PULSE MODULATOR TUBES 



819 



drop in the modulator tubes necessary to allow current Ip to pass. The 



E 



plate efficiency of the modulator is then simply —■ and the power dissipated 



in the modulator tube plate is IpCp during a pulse. The average power dis- 

 sipated in the plate is then IpCp multiplied by pulse length and by pulse 

 frequency. The heat storage capability of the plate is ordinarily great 

 enough that the average power is all that needs consideration. 



The conditions imposed on the modulator tube are somewhat analogous 

 to those of a class C amplifier at low frequency. The main difference is 

 that the angle of operation is very small, and there is usually no appreciable 

 backswing of plate voltage since the load is essentially a resistance. Typical 





Ip 



I 



Fig. 1 — Current and voltage relations in a pulse modulator tube. 



modulator circuits are shown in Fig. 2. It is sometimes found desirable to 

 employ a shunt inductance across the oscillator in the interest of a sharp 

 cutoff of the pulse on the oscillator, particularly where capacitances to 

 ground of various circuit components are appreciable. This results in an 

 additional current demand on the modulator tube since the current through 

 the inductance must also be supplied. The oscillator is often coupled to 

 the modulator circuit by means of a transformer in order that desirable im- 

 pedances are realized in each circuit. 



Design Considerations 



It was apparent on first consideration of the high-vacuum modulator 

 problem that use of oxide coated cathodes would be of enormous advantage 



