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Assessing the impact of hardware distortions on the formation of radar image

Keywords:

A. N. Kharlamov – Ph.D. (Eng.), Associate Professor, Moscow Aviation Institute (National Research University). E-mail: akharlam@mail.ru
V. M. Nuzhdin – Ph.D. (Eng.), Associate Professor, Moscow Aviation Institute (National Research University). E-mail: nio407@mai.ru
V. V. Egorov – Post-graduate Student, Moscow Aviation Institute (National Research University). E-mail: vsem_dobra@bk.ru


For obtaining highly informative radar images (RI) probing with short pulses and signals with the internal modulation, is used usually angular (LFM, FCM). However, during the compression of these signals the side lobes of the signaling function appears. Reducing the side lobes level (SLL) is a major challenge in the formation of radar images with a large dynamic range and in the selection of low brightness objects. However, there is a difficulty related with the fact that SLL directly depends not only on the structure of the probe signal, but also on the level of hardware noise and hardware system distortion. Correction of linear distortion, and nonlinear distortion (ND) of transmission path is not particularly difficult due to the deterministic nature of the signals. The task of correcting ND of the reception path, due to the random nature of the reflected signals, is much more complicated. Therefore, when designing analog portion of the receiver an efforts are made to provide the best possible linearity of the amplitude characteristics of its nodes. Obviously, however, that excessive requirements for very low ND lead to technical difficulties and high costs of the product. Target of this work – the development of measurement techniques and evaluation of the distortion level of receiver. In this paper we investigate the effect on signaling function (SLL) hardware distortions in analog quadrature formation. ND products fall within bandwidth and can not be filtered before or after analog-to-digital conversion. Quadrature forming unit can generally be represented as a nonlinear inertial device model. To construct an analytical non-linear inertial model that would describe with sufficient accuracy the process of the quadrature signals forming is a very difficult task. The proposed technique allows to work for the assess on the basis of the measurements made on a fairly common laboratory equipment. It consists of that the input device is supplied by a harmonic oscillation frequency, which is detuned from the carrier frequency by an amount of ΔF. The range of responses to such impact is a set of harmonic frequencies, that are multiples of the ΔF value. The amplitudes of the harmonics do determine the level of distortion and depend on the degree of imbalance and non-linearity of the quadrature channels amplitude characteristics. The studies conducted in the mathematical calculations allowed to establish the connection between the level of parasite harmonics and SLL signaling function obtained at the existing level of ND in the system. Results acquired in the work allows, basing on the requirements to radar system parameters, to determine the need for adjustments or hardware upgrades to reduce ND and evaluate the limitations of the dynamic range of radar data and the possibility of its expansion by increasing the base of signal (increasing the probe pulse duration). Fields tests and experiments carried out in the work, have confirmed the adequacy of the resulting estimations.
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June 24, 2020
May 29, 2020

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