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Results of investigations performed in the last years and devoted to peculiarities of signal and noisy characteristics formation in autodynes, which are widely used as the simplest transceivers in short-range radars (SRR). In the first section of the paper, the general equations for oscillator analysis, which take into consideration the inherent noises of the active element (AE) and noises of power supply (PS) under impact of emission reflected from the radar target. To obtain these general equations, the functional and equivalent circuits of the autodyne module are taken into consideration. The equivalent circuit is represented by parallel connection of the single oscillation circuit (OC), which includes the load conductance and averaged (over the oscillation period) «electronic» conductance of АE. At that, in the general case, depends on the bias DC voltage E, the amplitude A and the current frequency of high-frequency oscillations. The equivalent current generators and are connected in parallel to OC. The fist one represents the inherent noise of AE in the UHF oscillator, while the second rep[resents the impact of the proper emission reflected from the radar target, which delays on the time with respect to the current time. For the chosen model, assuming that quasi-harmonic oscillations exist on AE, and in accordance with the slowly-changing amplitude method, at first abbreviated differential equations of the perturbed oscillator are obtained. Then, to simplify an analysis, we perform the transfer to differential equations in variations for the autodyne response in the vicinity of the steady-state autonomous oscillator mode. After that, taking into account the smallness of the steady-state mode perturbation, the system of linearized differential equations is obtained for determination of the relative amplitude variations and frequency variations, as well as the relative value of the output auto-detecting signal. As a result of the equation system solution, we obtain equations for autodyne frequency variations, amplitude variations and current variations in the AE bias circuit. These variations, besides the useful signal components, contain the components caused by inherent noises of the oscillator and noises of PS. Then, we perform the numerical analysis of the autodyne noise characteristics using the sub-routine for modeling of orthogonal components of the normal random process included in the MathCAD packet. This numerical analysis shows that under conditions of strong feedback, the PS noises, as the natural oscillator noises, besides additive component of mentioned fluctuations, have additional multiplicative components. The last components cause the periodical non-stationary noise level at autodyne output at useful signal registration both on the base of frequency variations and amplitude variations. However, at signal registration in the power supply circuit of the UHF oscillator, the direct penetration of PS noises in this circuit may «mask» this mentioned noise non-stationarity. Such a behavior of noises is explained by periodic irregularity of the phase incursion of reflected emission. The derivative of the autodyne phase characteristic included in equations, for small frequency noises is, in a sense, the transfer coefficient of the frequency fluctuations, which are transformed then in phase noise owing to delay with respect to current time. These features of autodyne response formation should be taken into consideration in processing sections of the SRR signal. Using developed mathematical model of the autodyne, we perform then an analysis of internal oscillator parameters’ influence, such as its non-isochronity, non-isodromity and frequency detection, on the levels of amplitude and frequency noise, as well as the PS noise of autonomous oscillator. The account of PS noise influence in the developed autodyne model shows that these noises cause additional increase of fluctuations’ levels of frequency and amplitude of the РА oscillator as well as the mean value of DC current (voltage) of AE. Besides, formulas for calculation of the energy potential and the dynamic range of the autodyne are obtained. The energy potential is defined as the ratio of oscillator output power to a power of the minimally-detected signal obtained from the radar target at given signal/noise ratio at SRR output. If this ratio is chosen equaled to 1, then in this case, the potential is referred as limited, which can be obtained by the reverse value of the relative noise level. It is shown that the largest autodyne energy potential (of order 120...140 dB) is provided by signal registration on frequency variations; this parameter is by 30…40 dB below at signal registration on amplitude variations; at signal registration on the bias DC current (voltage) of AE, this parameter, to a great extent, is defined by PS noises penetrating in this circuit and may yield by additional 10…15 dB. Another important parameter of autodyne SRR is the dynamic range. This parameter has the specific limitation from above (concerning autodynes) in the level of reflected emission. This starts from requirement of absence of autodyne signal jumps at growth of reflected emission level and (or) at increase of distance to the radar target. The generalized parameter, which characterizes this limitation, is the external feedback parameter C fb of the autodyne, which should be less than 1. On the other hand (from below), the dynamic range depends on the autodyne inherent noise level, which defines the limited SRR energy potential as well. It is shown that to widen of the autodyne SRR dynamic range, it is necessary to increase the equivalent Q-factor of OC. For this, it is expedient to apply additional high-Q cavity in UHF oscillator. This solution promotes the parameters’ and characteristics’ improvement in the complex manner. It reduces not only the autodyne frequency deviation and, hence, the feedback parameter C fb value, but at that, the level of frequency noises decreases and the periodic non-stationarity phenomenon of the noise level in output signal is eliminated. For estimation of the dynamic range of autodyne SRR, it is supposed to introduce a concept of maximal distance to the reflected target expressed in the length of half-wavelength at exceeding of which the reflected emission with amplitude equaled to the noise level of the oscillator causes stochastic property in formation of the autodyne output signal. In he second section of the paper, the autodyne analysis task is concretized on the case of AE impedance characteristics examination with different «softness» index values. Calculations and analysis are fulfilled for oscillator output power, its autodyne and noise parameters and characteristics depending on the load conductance and oscillation amplitude. Results of such an analysis allow obtaining of required oscillation modes, for which the necessary performance characteristics of SRR are achieved. Investigation executed show that for a choice of the type and the AE operation mode for an autodyne providing the maximal value of the SRR energy potential, we should use the following recommendations. We must use the AE type and position of the operating point (in DC bias) so that it will provide the type of AE impedance characteristic, which is as close as possible to the rigid type. At that, it is necessary to settle the weak coupling mode between the oscillator and the load ensuring relatively high amplitude values in AE, when the output power is several times less than delivered by this oscillator to the load in the mode of optimal coupling. The oscillating system of the oscillator must provide the high values of natural Q -factor, while the AE must provide the minimal value of inherent noises. At that, it should be noted that the mode of the autodyne best energy potential differs from the mode of optimal coupling in power, as well as the maximal value of transfer coefficients of the autodyne response. It is shown that in the case of autodyne signal (in power) registration for the load variation from the under-coupling mode to the over-coupling mode, the autodyne signal is inverted. In the optimal coupling case, when the output power has a maximum, the autodyne sensitivity on output power variation is minimal, and the output response is caused by variations of frequency and amplitude only. Investigation results obtained in this paper, calculating relationships and plot of various functions versus the normalized oscillator load and the relative oscillation amplitude seem to us as useful for engineering calculation of parameters and characteristics, as well as for obtaining of optimal modes of autodyne UHF oscillators. At that, oscillators may be made on the base of both Gunn diodes and other AE both in the volumetric (waveguide) and in the hybrid-integrated implementation.
投稿的翻译标题Modern hybrid-integrated autodyne oscillators of microwave and mm-wave ranges and its applications. Part 10. Fundamentals of analysis and calculation of autodyne parameters taking noises into account
期刊Успехи современной радиоэлектроники
Published - 2018



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