REFLEX OSCILLATORS 545 



time for various electrons, (see (1) above) and aside from narrowing the 

 electronic tuning range, which may be inadmissable, the only way to reduce 

 the noise is to decrease the cathode current. This is usually inadmissable. 

 Thus, it appears that nothing much can be done about the noise in reflex 

 oscillators without sacrificing electronic tuning range. 



The seriousness of beating oscillator noise frcm a given tube depends, of 

 course, on the noise figure of the receiver without beating oscillator noise 

 and on the intermediate frequency. Usually, beating oscillator ncise is 

 worse at higher frequencies, partly because higher frequency oscillators have 

 greater electronic tuning (see (3) above). At a wavelength of around 

 1.25 cm, with a 60 mc I.F. amplifier, the beating oscillator ncise may be 

 sufficient so that were there no other noise at all the noise figure cf the 

 receiver would be around 12 db. 



Beating oscillator noise may be eliminated by use of a sharply tuned filter 

 between the beating oscillator and the crystal. This precludes use of elec- 

 tronic tuning. Beating oscillator noise may also be eliminated by use of a 

 balanced mixer in which, for example, the signal is fed to two crystals in the 

 same phase and the beating oscillator in opposite phases. If the LF. output 

 is derived so that the signal components from the two crystals add, the 

 output due to beating oscillator noise at signal frequencies will cancel out. 

 There is an increasing tendency for a number of reasons to use balanced 

 mixers and thus beating oscillator noise has become of less concern. 



XII. Build-up of Oscillation 



In certain applications, reflex oscillators are pulsed. In many of these 



; it is required that the r-f output appear quickly after the application cf 



' d-c power, and that the time of build-up be as nearly the same as possible 



: for successive applications of power. In this connection it is important to 



study the mechanism of the build-up of oscillations. 



In connection with build-up of oscillations, it is convenient to use complex 

 frequencies. Impedances and admittances at complex frequencies are 

 given by the same functions of frequency as those at real frequencies. 

 Suppose, for instance, the radian frequency is 



oj = ic — ja (12.1) 



This means the oscillations are increasing in amplitude. The admittance 

 !of a conductance G at this frequency is 



y = G 



The admittance of a capacitance C and the impedance of an inductance L are 



V = jo:C = juC + aC (12.2) 



Z = jcoL = jivL + aL (12.3) 



