THE RADAR RECEIVER 



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externally controlled as desired. It is also possible to synchronize this par- 

 ticular form of circuit at a submultiple of the externally available trigger 

 repetition frequency. 



Pentodes and other now available multi-element vacuum tubes, where the 

 multivibrator interstage coupling involves additional control elements, are 

 commonly employed in the modern radar receiver. Wave forms other than 

 the basic rectangular pulse forms appearing at the plate terminals of the 

 multivibrator circuit are available at various other points in the circuit and 

 are often employed in specific applications. 



One other basic form of pulse producing electronic circuits is known as 

 the ''blocking-oscillator" type: two typical examples of which are illustrated 

 in Fig. 48. Here the positive feedback of energy required to produce the 

 multivibrator characteristic is realized through the use of a single vacuum 



+ B 



\smj — 



Fig. 48. — Typical Blocking-Oscillator Circuit Forms. 



tube and a transformer feedback circuit. This form may be described as an 

 oscillatory vacuum tube circuit where the grid circuit is so arranged to be 

 driven negative after one or more cycles of operation. This results in an 

 intermittent oscillation and the production of nonsinusoidal wave forms 

 similar to those produced by the general multivibrator circuits previously 

 described. The basic advantage of the blocking oscillator circuit form is one 

 of economy of vacuum tubes and attendant power supply reduction. 



Typical Timing Wave Circuits 



The practical military equipment requirements of World War II with the 

 emphasis on compactness and low-power consumption has resulted in the 

 development of a myriad of specialized circuits which reflect the ingenuity 

 of the electronic circuit designer and the basic flexibility of the modern 

 vacuum tube. In general, however, these circuit developments are quite 

 similar operationally to the basic forms here described. 



Figure 49 illustrates a typical circuit arrangement of the sweep timing 

 I portion of a PPI indicator as employed in a naval search radar equipment. 



