[time-nuts] HP 5370B

Didier Juges didier at cox.net
Sat May 10 21:40:06 EDT 2008

This is exactly why I should not have started that thread.
But since I started this, I have to finish it.

I confused you with specification requirement and acceptance requirement.
Here is where this came from:

In our business (custom military hardware), we receive a specification from
the customer, we design the equipment, we then write a test procedure to
verify compliance to the spec. The customer reviews and approve the test

Up to that point, no problem.

In an ideal world (and I am sure in many other areas of business, like the
one where you operate), the acceptance criteria used in the test procedure
would start from the spec requirement, deduct the measurement uncertainty,
the accuracy of the test equipment and the calibration uncertainty, some
people will even put everything in the RMS blender, then what's left is the
allowable worst case measurement. For example, if you have to meet <0.20V of
temperature drift on a power supply, and you can measure temperature drift
with 0.05V total adjusted error, the test data should show less than 0.15V
drift for the equipment to be certified. Sounds easy enough. Even I
understand it.

In my real world of defence department equipment contracting (mostly
microwave, but not exclusively), there is no allowance for measurement or
calibration uncertainty except in some very unusual circumstances. The
acceptance criteria happens to be the same as the spec level 99% of the
time. There is nothing fraudulent about it. The customer reviews and
approves the test procedure and acceptance criteria, and either the customer
themselves or the on-site government inspector witnesses 100% of the testing
and certifies the test data.

I know it is tempting to think that we do that because we are either stupid
or crooks or both because then it makes everybody else look smarter, but
unfortunately that is not the case. We do that because if we applied all the
uncertainties, we would end up with a negative acceptable band, i.e. the
accuracy of the test equipment is insufficient to guaranty that the spec is
met if everything were at worst case.

So, in my real world, we collectively (customer and vendor) turn a blind
eye, telling ourselves that the probability that the equipment is worse than
measured is about the same as the probability that the equipment is better,
and everybody goes home happy, most of the time.

Now please leave the accusations of fraud at the door. Thank you.

I am done with this for now.

Didier KO4BB

> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Mike S
> Sent: Saturday, May 10, 2008 7:24 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] HP 5370B
> At 08:02 PM 5/10/2008, GandalfG8 at aol.com wrote...
> >  This has nothing to do with fraudulently claiming compliance, it  
> >just started as a well reasoned discussion of tolerances and the 
> >implications  thereof.
> There can be no other conclusion from the statements made.
> "Most microwave network analyzers have amplitude resolution 
> of 0.01dB, while their accuracy is just around 1dB in most 
> cases...I have had to argue too many times that a piece of 
> equipment with a 2dB p-p requirement on flatness was just 
> fine when it measured 2.01dB on the HP network analyzer. I 
> would not have gotten in that argument if the data had been 1.99dB."
> "I have never had a piece of equipment rejected because a reading was
> 1.99 for a spec of 2 max"
> The statements were made with regard to instrument 
> resolution/accuracy/precision. Clearly, measuring 2.01 (or 
> 1.99) on an instrument with an accuracy of 1 does not allow 
> compliance with a specification of 2 to be met.
> It was only later that the red herring of significant digits 
> was brought up. A specification of "2" is ambiguous in that 
> regard anyway.

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