602 BELL SYSTEM TECHNICAL JOURNAL 



5) Ptdse repetition rate — A range of 160 to 2100 pulses per second has 

 been covered by coded tubes. Experimental tubes with very short 

 gaps have been tested up to 10,000 pulses per second. However, 

 the design of tubes for practical operation in this region would entail 

 considerable effort. 



6) Operating voltage range — ^Although a given set of tubes may exhibit a 

 wide range of operating voltage on a laboratory test, the rated range 

 must be considerably less because of manufacturing variations and 

 changes during life. However, since most radar modulators operate 

 at a fixed power level, this limitation is not a serious one. 



7) Trigger requirements — The spark gap tubes require a high-voltage 

 low-current trigger supply. While this is more difficult to obtain 

 than the low- voltage supplies required by some other modulators, it 

 caused no real difficulty in practice. 



3) Time jitter — Although the time jitter of coded tubes is of the order of 

 one microsecond, experimental tubes have been made which have, 

 at the operating voltage, a jitter of the order of one hundredth of a 

 microsecond. 



9) Efficiency — The switching efficiency for all of the past applications of 

 fixed gaps has been in the range of 90 to 96 percent, which makes the 

 fixed gap one of the most efficient switching devices for radar. 



10) Simplicity of manufacture — Since the unit type of fixed gap has only 

 two elements of simple geometry, its manufacture is relatively easy. 



11) Dependability — The dependabihty of the fixed gap has been demon- 

 strated by its satisfactory performance in its extensive application. 



Acknowledgments 



The authors wish to acknowledge the valuable help given by many who 

 cooperated in this project. In particular, they would like to thank Dr. S. B. 

 Ingram for his criticisms and suggestions in regard to the development of 

 coded sealed gaps. Also, we are especially indebted to Messrs. H. W. 

 Weinhart and C Depew for their invaluable help in the design and construc- 

 tion of the large variety of gaps required for this development. 



