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2-12 Q10. What are the two basic types of transmitters? Q11. What controls transmitter pulse width? Q12. In addition to a flat top, what characteristics must a modulator pulse have? Q13. What type of modulator is most commonly used in modern radar systems? Q14. What three types of storage elements most often are used in modulators? Q15. What characteristic is determined by the time required for a voltage wave to travel from the input end of an artificial transmission line to the output end and back again? 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 figure 2-7, 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.