542 



BELL SYSTEM TECHNICAL JOURNAL 



on the r.f. output. The curves represent unloaded gain; no external load- 

 ing was added to increase the bandwidth. 



The performance of the close-spaced triode when used as a modulator 

 appears to be superior in some respects to that of the silicon crystal 

 motlulators which are used in the New York-Boston microwave relay 

 system.' 



Single tubes had from 5 to 9 db gain compared with from 8 to 11 db 

 loss for the crystals for corresponding power outputs. To get this per- 

 formance the beating oscillator drive was only 200 milliwatts, compared 

 with about 700 milliwatts for the crystal modulator. This reduction in 

 r.f. power requirements means considerable simplihcation in a repeater. 



6 8 10 12 



OUTPUT POWER IN DBM 



Fig. 16. — Compression data on seven 1553 triodes. 



Plate voltage 200V 



Plate current 14 Ma 



B.O. Power 200 MW 



Matched inputs and output 



To offset this, the tube requires power suppUes which are not necessary 

 for the crystals, but low voltage power supplies should be cheap. The 

 bandwidth of the tube modulator, 60 to 80 mc is less than the very wide 

 (500 mc) band of the crystal modulator but it is comparable with the 

 band width of the e.xtra i.f. stages needed to drive the crystal modulator. 

 The life of the tubes, although very little data are available as yet, will 

 probably be less than the practically indefinite life of the silicon point 

 contact modulators. 



Amplifier 



The performance of the close-spaced triode as an amplifier can best 

 be described by referring to its impedance match, gain, transmission band- 

 width and compression. 



