36 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



To improve the resolution, work was initiated some time ago by S. E. 

 Miller to obtain measuring equipment which would operate with much 

 shorter pulses. As a result, pulses about 5 or 6 millimicroseconds long 

 became available at a frequency of 9,000 mc. In a pulse of this length 

 there are less than 100 cycles of radio frequency energy, and the signal 

 occupies less than ten feet of path length in the transmission medium. 

 The RF bandwidth required is about 500 mc. In order to obtain such 

 bandwidths, traveling wave tubes were developed by J. R. Pierce and 

 members of the Electronics Research Department of the Laboratories. 

 The completed amplifiers were designed by W. W. Mumford. N. J. 

 Pierce, R. W. Dawson and J. W. Bell assisted in the design and construc- 

 tion phases, and G. D. Mandeville has been closely associated in all of 

 this work. 



2. PULSE GENERATION 



These millimicrosecond pulses have been produced by two different 

 types of generators. In the first equipment, a regenerative pulse gener- 

 ator of the type suggested by C. C. Cutler of the Laboratories was used.^ 

 This was a very useful device, although somewhat complicated and hard 

 to keep in adjustment. A brief description of it will permit comparisons 

 with a simpler generator which was developed a little later. 



A block diagram of the regenerative pulse generator is shown in Fig. 1. 

 The fundamental part of the system is the feedback loop drawn with 

 heavy lines in the lower central part of the figure. This includes a travel- 

 ing wave amplifier, a waveguide delay line about sixty feet long, a crystal 

 expander, a band-pass filter, and an attenuator. This combination forms 

 an oscillator which produces very short pulses of microwave energy. 

 Between pulses, the expander makes the feedback loop loss too high for 

 oscillation. Each time the pulse circulates around the loop it tends to 

 shorten, due to the greater amplification of its narrower upper part 

 caused by the expander action, until it uses the entire available band 

 width. A 500-mc gaussian band-pass filter is used in the feedback loop,^ 

 of this generator to determine the final bandwidth. An automatic gain 

 control operates with the expander to limit the pulse amplitude, thus 

 preventing amplifier compression from reducing the available expansion. 



To get enough separation between outgoing pulses for reflected pulse 

 measurements with waveguides, the repetition rate would need to be 

 too low for a practical delay fine length in the loop. Therefore a r2.8-mc 

 fundamental rate was chosen, and a gated traveling wave {\\\)v ampfifier 

 was used to reduce it to a 100-kc rate at the output. This amplifier is 

 kept in a cutoff condition for 127 pulses, and then a gate pulse restores 



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