DEVICE AND METHOD FOR ADJUSTING A BANDPASS FILTER, ESPECIALLY A COMBINER FILTER

Abstract: The invention relates to a frequency adjustment method for an RF bandpass filter, especially a combiner filter, wherein the center frequency of the passband of the RF bandpass filter (22) is adjusted in response to the RF power reflected from the input of the bandpass filter or passed through the bandpass filter. In the method a sample signal (Pr) proportional to the RF power reflected from the input of the bandpass filter or passed through the bandpass filter is mixed (29) with a signal of substantially the transmitting frequency, the mixing result is lowpass-filtered (24) and the bandpass filter (22) is adjusted in response to the lowpass-filtered mixing result (24a).

Description: Device and method for adjusting a bandpass filter, especially a combiner filter Field of the Invention The invention relates to a frequency adjustment method for an RF bandpass filter, especially a combiner filter, wherein the center frequency of the passband of the RF bandpass filter is adjusted in response to the RF power reflected from the input of the bandpass filter or passed through the Joandpass filter. Background of the Invention One known way of connecting several radio transmitters to the same antenna or antenna line is to connect each radio transmitter through a separate bandpass filter having its center frequency tuned to the transmitting frequency of the radio transmitter. Such bandpass filters are called combiner filters. The function of the combiner filter is to feed the transmitting signal of the respective radio trans¬ mitter to the antenna with the smallest possible losses and prevent the entrance of transmitting signals of different frequencies from other radio transmitters in the direction of the antenna to this particular radio transmitter as efficiently as possible. Conventionally, combiner bandpass filters are tuned fixedly to the transmitting frequencies of the radio transmitters. So it has not been possible to change the transmitting frequency of the radio transmitter without simultaneously changing the combiner filter or its tuning. In certain cases, however, it is desirable that the frequencies of radio transmitters can be changed simply and rapidly. One such case is the base station of a cellular mobile radio system to which pre¬ determined transmitting and receiving channels have been assigned. If the channel allocation of the system can be altered by varying the transmitting and receiving frequencies of the base stations, the channel capacity of the system can be utilized in a flexible and efficient way in varying conditions. Therefore combiner filters have been developed in which the center frequency changes automatically with the transmitting frequency. The adjustment of known automatically tunable combiner filters is based on the measurement of the RF power reflected from the input of the filter or the RF power passed through the combiner filter, and locking to a minimum/maximum value of the measured power. A problem with this adjustment method is the low adjusting accuracy and the narrow dynamic range. As the frequency selectivity of the entire system relies on the combiner filter, power signal com- ponents of other radio transmitters leaking from the output of the combiner filter to its input cause the minimum reflection suppression value of the reflec¬ tion suppression measurement at the filter input to be about 7 dB, which results in a narrow dynamic range for the measurement. In the adjustment method based on the measurement of the power passed through the combiner filter, the measuring dynamics of the maximum power value also remains low for the same reasons as above. In addition, the automatically tunable combiner filter based on this prior art adjustment method does not allow variation in the relative power levels between the radio transmitter, that is, the "mutual dynamics" is almost 0 dB, as a change in the power level of one transmitter affects immediately the power measurement in the adjustment circuits of the combiner filters of the other radio transmitters, thus causing adjustment error. Disclosure of the Invention The object of the invention is to provide a frequency adjustment method of a new type for RF bandpass filters, which can be applied in combiner bandpass filters to improve the accuracy and the dynamic range of the adjustment. This is achieved by means of a method of the type described in the introduction, wherein a sample signal proportional to the RF power reflected from the input of the bandpass filter or passed through the bandpass filter is mixed with a signal of sub- stantially the same frequency, the mixing result is lowpass-filtered, and the bandpass filter is adjusted in response to the lowpass-filtered mixing result. The basic idea of the invention is to increase the frequency selectivity of the measurement of the RF power passed through the filter or the reflected RF power by mixing the RF measurement signal (the sample signal) with a signal having the same fre¬ quency, that is, a transmitting-frequency signal, thus obtaining a substantially zero-frequency signal, that is, a DC signal having a level proportional to the RF power reflected from the input or the RF power passed through the filter. Power components leaking from the transmitting frequencies of other radio transmitters appear in the mixing result e.g. at a frequency of 300 kHz or at higher frequencies, which are filtered off in the lowpass filtration following the mixing. In this way the- invention provides a DC signal the level of which depends merely on the transmitting-frequency RF power component to be measured. As the transmitting signals of other radio transmitters do not affect the frequency adjustment of the automatically tuneable filter according to the invention, the invention provides a considerable improvement in the adjusting accuracy of the combiner filter and in the power dynamics of the measurement. The above-mentioned "mutual dynamics" is also improved as the mutual power levels of the radio transmitters can be varied without that the adjust¬ ment of the combiner filter is affected in any way. In the preferred embodiment of the invention, the "local oscillator signal" used in the mixing is derived directly from the transmitting signal of the radio transmitter, but the local oscillator signal can also be generated by means of an oscillator in- eluded in the adjustment circuit. In such a case, the desired transmitting frequency is instructed to the oscillator as channel data similar to that instructed to the radio transmitter itself. In this way a remote-controllable frequency adjustment of a very high accuracy can be obtained in the combiner filter, if desired. The effect of external loads on the adjustment of the combiner filter can be reduced by employing the measurement of the reflected RF power and the RF power passed through the filter simultaneously. This is based on the fact that different load situations have a different effect on these two quantities to be measured. The invention also relates to a device for adjusting an RF bandpass filter, especially a combiner filter, comprising means for generating an RF sample signal proportional to the RF power reflected from the input of the bandpass filter or passed through the bandpass filter; and adjusting means for adjusting the center frequency of the band- pass filter in response to the level of said sample signal. The device according to the invention further comprises mixing means for mixing the RF sample signal with a signal of substantially the same fre- quency; a lowpass filter connected to the output of the mixing means; and wherein the adjusting means is responsive to the level of the output signal of the lowpass filter for adjusting the bandpass filter. Brief Description of the Drawings In the following the invention will be de¬ scribed by means of illustrating embodiments with reference to the attached drawings, in which Figure 1 is a block diagram illustrating a typical transmitter system utilizing combiner filters; Figure 2 is a block diagram illustrating a bandpass filter adjusting circuit according to the invention in which power reflected from the input of the filter is measured; Figure 3 is a block diagram illustrating a bandpass filter adjusting circuit according to the invention in which power passed through the filter is measured; and Figure 4 is a block diagram illustrating a bandpass filter adjusting circuit in which both re¬ flected RF power and RF power passed through the filter are measured. Detailed Description of the Invention Referring to Figure 3, n radio transmitters Txl ... Txn having transmitting frequencies f ^ ... f n , respectively, are connected through bandpass filters 1^, 2 , .-• l n tuned to the respective frequencies, and then to a common summing point PI and further through an antenna line to a common transmitting antenna ANT. So a radio-frequency (RF) signal applied to the transmitting antenna ANT comprises the fre¬ quencies f- j _ ... f n of all the transmitters. The transmitting frequencies f^ ... f n are within the range 920-960 MHz, for instance. Such bandpass filters connecting several transmitters to a common antenna are generally referred to as combiner filters. The invention will be described below in connection with combiner filters, in which it can be particularly advantageously applied, but the inven¬ tion can also be applied in filters intended for other purposes when frequency selective power measurement is required in the frequency adjustment. Figure 2 shows a combiner filter utilizing the measurement of reflected power. The transmitting- frequency RF signal applied from the radio trans¬ mitter to an input IN is applied through a change¬ over switching device 30, a directional coupler device 21 or the like to a bandpass filter 22 and further to an output OUT in the bandpass filter. The output OUT may be connected e.g. to the antenna summing point PI of the type shown in Figure 1. The bandpass filter 22 is a narrow-band filter with a center frequency which is attempted to be adjusted as close as possible to the frequency of the trans¬ mitting carrier of the radio transmitter so that the signal to be transmitted is transferred to the antenna ANT with the smallest possible losses. The center frequency of the bandpass filter 22 is adjusted in response to the power of the RF signal component reflected from the input of the bandpass filter 22 in such a way that the reflected power com¬ ponent is minimized. For this purpose, the direc- tional coupler device 21 derives a sample signal Pr from the signal component reflected from the input of the bandpass filter 22, the sample signal being pro¬ portional to the power of the signal component. The sample signal Pr is then applied to the input of a mixer 29 through an adjustable gain amplifier 27. The directional coupler device 21 also derives a sample signal Pf from the outbound signal applied to the input, and the sample signal Pf is applied to a local oscillator input LO of the mixer 29 through an adjustable phase shifting device 25 and an adjustable gain amplifier 28. A predetermined phase relation is set between the sample signals Pf and Pr by means of the manually adjustable phase shifting device 25 on installing the filter. As the sample signals Pr and Pf are also equal in phase, the portion of the sample signal Pr proportional to the reflected power com¬ ponent appears in the mixing result present in the output IF of the mixer 29 as a substantially zero- frequency signal, i.e. as a DC signal. Power com- ponents leaked from other radio transmitters through the filter 22 to its input appear in the mixing result at a higher frequency, such as 300 kHz. The output signal of the mixer 29 is applied to an adjusting unit 23 through a lowpass filter 24, which filters the signal components of other radio trans¬ mitters from the mixing result. The level of the DC signal applied to the adjusting unit 23 is thus dependent merely on the amount of reflected trans¬ mitting-frequency power, which provides accurate fre- quency adjustment insensitive to interferences. The adjusting unit 23 tends to adjust the center fre¬ quency of the bandpass filter 22 so that the level of the DC signal 24a is minimized. The connection point between the adjustable phase shifter 25 and the adjustable amplifier 28 is connected to a power detector 26 which derives a dc voltage signal 26a from the sample signal Pf, the level of the dc voltage signal being proportional to the transmission power applied to the input IN. The adjusting unit 23 uses the signal 26a in the adjust¬ ment of the amplifiers 27 and 28 so that the dynamic ranges of the signals Pf and Pr applied to the mixer 29 and the output signal of the mixer 29 are optimal on each input power level. If desired, the power level information derived from the signal 26a can also be used as a reference value in the relative measurement of " the level of the signal 24a. The change-over switching device 30, which is controlled by the adjusting unit 23 by means of a control signal 46, may apply a transmitting-frequency local oscillator signal 41A to the directional coupler device 21 in place of the input signal IN. The local oscillator signal 41A- is generated by a local oscillator 41 included in the adjustment cir- cuit and controlled by an adjusting unit 43 by means of a control signal 45. The transmission channel used at each particular moment is instructed to the adjusting unit 43 by means of a signal CHDATA applied by the same control system that applies channel data to the radio transmitter itself. On receiving new channel data, the adjusting unit 43 applies the control signal 45 to the local oscillator 41 to cause it to assume a transmitting frequency corresponding to the new channel. The local oscillator signal is used in the absence of the actual transmission signal IN or when the combiner filter is to be tuned to the desired transmission channel by remote control. The use of the oscillator 41 improves the adjusting accuracy considerably in remote control operation. From the viewpoint of the operation of the adjusting circuit, the transmitting-frequency signal to be applied to the mixer may otherwise be derived from any source. Accordingly, the adjusting circuit may also be realized without the oscillator 41 or merely the oscillator may be used in the mixing procedure. In the last-mentioned case, the oscillator 41 may be connected directly to the mixer 29 and the switching device 30 may be omitted. Figure 3 shows an adjusting circuit according to the invention, in which the transmitting-frequency power passed through the bandpass filter 22 is measured. Components and blocks having the same oper¬ ation and structure in Figures 2 and 3 are indicated by the same reference numerals. In the following, only the differences between Figures 2 and 3 will be described. A directional coupler 31 or the like derives a sample signal Pf from the inputted trans¬ mission power. Correspondingly, ■ another directional coupler means 32 or the like is connected to the out- put of the filter 22 to derive a sample signal Pf" from the output signal of the bandpass filter. The sample signal Pf" is then applied through an adjust¬ able gain amplifier 27' to the input RF of a mixer 34. Similarly as in Figure 2, the mixer 34 mixes the transmitting-frequency sample signals Pf and Pf", so that a mixing result is obtained in the output IF of the mixer 34, in which a signal component propor¬ tional to the transmitting-frequency signal power passed through the filter 22 appears as a sub- stantially zero-frequency signal, that is, a DC signal. Any interfering components possibly caused by other radio transmitters appear at higher frequencies which are filtered off by a lowpass filter 35. So a DC signal 35a appears in the input of an adjusting unit 33, the level of the DC signal being dependent merely on the amount of transmitting-frequency power passed through the filter 22. The adjusting unit 33 adjusts the center frequency of the bandpass filter 22 so that the level of the signal 35a is at maximum. Figure 4 shows still another bandpass filter frequency adjustment circuit according to the inven¬ tion, in which measurement of both the reflected power and the power passed through the filter is em¬ ployed. The circuit diagram of Figure 4 is thus a combination of the circuit diagrams of Figures 2 and 3. A directional coupler means 44 derives a sample signal 'Pr from the reflected transmitting-frequency RF power reflected from the bandpass filter 22, and the sample signal Pr is then applied similarly as in Figure 2 through the phase shifting device 25 and the adjustable amplifier 28 to the input RF of the mixer 29, and so the mixer 29 mixes the sample signal Pr with the transmitting-frequency signal applied to the local oscillator input LO, thus producing a mixing result in the output IF, in which the component cor¬ responding to the reflected transmitting-frequency power is a substantially zero-frequency signal, that is a DC signal. This DC signal is filtered by the lowpass filter 24 so that a pure DC signal 24a is applied to the adjusting unit 43, the level of the signal 24a being dependent merely on the amount of transmitting-frequency power reflected from the input of the filter 22. Similarly as in Figure 3, the directional coupler means 32 in the output of the bandpass filter 22 separates a sample signal Pf" from the transmitting-frequency signal passed through the bandpass filter, and the sample signal Pf" is applied through an adjustable amplifier 27' to the input RF of the mixer 34. The mixer 34 mixes the signal Pf" with a transmitting-frequency signal applied to the local oscillator input LO, and so a mixing result appears in the output of the mixer 34, in which a substantially zero-frequency signal, that is, a DC signal corresponds to the signal Pf". High fre- quencies are filtered off the mixing result by means of the lowpass filter 35, so that a pure DC signal 35 having a level dependent merely on the amount of the transmitting-frequency power passed through the band¬ pass filter 22 is applied to the adjusting unit 43. Depending on the external load, the adjusting .unit 43 adjusts the bandpass filter either in such a way that the level of the signal 35a is at maximum or in such a way that the level of the signal 24a is at minimum. The adjusting unit 43 applies a certain criterium to select the signal 24a or 35a to be used in the adjustment in each specific case. It is also possible that the adjusting unit 43 uses the signals 35a and 24a simultaneously in the adjustment, thus aiming at keeping them at predetermined values. In Figure 4, the transmitting-frequency local oscillator signal Pf is applied from the directional coupler 44 to the mixers 29 and 34 and through the phase shifting device 25 and the amplifier 28 and the phase shifting device 25' and the amplifier 28' , respectively. The change-over switch 30 may connect either the transmitting signal or the local oscil¬ lator signal to the directional coupler 44. The figures and the description related to them are only intended to illustrate the present inven- tion. In their details, the method and the device according to the invention may vary within the scope of the attached claims.

Claims: Claims : 1. A frequency adjustment method for an RF bandpass filter, especially a combiner filter, wherein the center frequency of the passband of the RF bandpass filter is adjusted in response to the RF power reflected from the input of the bandpass filter or passed through the bandpass filter, c h a r a c ¬ t e r i z e d in that a sample signal proportional to the RF power reflected from the input of the band¬ pass filter or passed through the bandpass filter is 'mixed with a signal of substantially the same fre¬ quency, the mixing result is lowpass-filtered, and the bandpass filter is adjusted in response to the lowpass-filtered mixing result. 2. A method according to claim 1, c h a r a c ¬ t e r i z e d in that the mixing result is a DC signal. 3. A method according to claim 1 or 2, c h a r- a c t e r i z e d in that the sample signal is pro¬ portional to the reflected RF power, and that the bandpass filter is adjusted so that the level of said mixing result is at its minimum value. 4. A method according to claim 1 or 2, c h a r- a c t e r i z e d in that the sample signal is pro¬ portional to the RF power passed through the bandpass filter, and that the bandpass filter is adjusted so that the level of said mixing result is at its maxi¬ mum value. 5. A method according to claim 1 or 2, c h a r a c t e r i z e d by generating a first sample signal proportional to the reflected RF power, and a second sample signal proportional to the RF power passed through the bandpass filter; mixing each sample signal with a signal of substantially the same frequency for producing a first and a second mixing result; lowpass-filtering said mixing results; and adjusting the bandpass filter in response to both of said lowpass-filtered mixing results. 6. A device for adjusting an RF bandpass filter, especially a combiner filter, comprising means (21, 32, 44) for generating an RF sample signal (Pr, Pf" ) proportional to the RF power reflected from the input of the bandpass filter or passed through the bandpass filter; and adjusting means (23, 33, 43) for adjusting the center frequency of the bandpass filter (22) in response to the level of said sample signal (Pr, Pf"), c h a r a c t e r i z e d in that the device comprises mixing means (29, 34) for mixing the RF sample signal (Pr, Pf" ) with a signal of substantially the same frequency; a lowpass filter (24, 35) connected to the out- put of the mixing means (29, 34); and the adjusting means (23, 33, 43) being respons¬ ive to the level of the output signal (24a, 35a) of the lowpass filter (24, 35) for adjusting the band¬ pass filter. 7. A device according to claim 6, c h a r a c ¬ t e r i z e d by means (21, 25, 31) for deriving said signal of substantially the same frequency from the transmitting-frequency input signal of the band¬ pass filter. 8. A device according to claim 6, c h a r a c ¬ t e r i z e d by an adjustable oscillator means (41) for generating said signal of substantially the same frequenc . 9. A device according to claim 8, c h a r a c - t e r i z e d by a change-over switching means (30) for connecting an output signal (41A) of said oscillator means (41) as the local oscillator signal of the mixing means so that the filter is remote- controllable to a desired channel. 10. A device according to claim 6, 7, 8 or 9, c h a r a c t e r i z e d in that the device com¬ prises a first (44) and a second (32) means for generating a first RF sample signal (Pr) proportional to the RF power reflected from the input of the band¬ pass filter (22) and a second RF sample signal (Pf") proportional to the RF power passed through the band¬ pass filter (22), respectively; a first (29) and a second (34) mixing means for mixing the first (Pr) and the second (Pf") RF sample signal, respectively, with said signal of substan¬ tially the same frequency; and a first (24) and a second- (35) lowpass filter connected to the output of the first (29) and the second (34) mixing means; the adjusting means (43) being responsive to the levels of the output signals (24a, 35a) of both the first (24) and the second (35) lowpass filter for adjusting the bandpass filter. 12. A device according to claim 6, 7, 8 or 9, c h a r a c t e r i z e d in that the device com¬ prises means (21, 26) for generating a third sample signal (26a) proportional to the RF power applied to the bandpass filter; a level adjusting means (27) for adjusting the level of said RF sample signal (Pr, Pf"); a level adjusting means (28) for adjusting the level of said signal of substantially the same fre- quency; and control means (23, 33) for controlling said level adjusting means (27, 28) in response to the level of said third sample signal (26a).

Inventors: SAERKKAE VELI-MATTI, SAERKKAE, VELI-MATTI
Applicants: TELENOKIA OY
Application year: 1992
Main classification: H03H7
IPC codes: H03H11, H04B1