The HP 200A , first built in 1938, was the first product made by Hewlett-Packard and was manufactured in David Packard 's garage in Palo Alto, California .
121-463: It was a low-distortion audio oscillator used for testing sound equipment. It used the Wien bridge oscillator circuit, that had been the subject of Bill Hewlett 's masters thesis. It was also the first such commercial oscillator to use a simple light bulb as the temperature-dependent resistor in its feedback network. The light bulb was an inexpensive and effective automatic gain control that not only kept
242-430: A feedback loop ; an amplifier A {\displaystyle A} and an electronic filter β ( j ω ) {\displaystyle \beta (j\omega )} . The filter's purpose is to limit the frequencies that can pass through the loop so the circuit only oscillates at the desired frequency. Since the filter and wires in the circuit have resistance they consume energy and
363-466: A fractal surface, such as rocks or soil, and are used by navigation radars. A radar beam follows a linear path in vacuum but follows a somewhat curved path in atmosphere due to variation in the refractive index of air, which is called the radar horizon . Even when the beam is emitted parallel to the ground, the beam rises above the ground as the curvature of the Earth sinks below the horizon. Furthermore,
484-598: A magnitude and an angle, the above equation actually consists of two conditions: Equations (1) and (2) are called the Barkhausen stability criterion . It is a necessary but not a sufficient criterion for oscillation, so there are some circuits which satisfy these equations that will not oscillate. An equivalent condition often used instead of the Barkhausen condition is that the circuit's closed loop transfer function (the circuit's complex impedance at its output) have
605-403: A receiver and processor to determine properties of the objects. Radio waves (pulsed or continuous) from the transmitter reflect off the objects and return to the receiver, giving information about the objects' locations and speeds. Radar was developed secretly for military use by several countries in the period before and during World War II . A key development was the cavity magnetron in
726-444: A square , sawtooth or triangle wave . It consists of an energy-storing element (a capacitor or, more rarely, an inductor ) and a nonlinear switching device (a latch , Schmitt trigger , or negative resistance element) connected in a feedback loop . The switching device periodically charges the storage element with energy and when its voltage or current reaches a threshold discharges it again, thus causing abrupt changes in
847-424: A transmitter that emits radio waves known as radar signals in predetermined directions. When these signals contact an object they are usually reflected or scattered in many directions, although some of them will be absorbed and penetrate into the target. Radar signals are reflected especially well by materials of considerable electrical conductivity —such as most metals, seawater , and wet ground. This makes
968-430: 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. Oscillators are often characterized by the frequency of their output signal: There are two general types of electronic oscillators:
1089-458: A trimmer capacitor in series or parallel with the crystal. The Barkhausen criterion above, eqs. (1) and (2), merely gives the frequencies at which steady-state oscillation is possible, but says nothing about the amplitude of the oscillation, whether the amplitude is stable, or whether the circuit will start oscillating when the power is turned on. For a practical oscillator two additional requirements are necessary: A typical rule of thumb
1210-482: A different dielectric constant or diamagnetic constant from the first, the waves will reflect or scatter from the boundary between the materials. This means that a solid object in air or in a vacuum , or a significant change in atomic density between the object and what is surrounding it, will usually scatter radar (radio) waves from its surface. This is particularly true for electrically conductive materials such as metal and carbon fibre, making radar well-suited to
1331-406: A dynamo, what would now be called a parametric oscillator . The arc oscillator was rediscovered and popularized by William Duddell in 1900. Duddell, a student at London Technical College, was investigating the hissing arc effect. He attached an LC circuit (tuned circuit) to the electrodes of an arc lamp, and the negative resistance of the arc excited oscillation in the tuned circuit. Some of
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#17328687082081452-447: A feedback oscillator circuit will oscillate, the feedback loop is thought of as broken at some point (see diagrams) to give an input and output port (for accuracy, the output port must be terminated with an impedance equal to the input port). A sine wave is applied to the input v i ( t ) = V i e j ω t {\displaystyle v_{i}(t)=V_{i}e^{j\omega t}} and
1573-535: A full radar system, that he called a telemobiloscope . It operated on a 50 cm wavelength and the pulsed radar signal was created via a spark-gap. His system already used the classic antenna setup of horn antenna with parabolic reflector and was presented to German military officials in practical tests in Cologne and Rotterdam harbour but was rejected. In 1915, Robert Watson-Watt used radio technology to provide advance warning of thunderstorms to airmen and during
1694-401: A gain of 1+R f /R b and with a high input impedance. R 1 , R 2 , C 1 and C 2 form a bandpass filter which is connected to provide positive feedback at the frequency of oscillation. R b self heats and increases the negative feedback which reduces the amplifier gain until the point is reached that there is just enough gain to sustain sinusoidal oscillation without over driving
1815-401: A given phase change Δ ϕ {\displaystyle \Delta \phi } depends on the slope of the loop phase curve at ω 0 {\displaystyle \omega _{0}} , which is determined by the Q {\displaystyle Q} RC oscillators have the equivalent of a very low Q {\displaystyle Q} , so
1936-543: A nonlinear electronic oscillator model, the Van der Pol oscillator , was done by Balthasar van der Pol in 1927. He originated the term "relaxation oscillation" and was first to distinguish between linear and relaxation oscillators. He showed that the stability of the oscillations ( limit cycles ) in actual oscillators was due to the nonlinearity of the amplifying device. Further advances in mathematical analysis of oscillation were made by Hendrik Wade Bode and Harry Nyquist in
2057-406: A pair of poles on the imaginary axis . In general, the phase shift of the feedback network increases with increasing frequency so there are only a few discrete frequencies (often only one) which satisfy the second equation. If the amplifier gain A {\displaystyle A} is high enough that the loop gain is unity (or greater, see Startup section) at one of these frequencies,
2178-729: A physics instructor at the Imperial Russian Navy school in Kronstadt , developed an apparatus using a coherer tube for detecting distant lightning strikes. The next year, he added a spark-gap transmitter . In 1897, while testing this equipment for communicating between two ships in the Baltic Sea , he took note of an interference beat caused by the passage of a third vessel. In his report, Popov wrote that this phenomenon might be used for detecting objects, but he did nothing more with this observation. The German inventor Christian Hülsmeyer
2299-495: A proposal for further intensive research on radio-echo signals from moving targets to take place at NRL, where Taylor and Young were based at the time. Similarly, in the UK, L. S. Alder took out a secret provisional patent for Naval radar in 1928. W.A.S. Butement and P. E. Pollard developed a breadboard test unit, operating at 50 cm (600 MHz) and using pulsed modulation which gave successful laboratory results. In January 1931,
2420-467: A protracted legal battle over the rights to the "regenerative" oscillator circuit which has been called "the most complicated patent litigation in the history of radio". De Forest ultimately won before the Supreme Court in 1934 on technical grounds, but most sources regard Armstrong's claim as the stronger one. The first and most widely used relaxation oscillator circuit, the astable multivibrator ,
2541-698: A pulsed system, and the first such elementary apparatus was demonstrated in December 1934 by the American Robert M. Page , working at the Naval Research Laboratory . The following year, the United States Army successfully tested a primitive surface-to-surface radar to aim coastal battery searchlights at night. This design was followed by a pulsed system demonstrated in May 1935 by Rudolf Kühnhold and
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#17328687082082662-442: A rescue. For similar reasons, objects intended to avoid detection will not have inside corners or surfaces and edges perpendicular to likely detection directions, which leads to "odd" looking stealth aircraft . These precautions do not totally eliminate reflection because of diffraction , especially at longer wavelengths. Half wavelength long wires or strips of conducting material, such as chaff , are very reflective but do not direct
2783-407: A resistance that increases with temperature as the current through them increases. As the amplitude of the signal current through them increases during oscillator startup, the increasing resistance of these devices reduces the loop gain. The essential characteristic of all these circuits is that the nonlinear gain-control circuit must have a long time constant , much longer than a single period of
2904-542: A ring of active delay stages, such as inverters . Generally the ring has an odd number of inverting stages, so that there is no single stable state for the internal ring voltages. Instead, a single transition propagates endlessly around the ring. Some of the more common relaxation oscillator circuits are listed below: An oscillator can be designed so that the oscillation frequency can be varied over some range by an input voltage or current. These voltage controlled oscillators are widely used in phase-locked loops , in which
3025-551: A role in the invention of the oscillator. In the summer of 1912, Edwin Armstrong observed oscillations in audion radio receiver circuits and went on to use positive feedback in his invention of the regenerative receiver . Austrian Alexander Meissner independently discovered positive feedback and invented oscillators in March 1913. Irving Langmuir at General Electric observed feedback in 1913. Fritz Lowenstein may have preceded
3146-485: A small change in frequency. Therefore, the circuit's oscillation frequency is very close to the natural resonant frequency of the tuned circuit , and doesn't depend much on other components in the circuit. The quartz crystal resonators used in crystal oscillators have even higher Q {\displaystyle Q} (10 to 10 ) and their frequency is very stable and independent of other circuit components. The frequency of RC and LC oscillators can be tuned over
3267-662: A system might do, Wilkins recalled the earlier report about aircraft causing radio interference. This revelation led to the Daventry Experiment of 26 February 1935, using a powerful BBC shortwave transmitter as the source and their GPO receiver setup in a field while a bomber flew around the site. When the plane was clearly detected, Hugh Dowding , the Air Member for Supply and Research , was very impressed with their system's potential and funds were immediately provided for further operational development. Watson-Watt's team patented
3388-431: A ubiquitous audio generator in engineering laboratories worldwide from the 1950s to the 1990s. William R. Hewlett 's Wien bridge oscillator can be considered as a combination of a differential amplifier and a Wien bridge , connected in a positive feedback loop between the amplifier output and differential inputs. At the oscillating frequency, the bridge is almost balanced and has very small transfer ratio. The loop gain
3509-411: A vibrating quartz crystal . Crystal oscillators are ubiquitous in modern electronics, being the source for the clock signal in computers and digital watches, as well as a source for the signals generated in radio transmitters and receivers. As a crystal oscillator's “native” output waveform is sinusoidal , a signal-conditioning circuit may be used to convert the output to other waveform types, such as
3630-425: A wide range by using variable components in the filter. A microwave cavity can be tuned mechanically by moving one of the walls. In contrast, a quartz crystal is a mechanical resonator whose resonant frequency is mainly determined by its dimensions, so a crystal oscillator's frequency is only adjustable over a very narrow range, a tiny fraction of one percent. It's frequency can be changed slightly by using
3751-514: A wide region and direct fighter aircraft towards targets. Marine radars are used to measure the bearing and distance of ships to prevent collision with other ships, to navigate, and to fix their position at sea when within range of shore or other fixed references such as islands, buoys, and lightships. In port or in harbour, vessel traffic service radar systems are used to monitor and regulate ship movements in busy waters. Meteorologists use radar to monitor precipitation and wind. It has become
HP 200A - Misplaced Pages Continue
3872-855: A writeup on the apparatus was entered in the Inventions Book maintained by the Royal Engineers. This is the first official record in Great Britain of the technology that was used in coastal defence and was incorporated into Chain Home as Chain Home (low) . Before the Second World War , researchers in the United Kingdom, France , Germany , Italy , Japan , the Netherlands , the Soviet Union , and
3993-498: Is "almost" an oscillator; it can store energy in the form of electronic oscillations if excited, but because it has electrical resistance and other losses the oscillations are damped and decay to zero. The negative resistance of the active device cancels the (positive) internal loss resistance in the resonator, in effect creating a resonator circuit with no damping, which generates spontaneous continuous oscillations at its resonant frequency . The negative-resistance oscillator model
4114-403: Is a radiodetermination method used to detect and track aircraft , ships , spacecraft , guided missiles , motor vehicles , map weather formations , and terrain . A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna , a receiving antenna (often the same antenna is used for transmitting and receiving) and
4235-414: Is a product of the very high amplifier gain and the very low bridge ratio. In Hewlett's circuit, the amplifier is implemented by two vacuum tubes. The amplifier's inverting input is the cathode of tube V 1 and the non-inverting input is the control grid of tube V 2 . To simplify analysis, all the components other than R 1 , R 2 , C 1 and C 2 can be modeled as a non-inverting amplifier with
4356-452: Is a simplification for transmission in a vacuum without interference. The propagation factor accounts for the effects of multipath and shadowing and depends on the details of the environment. In a real-world situation, pathloss effects are also considered. Frequency shift is caused by motion that changes the number of wavelengths between the reflector and the radar. This can degrade or enhance radar performance depending upon how it affects
4477-451: Is as follows, where F D {\displaystyle F_{D}} is Doppler frequency, F T {\displaystyle F_{T}} is transmit frequency, V R {\displaystyle V_{R}} is radial velocity, and C {\displaystyle C} is the speed of light: Passive radar is applicable to electronic countermeasures and radio astronomy as follows: Only
4598-560: Is intended. Radar relies on its own transmissions rather than light from the Sun or the Moon, or from electromagnetic waves emitted by the target objects themselves, such as infrared radiation (heat). This process of directing artificial radio waves towards objects is called illumination , although radio waves are invisible to the human eye as well as optical cameras. If electromagnetic waves travelling through one material meet another material, having
4719-431: Is not limited to one-port devices like diodes; feedback oscillator circuits with two-port amplifying devices such as transistors and tubes also have negative resistance. At high frequencies, three terminal devices such as transistors and FETs are also used in negative resistance oscillators. At high frequencies these devices do not need a feedback loop, but with certain loads applied to one port can become unstable at
4840-400: Is often used in the feedback loop that provides a 'slow' gain reduction with amplitude. This stabilizes the loop gain at an amplitude below the saturation level of the amplifier, so it does not saturate and "clip" the sine wave. Resistor-diode networks and FETs are often used for the nonlinear element. An older design uses a thermistor or an ordinary incandescent light bulb ; both provide
4961-417: Is the range. This yields: This shows that the received power declines as the fourth power of the range, which means that the received power from distant targets is relatively very small. Additional filtering and pulse integration modifies the radar equation slightly for pulse-Doppler radar performance , which can be used to increase detection range and reduce transmit power. The equation above with F = 1
HP 200A - Misplaced Pages Continue
5082-409: Is to make the small signal loop gain at the oscillation frequency 2 or 3. When the power is turned on, oscillation is started by the power turn-on transient or random electronic noise present in the circuit. Noise guarantees that the circuit will not remain "balanced" precisely at its unstable DC equilibrium point ( Q point ) indefinitely. Due to the narrow passband of the filter, the response of
5203-623: The Nyquist frequency , since the returned frequency otherwise cannot be distinguished from shifting of a harmonic frequency above or below, thus requiring: Or when substituting with F D {\displaystyle F_{D}} : As an example, a Doppler weather radar with a pulse rate of 2 kHz and transmit frequency of 1 GHz can reliably measure weather speed up to at most 150 m/s (340 mph), thus cannot reliably determine radial velocity of aircraft moving 1,000 m/s (2,200 mph). In all electromagnetic radiation ,
5324-714: The RAF's Pathfinder . The information provided by radar includes the bearing and range (and therefore position) of the object from the radar scanner. It is thus used in many different fields where the need for such positioning is crucial. The first use of radar was for military purposes: to locate air, ground and sea targets. This evolved in the civilian field into applications for aircraft, ships, and automobiles. In aviation , aircraft can be equipped with radar devices that warn of aircraft or other obstacles in or approaching their path, display weather information, and give accurate altitude readings. The first commercial device fitted to aircraft
5445-1262: The United Kingdom , which allowed the creation of relatively small systems with sub-meter resolution. The term RADAR was coined in 1940 by the United States Navy as an acronym for "radio detection and ranging". The term radar has since entered English and other languages as an anacronym , a common noun, losing all capitalization . The modern uses of radar are highly diverse, including air and terrestrial traffic control, radar astronomy , air-defense systems , anti-missile systems , marine radars to locate landmarks and other ships, aircraft anti-collision systems, ocean surveillance systems, outer space surveillance and rendezvous systems, meteorological precipitation monitoring, radar remote sensing , altimetry and flight control systems , guided missile target locating systems, self-driving cars , and ground-penetrating radar for geological observations. Modern high tech radar systems use digital signal processing and machine learning and are capable of extracting useful information from very high noise levels. Other systems which are similar to radar make use of other parts of
5566-440: The electromagnetic spectrum . One example is lidar , which uses predominantly infrared light from lasers rather than radio waves. With the emergence of driverless vehicles, radar is expected to assist the automated platform to monitor its environment, thus preventing unwanted incidents. As early as 1886, German physicist Heinrich Hertz showed that radio waves could be reflected from solid objects. In 1895, Alexander Popov ,
5687-403: The linear or harmonic oscillator , and the nonlinear or relaxation oscillator . The two types are fundamentally different in how oscillation is produced, as well as in the characteristic type of output signal that is generated. The most-common linear oscillator in use is the crystal oscillator , in which the output frequency is controlled by a piezo-electric resonator consisting of
5808-414: The microwave range and above, since at these frequencies feedback oscillators perform poorly due to excessive phase shift in the feedback path. In negative-resistance oscillators, a resonant circuit, such as an LC circuit , crystal , or cavity resonator , is connected across a device with negative differential resistance , and a DC bias voltage is applied to supply energy. A resonant circuit by itself
5929-407: The reflective surfaces . A corner reflector consists of three flat surfaces meeting like the inside corner of a cube. The structure will reflect waves entering its opening directly back to the source. They are commonly used as radar reflectors to make otherwise difficult-to-detect objects easier to detect. Corner reflectors on boats, for example, make them more detectable to avoid collision or during
6050-464: The square wave typically utilized in computer clock circuits. Linear or harmonic oscillators generate a sinusoidal (or nearly-sinusoidal) signal. There are two types: The most common form of linear oscillator is an electronic amplifier such as a transistor or operational amplifier connected in a feedback loop with its output fed back into its input through a frequency selective electronic filter to provide positive feedback . When
6171-446: The tank circuit in LC oscillators will cause the oscillation frequency to change, so for a constant frequency these components must have stable values. How stable the oscillator's frequency is to other changes in the circuit, such as changes in values of other components, gain of the amplifier, the load impedance, or the supply voltage, is mainly dependent on the Q factor ("quality factor") of
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#17328687082086292-527: The "new boy" Arnold Frederic Wilkins to conduct an extensive review of available shortwave units. Wilkins would select a General Post Office model after noting its manual's description of a "fading" effect (the common term for interference at the time) when aircraft flew overhead. By placing a transmitter and receiver on opposite sides of the Potomac River in 1922, U.S. Navy researchers A. Hoyt Taylor and Leo C. Young discovered that ships passing through
6413-413: The 1920s went on to lead the U.K. research establishment to make many advances using radio techniques, including the probing of the ionosphere and the detection of lightning at long distances. Through his lightning experiments, Watson-Watt became an expert on the use of radio direction finding before turning his inquiry to shortwave transmission. Requiring a suitable receiver for such studies, he told
6534-403: The 1930s. In 1969 Kaneyuki Kurokawa derived necessary and sufficient conditions for oscillation in negative-resistance circuits, which form the basis of modern microwave oscillator design. Radar Radar is a system that uses radio waves to determine the distance ( ranging ), direction ( azimuth and elevation angles ), and radial velocity of objects relative to the site. It
6655-523: The 19th century. The current through an arc light is unstable due to its negative resistance , and often breaks into spontaneous oscillations, causing the arc to make hissing, humming or howling sounds which had been noticed by Humphry Davy in 1821, Benjamin Silliman in 1822, Auguste Arthur de la Rive in 1846, and David Edward Hughes in 1878. Ernst Lecher in 1888 showed that the current through an electric arc could be oscillatory. An oscillator
6776-535: The Barkhausen criterion, at which point the amplitude levels off and steady state operation is achieved, with the output a slightly distorted sine wave with peak amplitude determined by the supply voltage. This is a stable equilibrium; if the amplitude of the sine wave increases for some reason, increased clipping of the output causes the loop gain | A β ( j ω 0 ) | {\displaystyle |A\beta (j\omega _{0})|} to drop below one temporarily, reducing
6897-517: The DC voltage across the varactor changes its capacitance , which changes the resonant frequency of the tuned circuit. Voltage controlled relaxation oscillators can be constructed by charging and discharging the energy storage capacitor with a voltage controlled current source . Increasing the input voltage increases the rate of charging the capacitor, decreasing the time between switching events. A feedback oscillator circuit consists of two parts connected in
7018-770: The United States, independently and in great secrecy, developed technologies that led to the modern version of radar. Australia, Canada, New Zealand, and South Africa followed prewar Great Britain's radar development, Hungary and Sweden generated its radar technology during the war. In France in 1934, following systematic studies on the split-anode magnetron , the research branch of the Compagnie générale de la télégraphie sans fil (CSF) headed by Maurice Ponte with Henri Gutton, Sylvain Berline and M. Hugon, began developing an obstacle-locating radio apparatus, aspects of which were installed on
7139-620: The active device can no longer be considered a 'pure gain', and it will contribute some phase shift to the loop. An alternate mathematical stability test sometimes used instead of the Barkhausen criterion is the Nyquist stability criterion . This has a wider applicability than the Barkhausen, so it can identify some of the circuits which pass the Barkhausen criterion but do not oscillate. Temperature changes, other environmental changes, aging, and manufacturing tolerances will cause component values to "drift" away from their designed values. Changes in frequency determining components such as
7260-497: The active device itself, such as the interelectrode capacitance between output and input, make the device unstable. The input impedance of the active device falls with frequency, so it may load the feedback network. As a result, stable feedback oscillators are difficult to build for frequencies above 500 MHz, and negative resistance oscillators are usually used for frequencies above this. The first practical oscillators were based on electric arcs , which were used for lighting in
7381-399: The amplifier. If R 1 = R 2 and C 1 = C 2 then at equilibrium R f /R b = 2 and the amplifier gain is 3. When the circuit is first energized, the lamp is cold and the gain of the circuit is greater than 3 which ensures start up. The dc bias current of vacuum tube V1 also flows through the lamp. This does not change the principles of the circuit's operation, but it does reduce
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#17328687082087502-407: The amplifying device, the transistor , vacuum tube or op-amp . The maximum voltage swing of the amplifier's output is limited by the DC voltage provided by its power supply. Another possibility is that the output may be limited by the amplifier slew rate . As the amplitude of the output nears the power supply voltage rails, the amplifier begins to saturate on the peaks (top and bottom) of
7623-433: The amplifying device, the amplifier will act as a pure gain A {\displaystyle A} , but if the oscillation frequency ω 0 {\displaystyle \omega _{0}} is near the amplifier's cutoff frequency ω C {\displaystyle \omega _{C}} , within 0.1 ω C {\displaystyle 0.1\omega _{C}} ,
7744-497: The amplitude and phase of the sine wave after going through the loop v o = V o e j ( ω t + ϕ ) {\displaystyle v_{o}=V_{o}e^{j(\omega t+\phi )}} is calculated Since in the complete circuit v o {\displaystyle v_{o}} is connected to v i {\displaystyle v_{i}} , for oscillations to exist The ratio of output to input of
7865-519: The amplitude becomes large enough that the amplifier becomes nonlinear , generating harmonic distortion, technically the frequency domain analysis used in normal amplifier circuits is no longer applicable, so the "gain" of the circuit is undefined. However the filter attenuates the harmonic components produced by the nonlinearity of the amplifier, so the fundamental frequency component sin ω 0 t {\displaystyle \sin \omega _{0}t} mainly determines
7986-401: The amplitude of the output at equilibrium because the bias current provides part of the heating of the lamp. This electronics-related article is a stub . You can help Misplaced Pages by expanding it . Audio oscillator An electronic oscillator is an electronic circuit that produces a periodic, oscillating or alternating current (AC) signal, usually a sine wave , square wave or
8107-467: The amplitude of the signal drops as it passes through the filter. The amplifier is needed to increase the amplitude of the signal to compensate for the energy lost in the other parts of the circuit, so the loop will oscillate, as well as supply energy to the load attached to the output. To determine the frequency(s) ω 0 = 2 π f 0 {\displaystyle \omega _{0}\;=\;2\pi f_{0}} at which
8228-533: The arrest of Oshchepkov and his subsequent gulag sentence. In total, only 607 Redut stations were produced during the war. The first Russian airborne radar, Gneiss-2 , entered into service in June 1943 on Pe-2 dive bombers. More than 230 Gneiss-2 stations were produced by the end of 1944. The French and Soviet systems, however, featured continuous-wave operation that did not provide the full performance ultimately synonymous with modern radar systems. Full radar evolved as
8349-475: The beam path caused the received signal to fade in and out. Taylor submitted a report, suggesting that this phenomenon might be used to detect the presence of ships in low visibility, but the Navy did not immediately continue the work. Eight years later, Lawrence A. Hyland at the Naval Research Laboratory (NRL) observed similar fading effects from passing aircraft; this revelation led to a patent application as well as
8470-400: The circuit to a noise pulse will be sinusoidal, it will excite a small sine wave of voltage in the loop. Since for small signals the loop gain is greater than one, the amplitude of the sine wave increases exponentially. During startup, while the amplitude of the oscillation is small, the circuit is approximately linear , so the analysis used in the Barkhausen criterion is applicable. When
8591-410: The circuit will oscillate at that frequency. Many amplifiers such as common-emitter transistor circuits are "inverting", meaning that their output voltage decreases when their input increases. In these the amplifier provides 180° phase shift , so the circuit will oscillate at the frequency at which the feedback network provides the other 180° phase shift. At frequencies well below the poles of
8712-421: The components. Since at high frequencies the tank circuit has very small capacitance and inductance, parasitic capacitance and parasitic inductance of component leads and PCB traces become significant. These may create unwanted feedback paths between the output and input of the active device, creating instability and oscillations at unwanted frequencies ( parasitic oscillation ). Parasitic feedback paths inside
8833-484: The detailed shape of the output waveform, electronic circuit simulation computer programs like SPICE are used. A typical design procedure for oscillator circuits is to use linear techniques such as the Barkhausen stability criterion or Nyquist stability criterion to design the circuit, use a rule of thumb to choose the loop gain, then simulate the circuit on computer to make sure it starts up reliably and to determine
8954-408: The detection of aircraft and ships. Radar absorbing material , containing resistive and sometimes magnetic substances, is used on military vehicles to reduce radar reflection . This is the radio equivalent of painting something a dark colour so that it cannot be seen by the eye at night. Radar waves scatter in a variety of ways depending on the size (wavelength) of the radio wave and the shape of
9075-471: The detection process. As an example, moving target indication can interact with Doppler to produce signal cancellation at certain radial velocities, which degrades performance. Sea-based radar systems, semi-active radar homing , active radar homing , weather radar , military aircraft, and radar astronomy rely on the Doppler effect to enhance performance. This produces information about target velocity during
9196-411: The detection process. This also allows small objects to be detected in an environment containing much larger nearby slow moving objects. Doppler shift depends upon whether the radar configuration is active or passive. Active radar transmits a signal that is reflected back to the receiver. Passive radar depends upon the object sending a signal to the receiver. The Doppler frequency shift for active radar
9317-606: The device in patent GB593017. Development of radar greatly expanded on 1 September 1936, when Watson-Watt became superintendent of a new establishment under the British Air Ministry , Bawdsey Research Station located in Bawdsey Manor , near Felixstowe, Suffolk. Work there resulted in the design and installation of aircraft detection and tracking stations called " Chain Home " along the East and South coasts of England in time for
9438-538: The electric field is perpendicular to the direction of propagation, and the electric field direction is the polarization of the wave. For a transmitted radar signal, the polarization can be controlled to yield different effects. Radars use horizontal, vertical, linear, and circular polarization to detect different types of reflections. For example, circular polarization is used to minimize the interference caused by rain. Linear polarization returns usually indicate metal surfaces. Random polarization returns usually indicate
9559-555: The energy was radiated as sound waves by the arc, producing a musical tone. Duddell demonstrated his oscillator before the London Institute of Electrical Engineers by sequentially connecting different tuned circuits across the arc to play the national anthem " God Save the Queen ". Duddell's "singing arc" did not generate frequencies above the audio range. In 1902 Danish physicists Valdemar Poulsen and P. O. Pederson were able to increase
9680-473: The entire area in front of it, and then used one of Watson-Watt's own radio direction finders to determine the direction of the returned echoes. This fact meant CH transmitters had to be much more powerful and have better antennas than competing systems but allowed its rapid introduction using existing technologies. A key development was the cavity magnetron in the UK, which allowed the creation of relatively small systems with sub-meter resolution. Britain shared
9801-431: The feedback filter. Since the amplitude of the output is constant due to the nonlinearity of the amplifier (see Startup section below), changes in component values cause changes in the phase shift ϕ = ∠ A β ( j ω ) {\displaystyle \phi \;=\;\angle A\beta (j\omega )} of the feedback loop. Since oscillation can only occur at frequencies where
9922-441: The feedback loop: In addition to the feedback oscillators described above, which use two-port amplifying active elements such as transistors and operational amplifiers, linear oscillators can also be built using one-port (two terminal) devices with negative resistance , such as magnetron tubes, tunnel diodes , IMPATT diodes and Gunn diodes . Negative-resistance oscillators are usually used at high frequencies in
10043-461: The firm GEMA [ de ] in Germany and then another in June 1935 by an Air Ministry team led by Robert Watson-Watt in Great Britain. In 1935, Watson-Watt was asked to judge recent reports of a German radio-based death ray and turned the request over to Wilkins. Wilkins returned a set of calculations demonstrating the system was basically impossible. When Watson-Watt then asked what such
10164-601: The frequency produced into the radio range by operating the arc in a hydrogen atmosphere with a magnetic field, inventing the Poulsen arc radio transmitter , the first continuous wave radio transmitter, which was used through the 1920s. The vacuum-tube feedback oscillator was invented around 1912, when it was discovered that feedback ("regeneration") in the recently invented audion (triode) vacuum tube could produce oscillations. At least six researchers independently made this discovery, although not all of them can be said to have
10285-422: The loop gain (this is the " harmonic balance " analysis technique for nonlinear circuits). The sine wave cannot grow indefinitely; in all real oscillators some nonlinear process in the circuit limits its amplitude, reducing the gain as the amplitude increases, resulting in stable operation at some constant amplitude. In most oscillators this nonlinearity is simply the saturation (limiting or clipping ) of
10406-430: The loop, v o v i = A β ( j ω ) {\displaystyle {v_{o} \over v_{i}}=A\beta (j\omega )} , is called the loop gain . So the condition for oscillation is that the loop gain must be one Since A β ( j ω ) {\displaystyle A\beta (j\omega )} is a complex number with two parts,
10527-552: The narrow bandwidth of the crystal removes the harmonics from the output, producing a 'pure' sinusoidal wave with almost no distortion even with large loop gains. Since oscillators depend on nonlinearity for their operation, the usual linear frequency domain circuit analysis techniques used for amplifiers based on the Laplace transform , such as root locus and gain and phase plots ( Bode plots ), cannot capture their full behavior. To determine startup and transient behavior and calculate
10648-419: The nonlinear aspects of operation such as harmonic distortion. Component values are tweaked until the simulation results are satisfactory. The distorted oscillations of real-world (nonlinear) oscillators are called limit cycles and are studied in nonlinear control theory . In applications where a 'pure' very low distortion sine wave is needed, such as precision signal generators , a nonlinear component
10769-494: The ocean liner Normandie in 1935. During the same period, Soviet military engineer P.K. Oshchepkov , in collaboration with the Leningrad Electrotechnical Institute , produced an experimental apparatus, RAPID, capable of detecting an aircraft within 3 km of a receiver. The Soviets produced their first mass production radars RUS-1 and RUS-2 Redut in 1939 but further development was slowed following
10890-470: The oscillation. Therefore, over a single cycle they act as virtually linear elements, and so introduce very little distortion. The operation of these circuits is somewhat analogous to an automatic gain control (AGC) circuit in a radio receiver. The Wein bridge oscillator is a widely used circuit in which this type of gain stabilization is used. At high frequencies it becomes difficult to physically implement feedback oscillators because of shortcomings of
11011-446: The oscillator output amplitude constant, but it also kept the oscillator's loop gain near unity. The latter is a key technique for achieving a low distortion oscillator. Earlier, Larned Meacham had used light bulbs in bridge circuits to stabilize and linearize oscillators in 1938. The product code was chosen to give the impression that HP was an established company. A variation, the HP 200B,
11132-430: The oscillator's frequency can be locked to the frequency of another oscillator. These are ubiquitous in modern communications circuits, used in filters , modulators , demodulators , and forming the basis of frequency synthesizer circuits which are used to tune radios and televisions. Radio frequency VCOs are usually made by adding a varactor diode to the tuned circuit or resonator in an oscillator circuit. Changing
11253-426: The other port and show negative resistance due to internal feedback. The negative resistance port is connected to a tuned circuit or resonant cavity, causing them to oscillate. High-frequency oscillators in general are designed using negative-resistance techniques. Some of the many harmonic oscillator circuits are listed below: A nonlinear or relaxation oscillator produces a non-sinusoidal output, such as
11374-418: The others with a crude oscillator in late 1911. In Britain, H. J. Round patented amplifying and oscillating circuits in 1913. In August 1912, Lee De Forest , the inventor of the audion, had also observed oscillations in his amplifiers, but he didn't understand the significance and tried to eliminate it until he read Armstrong's patents in 1914, which he promptly challenged. Armstrong and De Forest fought
11495-520: The outbreak of World War II in 1939. This system provided the vital advance information that helped the Royal Air Force win the Battle of Britain ; without it, significant numbers of fighter aircraft, which Great Britain did not have available, would always have needed to be in the air to respond quickly. The radar formed part of the " Dowding system " for collecting reports of enemy aircraft and coordinating
11616-480: The output of the amplifier can no longer increase with increasing input, further increases in amplitude cause the equivalent gain of the amplifier and thus the loop gain to decrease. The amplitude of the sine wave, and the resulting clipping, continues to grow until the loop gain is reduced to unity, | A β ( j ω 0 ) | = 1 {\displaystyle |A\beta (j\omega _{0})|\;=\;1\,} , satisfying
11737-520: The output waveform. Although in the past negative resistance devices like the unijunction transistor , thyratron tube or neon lamp were used, today relaxation oscillators are mainly built with integrated circuits like the 555 timer IC . Square-wave relaxation oscillators are used to provide the clock signal for sequential logic circuits such as timers and counters , although crystal oscillators are often preferred for their greater stability. Triangle-wave or sawtooth oscillators are used in
11858-409: The phase changes very slowly with frequency, therefore a given phase change will cause a large change in the frequency. In contrast, LC oscillators have tank circuits with high Q {\displaystyle Q} (~10 ). This means the phase shift of the feedback network increases rapidly with frequency near the resonant frequency of the tank circuit. So a large change in phase causes only
11979-463: The phase shift is a multiple of 360°, ϕ = 360 n ∘ {\displaystyle \phi \;=\;360n^{\circ }} , shifts in component values cause the oscillation frequency ω 0 {\displaystyle \omega _{0}} to change to bring the loop phase back to 360n°. The amount of frequency change Δ ω {\displaystyle \Delta \omega } caused by
12100-401: The power supply to the amplifier is switched on initially, electronic noise in the circuit provides a non-zero signal to get oscillations started. The noise travels around the loop and is amplified and filtered until very quickly it converges on a sine wave at a single frequency. Feedback oscillator circuits can be classified according to the type of frequency selective filter they use in
12221-706: The primary tool for short-term weather forecasting and watching for severe weather such as thunderstorms , tornadoes , winter storms , precipitation types, etc. Geologists use specialized ground-penetrating radars to map the composition of Earth's crust . Police forces use radar guns to monitor vehicle speeds on the roads. Automotive radars are used for adaptive cruise control and emergency breaking on vehicles by ignoring stationary roadside objects that could cause incorrect brake application and instead measuring moving objects to prevent collision with other vehicles. As part of Intelligent Transport Systems , fixed-position stopped vehicle detection (SVD) radars are mounted on
12342-432: The radial component of the velocity is relevant. When the reflector is moving at right angle to the radar beam, it has no relative velocity. Objects moving parallel to the radar beam produce the maximum Doppler frequency shift. When the transmit frequency ( F T {\displaystyle F_{T}} ) is pulsed, using a pulse repeat frequency of F R {\displaystyle F_{R}} ,
12463-414: The response. Given all required funding and development support, the team produced working radar systems in 1935 and began deployment. By 1936, the first five Chain Home (CH) systems were operational and by 1940 stretched across the entire UK including Northern Ireland. Even by standards of the era, CH was crude; instead of broadcasting and receiving from an aimed antenna, CH broadcast a signal floodlighting
12584-410: The resulting frequency spectrum will contain harmonic frequencies above and below F T {\displaystyle F_{T}} with a distance of F R {\displaystyle F_{R}} . As a result, the Doppler measurement is only non-ambiguous if the Doppler frequency shift is less than half of F R {\displaystyle F_{R}} , called
12705-427: The roadside to detect stranded vehicles, obstructions and debris by inverting the automotive radar approach and ignoring moving objects. Smaller radar systems are used to detect human movement . Examples are breathing pattern detection for sleep monitoring and hand and finger gesture detection for computer interaction. Automatic door opening, light activation and intruder sensing are also common. A radar system has
12826-407: The scattered energy back toward the source. The extent to which an object reflects or scatters radio waves is called its radar cross-section . The power P r returning to the receiving antenna is given by the equation: where In the common case where the transmitter and the receiver are at the same location, R t = R r and the term R t ² R r ² can be replaced by R , where R
12947-411: The sine wave's amplitude back to its unity-gain value. Similarly if the amplitude of the wave decreases, the decreased clipping will cause the loop gain to increase above one, increasing the amplitude. The amount of harmonic distortion in the output is dependent on how much excess loop gain the circuit has: An exception to the above are high Q oscillator circuits such as crystal oscillators ;
13068-409: The sine wave, flattening or " clipping " the peaks. To achieve the maximum amplitude sine wave output from the circuit, the amplifier should be biased midway between its clipping levels. For example, an op amp should be biased midway between the two supply voltage rails. A common-emitter transistor amplifier's collector voltage should be biased midway between cutoff and saturation levels. Since
13189-491: The target. If the wavelength is much shorter than the target's size, the wave will bounce off in a way similar to the way light is reflected by a mirror . If the wavelength is much longer than the size of the target, the target may not be visible because of poor reflection. Low-frequency radar technology is dependent on resonances for detection, but not identification, of targets. This is described by Rayleigh scattering , an effect that creates Earth's blue sky and red sunsets. When
13310-569: The technology with the U.S. during the 1940 Tizard Mission . In April 1940, Popular Science showed an example of a radar unit using the Watson-Watt patent in an article on air defence. Also, in late 1941 Popular Mechanics had an article in which a U.S. scientist speculated about the British early warning system on the English east coast and came close to what it was and how it worked. Watson-Watt
13431-553: The timebase circuits that generate the horizontal deflection signals for cathode-ray tubes in analogue oscilloscopes and television sets. They are also used in voltage-controlled oscillators (VCOs), inverters and switching power supplies , dual-slope analog to digital converters (ADCs), and in function generators to generate square and triangle waves for testing equipment. In general, relaxation oscillators are used at lower frequencies and have poorer frequency stability than linear oscillators. Ring oscillators are built of
13552-879: The transmitter. The reflected radar signals captured by the receiving antenna are usually very weak. They can be strengthened by electronic amplifiers . More sophisticated methods of signal processing are also used in order to recover useful radar signals. The weak absorption of radio waves by the medium through which they pass is what enables radar sets to detect objects at relatively long ranges—ranges at which other electromagnetic wavelengths, such as visible light , infrared light , and ultraviolet light , are too strongly attenuated. Weather phenomena, such as fog, clouds, rain, falling snow, and sleet, that block visible light are usually transparent to radio waves. Certain radio frequencies that are absorbed or scattered by water vapour, raindrops, or atmospheric gases (especially oxygen) are avoided when designing radars, except when their detection
13673-611: The tube. The first of these was the Barkhausen–Kurz oscillator (1920), the first tube to produce power in the UHF range. The most important and widely used were the klystron (R. and S. Varian, 1937) and the cavity magnetron (J. Randall and H. Boot, 1940). Mathematical conditions for feedback oscillations, now called the Barkhausen criterion , were derived by Heinrich Georg Barkhausen in 1921. He also showed that all linear oscillators must have negative resistance. The first analysis of
13794-487: The two length scales are comparable, there may be resonances . Early radars used very long wavelengths that were larger than the targets and thus received a vague signal, whereas many modern systems use shorter wavelengths (a few centimetres or less) that can image objects as small as a loaf of bread. Short radio waves reflect from curves and corners in a way similar to glint from a rounded piece of glass. The most reflective targets for short wavelengths have 90° angles between
13915-467: The use of radar altimeters possible in certain cases. The radar signals that are reflected back towards the radar receiver are the desirable ones that make radar detection work. If the object is moving either toward or away from the transmitter, there will be a slight change in the frequency of the radio waves due to the Doppler effect . Radar receivers are usually, but not always, in the same location as
14036-608: Was a 1938 Bell Lab unit on some United Air Lines aircraft. Aircraft can land in fog at airports equipped with radar-assisted ground-controlled approach systems in which the plane's position is observed on precision approach radar screens by operators who thereby give radio landing instructions to the pilot, maintaining the aircraft on a defined approach path to the runway. Military fighter aircraft are usually fitted with air-to-air targeting radars, to detect and target enemy aircraft. In addition, larger specialized military aircraft carry powerful airborne radars to observe air traffic over
14157-406: Was built by Elihu Thomson in 1892 by placing an LC tuned circuit in parallel with an electric arc and included a magnetic blowout. Independently, in the same year, George Francis FitzGerald realized that if the damping resistance in a resonant circuit could be made zero or negative, the circuit would produce oscillations, and, unsuccessfully, tried to build a negative resistance oscillator with
14278-536: Was customized for Walt Disney , which bought eight units for use in the production of Fantasia . The circuit diagram is shown in Hewlett's 1939 patent. Through the 1940s and into the 1950s, the subsequent versions of the 200A covered different and wider frequency ranges. The latest version was the 200CD. It covered from the subaudio 5 Hz to the low end (Long Wave) of the AM radio band at 600 kilohertz. The 200CD became
14399-646: Was invented in 1917 by French engineers Henri Abraham and Eugene Bloch. They called their cross-coupled, dual-vacuum-tube circuit a multivibrateur , because the square-wave signal it produced was rich in harmonics , compared to the sinusoidal signal of other vacuum-tube oscillators. Vacuum-tube feedback oscillators became the basis of radio transmission by 1920. However, the triode vacuum tube oscillator performed poorly above 300 MHz because of interelectrode capacitance. To reach higher frequencies, new "transit time" (velocity modulation) vacuum tubes were developed, in which electrons traveled in "bunches" through
14520-748: Was sent to the U.S. in 1941 to advise on air defense after Japan's attack on Pearl Harbor . Alfred Lee Loomis organized the secret MIT Radiation Laboratory at Massachusetts Institute of Technology , Cambridge, Massachusetts which developed microwave radar technology in the years 1941–45. Later, in 1943, Page greatly improved radar with the monopulse technique that was used for many years in most radar applications. The war precipitated research to find better resolution, more portability, and more features for radar, including small, lightweight sets to equip night fighters ( aircraft interception radar ) and maritime patrol aircraft ( air-to-surface-vessel radar ), and complementary navigation systems like Oboe used by
14641-459: Was the first to use radio waves to detect "the presence of distant metallic objects". In 1904, he demonstrated the feasibility of detecting a ship in dense fog, but not its distance from the transmitter. He obtained a patent for his detection device in April 1904 and later a patent for a related amendment for estimating the distance to the ship. He also obtained a British patent on 23 September 1904 for
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