Stepwise switched filter

The invention relates to a resonator structure and radio frequency filter in which the resonating frequency of a transmission line resonator can be switched in a stepwise manner between at least three values. The switching is implemented as follows: a regulating element including a switch that has at least three states is arranged in connection with the resonator. The three states of the switch correspond to different values of the specific impedance and, hence, the resonating frequency of the transmission line resonator. The regulating element is in accordance with a known arrangement: it may be e.g. a coupling element formed of a strip line on the surface of a low-loss substrate or ceramic, or a side circuit including a capacitive and inductive element, coupled to the resonator. In the former example the switch is open in its first state, in its second state it grounds one end of the coupling element directly and in its other states it grounds the end of the coupling element through differently dimensioned transmission lines. In the latter implementation the switch is open in its first state, in its second state it forms at the side circuit a capacitive-inductive coupling in series and in its third state it bypasses the inductive element.

FIELD OF THE INVENTION 
The present invention relates to a resonator structure and a radio 
frequency filter, which comprise a transmission line resonator, preferably 
a helix, strip line, dielectric or air-insulated resonator, and a 
regulating element by means of which the specific impedance of said 
resonator structure and, hence, the resonating frequency of the 
transmission line resonator can be changed in a stepwise manner. 
BACKGROUND OF THE INVENTION 
In radio transceivers it is generally used duplex filters based on 
transmission line resonators to prevent the transmitted signal from 
entering the receiver and the received signal from entering the 
transmitter. Each multichannel radio telephone network has a specified 
transmission and reception frequency band. Also the difference between the 
reception and transmission frequencies during connection, ie. the duplex 
interval, complies with the network specifications. The frequency 
difference between the pass band and rejected band of an ordinary bandpass 
or band rejection filter is also called a duplex interval. It is possible 
to design a filter suitable for each network. Current manufacturing 
methods enable flexible and economic production of different 
network-specific filters. The frequency adjustment methods, or the 
so-called switching methods, aim at dividing the networks into blocks, 
thereby making it possible to cover the whole frequency band by one 
smaller filter designed for one block only. The filter is always switched 
to the block in use, in other words, adjusted to the frequency range used. 
Filter switching or frequency adjustment is based on changing the specific 
impedance and, hence, the resonating frequency of transmission line 
resonators included in the filter. The specific impedance is determined by 
the dimensions of the transmission line resonator and the grounded metal 
casing surrounding it as well as by regulation couplings arranged in the 
vicinity of the resonator. In prior art it is known a method for adjusting 
the resonating frequency of a transmission line resonator by placing a 
transmission line (FIG. 1) near the transmission line resonator, thereby 
producing an electromagnetic coupling M1 between it and the transmission 
line resonator, whereby the transmission line is called a coupling 
element. The electrical characteristics of the coupling element determine 
how the resonating frequency of the resonator is changed. 
It is known to build a switched resonator, ie. one whose resonating 
frequency can be changed, by arranging, as shown in FIG. 1, a switch SW1 
near a coupling element KE1, which, when it closes, grounds one end of the 
coupling element. Then the resonating frequency of the transmission line 
resonator SR is higher than with the switch SW1 open. With one coupling 
element and a two-state switch connected to it, it is possible to change 
the resonating frequency of the resonator only from one value to another. 
This kind of system is called two-step switching. 
In some cases it is preferable that one frequency can be selected out of 
three or more alternatives for the resonating frequency. Then we are 
talking about switching in three or more steps. A conventional embodiment 
of multiple-step switching is presented in the Finnish Patent FI-88442 
(U.S. Pat. No. 5,298,873) and it is illustrated in FIG. 2. In the method, 
two or more coupling elements KE1, KE2 and corresponding switches SW1, SW2 
are placed in the vicinity of a transmission line resonator SR. The 
electromagnetic coupling between the coupling element 1 and the 
transmission line resonator is marked M1, and the coupling between the 
coupling element 2 and the transmission line resonator is marked M2. When 
all switches are open, the resonating frequency of the resonator has a 
certain value f1. When one switch is closed, the value of the resonating 
frequency becomes f2. By closing another switch the frequency is changed 
to a third value f3. The number of alternatives for the resonating 
frequency values is determined by the number of coupling elements and 
switches. 
It is a disadvantage of the conventional arrangement that each coupling 
element and switch take room in the vicinity of the resonator, whereby 
resonators and filters consisting of them cannot be built very small. Size 
is of great importance, since the filters are used in small and 
lightweight mobile phones. In addition, the more coupling elements are 
used, the more the electromagnetic coupling between the resonator and the 
coupling elements affects the resonator's Q value. In the manufacturing 
process there also occurs certain deviation in the dimensioning of 
coupling elements, which results in variation in resonator 
characteristics, which is difficult to manage. The more coupling elements 
in one resonator, the greater the effect of the process deviation. 
SUMMARY OF THE INVENTION 
In the present invention the disadvantages mentioned above have been 
avoided. This is achieved by placing in the vicinity of the transmission 
line resonator one regulating element including a switch with at least 
three states. The switch changes the electrical characteristics of the 
regulating element. The three or more states of the switch correspond to 
the various electrical characteristics of the regulating element and, 
hence, the various specific impedance values of the resonator structure 
and so the various resonating frequencies. 
It is characteristic of the invention that a regulating element is placed 
in the vicinity of the transmission line resonator, including a switch 
with at least three states which correspond to the various specific 
impedance values of the resonator structure. 
The regulating element may be any of many alternatives included in prior 
art, such as a coupling element implemented as a strip line or a side 
circuit connected to the transmission line resonator. One preferable 
embodiment is a coupling element formed in the manufacturing process 
simultaneously with other strip line circuits included in the resonator 
and/or filter structure. It is characteristic of this embodiment that by 
changing the state of the switch connected to the coupling element the 
impedance of the coupling element is changed, which, in turn, changes the 
resonator's specific impedance and, hence, the resonating frequency. 
Since, according to the invention, there are at least three coupling 
element impedance values selectable by the switch, the system can be used 
to implement switching in three or more steps by using only one coupling 
element and one switch.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Prior art couplings (FIGS. 1 and 2) were already described above, so the 
invention will be described below referring mainly to FIGS. 3 to 8. 
FIG. 3 shows a wiring diagram of an embodiment of the present invention. 
The wiring diagram includes a transmission line resonator SR and a 
coupling element KE3 placed near it, which through an electromagnetic 
coupling M3 has an effect on the resonating frequency of the resonator. A 
three-state switch SW3 is connected to the coupling element and it is 
either open, as shown, or grounds one end of the coupling element directly 
or grounds one end of the coupling element through a transmission line 
SL1. 
In the first state the switch SW3 is open and the coupling element KE3 has 
an effect on the resonator's resonating frequency through the coupling M3. 
The resonating frequency has a value f1 which depends on the dimensioning 
of the transmission line resonator and the coupling element. In the second 
state the switch SW3 grounds one end of the coupling element directly, 
whereby the specific impedance of the resonator structure changes and the 
resonating frequency will have a value f2 which is higher than f1 
according to the principle presented in the patent FI-88442 (U.S. Pat. No. 
5,298,873). In the third state the switch SW3 grounds one end of the 
coupling element through a transmission line SL1, whereby the specific 
impedance of the resonator structure again changes and the resonating 
frequency will have a value f3 which is higher than f1 but lower than f2. 
According to the principle described it is also possible to implement 
switching in more steps. Then a switch will be used that has more than 
three states. Each state corresponds to a different impedance value e.g. 
so that the switch grounds one end of the coupling element through 
transmission lines dimensioned differently. FIG. 6 is the wiring diagram 
of an embodiment in which the states of a switch SW5 correspond to the 
groundings through differently dimensioned transmission lines SL3, SL4, 
SL5. The switch SW5 is not open in any of the states, and none of its 
states corresponds to the direct grounding of an end of the coupling 
element KE4. One of the states of the switch may be an open state (FIG. 7) 
and one of the states may be a direct grounding (FIG. 8). but neither of 
these is necessary from the point of view of the invention. 
All components shown in the wiring diagrams--the transmission line 
resonator, the coupling element connected to it, the three-state switch 
and the transmission line--are known as such, and their technical 
implementation is not difficult to a person skilled in the art. The 
transmission line resonator is preferably a helix resonator formed of a 
conductor wound into a cylindrical coil or a hole plated with a conductive 
coating in a dielectric (e.g. ceramic) block. The coupling element and the 
transmission line are preferably strip lines formed on a low-loss 
substrate or on the surface of a ceramic. The three-state switch is 
preferably a PIN diode or a coupling comprising several PIN diodes. An 
embodiment implemented with strip lines is particularly preferable, 
because the strip lines can be manufactured simultaneously with other 
strip lines included in the filter structure and no other separate 
components apart from the switch diodes are needed in the coupling. 
FIG. 5 shows a printed board used in the technical implementation of the 
first embodiment according to FIG. 3. It is a printed board for a 
comb-structured helix filter, in which each vertical branch is surrounded 
by a conductor wound into a cylindrical coil, ie. a helix (not shown). The 
printed board made of a low-loss substrate serves as a supporting element 
for the filter structure, and conductors and coupling pads required by 
electrical operation are formed on its surface with conventional methods. 
The conductor GND shaped like a broad T in the upper part of the branch 
makes a galvanic coupling to the ground potential for the coupling element 
KE3. A three-port component including two PIN diodes in a common-cathode 
coupling is attached to the coupling pads KT1, KT2, and KT3 below the 
coupling element. This component acts as a three-state switch SW3 in such 
a manner that the coupling functions are implemented with DC bias voltages 
connected to the ports. When the potential of the common cathode is higher 
than that of either anode the switch is open. When the potential of the 
common cathode is lower than that of one of the anodes the switch connects 
said anode to the common cathode. 
A transmission line SL1 begins at a coupling pad marked KT2, having one end 
connected to the ground potential through a resistor attached to the 
coupling pads KT4 and KT7 and through a capacitor attached to the coupling 
pads KT5 and KT6. A corresponding grounding is arranged at the coupling 
pad KT3 without a transmission line. 
FIG. 4 shows the wiring diagram of an alternative embodiment of the present 
invention. The wiring diagram includes a transmission line resonator SR 
and a side circuit which is galvanically coupled to it and includes a 
capacitive element C1, a transmission line SL2 and, according to the 
invention, a three-state switch SW4. In this embodiment only those 
transmission line resonators may be used where it is possible to have 
galvanic couplings at two locations for a side circuit. The transmission 
line resonator SR is preferably a helix resonator and the side circuit is 
formed of strip lines and separate components on a printed board which 
serves as a supporting structure for the helix resonator. Galvanic 
couplings are formed by soldering the strip line extending to the edge of 
the support branch to the resonator conductor. 
Also in this embodiment the switch SW4 is preferably a common cathode 
coupling with two PIN diodes for which it is arranged bias voltagas, using 
strip lines on the surface of the printed board that serves as a 
supporting structure for the resonator. The switch is either open, as 
shown, or connects the capacitance C1 and the transmission line SL2 in 
series or bypasses the transmission line SL2 altogether. At lower radio 
telephone frequencies the capacitive element C1 is preferably a separate 
component, but at frequencies exceeding 1000 MHz it may also comprise 
strip lines on a printed board. 
The invention has been described above only in connection with two 
frequency changing principles, but in no way is the invention limited to 
these two embodiments, but the multi-state stepwise switching of a 
coupling element or side circuit according to the invention can be 
employed in the implementation of many known frequency changing 
principles. What is essential from the point of view of all the 
embodiments is that the regulating element used for changing the 
resonating frequency is, as mentioned above, a switch having at least 
three states and providing versatile possibilities for the use of the 
regulating element, however simple. 
The advantages of the invention compared to prior art methods are based on 
reduced need for space, among other things. The placement of one coupling 
element in the field of the transmission line resonator can easily be done 
also in the small filters required by hand phones. One coupling element 
also affects the resonator's Q value considerably less than the use of 
many coupling elements according to prior art. With the use of one 
coupling element only, the space available for the physical implementation 
of the coupling is, in the case of three-step switching, twice as big as 
in a conventional arrangement, and, in the case of switching in more 
steps, even bigger. Then the coupling can be made very stable and 
dimensioning deviation occurring in the manufacturing process will not 
result in great differences between individual filters. 
Small filters according to the invention, capable of switching in three or 
more steps, have a wide range of application e.g. in hand-held phones of 
mobile telephone systems.