INTERMITTENT BEHAVIOR IN OSCILLATORS 3 



from zero to infinity. Similarly the filter is assumed to consist of linear 

 circuit elements and to have a definite curve of loss versus frequency. Asso- 

 ciated with this loss characteristic is some specific phase characteristic* 

 The limiter is assumed to have a loss which is independent of frequency but 

 which is explicitly related to the input (or output) voltage. 



Although amplifiers having the ideal performance indicated are not physi- 

 cally realizable there are no new or unfamiliar concepts involved. Similarly 

 the performance of passive networks, such as constitute the filter, has been 

 extensively studied and is well understood. It is therefore appropriate to 

 devote the following section to the third function. 



LIMITER 



Fig. 1 — Functional block diagram of an oscillator. 



III. Types of Limiters 



The limiters which are now in common use may be separated into four 

 relatively distinct groups. 



1. \'acuum tubes in which the gain is decreased by simple overload as the 

 level of oscillation rises. This is the most common form of limiter. 



2. \'aristors in which the impedance depends upon the instantaneous value 

 of current. Copper oxide, thy rite, and electronic diodes are examples. 



3. Thermistors in which the resistance depends upon the rms value of 

 current but does not vary appreciably during any one cycle. Carbon and 

 tungsten filament lamps are the most common examples. 



4. Vacuum tubes in which the gain is reduced by application of a bias 

 which depends upon the level of oscillation. Usually the bias is developed 

 by rectifying a portion of the output. 



The limiters of the first two groups depend for their operation upon the 

 generation of harmonic voltages and currents. The limiters of the second 



^ H. W. Bode, "Relations Between Attenuation and Phase in Feedback Amplifier De- 

 sign," Bell Sys. Tech. Jour., Vol. 19, pp. 421-457, July 1940. 



