778 BELL SYSTEM TECHNICAL JOURNAL 



tion through R% . The plate current of Vi flowing through the common 

 cathode resistor R^ results in a large effective bias applied to Vo which con- 

 tinues to maintain V2 in a cut-off condition. If a negative pulse of relatively 

 short duration is impressed upon the grid of Vi this tube will be driven to- 

 ward cut-off with an attendant increase in the plate potential of Vi . This 

 positive increase in voltage will be impressed upon the grid of V2 causing V2 

 to conduct plate current. The resulting decrease in the potential at the 

 plate of V2 further decreases the grid potential of Vi through the coupling 

 condenser C2 . This action progresses until Vi is driven beyond plate cur- 

 rent cut-off and V2 is conducting. This condition remains as long as the 

 discharge of the condenser C2 through Rs will maintain the grid of Vi at a 

 net negative potential. When the condenser C2 has discharged sufficiently 

 to allow the grid of Vi to increase above the cut-off value, Vi will again 

 conduct and the resultant action will reduce and eventually cut off the plate 

 current of V2 . The duration of the cycle of operation is here shown to be 

 dependent on the time constant of the circuit R3 d and may accordingly 

 be controlled as desired by proper selection of these elements. The return 

 of the grid of Vi to a very high positive voltage point in the circuit has a 

 definite advantage which may be considered as follows: A variation of the 

 grid voltage of V\ required to cut off the plate current will influence the time 

 duration of the cycle of operation. Here the time rate of change of the 

 grid voltage has been made extremely large by the choice of the return to the 

 high-voltage supply. Thus, an order of magnitude increase in the duration 

 stability of the circuit is achieved. 



A further modification of the trigger circuit furnishes the third general 

 type of multivibrator employed in the radar receiver field. This circuit 

 form, called the "free-running" type, has the property of presenting two 

 unstable limiting conditions and accordingly will produce sustained oscilla- 

 tions of a nonsinusoidal form. Figure 47c illustrates this circuit arrange- 

 ment. The essential circuit change over that given in Fig. 47b, is seen to be 

 the elimination of the stable equilibrium condition of Fi by the absence of a 

 positive potential on the grid of Vi . 



In the free-running type of multivibrator shown, the duration of operation 

 of a particular tube is related to the time of discharge of the coupling con- 

 denser and the grid resistance associated with the tube. If a different time 

 constant is chosen for each tube circuit, an unsymmetrical wave form, i.e. — 

 a pulse- to-no-pulse interval ratio other than one, can be produced. In gen- 

 eral, the free-running multivibrator is seldom used in this basic form because 

 of the limited repetition-rate stability of this circuit. It is customary, how- 

 ever, to trigger this free-running type of multivibrator with short-duration 

 pulses having a slightly higher repetition-rate than that determined by the 

 multivibrator circuit constants. In this manner the repetition-rate may be 



