Band-pass electronic filter

passiveRF

Components for telecommunication systems in the range of 320 MHz up to 2500 MHz Application and description In telecommunication system for example in cordless telephones and mobile transceivers there is a need of high selectivity RF filter circuits. We have developed a series of helix filters providing an improved performance in such applications. At very high frequencies conventional resonant circuits are not as good as, for instance, coaxial line circuits, cavities or Lecher lines. Mechanical dimensions of a coaxial line circuit can be reduced by winding the inner conductor in the shape of a helix. The stretched length of such helix inner conductor is approximately equal to the quarter wave length. The adjustment is carried out by moving a metal core adjuster; when at the top end of the helix, it causes an increase in capacitance; when in the middle of the helix, it causes a reduction in inductance. In the case of capacitive tuning the metallic adjuster screw is connected to the screening can. The bottom end of the helix carries maximum current, so this is the best area for inductive output or input. For these purposes, either a winding tap is used or the helix winding can be continued by a path on the printed circuit board, earthed at its end. The printed path has a certain inductance and its addition increases the inductance of the helix, i. e. lowers its frequency. The joint of helix and printed path can be used to create a transformer or for matching the impedance of other elements in the system.