Frequency counters

Chapter Frequency counters

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • Frequency countersThe measurement of energy used by your home is an application to which digital me-tering is well suited. It’s easier to read the drum type, digital kilowatt-hour meter thanto read the pointer type meter. When measuring frequencies of signals, digital meteringis not only more convenient, but far more accurate.The frequency counter measures by actually counting pulses, in a manner similarto the way the utility meter counts the number of turns of a motor. But the frequencycounter works electronically, without any moving parts. It can keep track of thousands,millions or even billions of pulses per second, and it shows the rate on a digital displaythat is as easy to read as a digital watch. It measures frequency directly by tallying upthe number of pulses in an oscillating wave, even when the number of pulses per sec-ond is huge.The accuracy of the frequency counter is a function of the lock-in time. Lock-in isusually done in 0.1 second, 1 second or 10 seconds. Increasing the lock-in time by a fac-tor of 10 will cause the accuracy to be good by one additional digit. Modern frequencycounters are good to six, seven or eight digits; sophisticated lab devices will show fre-quency to nine or ten digits.Other specialized meter typesThe following are some less common types of meters that you might come across inelectrical and electronic work.VU and decibel metersIn high-fidelity equipment, especially the more sophisticated amplifiers (“amps”), loud-ness meters are sometimes used. These are calibrated in decibels, a unit that you willsometimes encounter in reference to electronic signal levels. A decibel is an increase ordecrease in sound or signal level that you can just barely detect, if you are expecting thechange.Audio loudness is given in volume units (VU), and the meter that indicates it iscalled a VU meter. Usually, such meters have a zero marker with a red line to the rightand a black line to the left, and they are calibrated in decibels (dB) above and below thiszero marker (Fig. 3-12). The meter might also be calibrated in watts rms, an expressionfor audio power.As music is played through the system, or as a voice comes over it, the VU meterneedle will kick up. The amplifier volume should be kept down so that the meter does-n’t go past the zero mark and into the red range. If the meter does kick up into the redscale, it means that distortion is probably taking place within the amplifier circuit.Sound level in general can be measured by means of a sound-level meter, cali-brated in decibels (dB) and connected to the output of a precision amplifier with a mi-crophone of known, standardized sensitivity (Fig. 3-13). You have perhaps heard that avacuum cleaner will produce 80 dB of sound, and a large truck going by might subjectyour ears to 90 dB. These figures are determined by a sound-level meter. A VU meter isa special form of sound-level meter.Other specialized meter types57