[time-nuts] HP 5370B

[Didier Juges]

The point is that in my 30 years experience is selling equipment to
government and prime contractors, I have never had a piece of equipment
rejected because a reading was 1.99 for a spec of 2 max, but I have seen
many cases where an instrument reading 2.01 (or, God forbid 2.001 dB as some
can display) being argued as being unacceptable. We all know that a reading
1.99 does not guaranty that the equipment is actually better than 2, but I
have never had to make that argument.

I am not saying that a reading 2.001 is always acceptable when the spec says
2 max, but in most cases, make the same reading again a while later and
chances are good that you will be under 2 (or maybe way out).

If you read 2.01 for a spec of 2 max and the instrument is 1% acurate, you
have not established that the equipment was out of spec because the reading
could be off by 0.02. Now, there are other standards and practices. If you
specify a parameter to be 2 max for instance, it is not the same as 2.0 max.
The accepted practice is to round to the required significant digit, so that
if the spec is 2 max, a reading of 2.4 would be rounded to 2 and be
acceptable, but it would not be acceptable if the spec was 2.0 max. Not all
customers interpret it that way, but most do. 

Please note that these problems are more pronounced with microwave signals
than with things that are more easily measured, such as DC voltage. In the
microwave world, It is always extremely frustrating to try and correlate the
accuracy claims. You can take two instruments that are both 1% accurate and
with calibration tracable to NIST, and yet they will be off by several %
from each other. This is routine. There are simply too many factors
involved, such as VSWR, connector problems and so on.

Didier

> -----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 12:36 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] HP 5370B
> 
> At 10:43 AM 5/10/2008, Didier Juges wrote...
> (corrected for top-posting)
> >At 10:18 AM 5/10/2008, Mike S wrote...
> > > At 09:22 AM 5/10/2008, Didier Juges wrote...
> > > >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. Go figure.
> > >
> > > I figure you're both wrong. If the measuring instrument 
> is no better 
> > > than 1 dB absolute, you can't expect to do it - a flat line 
> > > measurement could represent an actual + or - 1 db, the 
> limit of the 
> > > spec.
> 
> >I knew would get comments on that. I simply did not provide enough 
> >information. I thought most everyone who has done it would 
> know what I 
> >was refering to.
> >
> >Acuracy of these instruments is not one number, it is a 3 
> dimensional 
> >chart, and time is the 4th dimension.
> 
> Exactly what (real world, not something made up to match the 
> claim like
> -.02 to -1.00) accuracy spec allows a reading of 2.01 to meet 
> a spec of max 2? Please tell us what you're referring to. I 
> don't claim to be an expert at math or statistics, but I just 
> don't see it.
> 
> I understand that there is the _possibility_ that the DUT is 
> in spec due to measurement uncertainty, but to say it's "just 
> fine" (i.e. 
> guaranteed to  be within spec) can't be claimed. Aren't you 
> measuring so you can guarantee the spec is met? If you say 
> 2.01 is as acceptable as 1.99, then what about 2.02? 2.03?...999.00?
> 
> You seem to be trying to make the point that a measurement 
> difference of .02 is meaningless if the accuracy is some 
> amount greater than that. 
> That's a losing argument, since there has to be _some_ 
> defined point beyond which the spec can't be guaranteed. For 
> your example, it might be impossible except for a perfect 
> measurement, as in the example I gave. In any case, the 
> measurement would always have to be less than the 
> specification. How much less would depend on the accuracy. 
> Maybe it's 1.50, maybe it's 1.49, but it's a pass/fail test, 
> either the specification is guaranteed by the measurement, or 
> it isn't (excepting statistical specs). 
> 
> 
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