Frequency Meter.

Frequency Meter

Frequency measurement

                               An important electrical quantity with no equivalent in DC circuits is frequency. Frequency measurement is very important in many applications of alternating current, especially in AC power systems designed to run efficiently at one frequency and one frequency only. If the AC is being generated by an electromechanical alternator, the frequency will be directly proportional to the shaft speed of the machine, and frequency could be measured simply by measuring the speed of the shaft. If frequency needs to be measured at some distance from the alternator, though, other means of measurement will be necessary. 

One simple but crude method of frequency measurement in power systems utilizes the principle of mechanical resonance. Every physical object possessing the property of elasticity (springiness) has an inherent frequency at which it will prefer to vibrate. The tuning fork is a great example of this: strike it once and it will continue to vibrate at a tone specific to its length. Longer tuning forks have lower resonant frequencies: their tones will be lower on the musical scale than shorter forks. 

Imagine a row of progressively-sized tuning forks arranged side-by-side. They are all mounted on a common base, and that base is vibrated at the frequency of the measured AC voltage (or current) by means of an electromagnet. Whichever tuning fork is closest in resonant frequency to the frequency of that vibration will tend to shake the most (or the loudest). make a collection of “tuning forks” out of a strip of sheet metal cut in a pattern akin to a rake, and you have the vibrating reed frequency meter:

Vibrating reed frequency meter diagram. 

Vibrating reed frequency meter front panel

The user of this meter views the ends of all those unequal length reeds as they are collectively shaken at the frequency of the applied AC voltage to the coil. The one closest in resonant frequency to the applied AC will vibrate the most, looking something like Figure above. 

Vibrating reed meters, obviously, are not precision instruments, but they are very simple and therefore easy to manufacture to be rugged. They are often found on small engine-driven generator sets for the purpose of setting engine speed so that the frequency is somewhat close to 60 (50 in Europe) Hertz. 

Vibrating Reed Frequency Meter 

                                               The vibrating reed frequency meter is one of the simplest devices for indicating the frequency of an AC source. It is used on power panels to monitor the frequency of AC. Figure 1 is a simplified diagram of one type of vibrating frequency meter. 

It is used to measure the applied frequency. Vibrating Reed Frequency meters indicate the supply frequency by means of individual reeds, when rated voltage ± 20% is applied across the terminals of the meter, the particular reed, whose natural frequency of vibration coincides with the supply frequency, vibrates with full amplitude. In case the supply frequency falls midway between two reeds both the reeds vibrate, at reduced amplitude. The amplitude of vibration at the tip of reeds is high enough for a distinct indication. The instrument can be operated at 110V, 220V, or 440V. Terminal screws with washers are provided for the selected operating voltage & remaining two terminals are covered with insulated caps. In case of RFD96 and RFD144 instruments, two sets of reeds with the same scale range can be offered, giving two independent meters in one casing. This facilitates in comparing the frequency of two voltage supplies for paralleling operation meter.The current, whose frequency is to be measured, flows through the coil and exerts maximum attraction on the soft iron armature twice during each cycle (Figure 1). The armature is attached to the bar, which is mounted on a flexible support. Reeds having natural vibration frequencies of 110, 112, 114 and so on up to 130 hertz are mounted on the bar (view B). The reed having a frequency of 110 hertz is marked 55 hertz. The one with 112 hertz is marked 56 hertz. The one with 120 hertz is marked 60 hertz, and so forth. 

When the coil is energized with a current having a frequency between 55 and 65 hertz, all the reeds are vibrating slightly. But the reed having a natural frequency closest to that of the energized current whose frequency is to be measured vibrates more. The frequency is read from the scaled value opposite the reed having the greatest vibration. In some instruments, the reeds are the same lengths but are weighted by different amounts at the top so that they will have different natural rates of vibration. 

The indicator dial of Figure 1 view C shows an end view of the reeds. If the current has a frequency of 60 hertz, the reed marked 60 hertz will vibrate the greatest amount, as shown. 

Resonant frequency meter

While reed-type meters are imprecise, their operational principle is not. In lieu of mechanical resonance, we may substitute electrical resonance and design a frequency meter using an inductor and capacitor in the form of a tank circuit (parallel inductor and capacitor). See Figure below. One or both components are made adjustable, and a meter is placed in the circuit to indicate maximum amplitude of voltage across the two components. The adjustment knob(s) are calibrated to show resonant frequency for any given setting, and the frequency is read from them after the device has been adjusted for maximum indication on the meter. Essentially, this is a tuneable filter

 circuit which is adjusted and then read in a manner similar to a bridge circuit (which must be balanced for a “null” condition and then read). 
This technique is a popular one for amateur radio operators (or at least it was before the advent of inexpensive digital frequency instruments called counters), especially because it doesn't require direct 
connection to the circuit. So long as the inductor and/or 

capacitor can intercept enough stray field (magnetic or resonant electric, respectively) from the circuit under test to cause the meter to indicate, it will work. 

In frequency as in other types of electrical measurement, the most accurate means of measurement are usually those where an unknown quantity is compared against a known standard, the basic instrument doing nothing more than indicating when the two quantities are equal to each other. This is the basic principle behind the DC (Wheatstone) bridge circuit and it is a sound metrological principle applied throughout the sciences. If we have access to an accurate frequency standard (a source of AC voltage holding very precisely to a single frequency), then measurement of any unknown frequency by comparison should be relatively easy. 

Moving-Disk Frequency Meter 
Moving-disk frequency meters are most commonly out-of-circuit meters. They can be used to spot check the frequency of power sources or equipment signals. A moving-disk frequency meter is shown in figure 2. One coil tends to turn the disk clockwise, and the other, counterclockwise. Magnetizing coil A is connected in series with a large value of resistance. Coil B is connected in series with a large inductance and the two circuits are supplied in parallel by the source. 

For a given voltage, the current through coil A is practically constant. 

However, the current through coil B varies with the frequency. 

At a higher frequency the inductive reactance is greater and the current 

through coil B is less; the reverse is true at a lower frequency.

The disk turns in the direction determined by the stronger coil.

  A perfectly circular disk would tend to turn continuously. This is not desirable, and so the disk is constructed so that it will turn only a certain amount clockwise or counter clockwise about the center position, which is commonly marked 60 hertz on commercial equipment. To prevent the disk from turning more than the desired amount, the left half of the disk is mounted so that when motion occurs, the same amount of disk area will always be between the poles of coil A. Therefore, the force produced by coil A to rotate the disk is constant for a constant applied voltage. The right half of the disk is offset, as shown in the figure. When the disk rotates clockwise, an increasing area will come between the poles of coil B; when it rotates counter clockwise, a decreasing area will come between the poles of coil B. The greater the area between the poles, the greater will be the disk current and the force tending to turn the disk. 

If the frequency applied to the frequency meter should decrease, the reactance offered by L would decrease and the field produced by coil B would increase. The field produced by coil A would remain the same. Thus, the force produced by coil B would tend to move the disk and the pointer counter clockwise until the area between the poles was reduced enough to make the two forces equal. The scale is calibrated to indicate the correct frequency. 

If the frequency is constant and the voltage is changed, the currents in the two coils-and therefore the opposing forces-change by the same amount. Thus, the indication of the instrument is not affected by a change in voltage. 

For that frequency standard, we turn our attention back to the tuning fork, or at least a more modern variation of it called the quartz crystal. Quartz is a naturally occurring mineral possessing a very interesting property called piezoelectricity. Piezoelectric materials produce a voltage across their length when physically stressed, and will physically deform when an external voltage is applied across their lengths. This deformation is very, very slight in most cases, but it does exist. 

Quartz rock is elastic (springy) within that small range of bending which an external voltage would produce, which means that it will have a mechanical resonant frequency of its own capable of being manifested as an electrical voltage signal. In other words, if a chip of quartz is struck, it will “ring” with its own unique frequency determined by the length of the chip, and that resonant oscillation will produce an equivalent voltage across multiple points of the quartz chip which can be tapped into by wires fixed to the surface of the chip. In reciprocal manner, the quartz chip will tend to vibrate most when it is “excited” by an applied AC voltage at precisely the right frequency, just like the reeds on a vibrating-reed frequency meter. 

Chips of quartz rock can be precisely cut for desired resonant frequencies, and that chip mounted securely inside a protective shell with wires extending for connection to an external electric circuit. When packaged as such, the resulting device is simply called a crystal (or sometimes “xtal”). The schematic symbol is shown in Figure below. 

Electrically, that quartz chip is equivalent to a series LC resonant circuit. (Figure below) The dielectric properties of quartz contribute an additional capacitive element to the equivalent circuit. 

The “capacitance” and “inductance” shown in series are merely electrical equivalents of the quartz's mechanical resonance properties: they do not exist as discrete components within the crystal. The capacitance shown in parallel due to the wire connections across the dielectric (insulating) quartz body is real, and it has an effect on the resonant response of the whole system. A full discussion on crystal dynamics is not necessary here, but what needs to be understood about crystals is this resonant circuit equivalence and how it can be exploited within an oscillator circuit to achieve an output voltage with a stable, known frequency. 

Crystals, as resonant elements, typically have much higher “Q” (quality) values than tank circuits built from inductors and capacitors, principally due to the relative absence of stray resistance, making their resonant frequencies very definite and precise. Because the resonant frequency is solely dependent on the physical properties of quartz (a very stable substance, mechanically), the resonant frequency variation over time with a quartz crystal is very, very low. This is how quartz movement watches obtain their high accuracy: by means of an electronic oscillator stabilized by the resonant action of a quartz crystal. 

Digital Frequency Meter

Frequence can be measured with a variety of electric and electronic devices. Electronically, frequence can be measured with such devices as digital frequency counters and heterodyne frequence meters. These devices are capable of measuring a wide range of frequencies extending to hundreds of megahertz.

Electric frequence meters can only measure a narrow range of frequencies in the power frequency range.

A digital frequency meter measures an unknown frequency by counting the number of cycles the frequency produces in a precisely controlled period of time. The counter circuit is incremented one count for each cycle. At the end of the time period, the final count, which represents the frequency, is displayed by the digital readout. For the next sampling of the unknown frequency, the counter is cleared, the time period is started over, and the final count in the counter is again displayed. If the measured frequency is stable, the readout does not change from sample to sample. Because the range switch selects the time period and places the decimal point in the readout, the indicated frequency is in the units specified by the range switch. When the time period is 1-ms, the readout is kilohertz and the range switch indicates kilohertz. For example, if the count at the end of the 1-ms period is100, the unknown (measured) frequency must be 100khz because 100 counts per millisecond is equal to

100,000 counts per second

The block diagram of the basic of a digital frequency meter is shown in fig 4 below.

The unknown frequency signal is fed to a Schmitt trigger.

The signal may be amplified before being applied to Schmitt trigger. In a Schmitt trigger, the signal is converted into a square wave with very fast rise and fall times, then differentiated and clipped. As a result the output from a Schmitt trigger is a train of pulses, one pulse, for each cycle of the signal.

OPERATIONAL PRINCIPLE OF DIGITAL INSTRUMENTS 

The output pulses from the Schmitt trigger are fed to start-stop gate. When this gate opens (start), the input pulses pass through this gate and are fed to an electronic counter which starts registering the input pulses when the gate is closed (stop), the input of pulses to counter ceases and it stops counting.

The counter displays the number of pulses that have passed through it in the time interval between start and stop. If this interval is known, the pulse rate and hence the frequency of the input signal can be known.

Suppose F is the frequency of unknown signal, N the number of counts displayed by counter and t is the time interval between start and stop of gate. Therefore frequency of unknown signal can be given as F=Nit(HZ)

Electronic LED Frequency Meter 

It is used to measure the applied frequency, functionally similar to vibrating reed meter. The meter incorporates light emitting diodes LEDs which are used instead of vibrating reeds to indicate applied frequency. The frequency is determined by blinking LEDs. When applied frequency matches with the frequency marked on the dial, that particular LED blinks with maximum intensity. When the applied frequency is mid way between two adjacent LEDs, both LEDs blink. When the meter is switched on all the LEDs glow sequentially for a period of 1 to 1.5 seconds, thereafter it will indicate the applied frequency.

All voltage ranges are available in single meter. Terminal screws with washers are provided only for specific voltage as per requirement & remaining two terminals are covered with insulated caps. 

In case of 96mm & 144mm instruments, two sets of LEDs with the same scale range can be offered, giving two independent meters in one casing (Dual Scale). This facilitates in comparing the frequency of two voltage supplies for paralleling operation.