Passive microwave filter tuning using bond wires

A practical method is proposed to tune microwave planar filters. When the circuit is fabricated, a number of additional capacitive patches and inductive lines are patterned close to the filter. These patches and lines can be connected to the filter by bond wires to tune the filter response. By replacing the traditionally used tuning screws with these bond wires, the package for a microwave filter can be much more compact, and the mechanical reliability of the tuned filters can be improved. To demonstrate the method, a three-pole microstrip filter is fabricated on a Duroid substrate. The additional capacitive patches and inductive lines are also patterned on the same substrate close to the filter. By connecting the patches and the lines to the filter with bond wires, the centre frequency of the three-pole filter is easily tuned by -7% and +7%, respectively, and can be tuned to an arbitrary value in a wider range. The return loss of the filter is tuned from 15 dB to better than 20 dB in the passband at the same time. It is also found that the bond wire has introduced negligible additional loss.