REFLEX OSCILLATORS 583 



fa ■ ^^ e might equally well have illustrated this by an example in which 

 the tuner was cooling. In order to avoid overshoot it is necessary that no 

 thermal impedance exist between the heat source and the expanding element. 

 Thus, as an example of a tuner in which overshoot will occur, one may cite 

 an expanding strut in the form of a tube heated by a resistance heater in- 

 ternal to the tube. In order to transfer heat from the resistance heater to 

 the tubing the former must of necessity operate at the higher temperature. 

 Hence, in the example given above, when the power is switched to the sus- 

 taining value heat will continue to be transferred to the tubing until the 

 heater and the tubing arrive at the same temperature. To minimize the 

 thermal impedance the heat should be generated within the body of, or on 

 the surface of, the expanding element. The resistance heating by current 

 passing through the expanding element illustrates the first case and heating 

 by electron bombardment the second. 



The second principle is quite obvious when once stated. If a rapid shift 

 infrequency is to be obtained at any point within the required tuning range, 

 then the potential tuning range must be considerably in excess of that re- 

 quired. Thus, if the tube is operating near one of the required frequency 

 limits and one demands that it go to the limit, the shift will progress very 

 slowly in the absence of excess range. If, however, it is possible to overdrive 

 the limit, the time required will be materially shortened. On the basis of 

 actual tube design this requires that the safe maximum or full on power into 

 the tuner must be considerably in excess of the power required to hold the 

 tuner at the frequency band limit nearest the full on condition. The tuning 

 mechanism must be capable of continuous operation under the full on condi- 

 tion in case this accidentally persists. Further, the power required to hold 

 the tuner at the other end of its band must be considerably in excess of zero 

 in order that rapid cooling may occur near this limit. It is not necessary 

 that the tuner produce motion for power inputs outside the band limits; 

 the essential condition is that the rates of heating and cooHng near these 

 limits should have values considerably greater than zero. 



It is always possible to purchase heating speed by the expenditure of 

 power in available over-drive. The cooling speed, on the other hand, is con- 

 trolled by the temperature difference between the source and sink, the heat 

 capacity of the tuner and the mechanism of cooling. Two methods of cool- 

 ing are available, conduction and radiation. For small amounts of motion 

 and in circumstances where large forces are not required, conduction cooling 

 can provide a satisfactory answer. In cases in which a large amount of 

 motion is required, as in the 2K45, conduction coohng imposes a number 

 of serious restrictions. The expanding element must be made from a ma- 

 terial having a large coefhcient of expansion and necessarily must be long. 

 Unfortunately, alloys having large expansion coefficients are very poor con- 



