The high noise level of IMPATT diode oscillators represents a major drawback in the application of these oscillators to modern microwave systems. Usually, two well known techniques are used to overcome this disadvantage: stabilization by means of a high Q-cavity and synchronization by means of injection of a low power low noise signal. However, both methods are effective only in a narrow frequency band. In this paper a new wide band technique for the reduction of IMPATT oscillator noise is described. It is based on the modulation of the oscillator bias current by the detected amplitude fluctuations and in principle allows for the suppression of the AM noise to any desired extent. Moreover, due to the strong correlation existing in IMPATT oscillators between AM and FM noise components, by modulation feedback simultaneous suppression of that part of the FM noise is possible, which is correlated with the AM noise.The AM and FM noise spectra of a free running microwave oscillator can be derived from the application of perturbation theory to the realistic oscillator equivalent circuit shown in fig. la. It consists of the series connection of the load impedance ZL(w), the impedance ZD(A,I) of the active element, which depends on the RF current amplitude A and on the bias current I, and the noise voltage generator e(t). jx Zwl) fig. 1 a) IMPATT active load~X dZ. it0 oscillator equivalent element dw circuit; b) represent-ZD(A) Z,W) _ -ation of the disturb-=RDA)+ LRLW). A / ed oscillator in the jXD) jXLw) AA complex plane e(t) R a) b) In the complex plane (fig. Ib) the amplitude Ao and frequency wo of the ideal non-disturbed oscillator are determined by the intersection point of the locus of the load and the impedance line of the active element, along which A increases and the d.c. current I is kept constant. Fluctuations of the amplitude and frequency by AA and Aw in non-ideal disturbed oscillators produce impedance deviations (aZD/aA)AA and (dZL/dw)Aw from the ideal operating point. These deviations are balanced by the noise impedance e(t)/AO, by which e(t) is represented in the complex plane. From this balance condition the following expressions result for the noise spectra (see ref. /1/): 2S (w0+Q) AM-Noise: SAA(Q) --.0 ee o (i) As --A04 I|aZD/aA I "sin" (e -a)
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