AFMETCAL Newsletter - Wright-Patterson Air Force Base

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Page 22AFMETCAL NEWSLETTERApril 2011Volume 31, Issue No. 3From The Bench (continued)(Continued from page 21)Doing Phase With The 33220A (continued)The old 3325A had a Phase Zero function, allowingany phase position to become the new zero degreereference point. The 33220A doesn’t have that.But the easy way to verify the 33220As phase accuracybefore you use it for any traceable phase offset,since I do not believe it is a calibrated function,would be to use a 53131A or 53132A Counter inPhase mode, set the 33220A to zero degrees and runit to Counter CHA , and your second generator withlocked time-base, run it to Counter CHB, and thenwhatever your 53131A counter reads, enter the samenumber into its OFFSET Function, but with the oppositepolarity. Phase measurements come out almostentirely nominal, that is, when measuringphase between two square-waves. Using squarewaves minimizes the trigger slew error characteristicof measuring phase between sine or triangle waves(slew error being the slight differences in time whenPictured above:Mr. Scott WeissAndersen PMELtriggering along a slant or a slope if the vertical amplitude of the trigger should vary even by slight amounts,but if the waveform has a perfectly vertical edge, such as with a square wave, then no slew error can theoreticallyexist as differences in the vertical trigger result in no horizontal time displacement).You can also verify phase accuracy using one of those new generation high accuracy digital oscilloscopes,such as the LeCroy LC584 or the Tektronix TDS5054. You might be able to do the job with cursors andzoom functions, but it’s probably easier to just use DELAY – set your zero degree reference, where your twogenerators coincide, using 0.0 delay and then check your cardinal phase points by loading in target DELAYs,and then adding or subtracting 90, 180 and 270 degrees to your 33220A degree setting. For instance, if youare using a phase signal of 100KHz, well, that is a period of 10uS. So your 90, 180, and 270 degree delay targetswould be 2.5, 5.0 and 7.5uS’s respectively. If you take your Period of 10uS and divide it by 360, thisgives you your time per degree, which in this case is 27.7777nS per degree. Again, my 33220A was hittingthe delay targets right on the line, and what jitter I was getting was probably from the 3325A which I was usingas the second reference generator (the jitter we see on delayed signals is classic Phase Noise, and if youcan actually watch it squirm back and forth, then it is probably phase noise in the 1Hz to 100Hz offset rangewhere phase noise tends to be highest).If you are unsure how to calculate tolerances of your digitizing scope or are unsure of its accuracy, simplytransfer the accuracy of a known accuracy waveform, such as measuring the period of your 100KHz signal -whether you use cursors or set in a delay of 10 uS, you will find that these scopes return readings that are virtuallynominal.(Continued on page 23)
Page 23AFMETCAL NEWSLETTERApril 2011Volume 31, Issue No. 3From The Bench (continued)(Continued from page 22)Doing Phase With The 33220A (continued)So if your lab owns 33220As then you shouldn’t have to worry too much about nursing along your 3325As.But if buying a new 33220A, remember to ask for the Time Base Option, as the standard model without a time-base Input/Output is virtually useless. So many checks nowadays requires that all the instruments be timebase phase locked together…so whatever the extra cost of the Time Base Option, it is well worth it.Scott WeissPMEL Technician, Andersen AFBEditor’s Note: Chief Niemann would like to remindall technicians that the contents of this article (orany newsletter article) do not replace the requirementto use valid technical data prior to attemptingrepair or calibration of any item of TMDE.You have a Starrett 120A caliper in for calibration.The crystal is yellow, the depth rod must be re-zeroedfor depth measurements, the pointer is at the 3o’clock position, and it is dirty. You clean it up aStarrett 120 Caliper RepairPicture above:Starrett 120A Caliperlittle and start the calibration. You find one or two outside measurements do not meet specifications. Youhave several options, the customer takes a limitation, you NRTS it, or you can repair it.A 6 inch Starrett 120 caliper can cost from $162 to $251. One with long jaws is $372. The 9 and 12 inchcalipers can run from $253 to $331. You can repair these calipers with less than $20 in parts and some time.With today’s budget cuts it does not make sense to NRTS a caliper that can easily be repaired.I have been repairing dial calipers and indicators for over 10 years. Caliper repair is straightforward and theStarrett 120 series is the easiest. The Brown and Sharpe and Mitutoyo calipers can be repaired without muchdifficulty; you just need to understand how they work.You will need a couple of needle pullers, a crystal press, and a few parts. I will list part numbers at the end ofthese instructions.Most of the Starrett needles are pressed on the pinion very tight and the first time you remove the needle youwill need a Mitutoyo hand/needle remover. If a needle is not pressed on too tight Bergeon makes several pullersthat work well. The Presto 7 has four pin diameters that will work for small needles through the Starrettneedles. The only issue is the smallest pin will break leaving a short stub. However, after it breaks it will stillwork for smaller needles.(Continued on page 24)
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