FLUKE 3034669_6003_ENG_A_W 电路图.pdf

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1、 Application Note Fluke 8845A/8846A statistics and histograms F r o m t h e F l u k e D i g i t a l L i b r a r y w w w . f l u k e . c o m / l i b r a r y “What is the output voltage of the power supply?” This is one of the most common measurements made with digital multimeters. Similar ques- tions

2、 arise about the frequency of oscillators, the value of resistors, operating temperature, or the input to a voltage controlled oscillator. Its easy to use a digital multimeter to take any of these readings. But taking a single reading gives you only part of the story. An equally important ques- tion

3、 is: “How stable is the output of the power supply?” We can frame these questions more precisely in statistical terms: What is the average value of the output, and what is the standard deviation? The Fluke 884X Multimeter has integrated analysis functions that provide the answers to both questions.

4、What do averages and standard deviations tell you? Averaging is a very common cal- culation. Its used for test scores, scientific results and sports sta- tistics. Its used to quantify per- formance over time (for example: average number of goals per season). Its also used to quan- tify typical perfo

5、rmance of a population (for example: average number of goals per player). In engineering, were interested in both these characteristics: How will the output of my circuit change over time, temperature, or humidity? If I build 100 of these circuits, how can I expect each unit to perform? Inherent in

6、each of these questions is the desire to predict performance. But we need more information to be able to use the average in this way. Say the temperature in a particular city has averaged around 21 C (70 C) for the past ten years. If we wake up the next day expect- ing the weather to be warm, we cou

7、ld be very surprised. This is because the temperature can vary over a wide range and still average 21 C. RadioFans.CN 收音机爱 好者资料库 2 Fluke Corporation Fluke 884x statistics and histograms So we need to know how widely the temperature varies around the average. Knowing the variance will tell us the ext

8、ent to which we can expect “average” performance in the future, or from other members of a population. This is the key to determining engineering specifications uncertainties and tolerances. Standard deviation (s) mea- sures the variation or spread of a data set. In formal, mathemati- cal terms, sta

9、ndard deviation is the square root of the variance in a set of data. A low standard deviation indicates a tight group- ing around the average, and a high standard deviation indicates more “randomness.” Figure 1a. Normal distribution with average = 10 and standard deviation = 0.02. Figure 1b. Normal

10、distribution with average = 10 and standard deviation = 0.01. Figure 1a and 1b show normal data distributions that are typi- cal of measurement data. Figure 1a shows a set of measurements with standard deviation of 0.02, while Figure 1b shows a set of measurements with a standard deviation of 0.01.

11、There is less uncertainty in the measurement with the lower standard devia- tion and thus we can be confi- dent setting tighter tolerance. How does the Fluke 884X deliver statistics? The 884x has integrated statis- tics processing and can calculate and display both average and standard deviation of

12、mea- surements. The data can be displayed either numerically, or thanks to the pixel display, graphically. The 8845A/8846A can perform statistics on the parameters listed in Table 1. dc voltageresistance ac voltagecapacitance ac voltage in dBfrequency dc currenttemperature ac currentperiod Table 1.

13、The 884X can perform statistics on these measurements. To illustrate how the 884X generates statistics, we will examine how a dc voltage changes over time. We will set up the 884X to measure dc voltage, and report the average and standard deviation. First, select the measurement on which the statist

14、ics will be performed; for this example, dc volts. Then, press the ANALYZE button on the front panel to bring up a menu of math func- tions as shown in Figure 2. There are two selections on this menu that deliver statistical measure- ments, the STATS function and the HISTOGRAM function. Lets look at

15、 the STATS function first. The STATS function As soon as you press the STATS softkey, the meter begins calcu- lating the average and standard deviation. It also stores the mini- mum and maximum measured values. You can reset the statis- tics function and start over again at any time by pressing the

16、RESTART softkey. You can also set a number of measurements for the meter to take, by pressing the #SAMPLES softkey and specifying a number from 2 to 5000. The statistics calculations will automatically stop once the number of read- ings has been reached as shown in Figure 3. The HISTOGRAM function L

17、ike the STATS function, the HISTOGRAM function conveys the average and standard devia- tion of a set of measurements. But the histogram mode delivers the information in a graphical representation that quickly and clearly indicates the distribution, or “spread,” of the measurements. For example, most

18、 measurements follow a normal distribution. That means the histogram should be symmetrical around the aver- age and almost all of the mea- surements (99.7 %) should be within three standard deviations. A lopsided, broad, or bimodal (two-peaked) distribution would indicate unexpected instability or e

19、rror in the unit under test. #SAMPLESSTOP Min Max 1.285972 1.615092 Avg SD 1.610871 30.4891m # / 182 50000 ANALYZE STATS HISTO GRAM TREND PLOTSTATSMATH + 0.001348 V- ANLZ MAN Figure 2. The 884X menus link directly to STATS and HISTOGRAM. Figure 3. Sample Min/Max/Avg/SD statistics. RadioFans.CN 收音机爱

20、好者资料库 3 Fluke Corporation Fluke 884x statistics and histograms Figure 4 shows an example of the histogram display. The his- togram consists of 10 bars. The height of each bar represents the relative frequency of a range, that is, how often a range of readings occurs compared to the other ranges. The

21、 center of the display represents the average measurement. The bars on either side of the average represent measurements within one stan- dard deviation from the average. The second bars from the aver- age represent measurements between one and two standard deviations, and so on, out to five standar

22、d deviations. Like the STATS function, the HISTOGRAM function starts as soon as the HISTOGRAM softkey is pressed. The calculations can be initialized and restarted by pressing the RESTART sofkey. Figure 4. Histogram display. Fluke Corporation PO Box 9090, Everett, WA USA 98206 Fluke Europe B.V. PO B

23、ox 1186, 5602 BD Eindhoven, The Netherlands For more information call: In the U.S.A. (800) 443-5853 or Fax (425) 446-5116 In Europe/M-East/Africa +31 (0) 40 2675 200 or Fax +31 (0) 40 2675 222 In Canada (800)-36-FLUKE or Fax (905) 890-6866 From other countries +1 (425) 446-5500 or Fax +1 (425) 446-5116 Web access: 2007 Fluke Corporation. All rights reserved. Specifications subject to change without notice. Printed in U.S.A. 5/2007 3034669 A-EN-N Rev A Fluke. Keeping your world up and running. RadioFans.CN 收音机爱 好者资料库