260 BELL SYSTEM TECHNICAL JOURNAL 



pulse on the calibrated screen of an oscilloscope. In a simple but suitable 

 peak voltmeter it is arranged to charge a condenser through a diode to the 

 peak voltage which may then be measured with a high resistance DC 

 voltmeter. 



The peak DC current, I, drawn by the magnetron is measured by passing 

 the current through a known resistance, usually one or two ohms, and deter- 

 mining the peak voltage developed across the resistance by means of a peak 

 voltmeter or calibrated oscilloscope. 



The average DC current drawn by the magnetron is the current measured 

 on a DC meter connected in one leg of the pulsing circuit, as shown in Fig. 39. 

 The pulse duration, t, as its name implies, is the length of the time during 

 which the voltage, usually measured near the top of the pulse, is maintained 

 across the magnetron. It may be determined from the pulse presentation 

 on an oscilloscope having a calibrated sweep or it may be calculated as indi- 

 cated below when other parameters are known. 



The pulse recurrence rate, pps, is the repetition frequency at which the 

 voltage pulse is applied and is determined by the frequency of the calibrated 

 primary oscillator driving the pulser or modulator circuit. 



The duty cycle, defined as the fraction of time the pulsed magnetron 

 operates, may be determined as the ratio of average to peak DC current or 

 as the product of the pulse duration and the pulse recurrence rate. 



The peak input power is the product of the peak DC voltage and the peak 

 DC current. 



The average input power is the product of the peak input power and the 

 duty cycle. 



The voltage and current pulse shapes as observed with oscilloscopes are of 

 importance in studying the spectrum and moding characteristics of the 

 magnetron under test. 



The dynamic V-I plot, or dynamic performance chart, is viewed on an 

 oscilloscope in which at any instant the vertical deflection is proportional to 

 peak DC voltage and the horizontal deflection proportional to peak DC 

 current. Three such plots are shown on Fig. 38 and their usefulness is 

 indicated in the corresponding text. 



The average output power is the average centimeter wave power delivered 

 to the useful load. The simplest and most foolproof method of measuring 

 this power is to absorb the energy in a column of water. From a determina- 

 tion of the rate of water flow and its temperature rise the power may be 

 calculated readily. The water column terminating the coaxial line or wave 

 guide of the test apparatus is made reflectionless either by tapering it or 

 preceding it by a quarter wavelength matching plate of proper dielectric 

 constant, analogous to the optical quarter wave plate. 



The peak output power may be calculated as the average output power 

 divided by the duty cycle. 



