IF Filter
Design problem:
A filter should be designed to be placed between the IF output of a mixer and input of an IF amplifier.
The RF and LO inputs of the mixer are at 8.2 GHz and 6 GHz, respectively.
The filter should be such that it is matched at 2.2 GHz (IF) and rejects the RF and LO signals appearing at the IF output of the mixer. The filter should have minimum number of components and its overall size should also be minimum.
The DC decoupling capacitor at the input of the IF amplifier should also appear in the filter structure.
A theoretical approach to this problem is complicated because the DC block capacitor makes the filter band-pass. However, at each stage of a band-pass filter there will be two components which contradicts to the minimum number of components requirement. Therefore a direct approach is preferred. The topology of the filter is determined, proper initial conditions are defined for the components and optimization goals are defined.
A four stage topology is as in the figure. Optimization is done and the following responses are obtained.
The lumped component values of the optimization results are not of the standard values. Therefore, standard values close to the optimization results are placed and the final response is obtained.
LAYOUT
The filter layout is defined on RO3003 of Rogers Corp.
The lumped components are of 0603 package.