34 BELL SYSTEM TECHNICAL JOURNAL 



per cycle) ; the positive pulses being used for keying the transmitter. These 

 pulses are rich in odd harmonics of the base oscillator frequency. By 

 rectifying these pulses to reverse the negative pulses, even harmonics of 

 the base frequency are obtained and the 18th harmonic (29.5 kc) is se- 

 lected by means of a filter. This harmonic frequency and the original 

 base frequency are applied to two phase shifters whose shafts are geared 

 together in the ratio of 18 to 1. Since one revolution of the one speed 

 phase shifter corresponds to 100,000 yards, one revolution of the 18-speed 

 unit corresponds to only 5550 yards with the result that range errors caused 

 by non-linearity of this phase shifter are reduced by a factor of 18. The 

 phase shifters employed are similar to those designed by Bell Telephone 

 Laboratories for use in a phase measuring bridge^ and are linear to within 

 ±1.5 degrees or about 0.4 per cent. The possible range error introduced 

 by imperfections in the 18-speed phase shifter was therefore only 23 yards, 

 well within the design requirements. It remains to be shown how this 

 accurate range information was applied to the indicator. 



The output of the 18-speed phase shifter in the range unit is connected 

 to the Control and Indicator where the phase shifted sine wave is used to 

 generate short, rectangular pulses of about 600 yards duration. One pulse 

 is produced for each cycle of the 29.5 kc wave so that 18 of them occur during 

 the 100,000-yard sweep interval. It is desired that only one of these pulses 

 appear as a range notch on the indicator screen and this pulse is selected 

 from the others by a pedestal pulse generated from the output of the one 

 speed phase shifter. It will be noted that as the phase shifters are rotated 

 by means of the range unit hand crank, the desired pulse from the 18-speed 

 phase shifter will remain substantially centered on the one-speed pedestal 

 pulse. After further shaping, the selected pulse is mi.\ed with the received 

 signals in the second video amplifier and is then appHed to the vertical 

 plates of the cathode ray indicator to form the "range notch". The range 

 notch is also transmitted to the Train Indicator and Train Meter where it 

 is used to prevent any signal from affecting those instruments except the 

 one being tracked by the range operator. 



Since it is desired to have the range notch appear in the center of the 

 100,000-yard sweep on the indicator, the sweep trigger pulse must occur 

 50,000 yards in advance of the notch. This trigger is obtained by se- 

 lection of another pulse from the accurate phase shifter, this time using a 

 one-speed pedestal produced by an input of reversed phase. The pulse 

 thus selected is used as a trigger for starting a saw-tooth sweep wave with a 

 duration corresponding to 100,000 yards radar range. Expansion of the 

 center portion of this sweep is obtained by adding to this wave a second 



" L. A. Meacham, U. S. Patent 2004613. 



