576 



ANTENNAS AND RF COMPONENTS 



in a rearrangement of atoms among the various energy levels. In this 

 manner, it is possible to invert the populations of pairs of energy levels 

 connected by microwave transitions. The feasibility of such a device has 

 not yet been demonstrated experimentally. 



Fig. 10-46 Single Resonant Circuit with 

 Variable Reactance. 



MAVARS (Modulating Amplification Using Variable React- 

 ance). In a MAVAR amplifier, three resonant circuits are coupled 

 through a common nonlinear reactive element, so that for certain choices of 

 resonant frequencies for the circuits, power is pumped from a generator 

 in the highest frequency circuit into the lower frequency circuits and 



amplification of a low power signal 

 at one of these lower frequencies 

 results. Because they depend for 

 their operation upon the variation of 

 a circuit parameter, they are often 

 called parametric amplifiers. The 

 principle of operation can be il- 

 lustrated with the single resonant 

 circuit shown in Fig. 10-46.^^ Sup- 

 pose that this circuit is oscillating at 

 its resonant frequency /s. It is possible to introduce additional energy into 

 this circuit and thus amplify the oscillating signal by separating the ca- 

 pacitor's plates when it is charged and pushing them together again when 

 the voltage across the capacitor is zero. This pumping action is analogous 

 to the manner in which a child pumps a swing. The frequency at which the 

 capacitor is pumped must be exactly twice the resonant frequency of the 

 original circuit/s, since the capacitor value is increased by pushing together 



its plates every time the voltage 

 across the variable capacitor is zero, 

 or twice each cycle. The periodic 

 variation of the capacitor would not 

 be obtained in practice by physically 

 moving the plates but rather by 

 applying a large voltage of the 

 proper frequency to a nonlinear 

 capacitor whose incremental capaci- 

 tance varied with applied voltage. 

 An external resonant circuit which 

 would supply this pumping fre- 

 quency is indicated with dotted lines 

 in Fig. 10-47. 



69Neither this circuit nor the one in Fig. 10-47 is an exact representation of an equivalent 

 circuit of practical parametric devices; these two are used to illustrate the principles involved. 



Fig. 10-47 Equivalent Electrical Circuit 



of Parametric Amplifier with Nonlinear 



Capacitance Element. 



