in a while, and wondering if anyone else has seen it.  When doing the software cal on an HP 53131A with 010 or 012 time base, every once in a while I get one that has an offset.  I am using my Rubidium, and I have two identical units that have both been warming up at least 72 hours (last time I ran into this I had about 6 units all with OPT 010 or 012).  I unsecure the cal mode, input the 10 MHz, and when it is done, I have a little offset.  Like with resolution of 0.1 mHz (milli Hertz), I get an offset of about 25.5 mHz (not MHz).  I have two counters.  One cals fine, and with the same resolution, I get a predictable one or two counts of offset just due to instability/noise, etc.  But the second unit, I get the above offset.  I re-ran the software cal and get about the same results on both counters.  Same thing happened on the last one (a few m onths ago).  I recall on the other one, I repeated the SW cal a few times, and it did the same thing each time.  So I don't believe I am doing anything wrong.  I tried adding a feedthrough term, and no effect.  I think there is something wrong with the counter.  Maybe a noisy A/D circuit or something (maybe a leaky capacitor somewhere??).  Just wondering if anyone else has had that problem.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind."
Lord Kelvin (1824-1907)
from lecture to the Institute of Civil Engineers, 3 May 1883

PMEL Whore

#1
07-12-2011 -- 06:12:38
What if you took a sig gen, tied it to your rubidium and then input a 10 MHz signal that is offset by the offset you are seeing, does that solve the problem, basically lie to the UUT.
I'll try being nicer if you'll try being smarter.

Hawaii596

#2
07-12-2011 -- 07:36:23
I actually thought of trying that.  One thought I have is that the offset may be a multiple of power line frequency (possibly indicating a leaky capacitor somewhere).  Problem is, the incoming shelves are full.  So I have to set it aside.  One other experimental thought I have is try reducing the resolution and seeing if I get the same approximate three digits of offset, but pushed over by what ever resolution I set to (for example, if I change resolution to 10 Hz, if the offset changes to 2550 Hz (still corresponding to the bottom three digits), which would still lean toward a leaky cap or equivalent, but associated with A/D circuitry..... this, once I have actual time to work on it.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind."
Lord Kelvin (1824-1907)
from lecture to the Institute of Civil Engineers, 3 May 1883

andyc_more

#3
01-10-2014 -- 09:27:13
Yes - I've seen this ( I have a 53131A and 53181A ).
It looks like the auto calibrate function is a little buggy.
It has also been noticed here http://gerrysweeney.com/diy-hpagilent-53131a-010-high-stability-timebase-option/

The video has a section which shows the auto calibration procedure performing the magic
( a binary search - and a dac to control the OCXO )

The calibration will even fail sometimes when it should not.

andyc_more

#4
01-10-2014 -- 09:47:45
Also - be aware that some rubidium oscillators 'sweep' around the 10.000000Mhz
As far as I recall the 10Mhz is locked to a microwave source which is swept in frequency around the absorption line for rubidium plasma. A light cell detects a dip in the absorption - which indicates the correct frequency - so you're always going to get 10Mhz +/- some very small amount swept at some rate (416.6Hz has been mentioned)
I've had better results manually adjusting a very good ocxo to match the rubidium , and then using the ocxo to calibrate the 53131A.
Standard rubidium oscillators are not very good over the short term - and I think the 53131A calibration method is too short.

Hawaii596

#5
01-10-2014 -- 16:26:24
This has been a while now.  But I feel like the Rubidium is not the culprit.  We use it all the time on many counters.  It's short and long term stability is quite a bit better.  If I recall that day, I had a stack of 53131A's and 53132A's.  They all cal'd perfectly except that one.  Seemed like it was something weird in the 53131A/2A.  I don't think that particular unit is around.  Just a strange mystery.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind."
Lord Kelvin (1824-1907)
from lecture to the Institute of Civil Engineers, 3 May 1883

andyc_more

#6
01-11-2014 -- 03:16:31
If it was repeatable then definitely a fault with the counter - the issues I've seen usually clear after a couple of recal attempts.
A nice page comparing the short/medium/longterm stability of certain rubidium standards
is at http://www.ke5fx.com/rb.htm
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