Dynatron oscillator
Encyclopedia
A dynatron oscillator is an electronic
circuit that uses negative resistance
to keep an LC tank circuit
oscillating.
In theory, if an ideal capacitor
is connected in parallel with an ideal inductor
, they form a resonant circuit that, once it begins oscillating, will oscillate forever as the energy is transferred back and forth between the capacitor and the inductor.
In practice, however, the two components are not ideal. Real inductors and capacitors are equivalent to an ideal component in parallel (or in series) with a resistance; a real resonant circuit is equivalent to an ideal capacitor, inductor, and resistor connected in parallel.
If a negative resistance equal in magnitude to this positive resistance can be connected in parallel with the above circuit, then the two resistances will cancel and the circuit will oscillate forever as described earlier.
Although no passive negative resistors exist, some active devices exhibit a negative resistance
characteristic. Examples of such devices are some tetrode
thermionic valves (or tubes) and the tunnel diode
. Using these devices to provide the negative resistance causes the tuned circuit to oscillate as long as power is provided to the active device. The power supplied compensates for resistive losses and power extracted from the oscillator. The oscillation is self starting as the resonant circuit is excited by the inherent noise of the active device.
The negative resistance exhibited by tetrodes is due to secondary emission
from the anode
when operated at a lower voltage than the screen grid. Secondary emission is usually an unwanted phenomenon, and more modern valves are treated to minimize it. Older valves make good dynatron oscillators.
This type of oscillator is commonly known as a 'dynatron' oscillator, though, technically, only the version using the tetrode valve is a true dynatron oscillator. Later models of tetrodes were treated to reduce the (usually undesirable) secondary emission characteristics of the plate, making them less suitable for the dynatron oscillator. An oscillator that uses the negative-resistance characteristic of screen grid secondary emission in a pentode
is called a "transitron oscillator".
M. G. Scroggie
described the dynatron oscillator in detail in earlier editions of his Radio Laboratory Handbook (later Radio and Electronic Laboratory Handbook). The type 77 pentode
was a good dynatron tube. The main advantage of this type of oscillator was extreme simplicity: a very simple dynatron circuit could be built without resonant
elements, and when a combination of inductor
and capacitor
was connected would oscillate at its natural frequency
; a very wide frequency range could be covered changing only the tuned circuit.
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
circuit that uses negative resistance
Negative resistance
Negative resistance is a property of some electric circuits where an increase in the current entering a port results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors are...
to keep an LC tank circuit
LC circuit
An LC circuit, also called a resonant circuit or tuned circuit, consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C...
oscillating.
In theory, if an ideal capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
is connected in parallel with an ideal inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
, they form a resonant circuit that, once it begins oscillating, will oscillate forever as the energy is transferred back and forth between the capacitor and the inductor.
In practice, however, the two components are not ideal. Real inductors and capacitors are equivalent to an ideal component in parallel (or in series) with a resistance; a real resonant circuit is equivalent to an ideal capacitor, inductor, and resistor connected in parallel.
If a negative resistance equal in magnitude to this positive resistance can be connected in parallel with the above circuit, then the two resistances will cancel and the circuit will oscillate forever as described earlier.
Although no passive negative resistors exist, some active devices exhibit a negative resistance
Negative resistance
Negative resistance is a property of some electric circuits where an increase in the current entering a port results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors are...
characteristic. Examples of such devices are some tetrode
Tetrode
A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode and an additional screen grid which significantly changes its behaviour.-Control grid:...
thermionic valves (or tubes) and the tunnel diode
Tunnel diode
A tunnel diode or Esaki diode is a type of semiconductor diode which is capable of very fast operation, well into the microwave frequency region, by using quantum mechanical effects....
. Using these devices to provide the negative resistance causes the tuned circuit to oscillate as long as power is provided to the active device. The power supplied compensates for resistive losses and power extracted from the oscillator. The oscillation is self starting as the resonant circuit is excited by the inherent noise of the active device.
The negative resistance exhibited by tetrodes is due to secondary emission
Secondary emission
Secondary emission in physics 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 primary particles are often charged particles like electrons or ions. If the secondary...
from the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
when operated at a lower voltage than the screen grid. Secondary emission is usually an unwanted phenomenon, and more modern valves are treated to minimize it. Older valves make good dynatron oscillators.
This type of oscillator is commonly known as a 'dynatron' oscillator, though, technically, only the version using the tetrode valve is a true dynatron oscillator. Later models of tetrodes were treated to reduce the (usually undesirable) secondary emission characteristics of the plate, making them less suitable for the dynatron oscillator. An oscillator that uses the negative-resistance characteristic of screen grid secondary emission in a pentode
Pentode
A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid vacuum tube , which was invented by the Dutchman Bernhard D.H. Tellegen in 1926...
is called a "transitron oscillator".
M. G. Scroggie
M. G. Scroggie
Marcus Graham Scroggie, B.Sc., F.I.E.E., was a British technical author active in the fields of radio and electronics.- Early life and education :...
described the dynatron oscillator in detail in earlier editions of his Radio Laboratory Handbook (later Radio and Electronic Laboratory Handbook). The type 77 pentode
Pentode
A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid vacuum tube , which was invented by the Dutchman Bernhard D.H. Tellegen in 1926...
was a good dynatron tube. The main advantage of this type of oscillator was extreme simplicity: a very simple dynatron circuit could be built without resonant
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
elements, and when a combination of inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
and capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
was connected would oscillate at its natural frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
; a very wide frequency range could be covered changing only the tuned circuit.