Golubev V.N., Baluykov V.M., Chumikov E.E., Lankov A.Yu.

The improvement of the radio electronics element base, which is the basis of direction finder (DF) receiver, made it possible to apply new principles to the receiver construction. First of all it concerns analog-digital signal converters (ADC). The improvement of such ADC parameters as maximum signal digitization frequency, maximum input signal frequency, etc. provide wide dynamic range of linearity, stimulates the development of so-called "computerized" high frequency (HF) band receivers. In such receiver the set of signals (1,5-30 MHz) is being directly sent from antenna output to ADC input through the wideband amplifier. All the other operations such as frequency scanning, signal (set of signals) filtering and its analysis is being processed in signal processing devices using digital methods. Thus high frequency tuning accuracy (does not exceed values of units of Hertz), the opportunity of automatic adaptation to the level of input signals, its filtration and high selectivity, produced by signal processing methods, is provided. The purpose of the research is to analyze the image channel (IC) in the linear path of the receiver constructed using direct digital transform and designed to form bearing characteristics in multi channel systems and to determine the functionality factors of such receiver in the real interference situation. The simplified skeleton diagram of the receiver with ADC at its input is shown in figure 1. The input oscillation is being filtered in bandpass filter and afterwards sent to the signal processing device. In some cases the analog amplifier may be installed between the bandpass filter and ADC to produce the necessary sensitivity of the receiver. The presence of IC of mFIC + nFh = Fi type in superheterodyne receiver (where FIC - IC frequency, Fh - heterodyne frequency, m and n - positive and negative integer numbers, Fi - intermediate frequency) is the result of the receiver unlinearity. The IC characteristics for the case of ADC at the input are listed in table 1. There are 8 such ICs, including 2 second-number channels, 2 third-number channels, and 4 fourth-number channels The most dangerous frequencies of IC are: Fs/2; Fs/3; Fs/4 (where Fs - signal frequency). If any filtration in the analog path is missing, the interferences influence the receiving of the desired signal in all ICs simultaneously, tuning any frequency of the frequency range. The most interesting case is using ADC at the input of the receiving path in short-wave band without desired signal filtering at the input of the receiver in HF band.