FM band stop filter (notch filter)

       FM band stop filter (notch filter). Usually used for satellite receiving front-end, SDR software radio receiving front-end. Too strong FM signal may cause the LNA to be blocked, affecting the quality of the signal to be received, so an FM band stop filter is needed to filter out the local FM that is too strong. Signal.
 
1. What is an elliptical filter?
       Elliptic filter, also known as Cauer filter, is a filter with ripples in the passband and stopband. Compared with other types of filters, elliptical filters have the smallest passband and stopband fluctuations under the same order. Its fluctuations in the passband and stopband are the same.
 
2. Characteristics of several common filters:
at the same order:
1) Butterworth filter has the flattest passband and slows down the stopband; 2) Chebyshev filter passes With equal ripple, the stopband drops quickly; the error between the frequency response curve of the cut and the ideal filter is minimal. 3) The elliptical filter has equal ripples in the passband (flat or equal ripples in the stopband), and the stopband drops the fastest. 4) The passband of the Bessel filter has equal ripples, and the stopband decreases the slowest. In other words, the frequency selection characteristics of the amplitude-frequency characteristics are the worst. However, the Bessel filter has the best linear phase characteristics;
 
★Use simulation software to simulate the filter:
 
 
★Get the simulation results:
It can be seen that the filter can perform a large attenuation in the FM band well, The design purpose was achieved.
 
 
 
★Calculate impedance and draw PCB:
Here the CPWG model is used for calculation.
So please do not change the PCB copper layout and line width, as well as the board thickness (1.2mm) and the placement of components unless you know how to deal with impedance.
 
 
 
★Make the physical PCB and weld it:
 
 
 
 
★Connect the welded filter module to the vector network analyzer, conduct actual measurement, and evaluate the effect:
Here we measure the S21 parameters of the filter.
 
 
★Connect the welded filter module to the vector network analyzer, conduct actual measurement, and evaluate the effect:
Here we measure the S21 parameters of the filter, connect the input of the filter to network branch port 1, and the output connection port 2, start Measurement.
 
 
 
★By observing the S21 parameters, this filter has good effect.
Some people may ask why the insertion loss is different from the simulated one. The simulated insertion loss is 0.several db, while the measured insertion loss is several db, and the stopband frequency has changed slightly. Variety? 
Obviously, simulation and reality cannot be completely consistent. Simulation uses an idealized model for simulation, and reality is subject to the parameter errors of components, materials, plates, etc., which is a normal phenomenon.
It can be seen from the S21 parameters that the effect of this filter is already very good.
 
 
 
★Here is another measured picture of S21 parameters in the entire range of 9KHz~4GHz.
The higher the frequency, the greater the insertion loss. This is also a normal phenomenon and is caused by the components themselves.