A frequency meter is an instrument that displays the frequency of a periodic electrical signal. Various types of mechanical frequency meters were used in the past, but since the 1970s these have almost universally been replaced by digital frequency counters.
One of the most basic forms of frequency meter is the vibrating reed meter or tuned reed meter. This consists of an electromagnet coil carrying the signal positioned near the end of a tuned metal reed or tuning fork-type arrangement. As the signal travels through the coil it creates a magnetic field with the sample frequency, which pushes and pulls on the reed, or a small piece of metal or a magnet connected to it. The reed is shaped to vibrate at a particular frequency, and if the signal in the magnet is close to it, it will begin to vibrate. [1] Multiple reeds can be positioned on a single electromagnet through various mechanical connections and the frequency of the signal can be determined by seeing which reed is vibrating the most. [2]
Similar systems, "reed receivers" were also used in early radio control systems; when the reed vibrated with enough amplitude it would cause an electrical contact to close and actuate the controls. [3]
More advanced systems were of the deflection type, ordinarily used for measuring low frequencies but capable of being used for frequencies as high as 900 Hz. There are two common types, the BTH resonance frequency meter and the Weston frequency meter. Both use electrical resonance to create a magnetic field to move a pointer, differing in their exact construction.
The BTH meter, named for the British Thomson-Houston heavy industrial firm, consists of a magnet coil connected to the input signal. Running through the center of the magnet is an iron core which extends past the end of the coil and is curved and tapered roughly like a sabre. At the other end of the core is a second coil that is allowed to move closer or further from the fixed input coil. This moving coil is connected to a capacitor to produce an LC circuit tuned to a particular frequency. [4]
Because the iron core passes through the moving coil, and the core is tapered, the inductance of the LC circuit changes as the coil moves closer or further from the fixed input coil. When a signal is applied to the input coil, the moving coil sees a force toward or away from the input coil, and begins to move until the resulting resonance of the LC+core is the same frequency as the input signal. Normally the moving coil is suspended from a pivot above it, so the linear motion along the core causes the coil, and an attached pointer, to rotate over a dial. [5]
The Weston frequency meter also uses tuned circuits, but in this case it is the relative inductance between two such circuits that creates the meter's movements. The system uses coils with open centers where the moving portion of the meter is positioned. Each coil has a partner that is electrically connected so that the resulting field between them is uniform like in a solenoid. Two such paired coils are used, arranged at right angles so that the resulting assembly looks like a hash mark, #, when viewed from the side. [6]
One of the pairs of coils are connected to inductors and resistors while the second does not have any inductors. This causes the current in the inductor side to change as the signal frequency varies away from the selected base frequency, while the field in the second set of coils does not. This causes the currents in the two sets of loops to vary in relation to each other, and the resulting magnetic field between them as well. A small magnet inserted in the open center turns to align itself with the resulting field. [7]
Earlier meters using mechanical displays were limited to frequencies on the order of 1000 Hz, although higher frequency examples were known. For radio frequency signals, these systems were generally far too slow to react, and new meters were introduced.
The absorption wavemeter is a simple system consisting of a single tunable LC circuit and a separate voltmeter or ammeter. The user tunes the LC circuit, typically through a variable capacitor, until the voltage in the circuit suddenly drops. This indicates that the local LC circuit is tuned to a frequency equal to that of the tested signal, at which point it begins to absorb the energy of the signal and cause the circuit's current to drop.
A unique form of absorption wavemeter developed in the 1960s for use with microwave systems or other short-wavelength sources. Unlike other designs, the cavity absorption wavemeter is mechanical in nature. [8]
The system consists of a cylindrical container with an internal piston that travels up and down within the cylinder. The position is normally controlled with a screw at the top of the device, or alternately, the cylinder itself forms a screw that carries the piston up and down as it turns. They were sometimes known as "gumball machines" due to their general shape. [8]
The signal to be tested is fed in through a waveguide on one side of the device below the piston, while a microwave power detector is connected to another waveguide on the opposite side. The user then turns the control until the output voltage suddenly drops. This occurs when the dimensions of the cavity are an exact multiple of the microwave wavelength and it begins to resonate. This causes some of the signal to dissipate into the cavity, lowering the output. [8]
A heterodyne is a signal that is created via the mixture of two other signals. The output is the difference of the two signals, typically on a very different frequency than the inputs. A simple example of a heterodyning is used to tune pianos; a tuning fork is used to produce a known-good frequency and then the matching key on the piano is stuck. The two signals mix and an audible "whaa-whaa" or "beating" can be heard at a much lower frequency, often a few Hertz. The piano string is then adjusted until the beating disappears, meaning the two frequencies are (close to) equal.
The same system can be used to measure an unknown radio frequency. In this case the tuning fork is replaced by a small radio transmitter, the local oscillator (LO), tuned to a frequency close to that of the one to be measured. As long as they are relatively close the resulting heterodyne signal will be audible. The user can then change the frequency of their LO radio until the audible signal disappears, in same fashion as tuning a piano. The unknown frequency is then equal to that of their LO, which can typically be read off the tuning dial. Alternately, the beat frequency can be fed into one of the frequency meters above, allowing accurate adjustment of the tuner by examining the motion of the dial.
An electromagnetic coil is an electrical conductor such as a wire in the shape of a coil. Electromagnetic coils are used in electrical engineering, in applications where electric currents interact with magnetic fields, in devices such as electric motors, generators, inductors, electromagnets, transformers, and sensor coils. Either an electric current is passed through the wire of the coil to generate a magnetic field, or conversely, an external time-varying magnetic field through the interior of the coil generates an EMF (voltage) in the conductor.
An electronic oscillator is an electronic circuit that produces a periodic, oscillating or alternating current (AC) signal, usually a sine wave, square wave or a triangle wave, powered by a direct current (DC) source. Oscillators are found in many electronic devices, such as radio receivers, television sets, radio and television broadcast transmitters, computers, computer peripherals, cellphones, radar, and many other devices.
A relay is an electrically operated switch. It consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals. The switch may have any number of contacts in multiple contact forms, such as make contacts, break contacts, or combinations thereof.
A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency. It was long believed to have been invented by US engineer Edwin Armstrong, but after some controversy the earliest patent for the invention is now credited to French radio engineer and radio manufacturer Lucien Lévy. Virtually all modern radio receivers use the superheterodyne principle.
A Tesla coil is an electrical resonant transformer circuit designed by inventor Nikola Tesla in 1891. It is used to produce high-voltage, low-current, high-frequency alternating-current electricity. Tesla experimented with a number of different configurations consisting of two, or sometimes three, coupled resonant electric circuits.
In communications and electronic engineering, an intermediate frequency (IF) is a frequency to which a carrier wave is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called heterodyning, resulting in a signal at the difference or beat frequency. Intermediate frequencies are used in superheterodyne radio receivers, in which an incoming signal is shifted to an IF for amplification before final detection is done.
In physics and engineering, the quality factor or Q factor is a dimensionless parameter that describes how underdamped an oscillator or resonator is. It is defined as the ratio of the initial energy stored in the resonator to the energy lost in one radian of the cycle of oscillation. Q factor is alternatively defined as the ratio of a resonator's centre frequency to its bandwidth when subject to an oscillating driving force. These two definitions give numerically similar, but not identical, results. Higher Q indicates a lower rate of energy loss and the oscillations die out more slowly. A pendulum suspended from a high-quality bearing, oscillating in air, has a high Q, while a pendulum immersed in oil has a low one. Resonators with high quality factors have low damping, so that they ring or vibrate longer.
A crystal radio receiver, also called a crystal set, is a simple radio receiver, popular in the early days of radio. It uses only the power of the received radio signal to produce sound, needing no external power. It is named for its most important component, a crystal detector, originally made from a piece of crystalline mineral such as galena. This component is now called a diode.
The Hartley oscillator is an electronic oscillator circuit in which the oscillation frequency is determined by a tuned circuit consisting of capacitors and inductors, that is, an LC oscillator. The circuit was invented in 1915 by American engineer Ralph Hartley. The distinguishing feature of the Hartley oscillator is that the tuned circuit consists of a single capacitor in parallel with two inductors in series, and the feedback signal needed for oscillation is taken from the center connection of the two inductors.
A tuned radio frequency receiver is a type of radio receiver that is composed of one or more tuned radio frequency (RF) amplifier stages followed by a detector (demodulator) circuit to extract the audio signal and usually an audio frequency amplifier. This type of receiver was popular in the 1920s. Early examples could be tedious to operate because when tuning in a station each stage had to be individually adjusted to the station's frequency, but later models had ganged tuning, the tuning mechanisms of all stages being linked together, and operated by just one control knob. By the mid 1930s, it was replaced by the superheterodyne receiver patented by Edwin Armstrong.
In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. The antenna intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, and the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, and finally recovers the desired information through demodulation.
A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies. The oscillations in a resonator can be either electromagnetic or mechanical. Resonators are used to either generate waves of specific frequencies or to select specific frequencies from a signal. Musical instruments use acoustic resonators that produce sound waves of specific tones. Another example is quartz crystals used in electronic devices such as radio transmitters and quartz watches to produce oscillations of very precise frequency.
A Colpitts oscillator, invented in 1918 by Canadian-American engineer Edwin H. Colpitts, is one of a number of designs for LC oscillators, electronic oscillators that use a combination of inductors (L) and capacitors (C) to produce an oscillation at a certain frequency. The distinguishing feature of the Colpitts oscillator is that the feedback for the active device is taken from a voltage divider made of two capacitors in series across the inductor.
In electronics, a choke is an inductor used to block higher-frequency alternating currents (AC) while passing direct current (DC) and lower-frequency ACs in a circuit. A choke usually consists of a coil of insulated wire often wound on a magnetic core, although some consist of a doughnut-shaped ferrite bead strung on a wire. The choke's impedance increases with frequency. Its low electrical resistance passes both AC and DC with little power loss, but its reactance limits the amount of AC passed.
An absorption wavemeter is a simple electronic instrument used to measure the frequency of radio waves. It is an older method of measuring frequency, widely used from the birth of radio in the early 20th century until the 1970s, when the development of inexpensive frequency counters, which have far greater accuracy, made it largely obsolete. A wavemeter consists of an adjustable resonant circuit calibrated in frequency, with a meter or other means to measure the voltage or current in the circuit. When adjusted to resonance with the unknown frequency, the resonant circuit absorbs energy, which is indicated by a dip on the meter. Then the frequency can be read from the dial.
A variety of types of electrical transformer are made for different purposes. Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional parts.
Analogue filters are a basic building block of signal processing much used in electronics. Amongst their many applications are the separation of an audio signal before application to bass, mid-range, and tweeter loudspeakers; the combining and later separation of multiple telephone conversations onto a single channel; the selection of a chosen radio station in a radio receiver and rejection of others.
A mechanical filter is a signal processing filter usually used in place of an electronic filter at radio frequencies. Its purpose is the same as that of a normal electronic filter: to pass a range of signal frequencies, but to block others. The filter acts on mechanical vibrations which are the analogue of the electrical signal. At the input and output of the filter, transducers convert the electrical signal into, and then back from, these mechanical vibrations.
A reed receiver or tuned reed receiver (US) was a form of multi-channel signal decoder used for early radio control systems. It uses a simple electromechanical device or 'resonant reed' to demodulate the signal, in effect a receive-only modem. The encoding used is a simple form of frequency-shift keying.
This glossary of electrical and electronics engineering is a list of definitions of terms and concepts related specifically to electrical engineering and electronics engineering. For terms related to engineering in general, see Glossary of engineering.
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