Pulse forming network
Encyclopedia
A Pulse Forming Network (PFN) accumulates electrical energy over a comparatively long time, then releases the stored energy in the form of a relatively square pulse
of comparatively short duration for various pulsed power
applications. In practice, a PFN is charged by means of a high voltage
power source, then rapidly discharged into a load
via a high voltage switch
, such as a spark gap
or hydrogen thyratron
. The load may be a high power microwave
oscillator such as a klystron
or magnetron, a flashtube
, or even an electromagnet
. Depending upon the application, the output pulse repetition rate may range from a fraction of a Hertz
to over 10kHz.
s and inductor
s. These components are interconnected (as a "ladder network
") that behaves similarly to a length of transmission line
. For this reason, a PFN is sometimes called an "artificial, or synthetic, transmission line". Electrical energy is initially stored within the charged capacitors of the PFN.
Sometimes an actual length of transmission line is used as the pulse forming network. This can give substantially flat topped pulses at the inconvenience of using of a large length of cable.
A Blumlein
transmission line is a particular configuration of transmission lines used to create high-voltage pulses with short rise and fall times. Its principle is closely related to a pulse-forming transmission line
discharge, although a Blumlein's output voltage is the same as the charging voltage whereas the Pulse-forming transmission line outputs half the charging voltage.
of short duration and high power. This high power pulse becomes a brief source of high power to the load.
Sometimes a specially designed pulse transformer is connected between the PFN and load. This technique improves the impedance
match between the PFN and the load so as to improve power transfer efficiency. A pulse transformer is typically required when driving higher impedance devices such as klystrons or magnetrons from a PFN. Because the PFN is charged over a relatively long time and then discharged over a very short time, the output pulse may have a peak power of megawatts or even terawatts.
The combination of high voltage source, PFN, HV switch, and pulse transformer (when required) is sometimes called a "power modulator" or "pulser".
Square wave
A square wave is a kind of non-sinusoidal waveform, most typically encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels...
of comparatively short duration for various pulsed power
Pulsed power
Pulsed power is the term used to describe the science and technology of accumulating energy over a relatively long period of time and releasing it very quickly thus increasing the instantaneous power.-Overview:...
applications. In practice, a PFN is charged by means of a high voltage
High voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...
power source, then rapidly discharged into a load
External electric load
If an electric circuit has a well-defined output terminal, the circuit connected to this terminal is the load....
via a high voltage switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
, such as a spark gap
Spark gap
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the gap's breakdown voltage, a spark forms,...
or hydrogen thyratron
Thyratron
A thyratron is a type of gas filled tube used as a high energy electrical switch and controlled rectifier. Triode, tetrode and pentode variations of the thyratron have been manufactured in the past, though most are of the triode design...
. The load may be a high power microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
oscillator such as a klystron
Klystron
A klystron is a specialized linear-beam vacuum tube . Klystrons are used as amplifiers at microwave and radio frequencies to produce both low-power reference signals for superheterodyne radar receivers and to produce high-power carrier waves for communications and the driving force for modern...
or magnetron, a flashtube
Flashtube
A flashtube, also called a flashlamp, is an electric arc lamp designed to produce extremely intense, incoherent, full-spectrum white light for very short durations. Flashtubes are made of a length of glass tubing with electrodes at either end and are filled with a gas that, when triggered, ionizes...
, or even an electromagnet
Electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...
. Depending upon the application, the output pulse repetition rate may range from a fraction of a Hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
to over 10kHz.
Implementation
A PFN consists of a series of high voltage energy storage capacitorCapacitor
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...
s and 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...
s. These components are interconnected (as a "ladder network
Electrical network
An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...
") that behaves similarly to a length of transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
. For this reason, a PFN is sometimes called an "artificial, or synthetic, transmission line". Electrical energy is initially stored within the charged capacitors of the PFN.
Sometimes an actual length of transmission line is used as the pulse forming network. This can give substantially flat topped pulses at the inconvenience of using of a large length of cable.
A Blumlein
Alan Blumlein
Alan Dower Blumlein was a British electronics engineer, notable for his many inventions in telecommunications, sound recording, stereo, television and radar...
transmission line is a particular configuration of transmission lines used to create high-voltage pulses with short rise and fall times. Its principle is closely related to a pulse-forming transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
discharge, although a Blumlein's output voltage is the same as the charging voltage whereas the Pulse-forming transmission line outputs half the charging voltage.
Uses of PFNs
Upon command, a high voltage switch then transfers the energy stored within the PFN into the load. When the switch "fires" (closes), the network of capacitors and inductors within the PFN creates an approximately square output pulseSquare wave
A square wave is a kind of non-sinusoidal waveform, most typically encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels...
of short duration and high power. This high power pulse becomes a brief source of high power to the load.
Sometimes a specially designed pulse transformer is connected between the PFN and load. This technique improves the impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
match between the PFN and the load so as to improve power transfer efficiency. A pulse transformer is typically required when driving higher impedance devices such as klystrons or magnetrons from a PFN. Because the PFN is charged over a relatively long time and then discharged over a very short time, the output pulse may have a peak power of megawatts or even terawatts.
The combination of high voltage source, PFN, HV switch, and pulse transformer (when required) is sometimes called a "power modulator" or "pulser".
See also
- Pulse-code modulationPulse-code modulationPulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...
- Pulse-position modulationPulse-position modulationPulse-position modulation is a form of signal modulation in which M message bits are encoded by transmitting a single pulse in one of 2^M possible time-shifts. This is repeated every T seconds, such that the transmitted bit rate is M/T bits per second...
- Pulse-width modulationPulse-width modulationPulse-width modulation , or pulse-duration modulation , is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches....
- Pulse-density modulationPulse-density modulationPulse-density modulation, or PDM, is a form of modulation used to represent an analog signal with digital data. In a PDM signal, specific amplitude values are not encoded into pulses of different size as they would be in PCM. Instead, it is the relative density of the pulses that corresponds to...
- CrossatronCrossatronIn electronics, a crossatron is a high-power pulsed modulator device, a cold cathode gas-filled tube that combines the best features of thyratrons, vacuum tubes, and power semiconductor switches...
- radarRadarRadar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
modulators - missile operationMissileThough a missile may be any thrown or launched object, it colloquially almost always refers to a self-propelled guided weapon system.-Etymology:The word missile comes from the Latin verb mittere, meaning "to send"...
s - laserLaserA laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s - pulsed powerPulsed powerPulsed power is the term used to describe the science and technology of accumulating energy over a relatively long period of time and releasing it very quickly thus increasing the instantaneous power.-Overview:...
- particle accelerators
External links and references
- Eric Heine, "Conversion". NIKHEF Electronic Department, Amsterdam, The Netherlands.
- Riepe, Kenneth B., "High-voltage microsecond pulse-forming network". Review of Scientific Instruments Vol 48(8) pp. 1028–1030. August 1977. (Abstract)
- Glasoe, G. NorrisG. N. GlasoeG. Norris Glasoe was an American nuclear physicist. He was a member of the Columbia University team which was the first in the United States to verify the European discovery of the nuclear fission of uranium via neutron bombardment. During World War II, he worked at the MIT Radiation Laboratory...
, Lebacqz, Jean V., "Pulse Generators", McGraw-Hill, MIT Radiation Laboratory Series, Volume 5, 1948.