In radio, a detector is a device or circuit that extracts information from a modulated radio frequency current or voltage. The term dates from the first three decades of radio (1888-1918). Unlike modern radio stations which transmit sound (an audio signal) on an uninterrupted carrier wave, early radio stations transmitted information by radiotelegraphy. The transmitter was switched on and off to produce long or short periods of radio waves, spelling out text messages in Morse code. Therefore, early radio receivers did not have to demodulate the radio signal, but just distinguish between the presence or absence of a radio signal, to reproduce the Morse code "dots" and "dashes". The device that performed this function in the receiver circuit was called a detector.[1] A variety of different detector devices, such as the coherer, electrolytic detector, magnetic detector and the crystal detector, were used during the wireless telegraphy era until superseded by vacuum tube technology.
After the invention of amplitude modulation (AM) enabled the development of AM radiotelephony, the transmission of sound (audio), during World War 1, the term evolved to mean a demodulator, (usually a vacuum tube) which extracted the audio signal from the radio frequency carrier wave. This is its current meaning, although modern detectors usually consist of semiconductor diodes, transistors, or integrated circuits.
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In a superheterodyne receiver the term is also sometimes used to refer to the mixer, the tube or transistor which converts the incoming radio frequency signal to the intermediate frequency. The mixer is called the first detector, while the demodulator that extracts the audio signal from the intermediate frequency is called the second detector. In microwave and millimeter wave technology the terms detector and crystal detector refer to waveguide or coaxial transmission line components, used for power or SWR measurement, that typically incorporate point contact diodes or surface barrier Schottky diodes.
One major technique is known as envelope detection. The simplest form of envelope detector is the diode detector that consists of a diode connected between the input and output of the circuit, with a resistor and capacitor in parallel from the output of the circuit to the ground to form a low pass filter. If the resistor and capacitor are correctly chosen, the output of this circuit will be a nearly identical voltage-shifted version of the original signal.
An early form of envelope detector was the crystal detector, which was used in the crystal set radio receiver. A later version using a crystal diode is still used in crystal radio sets today. The limited frequency response of the headset eliminates the RF component, making the low pass filter unnecessary.
More sophisticated envelope detectors include the grid-leak detector, the plate detector, the infinite-impedance detector, transistor equivalents of them and precision rectifiers using operational amplifiers.
A product detector is a type of demodulator used for AM and SSB signals, where the original carrier signal is removed by multiplying the received signal with a signal at the carrier frequency (or near to it). Rather than converting the envelope of the signal into the decoded waveform by rectification as an envelope detector would, the product detector takes the product of the modulated signal and a local oscillator, hence the name. By heterodyning, the received signal is mixed (in some type of nonlinear device) with a signal from the local oscillator, to give sum and difference frequencies to the signals being mixed, just as a first mixer stage in a superhet would produce an intermediate frequency; the beat frequency in this case, the low frequency modulating signal is recovered and the unwanted high frequencies filtered out from the output of the product detector. Because the sidebands of an amplitude-modulated signal contain all the information in the carrier displaced from the center by a function of their frequency, a product detector simply mixes the sidebands down into the audible range so that the original audio may be heard.
Product detector circuits are essentially ring modulators or synchronous detectors and closely related to some phase-sensitive detector circuits. They can be implemented using something as simple as ring of diodes or a single dual-gate Field Effect Transistor to anything as sophisticated as an Integrated Circuit containing a Gilbert cell. Product detectors are typically preferred to envelope detectors by shortwave listeners and radio amateurs as they permit the reception of both AM and SSB signals. They may also demodulate CW transmissions if the beat frequency oscillator is tuned slightly above or below the carrier.
AM detectors cannot demodulate FM and PM signals because both have a constant amplitude. However an AM radio may detect the sound of an FM broadcast by the phenomenon of slope detection which occurs when the radio is tuned slightly above or below the nominal broadcast frequency. Frequency variation on one sloping side of the radio tuning curve gives the amplified signal a corresponding local amplitude variation, to which the AM detector is sensitive. Slope detection gives inferior distortion and noise rejection compared to the following dedicated FM detectors that are normally used.
A phase detector is a nonlinear device whose output represents the phase difference between the two oscillating input signals. It has two inputs and one output: a reference signal is applied to one input and the phase or frequency modulated signal is applied to the other. The output is a signal that is proportional to the phase difference between the two inputs.
The phase-locked loop detector requires no frequency-selective LC network to accomplish demodulation. In this system, a voltage controlled oscillator (VCO) is phase locked by a feedback loop, which forces the VCO to follow the frequency variations of the incoming FM signal. The low-frequency error voltage that forces the VCO's frequency to track the frequency of the modulated FM signal is the demodulated audio output. The phase-locked loop detector should not be confused with the phase-locked loop frequency synthesizer, which is often used in digitally-tuned AM and FM radios to generate the local oscillator frequency.
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Velocity can be measured using several methods. The simplest is to measure how much time it takes for a particle to travel a certain distance, using precise time-of-flight detectors. Another method looks at how much a particle ionises the matter that it passes through, as this is velocity-dependent and can be measured by tracking devices.
Collating all these clues from different parts of the detector, physicists build up a snapshot of what was in the detector at the moment of a collision. The next step is to scour the collisions for unusual particles, or for results that do not fit current theories.
Uniden R-Series radar detectors offer some of the best performing radar/laser detectors available in the market today. Our radar/laser detectors feature cutting edge technologies, such as industry-leading radar detection range and accuracy with dual horn antennas providing directional arrows to indicate radar signal source (R8). Other features include a low-noise amplifier and DSP chip to analyze signal received in matter of nano seconds; an internal GPS receiver to automatically or manually mark/mute locations, advanced K/Ka band filtering to ignore false alerts and vehicle blind spot monitoring/collision avoidance systems. All radar detectors come with free updates for the firmware and the Speed/Red-light Camera location database allowing you to keep your radar detector up-to-date.
Upon transfer of ownership, all residential properties in the City of Fitchburg must be equipped with hard-wired Photoelectric smoke detectors with a battery backup. Ionized Detectors of any kind are not allowed under Massachusetts Building and Fire Codes.
After the neutrinos pass through the near detector, they will morph from one of their three types into another as they speed through the earth to the much larger detector, called the DUNE far detector, at Sanford Underground Research Facility in Lead, South Dakota.
Massachusetts General Laws Chapter 148, Section 26F requires that all properties being sold be inspected by the Fire Department for compliance with smoke detector regulations. For more information about smoke detector regulations, please visit the Commonwealth of Massachusetts website.
There are a number of different types and brands of CO detectors on the market today. They can be most easily characterized by whether they operate on household current or batteries. Underlying this, in most cases, is the type of sensor employed in the detectors operation. Detectors using household current typically employ some type of solid-state sensor which purges itself and resamples for CO on a periodic basis. This cycling of the sensor is the source of its increased power demands. Detectors powered by batteries typically use a passive sensor technology which reacts to the prolonged exposure to CO gas. e24fc04721