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Frequency Meter

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Frequency Meter an instrument for measuring the frequency of periodic processes (oscillations). The frequency of mechanical vibrations is usually measuredby means of mechanical vibration frequency meters and by electrical meters equipped with transducers to convert the mechanical vibrations into electrical oscillations.

Figure 1. Electromechanical vibrationfrequency meter: (a) scale, registering areading of 50 Hz, (b) diagram of theinstrument; (1) electromagnet winding, (2)electromagnet armature, (3) base of thefrequency meter, (4) elastic supports, (5)reeds

The simplest mechanical vibration frequency meter operates on the resonance principle and consists of a series of flexible reeds fastenedat one end to a common base. The lengths and masses of the reeds are chosen in such a way that their natural vibration frequencies forma specified discrete scale, from which the value of the frequency being measured is determined. When mechanical vibrations act on thebase of the meter, they cause the flexible reeds to vibrate; the highest vibration amplitude is observed on the reed whose natural vibrationfrequency is equal or close to the value of the frequency being measured.

The frequency of electrical oscillations is measured by means of electromechanical, electrodynamic, electronic, moving-iron, and moving-coil frequency meters. The simplest electromechanical type consists of an electromagnet and a series of flexible reeds (as in themechanical frequency meter) on a common base that is attached to the armature of the electromagnet (Figure 1). The electricaloscillations being measured are fed to the winding of the electromagnet; the armature vibrations thereby produced are transmitted to thereeds, and the value of the frequency being measured is determined from the vibrations.

The principal element in electrodynamic frequency meters is a ratio meter with an oscillatory circuit in one of its branches that ispermanently tuned to the average frequency for the measurement range of the given instrument (Figure 2). When connected to an ACcircuit, the moving part of the ratio meter is deflected by an angle proportional to the phase shift between the currents in the windings of theratio meter, which depends on the ratio of the frequency being measured to the resonance frequency of the oscillatory circuit. Themeasurement error of electrodynamic frequency meters ranges from 10–1 to 5 × 10–2.

Figure 2. Schematic diagram of an electrodynamicfrequency meter: (W) fixed coil of a ratio meter consisting oftwo identical parts, designed to create a uniform magneticfield; (W1) and (W2) moving coils rigidly secured together atan angle of 90°, which interact with coil W; (C), (L), and (R) electrical capacitance, inductance, and resistance,respectively, of the oscillatory circuit; (C1) capacitor toproduce a phase shift of 90° between U and l1; (U) voltagewhose frequency is being measured; (l) and (l1) currents inthe branches of the ratio meter

The frequency of electromagnetic oscillations in the radio-frequency and microwave-frequency ranges is measured by means of electronicfrequency meters (wavemeters), such as the resonant, heterodyne, and digital types.

The operation of a resonant-type frequency meter is based on the comparison of the frequency being measured with the frequency ofnatural oscillations in an electrical circuit (or a microwave resonator) that is tuned to resonance with the frequency being measured. Themeter consists of an oscillatory circuit with a coupling loop that picks up the electromagnetic oscillations (radio waves), a detector, anamplifier, and a resonance indicator (Figure 3). During measurement, the circuit is tuned by means of a calibrated capacitor (or the plungerof a microwave resonator) to the frequency of the electromagnetic oscillations being picked up until resonance is achieved, as shown bythe greatest deflection of the pointer on the indicator. The measurement error ranges from 5 × 10–3 to 5 × 10–4.

In heterodyne frequency meters, the frequency being measured is compared with a known frequency (or one of its harmonics) producedby an oscillator, or heterodyne. As the heterodyne frequency is tuned to the frequency of the oscillations being measured, beats occur atthe output of a mixer (in which the frequencies are compared); after amplification the beats are indicated by the pointer on an instrument,by an earphone, or sometimes by an oscilloscope. The relative error of heterodyne frequency meters ranges from 5 × 10–4 to 5 × 10–6.


Figure 3. Schematic diagram of a resonant-type frequency meter: (Lc) coupling loop, (L) and (C) oscillatory circuit (C is a calibratedvariable capacitor), (D) detector (semiconductor diode), (A) amplifier, (M)indicator (microammeter or millivoltmeter)

Digital frequency meters (frequency counters) are now widely used. Their operation involves a count of the number of periods in theoscillations being measured during a specified time interval. Frequency counters consist of a device that converts the sinusoidal voltage ofthe frequency being measured into a train of unidirectional pulses, a gate for the pulses that opens for a certain time interval (usually from10–4 to 10 sec), an electronic counter that registers the number of pulses at the gate output, and a digital display. Modern digital frequencycounters operate over a frequency range from 10–4 to 109 hertz with a relative measurement error from 10–9 to 10–11 and a sensitivity of 10–2 volt. Such devices are used primarily for testing radio equipment and, with various measuring transducers, for measuringtemperature, vibrations, pressure, strain, and other physical quantities.

Primary and secondary frequency standards, which have an error in the range from 10–12 to 5 × 10–14, function as a type of high-accuracyreference frequency meters. The rotational speed of shafts in machines and mechanisms is measured with a tachometer.

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