Radar transmitter-HV engineering, crowbar design and environmental qualification — A case study

This paper discusses the total redesign and re-engineering of a 75 KW pulsed TWT based transmitter for Phased array radar. The transmitter initially designed and developed, had the following limitations: 1. The transmitter was operational for only eight hours on continuous run, while the requirement was for 48 hour endurance. Redesign of collector transformer including modification of the core dimensions was carried out by analysing heat dissipation problem. 2. The system was not operating at sub-zero temperatures whereas the system demanded operation right from −20 deg to +55 deg. 3. The transmitter design used an imported spark gap and used liquid dielectric as the medium for insulation and cooling the crow bar hardware requiring too much maintenance efforts and also ceramic (HV) bushings were used for HV connectivity adding to the complexity. 4. The transmitter operation at all spot frequencies required readjustments and fine tuning at pre-regulator stage to deliver the required power output which put a restriction on the automatic frequency change over during frequency agility operation. 5. Better MTI Cancellation required improvement of modulator pulse shape. 6. Introduction of high voltage connectors at module level avoiding the HV exposure to open air thus avoiding corona inception. 7. Also for ease of maintenance and improved reliability of the transmitter, modularization, good mechanical engineering, thermal design and packaging were essential. This case study fully analyses the above drawbacks of the transmitter and provides detailed account of the re-design and improvements carried out on the TWT based transmitter for Phased Array Radar. The HV engineering improvements, crow-bar design and inverter-pre-regulator testing and integration to develop 37KV to operate under sub-zero temperatures; miniaturization and packaging will be presented in this paper. It also enabled the authors to replace the imported spark gap with an indigenous one the- - reby saving lot of foreign exchange and contributing to the self-reliance. The High Voltage power supplies are critical sub-systems which are designed as a combination of high frequency switching type for cathode (37 KV) and mains frequency step up transformer type for collector (26 KV). The accurate maintenance of cathode voltage was achieved through on-off regulation scheme using A to D converter. The collector supply is unregulated, 6 KW floating at 37 KV. The modulator design is based on two regulated supplies and has a switching circuit with two active switches to generate flat pulse with 100 nano Sec rise and fall time to support 10 micro sec pulse width and staggered PRF around 2 KHz for better MTI improvement of the radar. Partition of LV and HV power supplies, modulator housing based on Faraday cage for better high voltage insulation capability. For good EMI compatibility in the transmitter, separation of high power switching circuits and sensitive feedback and the logic circuits was resorted to. Grounding scheme for LV and HV stages has been appropriately carried out for avoiding mutual interference. This transmitter is produced by production agency (BEL) through technology transfer and is used in two programmes-Weapon Locating Radar for Army and Rajendra Radar for Akash Missile system for IAF. The aim of this paper is to present the modifications in the cathode power supply, modulator, crow-bar redesign, HV engineering and modularization of the transmitter which resulted in considerable improvement in the performance of the transmitter for Weapon Locating Radar, which was subsequently used in Battery Level Radar III for Akash missile system also.