2-13When the coil is energized by a current with a frequency between 55 and 65 hertz, all the reeds arevibrated slightly; but, the reed having a natural frequency closest to that of the energizing current vibratesthrough a larger amplitude. The frequency is read from the scale value opposite the reed having thegreatest amplitude of vibration.In some instruments, the reeds are the same length; but they are weighted by different amounts at thetop so they will have different natural rates of vibration. An end view of the reeds in the indicator isshown in view C. If the energizing current has a frequency of 60 hertz, the reed marked 60 will vibrate thegreatest amount, as shown. View D shows a hand-held vibrating-reed frequency meter mounted on thecasing of a motor-generator.Tuned CircuitsTUNED CIRCUITS are used as filters for the passage or rejection of specific frequencies.BANDPASS FILTERS and BAND-REJECT FILTERS are examples of this type. Tuned circuits havecertain characteristics that make them ideal for certain types of filters, especially where a high degree ofselectivity is desired. A series-tuned circuit offers a low impedance to currents of the particular frequencyto which the circuit is tuned and a relatively high impedance to currents of all other frequencies. Aparallel-tuned circuit, on the other hand, offers a very high impedance to currents of its natural, orresonant, frequency and a relatively low impedance to others. If you feel you need to review the subject oftuned circuits at this time, refer to NEETS, Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits, for more information on these circuits and their applications.AUDIO FREQUENCIESFrequency measurements in the af range can be made by the comparison method or the direct-reading frequency meter. Frequency comparisons can be made by the use of a calibrated af generator inconjunction with either an oscilloscope or a modulator and a zero-beat indicating device. Direct-readingfrequency measurements can be made by instruments using series, frequency-selective electricalnetworks, bridge test sets having null indicators, or counting-type frequency meters.Heterodyne Frequency MetersHeterodyne frequency meters are available in several varieties. They measure the frequency of theunknown signal by matching the unknown signal with a locally generated signal of the same frequencyobtained from a calibrated, precision oscillator. This method is normally referred to as zero beating.When a perfect frequency match is obtained, it is indicated by the absence of a beat note (zero beat). Thetechnician generally uses a set of headphones to detect a zero-beat condition in the equipment beingtested.The basic heterodyne meter (figure 2-9) is a calibrated variable oscillator, which heterodynes againstthe frequency to be measured. Coupling is accomplished between the frequency meter and the output ofthe equipment under test. (NOTE: This coupling should be in accordance with the step-by-stepprocedures listed in the technical manual for the frequency meter.) The calibrated oscillator is then tunedso that the difference between the oscillator frequency and the unknown frequency is in the af range. Thisdifference in frequency is known as the BEAT FREQUENCY. As the two frequencies are brought closerto the same value, the tone in the headset will decrease in pitch until it is replaced by a series of rapidclicks. As the process is continued, the clicks will decrease in rapidity until they stop altogether. This isthe point of zero beat; that is, the point at which the frequency generated in the oscillator of the frequencymeter is equal to the frequency of the unknown signal being measured.
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