The present invention relates to a resonator device including a transmission line resonator and a coupling element for controlling the frequency response of the resonator device. The present invention has application in radio frequency filters.
In radio transceivers duplex filters based on transmission line resonators are generally used to prevent access of a signal to be transmitted to the receiver and that of the received signal to the transmitter. Each multi-channel radio phone network has a transmission and reception frequency band specified for it. The difference of the reception frequency and the transmission frequency during connection, the duplex interval, is also consistent with the network specification. Hence, for each network such a duplex filter should be designed which is just appropriate for it. It is not, however, economical to design a variety of different duplex filters for different radio phone networks, but the stop bands and pass bands of the filter are made, as far as possible, adjustable to some extent, whereby such filters are also suitable for use with greater or smaller bandwidths than those serving as basis for the original design. Mostly, there is no great need to adjust the stop bands or passbands, and any desired new bandwidth is thus achieved simply by increasing or decreasing the coupling between the resonator circuits in the filter. The number of resonators may then be left unchanged.
A helical coil resonator is a transmission line resonator which is widely used in high frequency range filters. A quarter-wave resonator comprises inductive elements, which are a wire wound to form a cylindrical coil, one end thereof being short-circuited, and a conductive shell encircling the coil. The conductive shell is connected to the low impedance, short-circuited end of the coil. The capacitive element of the resonator is formed between the open end of the coil and the conductive shell around the coil. A coupling to the resonator can be made either capacitively at the upper end of the resonator coil in which the electric field is strong, or inductively at the lower end of the coil in which the magnetic field is strong, or a coupling aperture may be used. The last mentioned system is used between two resonators. An inductive coupling is provided when a wire to be connected is terminated with a coupling link placed in a strong magnetic field in a resonator. The coupling is more effective the larger the coupling link and the stronger the magnetic field of the resonator acting in the coupling link.
A coupling to a resonator may also be made by connecting a wire to be coupled directly to a resonator coil, most often to the first turn thereof. This method is called tapping. The tapping point determines the input impedance detected by the wire to be coupled in the direction of the resonator, and it can be defined either by testing or by calculation. A drawback in a coupling made by tapping is that, because of the fixed direct contest the input impedance and thus, the strength of the coupling, cannot be controlled at all.
An adjustable inductive coupling can, as is well known in the art, be implemented using a so-called wire link, referentially depicted in FIGS. 1A and 1B. FIG. 1A shows a resonator in top view, and FIG. 1B is a side view. Reference numeral 1 in the figures refers to a helical coil provided with a straight leg part 2 inserted in a hole made in a circuit board 3 and soldered to the metallized cover of the board surface, and becoming grounded thereby. The metallization is shown with one solid line. Only a few lowermost turns of the coil are shown. The wire link 4, shown in top view in FIG. 1A, is a bent piece of wire, the ends 5, 6 thereof being bent towards the circuit board 3, and at both ends it is inserted into the holes borred in the circuit board 3, FIG. 1B. In wave soldering, one end 5 is soldered to the metallized surface of the opposite side of the circuit board viewed from the resonator 1 and is grounded thereby. The other end is soldered to the wire strip 8 on the surface of the circuit board 3 facing the resonator, by which the radio frequency signal is conducted to the wire link 4. The self-inductance of the wire link 4 forms an inductive element by which a resonance is made via the electromagnetic field to the resonator 1. The self-inductance is determined by the thickness and length of the wire. The wire link 4 is located in the immediate vicinity of a first turn of the resonator coil 1 located on the same circuit board 3, FIG. 1A, and in The direction therewith, FIG. 1B. The nodes 7 and 9 on the ends of the wire link 4 keep it in the right position during the wave soldering, thus preventing the wire from sliding too far through the circuit board 3. The mutual inductance between the wire link 4 and the resonator 1, and hence The coupling, is adjusted by pressing the link towards the circuit board, or off therefrom, in the direction of arrow A, FIG. 1B.
This prior art approach is encumbered with certain drawbacks. Depending on the position and size of the resonator coil, a number of wire link designs of different thicknesses and shapes are needed in order to implement a desired coupling and adjustment. Adjusting the position of a wire link attached to a circuit board in the tuning phase of a filter is difficult because, firstly, the wire may be thick and therefore rigid, and secondly, when bending a wire, the foil of the circuit board may easily break. If no holes are used and the wire link is soldered to the wire pads on the surface of the circuit board 3, the wire pad foil can be torn off from the surface. In most instances, it is not desirable to have any projecting parts on the outer surface of The filter, or as in the present instance, on the outer surface of the circuit board.