Pulse-forming network

A PFN consists of a series of high-voltage energy-storage capacitors and inductors. 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 by a high-voltage DC power supply. When the PFN is discharged, the capacitors discharge in sequence, producing an approximately rectangular pulse. The pulse is conducted to the load through a transmission line. The PFN must be impedance-matched to the load to prevent the energy reflecting back toward the PFN. ==Transmission-line PFNs==

A length of transmission line can be used as a pulse-forming network. This can give substantially flat-topped pulses at the inconvenience of using of a large length of cable. In a simple charged transmission-line pulse generator (animation, right) a length of transmission line such as a coaxial cable is connected through a switch to a matched load RL at one end, and at the other end to a DC voltage source V through a resistor RS, which is large compared to the characteristic impedance Z0 of the line. and is widely used today in PFNs. In the Blumlein generator (animation, right), the load is connected in series between two equal-length transmission lines, which are charged by a DC power supply at one end (note that the right line is charged through the impedance of the load). To trigger the pulse, a switch short-circuits the line at the power-supply end, causing a negative voltage step to travel toward the load. Since the characteristic impedance Z0 of the line is made equal to half the load impedance RL, the voltage step is half-reflected and half-transmitted, resulting in two symmetrical opposite-polarity voltage steps, which propagate away from the load, creating between them a voltage drop of V/2 − (−V/2)= V across the load. The voltage steps reflect from the ends and return, ending the pulse. As in other charge-line generators, the pulse duration is equal to 2D/c, where D is the length of the individual transmission lines. A second advantage of the Blumlein geometry is that the switching device can be grounded, rather than located in the high-voltage side of the transmission line as in the typical charged line, which complicates the triggering electronics. ==Uses of PFNs==

Upon command, a high-voltage switch 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 pulse 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 a high-voltage source, PFN, HV switch, and pulse transformer (when required) is sometimes called a "power modulator" or "pulser". ==See also==