Calculation of the bandpass filter with the minimum requirements for the spurs jamming implementation
This article analyzes a model of a symmetric trapezoidal bandpass filter. The main purpose of this article is to determine the maximum possible shape factor of a filter which provides the required level for filtering spurs. A comparison between the determined value and the one of the real filter allows us to define whether the real filter meets the filtering requirements.If the shape factor of the real filter is less than the calculated one, it means that the filter has a steeper AFC graph and will completely suppress the interference. The trapezoid that was chosen as the basis of the model corresponds to the bandwidth of the filter and all the frequencies that are within the trapezoid will pass through the filter and the ones that outside will be rejected. To determine the maximum shape factor of a filter for given input data it is necessary to find the coordinates of the vertices of an isosceles trapezoid located as follows: level/frequency of the frequency converter lies on the smaller base of the trapezoid and spurs lie either on the legs of the trapezoid or behind them outside the trapezoid. To solve this problem a parametric representation of the coordinates of the vertices is given. There are three different types of parameterization depending on the spurs location on the spectral chart: by the lower bandwidth, by the upper bandwidth or by the relative bandwidth.