The choice of filter type can be selected based on indicators such as the bandwidth of the filter design and the specific application. For narrowband filters with a relative bandwidth of less than 20%, the design method of narrowband filters should be used. For wideband filters with a relative bandwidth of 40% or more, the design method of wideband filters should be used; Between the middle is a medium bandwidth filter. It can be seen from the above indicators that the filter is a narrowband bandpass filter.
The Butterworth filter is designed to have the largest flat amplitude response in the passband. The advantage of the Chebyshev filter is that the out-of-band rejection is good, but there is some fluctuation in the band; the filter requirements of this paper are designed. The out-of-band near-end suppression is good (can be implemented with a Chebyshev filter or an elliptic function filter, but the LC filter is used for later analysis, and the value of the Chebyshev-type circuit element is used for the LC filter. Too small, difficult to achieve, this can be explained by software simulation), it can be seen that the elliptic function filter is more suitable.
Microstrip filters can be applied over a wide frequency range (from a few hundred MHz to tens of GHz) by using different substrate materials; coaxial filters are difficult to achieve due to their small size; waveguide filters At small signal levels, its frequency is basically 8~100 GHz; ceramic dielectric filter is bulky, shape factor and quality factor are small; LC filter is suitable for this filter frequency band, and it is easier to make and debug.
In summary, the design uses an elliptic function LC bandpass filter for design. The design indicators are as follows:
The center frequency is 450 MHz; the operating bandwidth is 20 MHz; the in-band insertion loss is "3 dB; the out-of-band rejection is -40 dB.
1 About the choice of filter order N
The filter order number N is an important parameter, and its choice directly affects the performance of the filter, especially the insertion loss and out-of-band characteristics of the filter, so the choice of N is very important.
Let's briefly explain some of the concepts and formulas that may be used.
For the terminal short-circuit filter, the filter order number N should satisfy the following relationship:
For a bandpass filter, the relative bandwidth is less than 20%, so it belongs to a narrowband filter.
It can be calculated from the above formula: N must be greater than or equal to 3 to satisfy the above approximate index.
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