Pulse forming networks are similar to an artificial transmission line in that it stores energy between pulses and produces a nearly rectangular pulse. The pulse-forming network in view B of the figure below consists of inductors and capacitors so arranged that they approximate the behavior of an artificial transmission line.
Each capacitor in the artificial transmission line, shown in view A, must carry the high voltage required for the modulator pulse. Because each capacitor must be insulated for this high voltage, an artificial transmission line consisting of many sections would be bulky and cumbersome. Pulse forming networks , as shown in view B of the picture below, can carry high voltage but does not require bulky insulation on all of its capacitors. Only series capacitor C1 must have high-voltage insulation. Because the other capacitors are in parallel with the corresponding inductors, the modulator- pulse voltage divides nearly equally among them. Thus, except for C1, the elements of the pulse forming networks are relatively small.
Pulse forming networks are often insulated by immersing each circuit element in oil. The network is usually enclosed in a metal box on which the pulse width, characteristic impedance, and safe operating voltage of the network are marked. If one element in such a network fails, the entire network must be replaced.
Modulator storage elements.
Modulator Switching Devices
The voltage stored in a storage-element capacitor, artificial transmission line, or pulse-forming network must be discharged through a MODULATOR SWITCHING DEVICE. The modulator switching device conducts for the duration of the modulator pulse and is an open circuit between pulses. Thus, the modulator switch must perform the following four functions:
1. Close very quickly and then reach full conduction in a small fraction of a microsecond
2. Conduct large currents (tens or hundreds of amperes) and withstand large voltages (thousands of volts)
3. Cease conducting (become an open circuit) with the same speed that it starts to conduct
4. Consume only a very small fraction of the power that passes through it.
These switching and conducting requirements are met best by the THYRATRON tube. The thyratron tube is normally held below cutoff by a negative grid voltage and conducts when a positive trigger pulse is applied to its grid. Once fired, the thyratron tube continues to conduct as long as the storage element (artificial transmission line or pulse-forming network) is discharging.
During discharge of the storage element, the gas in the thyratron tube is highly ionized. While the storage element discharges, the plate-to-cathode resistance of the thyratron is practically zero. When the storage element is completely discharged, current ceases to flow through the thyratron and the gases become deionized; the negative grid bias regains control, and the thyratron is cut off (the modulator switch opens).
Most radar modulators use a high-voltage, dc power supply. Typical dc power supplies for radar modulators use a half-wave rectifier, a full-wave rectifier, or a bridge rectifier.
The modulator charging impedance, shown in the figure below, prevents the dc power supply from becoming short-circuited when the modulator switch closes. When the modulator switch is open, the charging impedance also controls the rate at which the storage element charges. When the charging impedance is small, the storage element charges rapidly.
Modulator charging impedance.
Many different kinds of charging impedance and charging circuits are used in radar modulators.
The type of charging impedance and charging circuit used depends on the following five elements:
1. The type of power supply (ac or dc)
2. The type of storage element
3. The amount of modulator pulse voltage required
4. The pulse-repetition rate
5. The frequency of the available ac supply voltage