Design of high performance TWT based C-Band transmitter for high altitude operation

This paper presents the design issues and challenges in the realization of high performance Traveling Wave Tube (TWT) based Transmitter for Weapon Locating Radar for the detection of targets having very small RCS viz. mortar shell, gun shells etc and to locate the origin of the weapon. The design goal was to achieve stable operation of the transmitter having spectrum purity of near carrier noise of -75dBc/Hz at 100 Hz away from the carrier while delivering 60 kW (minimum) of peak power and 3 kW of average power across the bandwidth of 500MHz at C-Band. These requirements translates into High Voltage Power Supply (HVPS) design, i.e., Cathode Power Supply -39 kVDC, 6.0 kW and Collector power supply 30 kVDC, 16.5 kW with stringent pulse to pulse regulation of the order of 0.0004% under adverse environmental conditions viz. reliable operation of high voltage power supplies under 16500 ft operating conditions. The realization of critical units, specific high voltage engineering to enhance the reliability is discussed in addition to design of the transmitter. High voltage power supply design issues, Control and protection strategy of the transmitter and the realization of a fast solid state FDM (Floating Deck Grid Modulator) has been discussed. The development & testing of transmitter has been completed. The test results & simulation results have been presented in this paper. Generally bulk energy storage capacitors are used to meet RF amplitude flatness requirements of the transmitter. The energy stored in these capacitors is typically of the order of 600 to 800 joules. TWT in general can handle energy of the order of 50 joules during arcing without causing permanent damage to the tube. Hence the effective energy diversion mechanism is required. The detailed analysis and protection employed have also been discussed in this paper.