WO2011150952A1

WO2011150952A1 - Tuning system of resonator filters

Info

Publication number: WO2011150952A1
Application number: PCT/EP2010/057510
Authority: WO
Inventors: Yngve Eriksen
Original assignee: Prism Microwave Oy
Priority date: 2010-05-31
Filing date: 2010-05-31
Publication date: 2011-12-08

Abstract

A tuning system of a radio-frequency resonator filter (210). The tuning is based on the analysis of the spectrum of at least the output signal (E out) of the filter. Snatches are taken typically from both input and output signals, the spectra of which are shifted to low frequencies and the signals (LFS) provided in this way are converted to digital ones. The analysis is done in the digital process in a signal processor (230), and the distortion caused by the filter is deduced from the shape of the spectrum corresponding to the output signal, in respect of the spectrum corresponding to the input signal, the tuning elements of the filter are then adjusted so that distortion is minimized.

Description

Tuning system of resonator filters

The invention relates to a method for tuning of a radio-frequency resonator filter and an arrangement which functions in accordance with that method.

Filters are generally used in radio devices in order to attenuate undesired fre- quency components and noise. Need for such a function rises e.g. in conjunction with the shift of the signal spectrum in the modulators and demodulators. A filter is required also between the power amplifier and antenna of a radio transmitter, by which filter the frequency components outside the transmitting band of the signal to be transferred are prevented from radiating into the environment. The passband of a filter should naturally cover the frequency range, in which the spectrum of the signal to be transferred is located, and only that range. If the passband is displaced from its optimum location, the shape of the signal spectrum changes, which means a distortion of the signal. At the manufacturing stage of a filter, a basic tuning is made for it, in which the passband is set at the right place and in addition the shape of the response curve is set in accordance with the specifications. However, during the use the tuning may change a little because of the influence of the ambient, for which reason it must be possible to check the tuning and rectify it when needed. This is laborious in practice and involves costs.

In this description, the filter being object of the tuning consists of at least one cav- ity resonator. An advantage of this kind of filters is relatively low losses, in which case the attenuation caused by them is low. In a multi-resonator filter the electromagnetic field of a signal is led to one resonator and output from another resonator. An electromagnetic coupling exists on the transmission path between successive resonators. Each resonator has a natural frequency, which usually differs from the natural frequencies of the other resonators. When tuning the filter, the natural frequencies of the resonators are changed by changing their electric size mechanically. There is a plurality of different adjusting structures for this purpose, beginning from the conventional tuning screws in the lid of the resonator cavities. There are also mechanisms by which the natural frequency of all resonators can be changed at the same time and the same amount, in which case the passband of the filter shifts, retaining its shape. In a simple method for tuning a resonator filter the level of the output signal of the filter is measured and the passband of the filter is shifted until the level of the output signal is at its maximum. A flaw of the method is that the tuning can remain inaccurate, because the signal level alone is not always a sufficient indicator of the optimum setting. In addition, the manual tuning means a slow tuning process.

Fig. 1 shows a solution for tuning a resonator filter, known from the publication US 2007/0133443. The filter 1 10 is fed by an analyzer to which also the filter response is led. On grounds of the response the analyzer calculates the values of the scattering parameters Sn , S21 , S12 and S22 for the filter. These measuring values S_m\ are compared with the reference values S_rj generated by the reference unit REF, which values correspond to the ideal filter. The comparison takes place between the MSE (mean square error) values calculated from the phase factor of said pa- rameters. On grounds of the differences the control unit 130 adjusts the tuning element of a resonator either inwards or outwards depending on the sign of the difference. The adjustment is continued until the difference is below a certain limit. This process is resonator-specific; it is then gone through N times, when the filter includes N resonators. By means of the method the tuning becomes automatic. An object of the invention is to implement the tuning of a resonator filter in a new and advantageous way. The method according to the invention is characterized in that which is specified in the independent claim 1 . The arrangement according to the invention is characterized by what is specified in the independent claim 5. Some advantageous embodiments of the invention are presented in the depend- ent claims.

The basic idea of the invention is the following: The tuning of a resonator filter is based on the analysis of the spectrum of at least the output signal of the filter. Snatches are taken typically from both input and output signals, the spectra of which are shifted to low frequencies and the signals provided in this way are con- verted to digital ones. The analysis is done in the digital process, and the distortion caused by the filter is deduced from the shape of the spectrum corresponding to the output signal, in respect of the spectrum corresponding to the input signal. The tuning elements of the filter are then adjusted so that the distortion is minimized.

An advantage of the invention is that the quality of the filtered signal improves compared with the case where the filter is tuned in a usual way. This is due to the fact that the distortions caused by the filter become rectified more accurately. Another advantage of the invention is that the the tuning of a filter takes place automatically and relatively fast, after the initialization. The invention is below described in detail. Reference will be made to the accompanying drawings where

Fig. 1 presents an example of the known tuning system of a resonator filter,

Fig. 2 presents as a block diagram an arrangement according to the invention, Fig. 3 presents as a flow diagram an example of the method according to the invention, and

Figs. 4a-c present an example of the rectification of the spectrum of the output signal.

Fig. 1 was already explained in conjunction with the description of the prior art. Fig. 2 shows as a block diagram an arrangement according to the invention. The tunable filter 210, which is a part of the transmission path of a radio-frequency signal, is visible in the figure. A signal source feeds the filter, and the transmission path naturally continues to a load. The filter may belong to the transmitter or receiver of a radio device. In the former case the source is for example the power amplifier of the transmitter, the load then being the antenna, and in the latter case the antenna can function as the source and the low-noise amplifier as the load. The signal propagating through the filter can be a signal, which relates to the normal operation of the device, or a test signal generated expressly for the tuning.

The arrangement according to the invention comprises a first 221 and second 222 directional coupler, a switch 240, demodulator 250, analog/digital converter 260 and signal processor 230. The first directional coupler is on the transmission path on the side of the filter input and the second directional coupler is on the transmission path on the side of the filter output. One input of the switch 240, which is in this case a change-over switch, is connected to the measurement port of the first directional coupler 221 , from which port the first measuring signal proportional to the input signal E_in of the filter is received. The other input of the change-over switch is connected to the measurement port of the second directional coupler 222, from which port the second measuring signal proportional to the output signal E_out of the filter is received. The state of the change-over switch is set by its control signal SWC so that the first and second measuring signal come to the output, or the common terminal, of the switch by turns. This output is connected to the demodulator 250, by which the measuring signals are converted into low frequency ones, or their spectra are shifted to a low frequency range. This can take place in two stages by shifting a spectrum first to a fixed intermediate frequency range us- ing a subcarrier with channel-specific frequency f_n and then to said low frequency range using a subcarrier with constant frequency f₀.

The output of the demodulator 250 is connected to the converter 260, by which the output signal LFS of the demodulator is converted into a discrete and digital one. The output of the converter again is connected to the digital signal processor, or more briefly processor 230, which implements the tuning process proper. The processor also generates the above-mentioned control signal SWC of the changeover switch so that the processor 'knows' the period, when the digital signal corresponding to the input signal of the filter comes in and the period, when the digital signal corresponding to the output signal of the filter comes in. The filter's control loop closes so that the processor is connected from its output side to the tuning elements of the filter 210. These elements are electrically adjustable; e.g. step motors are used for moving them. The output signals FLC of the processor adjust these tuning elements. The processor naturally includes a tuning program PRG, which functions congruent with the method according to the invention. The processor can also be connected to the data bus of the radio device in question.

For briefness, also the digital signal, which corresponds to the filter's input signal E_in, is called 'input signal', and also the digital signal, which corresponds to the filter's output signal E_out, is called 'output signal'. Fig. 3 shows as a flow chart an example of the method according to the invention. In the initialization stage the feed of a radio frequency signal is arranged for the filter and the program of the signal processor is started. In step 301 sample snatches are taken from the input and output signals and these snatches are stored in digital form into the memory of the processor. The number of the snatches to be collected is a pa- rameter value in the program. When the transmission system functions with time- division principle, the sample snatches of the signals can be of one time slot length. The snatches can be taken synchronously in certain time slots determined in the program or randomly in all time slots. In step 302 the spectra of the input and output signals is calculated, in other words the processor calculates it, e.g. by FFT method (Fast Fourier Transform). In step 303 the spectra are normalized and averaged to facilitate the analysis. After this, step 304, the value of at least one error quantity is determined from the spectra. The error quantity can be for example the difference of the spectrum widths of the input and output signals or the difference of the peak positions in the spectra of the input and output signals. The spectrum width is calcu- lated as the difference of for example its -3 dB positions or first zero positions. The value of the error quantity can be determined also from the spectrum of a mere output signal by calculating the difference of the energy contents of the spectrum parts on different sides of the center frequency of the channel. In the next step 305 the value(s) of the error quantity/-ies are interpreted. If they indicate a significant distortion in the output signal, the tuning elements of the filter are adjusted so that the distortion reduces (step 306). The distortion can be e.g. such that the spectrum of the output signal is weighed on either side from the center frequency of the channel, when the spectrum of the input signal is symmetrical. In a simple case the natural frequency of all resonators in the same direction is changed only a little by adjusting the tuning elements for this purpose. In more complex cases the processor controls the element adjusting the natural frequency in a different way in different resonators and/or controls also at least a part of the tuning elements, on which the strength of the couplings between the resonators depend.

After step 306 it is returned to step 301 , that is, new snatches of the input and output signals are stored in the memory of the processor, and it is continued from that step onwards. If it is found in step 305 that the value of the error quantity is below a certain limit or that it has not decreased significantly from the previous value, the tuning process is finished. It can be ended up directly to this state in the first control turn or after one or more control turns.

Figs. 4a-c show an example of the rectification of the spectrum of the output sig- nal. In Fig. 4a there is the spectrum 471 of the input signal, in Fig. 4b the spectrum 472 of the output signal before the rectification and in Fig. 4c the spectrum 474 of the output signal after the rectification. The spectrum 471 of the input signal is distributed symmetrically to both sides of the frequency f_c, which corresponds in the processor's calculation to the center frequency of the used radio frequency chan- nel. Also the main beam 473 of the spectrum of the input signal is marked by a dashed line in Figs. 4b and 4c. It is seen that in this example the spectrum 472 of the output signal before the rectification is weighed below the frequency f_c, and its width is smaller than the width of the main beam 473 of the spectrum of the input signal. In addition, the spectrum 472 is somewhat asymmetric also in respect of the center frequency of its own.

Fig. 4c shows that the spectrum 474 of the output signal after the rectification has almost the same width as the main beam of the spectrum of the input signal and that they have the same center frequency f_c. The processor has in this example adjusted the tuning elements of the filter so that its passband shifts a little upwards and widens slightly. In addition, such positions, by which the symmetry of the spectrum of the output signal improves, have been sough for the tuning elements. The search can take place at least partly through trial and error principle by repeating the control turns until the value of the error quantity minimizes.

The tuning system according to the invention has been described above. It can differ in the details from what is presented. The signal processor can be a unit of its own or it can be implemented in conjunction with a processor which runs also other functions. The comparison of the spectrum of the output signal can be done also to an ideal reference spectrum instead of the spectrum of the input signal. The required switch can be implemented e.g. by FET (Field Effect Transistor), PHEMT (Pseudomorphic High Electron Mobility Transistor) or MEMS (Micro Electro Me- chanical System) technique. The inventive idea can be applied in different ways within the scope defined by the independent claims 1 and 5.

Claims

1 . A method for tuning a resonator filter, in which method a radio-frequency signal is fed to the filter, an output signal (E_out) of the filter is analyzed and tuning elements of the filter are adjusted (306) until at least one error quantity used in the analysis of said ouput signal is minimized,

characterized in that said analysis concerns the spectrum of the output signal, and at least the difference of energy content of the parts of the spectrum of the output signal on different sides of channel's center frequency, the difference of the spectrum widths of the output signal and a reference signal and the difference of the peak positions in the spectra of the output signal and a reference signal are potential error quantities, whereupon the method comprises steps for:

- taking sample snatches at least from the output signal (E_out);

- shifting spectra of the sample snatches from radio-frequency range to low frequencies;

- converting resulted low frequency signal(s) into discrete and digital one(s);

- calculating (302, 303) from resulted digital signal a normalized and averaged spectrum of the output signal; and

- determining (304) a value of at least one error quantity using resulted spectrum to adjust said tuning elements.

2. A method according to claim 1 , characterized in that sample snatches are taken also from the input signal (E_in) of the filter, the spectra of these snatches are shifted to low frequencies, resulted low frequency signal is converted into discrete and digital and a normalized and averaged spectrum is calculated from resulted digital signal, in which case said reference signal is the digitized input signal.

3. A method according to claim 1 , characterized in that the spectrum of said reference signal is an ideal spectrum of the output signal stored in a memory.

4. A method according to claim 1 , characterized in that the tuning elements of the filter are adjusted at least partly through trial and error principle by repeating the control turns until the value of the error quantity minimizes.

5. An arrangement for tuning a resonator filter (21 0), which filter comprises electrically adjustable tuning elements, and the arrangement comprises a controller of these tuning elements, characterized in that said controller is a digital signal processor (230), and the arrangement further comprises

- a directional coupler (222) on output side of the filter; - a switch (240), an input of which is connected to measuring port of said directional coupler to take sample snatches from the output signal (E_out) of the filter;

- a demodulator (250), an input of which is connected to the output of said switch;

- an analog/digital converter (260), an input of which is connected to the output of the demodulator and an output of which is connected to said signal processor

(230); and

- a tuning program (PRG) in the signal processor for calculating and analyzing the spectrum of at least the digitized output signal and for adjusting the tuning elements of the filter on grounds of the results of the analysis.

6. An arrangement according to claim 5, characterized in that it further comprises a directional coupler (221 ) also on the input side of the filter, in which case said switch (240) is a change-over switch, second input of which is connected to measuring port of the above-mentioned directional coupler (221 ) to take sample snatches also from the input signal (E_in) of the filter for calculating also its spec- trum and for using in the tuning of the filter.

7. An arrangement according to claim 5, characterized in that said switch is implemented by FET, PHEMT or MEMS technique.