dynatron oscillator
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In electronics, the dynatron oscillator, invented in 1918 by
Albert Hull Albert Wallace Hull (19 April 1880 – 22 January 1966) was an American physicist and electrical engineer who made contributions to the development of vacuum tubes, and invented the magnetron. He was a member of the National Academy of Scienc ...
at
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, is an obsolete
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. The type kn ...
electronic oscillator An electronic oscillator is an electronic circuit that produces a periodic, oscillation, oscillating electronic signal, often a sine wave or a square wave or a triangle wave. Oscillation, Oscillators convert direct current (DC) from a power supp ...
circuit which uses a
negative resistance In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it. This is in contrast to an ordi ...
characteristic in early
tetrode A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate (called ''anode'' in British English). Th ...
vacuum tubes, caused by a process called
secondary emission In particle physics, secondary emission is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emi ...
. on Peter Millet'
Tubebooks
website
It was the first negative resistance vacuum tube oscillator. The dynatron oscillator circuit was used to a limited extent as
beat frequency oscillator In a radio receiver, a beat frequency oscillator or BFO is a dedicated oscillator used to create an audio frequency signal from Morse code radiotelegraphy ( CW) transmissions to make them audible. The signal from the BFO is mixed with the recei ...
s (BFOs), and
local oscillator In electronics, a local oscillator (LO) is an electronic oscillator used with a mixer to change the frequency of a signal. This frequency conversion process, also called heterodyning, produces the sum and difference frequencies from the frequenc ...
s in vacuum tube
radio receiver 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. Th ...
s as well as in scientific and test equipment from the 1920s to the 1940s but became obsolete around World War 2 due to the variability of secondary emission in tubes. Negative transconductance oscillators, such as the transitron oscillator invented by Cleto Brunetti in 1939, are similar negative resistance vacuum tube oscillator circuits which are based on negative
transconductance Transconductance (for transfer conductance), also infrequently called mutual conductance, is the electrical characteristic relating the current through the output of a device to the voltage across the input of a device. Conductance is the reciproca ...
(a fall in current through one grid electrode caused by an increase in voltage on a second grid) in a
pentode A pentode is an electronic device having five electrodes. The term most commonly applies to a three-grid amplifying vacuum tube or thermionic valve that was invented by Gilles Holst and Bernhard D.H. Tellegen in 1926. The pentode (called a ''tripl ...
or other multigrid vacuum tube. Gottlieb, 1997, ''Practical Oscillator Handbook'', p. 78-81
/ref> These replaced the dynatron circuit and were employed in vacuum tube electronic equipment through the 1970s.


How they work

The dynatron and transitron oscillators differ from many oscillator circuits in that they do not use
feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
to generate oscillations, but
negative resistance In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it. This is in contrast to an ordi ...
. A
tuned circuit An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ac ...
(resonant circuit), consisting of an
inductor An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...
and
capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...
connected together, can store electric energy in the form of oscillating currents, "ringing" analogously to a tuning fork. If a tuned circuit could have zero
electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallels ...
, once oscillations were started it would function as an
oscillator Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
, producing a continuous
sine wave A sine wave, sinusoidal wave, or just sinusoid is a curve, mathematical curve defined in terms of the ''sine'' trigonometric function, of which it is the graph of a function, graph. It is a type of continuous wave and also a Smoothness, smooth p ...
. But because of the inevitable resistance inherent in actual circuits, without an external source of power the energy in the oscillating current is dissipated as heat in the resistance, and any oscillations decay to zero. In the dynatron and transitron circuits, a vacuum tube is biased so that one of its electrodes has
negative differential resistance In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it. This is in contrast to an ordina ...
. This means that when the voltage on the electrode with respect to the cathode is increased, the current through it decreases. A tuned circuit is connected between the electrode and the cathode. The negative resistance of the tube cancels the positive resistance of the tuned circuit, creating in effect a tuned circuit with zero AC resistance. A spontaneous continuous
sinusoidal A sine wave, sinusoidal wave, or just sinusoid is a mathematical curve defined in terms of the '' sine'' trigonometric function, of which it is the graph. It is a type of continuous wave and also a smooth periodic function. It occurs often in m ...
oscillating voltage at the
resonant frequency Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
of the tuned circuit is generated, started by
electrical noise In electronics, noise is an unwanted disturbance in an electrical signal. Noise generated by electronic devices varies greatly as it is produced by several different effects. In particular, noise is inherent in physics, and central to the ...
in the circuit when it is turned on. An advantage of these oscillators was that the negative resistance effect was largely independent of frequency, so by using suitable values of
inductance Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The flow of electric current creates a magnetic field around the conductor. The field strength depends on the magnitude of the ...
and
capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are ...
in the tuned circuit they could operate over a wide frequency range, from a few hertz to around 20 MHz. Another advantage was that they used a simple single LC tuned circuit without the taps or "tickler" coils required by oscillators such as the
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or
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circuits.


Dynatron oscillator

In the dynatron a
tetrode A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate (called ''anode'' in British English). Th ...
tube is used. In some tetrodes the
plate Plate may refer to: Cooking * Plate (dishware), a broad, mainly flat vessel commonly used to serve food * Plates, tableware, dishes or dishware used for setting a table, serving food and dining * Plate, the content of such a plate (for example: ...
(anode) has negative differential resistance, due to electrons knocked out of the plate when electrons from the cathode hit it, called ''
secondary emission In particle physics, secondary emission is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emi ...
''. This causes a downward "kink" in the plate current vs. plate voltage curve ''(graph below, grey region)'' when the screen grid is biased at a higher voltage than the plate, as described below. This negative resistance was mostly a feature of older tubes, of 1940s or earlier vintage. In most modern tetrodes, to prevent
parasitic oscillation Parasitic oscillation is an undesirable electronic oscillation (cyclic variation in output voltage or current) in an electronic or digital device. It is often caused by feedback in an amplifying device. The problem occurs notably in RF, audio, ...
s the plate is given a coating which drastically reduces the unwanted secondary emission, so these tubes have virtually no negative resistance "kink" in their plate current characteristic, and cannot be used in dynatron oscillators. The tetrode wasn't the only tube which could generate dynatron oscillations. Early
triode A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 19 ...
s also had secondary emission and thus negative resistance, and before the tetrode was invented they were used in dynatron oscillators by biasing the
control grid The control grid is an electrode used in amplifying thermionic valves (vacuum tubes) such as the triode, tetrode and pentode, used to control the flow of electrons from the cathode to the anode (plate) electrode. The control grid usually consists ...
more positive than the plate. Hull's first dynatron oscillator in 1918 used a special "dynatron" vacuum tube of his own design ''(shown above)'', a triode in which the grid was a heavy plate perforated with holes which was robust enough to carry high currents. This tube saw little use as standard triode and tetrodes could function adequately as dynatrons. The term "dynatron" came to be applied to all negative resistance oscillations in vacuum tubes; for example the
split-anode magnetron The cavity magnetron is a high-power vacuum tube used in early radar systems and currently in microwave ovens and linear particle accelerators. It generates microwaves using the interaction of a stream of electrons with a magnetic field whil ...
was said to work by "dynatron oscillation". An advantage of the dynatron circuit was that it could oscillate over a very wide frequency range; from a few hertz to 20 MHz. It also had very good frequency stability compared to other LC oscillators of that time, and was even compared to
crystal oscillators A crystal oscillator is an electronic oscillator circuit that uses a piezoelectric crystal as a frequency-selective element. The oscillator frequency is often used to keep track of time, as in quartz wristwatches, to provide a stable clock ...
. The circuit became popular after the advent of cheap tetrode tubes such as the UY222 and UY224 around 1928. It was used in
beat frequency oscillator In a radio receiver, a beat frequency oscillator or BFO is a dedicated oscillator used to create an audio frequency signal from Morse code radiotelegraphy ( CW) transmissions to make them audible. The signal from the BFO is mixed with the recei ...
s (BFOs) for code reception and
local oscillator In electronics, a local oscillator (LO) is an electronic oscillator used with a mixer to change the frequency of a signal. This frequency conversion process, also called heterodyning, produces the sum and difference frequencies from the frequenc ...
s in
superheterodyne 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 carri ...
receivers as well as in laboratory
signal generator A signal generator is one of a class of electronic devices that generates electrical signals with set properties of amplitude, frequency, and wave shape. These generated signals are used as a stimulus for electronic measurements, typically used i ...
s and scientific research. RCA's 1931 prototype
television Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertisin ...
used two UY224 tubes as dynatron oscillators to generate the vertical deflection (28 Hz) and horizontal deflection (2880 Hz) signals for the CRT's deflection coils. However the dynatron had some drawbacks. It was found that the amount of secondary emission current from the plate varied unpredictably from tube to tube, and also within a single tube over its operating life; eventually it would stop oscillating. When replacing the tube, several might have to be tried to find one that would oscillate in a circuit. In addition, since dynatron oscillations were a source of instability in amplifiers, the tetrode's main application, tube manufacturers began applying a
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coating to the plate which virtually eliminated secondary emission. By 1945 the use of the dynatron circuit was declining.


Secondary emission

In an electron tube, when
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...
s emitted by the
cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
strike the
plate Plate may refer to: Cooking * Plate (dishware), a broad, mainly flat vessel commonly used to serve food * Plates, tableware, dishes or dishware used for setting a table, serving food and dining * Plate, the content of such a plate (for example: ...
, they can knock other electrons out of the surface of the metal, an effect called ''
secondary emission In particle physics, secondary emission is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emi ...
''. In a normal tetrode amplifier this is an unwanted effect, and the
screen grid A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate (called ''anode'' in British English). T ...
next to the plate is biased at a lower potential than the plate, so these ''secondary electrons'' are repelled and return to the plate due to its positive charge. However, if the screen grid is operated at a higher potential than the plate, the secondary electrons will be attracted to it, and return to ground through the screen grid supply. This represents a current of electrons ''I''G2 ''away'' from the plate, which reduces the net plate current ''I''P below the cathode current ''I''C :I_P = I_C - I_ \, Higher plate voltage causes the primary electrons to hit the plate with more energy, releasing more secondary electrons. Therefore, starting at the voltage at which the primary electrons have enough energy to cause secondary emission, around ''V''P = 10V, there is an operating region ''(grey)'' in which an increase in plate voltage causes more electrons to leave the plate than the additional electrons arriving at the plate, and therefore a net reduction in plate current.


Negative resistance

Since in this region an increase in plate voltage causes a decrease in plate current, the AC plate resistance, that is the differential output resistance of the tube, is negative: :r_P = < 0 \, As with other
negative differential resistance In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it. This is in contrast to an ordina ...
devices like the
tunnel diode A tunnel diode or Esaki diode is a type of semiconductor diode that has effectively "negative resistance" due to the quantum mechanical effect called tunneling. It was invented in August 1957 by Leo Esaki, Yuriko Kurose, and Takashi Suzuki ...
, this negative resistance can be used to create an oscillator. A parallel tuned circuit is connected in the plate circuit of the tetrode. The circuit will oscillate if the magnitude of the negative plate resistance is less than the parallel resistance ''R'' of the tuned circuit, including any load connected to the oscillator. :, r_P, < R \, The frequency of oscillation is close to the
resonant frequency Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
of the tuned circuit. :f = \sqrt \,


Design

As can be seen from the graphs, for dynatron operation the screen grid had to be biased at a considerably higher voltage than the plate; at least twice the plate voltage. The plate voltage swing is limited to the negative resistance region of the curve, the downward "kink", so to achieve the largest output voltage swing, the tube should be biased in the center of the negative resistance region. The negative resistance of older tetrode tubes was around 10kΩ - 20kΩ, and can be controlled by varying the
control grid The control grid is an electrode used in amplifying thermionic valves (vacuum tubes) such as the triode, tetrode and pentode, used to control the flow of electrons from the cathode to the anode (plate) electrode. The control grid usually consists ...
bias. If the magnitude of the negative resistance , ''r''P, is just small enough to start oscillation, just a little smaller than the positive resistance ''R'' of the tuned circuit, the oscillation frequency will be very stable, and the output waveform will be almost sinusoidal. If the negative resistance is made significantly smaller than the positive resistance, the voltage swing will extend into the nonlinear part of the curve, and the peaks of the sine wave output will be flattened ("clipped").


Transitron oscillator

The transitron oscillator, invented by Cledo Brunetti in 1939, (although a similar effect was observed in tetrodes by
Balthasar van der Pol Balthasar van der Pol (27 January 1889 – 6 October 1959) was a Dutch physicist. Life and work Van der Pol began his studies of physics in Utrecht in 1911. J. A. Fleming offered van der Pol the use of the Pender Electrical Laboratory at ...
in 1926, and Edward Herold described a similar oscillator in 1935) is a
negative resistance In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it. This is in contrast to an ordi ...
oscillator circuit using a
pentode A pentode is an electronic device having five electrodes. The term most commonly applies to a three-grid amplifying vacuum tube or thermionic valve that was invented by Gilles Holst and Bernhard D.H. Tellegen in 1926. The pentode (called a ''tripl ...
vacuum tube, in which, instead of the plate, the
screen grid A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate (called ''anode'' in British English). T ...
has negative resistance due to being coupled to the suppressor grid. See the circuit at right. In the transitron, the
screen grid A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate (called ''anode'' in British English). T ...
is biased at a positive voltage ''(battery B1)'' above the plate voltage while the suppressor grid is biased negatively ''(battery B2)'', at or below the cathode voltage. Therefore, all the electrons will be reflected by the negative suppressor grid and none will get through to the plate. The reflected electrons will instead be attracted to the screen grid, so the screen current will be high while the plate current will be zero. However, if the suppressor grid voltage is increased, as it approaches zero (the cathode voltage) electrons will begin to pass through it and reach the plate, so the number diverted to the screen grid, and thus the screen current, will decrease. Since the other grids don't take significant current the cathode current \scriptstyle I_\text is split between the plate \scriptstyle I_\text and the screen grid \scriptstyle I_\text: :I_\text = I_\text - I_\text \, The division of current between the screen grid and plate is controlled by the suppressor voltage. This inverse relationship is indicated by saying the
transconductance Transconductance (for transfer conductance), also infrequently called mutual conductance, is the electrical characteristic relating the current through the output of a device to the voltage across the input of a device. Conductance is the reciproca ...
between the screen and suppressor grid (the change in screen current Δ''I''G2 divided by the change in suppressor voltage Δ''V''G3) is negative. Since the suppressor grid voltage and not the screen grid voltage controls the screen current, if the suppressor and screen grid are coupled together with a capacitor (''C2'') so there is a constant potential difference between them, increasing the screen grid voltage will increase the suppressor voltage, resulting in a decrease in screen current. This means the screen grid has
negative differential resistance In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it. This is in contrast to an ordina ...
with respect to the cathode, and can be used to create oscillations. In the transitron circuit, the screen and suppressor grids are coupled with a bypass capacitor (''C2'') which has a low impedance at the oscillation frequency, so they have a constant potential difference. The parallel tuned circuit (''C1-L'') is connected between the screen grid and the cathode (through battery ''B1''). The negative resistance of the screen grid cancels the positive resistance of the tuned circuit, causing oscillations. As in the dynatron oscillator the control grid can be used to adjust the negative resistance. Since the transitron oscillator didn't depend on secondary emission it was far more reliable than the dynatron. However, because the screen grid is not designed to handle high power, the oscillator's output power is limited. Other tubes with multiple grids beside the pentode, such as the
hexode The pentagrid converter is a type of radio receiving valve (vacuum tube) with five grids used as the frequency mixer stage of a superheterodyne radio receiver. The pentagrid was part of a line of development of valves that were able to take an ...
and
pentagrid converter The pentagrid converter is a type of radio receiving valve (vacuum tube) with five grids used as the frequency mixer stage of a superheterodyne radio receiver. The pentagrid was part of a line of development of valves that were able to take an ...
tube, have been be used to make similar negative transconductance oscillators. Pentode tubes used in this circuit have a negative transconductance of only around -250 microsiemens, giving a negative resistance of -4000Ω. Tubes with more grids, such as the
pentagrid converter The pentagrid converter is a type of radio receiving valve (vacuum tube) with five grids used as the frequency mixer stage of a superheterodyne radio receiver. The pentagrid was part of a line of development of valves that were able to take an ...
, can be used to make transitron oscillators with higher transconductance, resulting in smaller negative resistance.


References

{{DEFAULTSORT:Dynatron Oscillator Electronic oscillators