Fluke 287 Fluke 287 and 289 Multimeter Users Manual - Page 53

selector to the menu item labeled Hz,%,ms. Next press the softkey labeled Hz. 8:10pm 06/13/07 60.050Hz Auto Range 0 100 200 300 400 500 VAC 123.45 VAC Menu Save Setup Figure 24. Frequency Display est22.eps As shown in Figure 24, the frequency of the input signal is displayed in the primary display. The volts or amps value of the signal is shown in the secondary display. The bar graph does not indicate frequency but indicates the volts or amps value of the input signal. True-rms Digital Multimeters Making Measurements Selection between a rising trigger c or falling trigger dedge is performed by pressing the softkey labeled c d. This softkey toggles the trigger setting between the two selections. The following are tips for frequency measurements: If a reading shows as 0 Hz or is unstable, the input signal may be below or near the trigger level. You can usually correct these problems by manually selecting a lower input range, which increases the sensitivity of the meter. If a reading seems to be a multiple of what you expect, the input signal may be distorted. Distortion can cause multiple triggerings of the frequency counter. Selecting a higher voltage range might solve this problem by decreasing the sensitivity of the meter. In general, the lowest frequency displayed is the correct one. Measuring Duty cycle Duty cycle (or duty factor) is the percentage of time a signal is above or below a trigger level during one cycle, as shown in Figure 25. The duty-cycle mode is optimized for measuring the on or off time of logic and switching signals. Systems such as electronic fuel injection systems and switching power supplies are controlled by pulses of varying width, which can be checked by measuring duty cycle. 43