As is well known to persons skilled in the art, filters having the desired properties can be realised by the appropriate interconnection of a number of resonators. The resonators are in the form of a transmission line resonator corresponding to the parallel connection of an inductance and a capacitance. It is also well known in the art in high frequency technology to use different types of resonators for different applications according to the conditions and the desired properties. Known resonator types include dielectric, helical, strip line and air-insulated rod resonators each having a relevant range of uses. For example, dielectric resonators and filters constructed therefrom are commonly used in high frequency technology and are useful in a number of applications because of their small size and weight, stability and power resistance. For instance, a dielectric filter, for use in a duplex filter, can be constructed from separate ceramic blocks or from one block provided with a number of resonators in which the coupling therebetween is accomplished electromagnetically within the ceramic material. A dielectric stop filter is usually composed of separate blocks, with coupling between the resonators via the dielectric material being prevented completely. A filter described above and used in the first end of the duplex filter may equally be constructed from helical, strip line or coaxial resonators. All of these are filter designs well known to a person skilled in the art, and therefore, they are not described herein any further detail.
Generally speaking a filter is an electrical circuit which passes certain frequencies and stops (or attenuates) other frequencies. For instance in telecommunications technology use filters which pass a desired range of frequencies while attenuating other frequencies--known as a bandpass filter and filters which attenuate a desired range of frequencies while passing other frequencies--known as a bandstop filter are commonly used.
It is known to persons skilled in the art to have a coupling between the resonators which is purely inductive or capacitive or a combination of these. The inductive coupling is generally made closer to the grounded (bottom) end of the resonator where the current is higher, whereas the current is substanstially zero at the open circuit (top) end of the resonator, where the impedance is high, and thus the coupling between the resonators is capacitive. It is known to a person skilled in the art to realize the coupling to, or between, the resonators either purely inductively or capacitively, or, in different ways, as a combination of these.
In radio transceiver systems e.g. in radio telephone systems the receive band is often at higher frequencies than the transmit band, and usually two bandpass filters are used as the filters in the receive and transmit sections of the transceiver. On the other hand, as the filter in the radiotransceivers transmitter section, it is also possible to use a bandstop filter instead of a bandpass filter, in which the resonators act as absorbing circuits at the resonance frequencies and pass lower frequencies and act as a low-pass filter. In the radiotransceiver's receive section receive it is possible to use a bandpass filter, in which the resonance frequencies of the resonators are located in the receive band, whereby they attenuate other frequencies, i.e. the filter acts as a bandpass filter. Usually the filters in the transmit and receive branch are different blocks, but they may be combined or can be a part of the same component block.
According to the present invention, there is provided a filter characterised in that, in a first mode, the filter is operable to attenuate the range of radio frequency signals, and, in a second mode, the filter is operable to pass the range of radio frequency signals. This has the advantage of having one filter that can be either a bandstop or bandpass filter, the filter type being selectable, in situ.
Such filters can be used, for example, in the transmit and receive branch of a radiotransceivers duplex filter, e.g. as a bandpass filter in the receive section which for the transmitter branch filter is changed into a bandstop filter. Thus the filters in the transmitter and receive section of a duplex filter can be manufactured more economically by making them with the same basic structure, whereby the size of the production batch increases, thus providing lower production costs.
A known bandstop filter is illustrated in FIG. 1 and comprises two resonaters RES1, RES2. A transmission line TL1, TL2 is galvanically coupled at a suitable point A, B to each resonator RES1 and RES2. Each coupling point A,B will determine the impedance level of each respective resonator RES1, RES2, and by suitably selecting this coupling point the resonator can be matched to the rest of the circuit. This matching, in which the coupling point forms a tap to the resonator, is called tapping and the coupling point A, B is called the tapping point. When helical resonators are used they are accordingly matched by tapping, whereby the connecting conductor is e.g. soldered to a certain point of the coil of the helical resonator, usually to the first turn of the coil. The resonators RES1, RES2 form a filter when the resonators are mutually coupled. The coupling can be made either capacitively or inductively or as a combination of these, depending on the desired filter. When the resonators have a reactive i.e. inductive coupling using coil or transmission line L, a bandstop filter is obtained which in this case is a low-pass filter. Then this reactive coupling is realized by a physical component L. This low-pass filter shown in FIG. 1 has transmission zeros at the resonance frequencies of the resonators RES1, RES2, so that the filter attenuates a signal at these resonance frequencies. To obtain a high-pass filter the transmission lines TL1, TL2 are replaced by capacitances. The filter input IN and the output OUT are obtained at the other end E, F of the transmission lines coupled to the. resonators.
When the resonators of the filter according to FIG. 1 are also coupled capacitively to each other, so that the capacitance substantially cancels the inductance, then we obtain a filter of the bandpass type, which acts as a bandpass filter at the stopband frequency of the bandstop filter. Further, when the coupling between the resonators RES1, RES2 is adjusted, we can shift the passband of the bandpass filter for instance so, that when the bandstop filter provided in the transmit branch is altered to be the bandpass filter provided in the receive branch, its passband is at slightly higher frequencies than the stopband of the bandstop filter.
The bandstop filter shown in FIG. 1 can be altered into a bandpass filter by having the inductive coupling L between the resonators RES1, RES2 and also a capacitive coupling C, preferably at the high impedance i.e. open-circuit, end of the resonators, as is shown in FIG. 2. When the resonators RES1, RES2 have inductive and capacitive couplings, the filter type i.e. either bandstop or bandpass is determined by the ratio of the capacitive and the inductive coupling, which provides a bandstop filter when the inductive coupling is dominant and a bandpass filter when the capacitive coupling is dominant. In the case of FIG. 2 a bandpass filter is obtained, in which the resonance frequencies of the resonators RES1, RES2 determine the frequency of the passband. When the capacitive coupling is strong, the capacitance cancels the inductance and a signal passes mainly through the capacitive coupling at passband frequencies of the filter, whereas the resonators RES1, RES2 appear as high impedances at the stopband frequencies thus attenuating the signal at these stopband frequencies. When we further provide the filter with adjusting components, known to a person skilled in the art, for shifting the resonance frequency of the resonators, we obtain a bandpass filter in which the passband is at slightly different frequencies than the stopband of the bandstop filter used as the basic component, i.e. the filter before being changed to a bandpass filter.