Michael Barge asks: >(1) Do most labs report an uncertainty measurement in the test report, on the data sheet, on a certificate of compliance? >(2) How did you generate the measurement of uncertainty for emission tests? For immunity tests? > AND MOST IMPORTANTLY >(3) What do you tell the customer when he is below the limit by less than the measurement uncertainty? When he is above by less?
---------------------------------------------------------------- Well Michael, the lab that I do most of my business with does not report the measurement of uncertainty in my reports, and I am very satisfied that they do not. I certainly do not include a value in the declaration of conformity, nor do I intend to ever. It would be ludicrous for our declaration to state that we are 96% confident that we comply. Either we comply or we don't. In my opinion, the measurement uncertainty is unnecessary and its inclusion can only lead to stricter limits being enforced. Enforced by the unknowing in the beginning and eventually by decree "because that's the way it's done now". I have delt with a lab that included the measurement of uncertainty in their report, it was 2 dB. And if we were closer than 2 dB to the limit they considered it a failure. Evidently, because their instrumentation was not sufficiently accurate to give them measurements that they could be confident in, our equiment suffered. I don't use that lab any more. My contention is that this measurement uncertainty is a lot of hog wash. The same as reporting frequencies to three decimal places and dB's to two decimal places. That's missing the original intent of the requirement, which was to not interfere with other's communications. The method used originally was a radio receiver, with front mounted rotary switches that dropped attennuation pads into the receive circuit (i.e. see the older Rhode & Schartz used by VDE). The level was read as the amount of attennuation needed to bring the incoming signal to a null. Thus one either passed or failed, with no reguard to fractions of dBs. As far as the frequency was concerned, it was read off a log scale similar to a radio tuner (before digital readouts), and one was lucky to get the reading down to 10 MHz in the upper scale. It wasn't until the introduction of the spectrum analyzer by HP around 1979-80 that we could get readings down to three decimal places. Why did HP pick three? If they had picked five, lab assistants and engineers would now be blindly writing values down to five decimal places. It doesn't make it more accurate, it only gives a feeling of being very scientific. One can get the diameter of a circle by using 3.14 for pie (?) just as well as using pie to 27 decimal places. Proponents of the measurement uncertainty practice should take a look at the way that the VDE engineers write down their measurements. They have a sheet of log paper and a pencil. They put the point of the pencil ot their best approximation of frequency and dB level on the log paper and make a large dot. They then pull down on the dot to make a tail, so that it will be easier to identify. Again, you either pass or you fail. The size of the pencil dot and the way the VDE engineer places the pencil on the paper greatly overshadows any measurement uncertainty there might have been in the system. One final comment, our instumentation has always been accurate, and we have always known how acurate it is. The accuracy is specified by the manufacturer and the instrumentation is kept within calibration. Also the regulatory bodies have been satisfied with that process. I have a hunch, and it is only a hunch, that this measurement uncertainty was developed by an EMI engineer who had no way of coming up with the reason for getting different measurements at different sites. The old "It passes at home by 2 dB and fails at the lab by 3 dB" syndrome. So in an attemp to explain EMI in logical terms (!!!) to his boss, he lumpted the variations of all factors (cable losses, antenna factors, amplifiers, spectrum analyzer, preselector, attennuating pads, connector lo sses, site attennuations, etc.) and came up with a fudge factor which he called "measurement uncertainty". Now he could go to his boss and explain why his measurements at home were different than the measurements at the test site. But I bet that he still had to fix his equipment. Enough ranting. Cheers, Gabriel Roy Hughes Network Systems, MD It's pretty obvious that the opinions are my own. Jim Eickler's invisible friend doesn't even talk to me any more.
