GB2389258A

GB2389258A - Filter arrangement in a multi-band mobile terminal device

Info

Publication number: GB2389258A
Application number: GB0307222A
Authority: GB
Inventors: Jyrki Portin; Raimo Klemetti; Vesa-Matti Salminen; Jarkko Ylisirnioe; Juha Isoaho; Hannu Pakonen; Marko Paasila
Original assignee: Microcell SA Luxembourg Zweigniederlassung Schweiz
Current assignee: Flextronics ODM Luxembourg SA Luxembourg Zweigniederlassung Schweiz
Priority date: 2002-04-02
Filing date: 2003-03-28
Publication date: 2003-12-03
Anticipated expiration: 2023-03-28
Also published as: GB2389258B; FI114763B; GB0307222D0; FI20020627A; FI20020627A0

Abstract

A terminal device (90, fig. 1) for mobile communications networks may operate in frequency bands such as 800MHZ for GSM, 1800MHZ for DCS, or 1900MHZ for PCS services. The receiver of the terminal device 90 includes at least two bandpass filters 21, 22, the overlapping passbands of which are further bandpass filtered by a third bandpass filter 40. Filter 40 may be a ceramic filter with good frequency stability. Because the edges of the passband 40a of the bandpass filter 40 are steep, the filters 21, 22 can be implemented using surface acoustic wave (SAW) filters, whose passbands may be less stable in response to temperature changes. The arrangement minimises filter costs as the final passband is determined by the passband of only the filter 40.

Description

GB 23B9258 A continuation (72) cont J u ha Isoa ho Hannu Pakonen Marko

Paasila (74) Agent and/or Address for Service: Hoffmann Eltle Sardinia House, Sardinia Street, 52 Lincoln's Inn Fields, LONDON,

WC2A 3LZ, United Kingdom

( - 2389258

FILTER ARRANGEMENT

Field of the technique

5 The invention relates to the filtering of a signal received by a tennina] device. More particularly, the invention relates to the filtering of a received signal in the receiver of a terminal device that is capable of operating in several frequency bands.

Background of the invention

At present, the terminal devices are being implemented such that they are capable of operating in several frequency bands. A greater number of bands allow for more capacity on the networks' which enables one to enhance and accelerate various services, and particularly their availability. Frequency bands t Apical of mobile 15 communication networks are e.g. 800 MHz, 900 MHz, 1800 MHz and 191)() MHz.

Of the receivers of the aforementioned so-called multi-band terminal devices, specific features are required. Since the terminal device antenna receives signal from an extremely broad frequency band, it is particularly important that from the received 20 signals, those disposed at non-desired frequencies are filtered omit. In fi1tcring the signals of the receivers of mobiles stations, various passband filter arrangements are typically used that pass through signals in a particular frequency band i.e. band-pass.

The width of the passbands of the filters being used in the filter arrangement as well 25 as their position in the frequency plane affect the implementation of the filter arrangement, as well as their functionality. Thus, in implementing each arrangement it is possible to use various components, thereby resulting In different arrangement and implementation costs. Good Dequency stability at venous temperatures is generally required of the filters. This means that the passband of the filter must remain the 30 same, i.e. the passband does not move regardless of even possibly big changes in the temperature. For example, the passband of typical, so-called SAW filters (SAW = Surface Acoustic Waves) changes due to temperature to such an e xtent that as such they are not readily suitable for all purposes. However, SAW filters may be used, as long as their features are taken into account as early as in the planning phase. The

( advantage associated with the SAW filters is their relatively affordahie purchasing price. 5 Summary of the invention

The objective of the invention is to eliminate or at least alleviate the disadvantages of prior art in the filtering of a signal received by a terminal device The invention

provides a method of filtering, a terminal device and a band-pass filter arrangement 10 for filtering a signal received in the terminal device.

The objective of the invention is to erhance the filter arrangement being used in the receivers of portable terminal devices in such a manner that the final passband of the output of the filter arrangement would be as steep as possible.

One further objective of the invention is that the arrangement is affordable in terms of costs and technically practicable for use in a portable tennirml device.

The aforementioned objectives are attained by a solution that has been described in 20 the independent claims.

According to a first aspect of the invention there is provided a filtering method for a frequency band received by a multi-band receiver of a terminal device utilizing wireless data transfer, the method comprising: The received frequency band is passband filtered to form at least two passbands, which passbands are further passband filtered; and wherein the lowermost frequencies of the lowest received passhand are Altered out in 30 order lo narrow the aforementioned passband, and the other fiequencies of the band are passed through; and/or - the uppermost frequencies of the uppermost-received passbant1 are filtered out in order to narrow the aforementioned passband, and the other frequencies of the band are passed through.

Advantageously, no more than two different passbands are narrowed Advantageously. the passband is narrowed no more than up to the middle of the 5 passband width of a non-narrowed passband.

Advantageously, the passbands are amplified prior to passband filtering them.

Advantageously, passbands are narrowed' at least a part of which are overlapping to 10 some extent.

Advantageously, a multi-band receiver is used to receive signal from DCS and PCS systems. 15 According to another aspect of the invention, a multiband receiver of a tenninal device utilizing wireless data transfer is provided, the receiver comprising: At least two passband filters, the passband ofthe first passband filter of which extends to a lower frequency than the passband of the second passband filter; For narrowing the passband of the first or second passband filter or both of the passbands, a third passband filter, in whose passband, one or the other or both of the following conditions are valid at a time: 25 - the lowermost pass frequency of the third passband filter is disposed in the passband of the first passband filter, - the uppermost pass frequency of the third passband filter is disposed in to passband of the second passband filter, 30 Advantageously, the first and second passband filters are interconnected in parallel.

Advantageously, the third passband filter is adapted to narrow the passband no more than up to the middle of the bandwidth of the passband.

( Advantageously, each passband filter is adapted to attenuate the frequencies disposed outside of its own passband.

5 Advantageously, the terminal device further comprises an amplifier between the first and third passband filter for amplifying the signals disposed in the passband of the first passband filter.

Advantageously, the terminal device further comprises an amplifier between the second 10 and third passband filter for amplifying the signals disposed in the passband of the second passband filter.

Advantageously, there are signals of the DCS and PCS system in the passbands of the first and second passband filter such that in both of the passbands there are signals of 15 different systems at the same time.

Advantageously, the third passband filter has been implemented using a ceramic filter Advantageously, the first and second passband filters are SAW filters.

Advantageously, the terminal device is a mobile station, such as a mobile phone.

According to a third aspect of the invention, a passband filter arrangement is provided, the arrangement composing: At least two passband filters, the passband of the first passband filter of which extends to a lower frequency than the passband of the second passband filter; For narrowing the passband of the first or second passband filter or both of the 30 passbands, a third passband filter, in whose passband, one or the other or both of the following conditions are valid at a time: - the lowermost pass frequency of the third passband filter is in the passband of the first passband filter,

l ! - the uppermost pass frequency of the third passband filter is in the passband of the second passband filter.

s The idea of the invention is to implement a filter arrangement that fulfils the above-

mentioned requirements. The requirements are attained using a filter arrangement in which the non-overlapping passbands of the first two passband filters are passed to the third passband filter which filters the lowermost frequencies from the lower frequency 10 band and the uppermost frequencies from We upper frequency band.

The arrangement according to the invention has several aclvantages. The aforementioned first two so-called front filters can be in terms of costs affordable SAW filters, which are easy to implement. Since the third filter filters the passbands 15 of both front filters, i.e. the third filter determines the final bands, in determining the filter parameters of the filters, there is no need to very carefully consider the possible moving ofthe passbands of the front filters, e.g. due to temperature.

List of figures In the following, the invention and its advantageous implementation modes are described in more detail with reference e.g. to Figures 14, in which Figure I represents a terminal device as seen from the front, 25 Figure 2 represents a principled block diagram of the terminal device, Figure 3 represents the filter arrangement of the receiver in more detail, Figures 4-6 represent different passbands of the filter arrangement.

Detailed description of the invention

Figure 1 shows one implementation mode of the terminal device 90 according to the invention, which comprises keys 9] and a display 92. The display as shown fimctions as a transceiver that in its operation applies e.g. a GSM, GPRS, UNITS or some other method known per se or some standard. The terminal device can in its operation apply

! a TDMA (Time Division Multiple Access), an FDMA (FDMA= Frequency Division Multiple Access), a COMA (CDMA - Code Division Multiple Access) or some other multiplexing method. The used multiplexing method has, however, no effect on the 5 functioning of the invention. The terminal device is preferably a so-called multi-band device. The terminal device may be a mobile station or some other terminal device, but the device type itself or the like is not substantial with regard to the invention. Substantial 10 is that the device is capable of receiving signal via the air path. Besides being capable of receiving signal, the subscriber terminal device DO as shown in Figure I is, in addition, capable of sending signal to the air path.

Figure 2 represents a principled block diagram illustrating one implementation mode 15 of the subscriber terminal device according to the invention. The terminal device functioning as the transceiver comprises on the receiving side a front filter block 20, an amplifier 30, a filter 40 and a demodulator 50. The terminal device comprises on the transmitting side a modulator 70 and an amplifier 80. The filters 20, 40 constitute a part of the filter arrangement of the receiver.

For transmitting and receiving signal, the terminal device comprises an antenna 10, which may be of any known type. The antenna may be in respect of type either internal or external. The tenninal device further comprises keys 91, a display 92, a handset 93 and a microphone 94.

Further, the terminal device comprises a control means 60 for controlling the blocks of the terminal device. The control means 60 receives from the microphone 94 and keys 91 signals that are further transmitted to the headphone 93 and to the display 92.

The control means 60 can be implemented e.g. by means of a microprocessor and a 30 software application.

the reception and nc direction the terniinal device functions in principle as follows.

The antenna 10 receives signals disposed at a very wide frequency that are filtered in the filter block 20. The filter block 2() is in principle a passband filter that may be

( implewcutcd e.g. by means of a SAW filter, in which case the purchasing costs of the filter are relatively small. The output of the filter block 20 is connected to the input of the arnplifer for amplifying the signals disposed in the passband of the filter.

The amplified signals are transmitted to the filter 40, which also is a passhand filter.

The signals disposed in the passband of the filter are transmitted to the demodulator 50 for demodulating, after which the demodulated signal is transmitted to the control means 60, which further processes the signal.

In the transmission i.e. tx direction the terminal device functions in principle as follows. The control means 60 processes the information signal received from the microphone 94 that is transmitted to the modulator 70 for demodulating. The signal received as a result of demodulating is amplified in the amplifier 80, after which the 15 amplified signal is transmitted to the radio path via the antenna 10.

The terminal device may function e.g. in the GSM (GSM - Global Systems for Mobile Communications, 900 MHz), DCS (DCS = Digital Cellular System, 1800 MHz) and PCS system (PCS = Personal Communications Services, 1900 MHz). More 20 specifically, the frequency band of the DCS system is in the uplink direction 1710-

1785 and in the downlink direction 1805-1880 MHz. The frequency band of the PCS system is in the uplink direction 1850-19lO and in the downlink direction 1930-1990 MHz. 25 It is assumed that the terminal device functions in three frequency ranges fin (900 MHz), fb2 (1800 MHz) and Fb3 (1900 MHz), of which the frequency range fib; is disposed relatively far off from the ranges fb2 arid fb3. Lnstead, of the aforewcationed frequency ranges fb2 and fb3 are relatively close to each other, and they may even be overlapping to some extent. While the Dequency ranges fb2 and fh3 would not be 30 overlapping, the passbands of the frequency ranges must be separated Tom one another preferably by applying the passband filtering.

Figure 3 represents the filter arrangement of the receiver comprising a filter block 20 and a filter 40. From Figure 3 it is seen that the filter block 20 comprises at least a

( first passband filter 21 and a second passband filter 22. The arrangement further comprises an amplifier block 30, which consists of amplifiers 31, 32. The output of the filter 21 is connected to the input of the amplifier 31, and the output of the filter 22 is connected to the input of the amplifier 32. The outputs of the amplifiers are connected and coupled to the input of the filter functiorung as the third filter 40.

The filters 21, 22 apply passband filtering to Me frequency band received by them.

The signals disposed in the pass frequency of both filters 21,22 are amplified, and the I O amplified signals are transmitted to the third filter 40, which further filters the signals disposed in the passbands of the filters 21,22, if necessary.

In the following, there is described the functioning of the filter arrangement in more detail with reference to Figure 4, which shows the passbards 21a, 22a, 40a of the 15 filters 21,22 and 40. The filter 21 passes through the signals received by the terminal device antenna that are disposed in the fist frequency band. The frequency foul forms the lowest limit of the passband of the filter 21, and the frequency f2 H forms the upper limit. The filter 22 passes through the signals received by the terminal device antenna that are disposed in the second frequency band. The frequency fig. forms the lowest 20 limit of the passband of the filter 22, and the frequency f22H forms the upper limit.

As can be seen from Figure 4, the lowermost pass frequency fin' of the second passband filter 22 is bigger than the uppermost pass frequency If,, of the first passband filter 21. In other words, the passbands of the first and second filter are not 25 overlapping.

The signals disposed in the passband of the first filter 21 are transmitted to the amplifier 31 to be amplified, and the signals disposed in the passband of the second filter Z2 are transmitted to the amplifier 32 to be amplified. After amplifying, the 30 signals are transmitted to the third passband filter 40 to be amplified, the lowermost pass frequency f40c of which is disposed in the probed of the first passband filter 21.

In other words, with regard to Figure 4, the following fonnula is valid: ( 1) f2 1 L C f40L < f22H

The uppermost pass frequency of the passband of the third passband filter 40 is disposed in the passband of the second passband filter 22. More particularly, this means a condition according to the following formula for the filter arrangement.

(2) f2:L < f40H < f22H The third passband filter 40 is preferably a ceramic filter, as its type is concemed, which makes it easy to implement. The tuning is easy because the passband of the 10 filter does not change due to changes in the temperature. A ceramic fitter makes it, In addition, possible to provide steep edges for the passband.

The lower band of the third passband filter 40 includes signals of troth the DCS and PCS system, The filter parameters of the third passband filter 40 have been chosen so I 5 that the passband of the filter can be made so as to be of the desired size. The filter 40 narrows the passbands of the first and second filter, in which case the width of both bands can be made to have the desired size. However, the third filter 40 needs not narrow both of the passbands 21 a, 22a to the same extent.

20 The arrangement according to the invention enables one to minimize the filter costs because the final passband is determined based on the passband of solely one filter, i.e. the third filter 40. In other words, in the filter arrangement one needs to use only one filter with good technical properties (filter 40) that can be accurately adjusted to the desired pass frequency without the adjust-rnent changing e.g. due to temperature.

25 Because the edges of the passband of a ceramic filter 40 are steep, the filters 21, 22 can be implemented using SAW filters that are affordable in terms of costs and whose technical values need not necessarily be so strict as those of the filter 40.

Figures 5 and 6 represent some possible passbands of the filter arrangement. Prom 30 Figure 5 can be seen that the lowermost pass frequency Blat. of the passband of the passband filter 40 is disposed in the passband of the first passband filter 21, just as in Figure 4.

/ Instead, the uppermost pass frequency of the passband of the third passband filter 40 is the same as the uppermost frequency of the passband of the second passband filter 22. The pass frequency o f the passband of the third passband filter 4() may, however, 5 be higher than the uppermost frequency of the passband of the second passband filter 22. With reference to Figure 5, the following fonnula is valid for the filter arrangement (3) f22l. < f40H = f22H As can be seen from Figure 6, the passband block 20 has formed three passbands 21a, 22a, 23a, of which the bands 21a and 23a are a little overlapping. Furthermore, for the passband filter 40 there is provided a passband 40a.

15 For the limiting frequencies of the passbands, the following formula is valid 4) f21L < f40L < f23L c f21H < f23H c f22L < f40H < f22H Because there now arc more than two passbarlds formed by the filter block, the filter 40 passes one band i.e. the middlemost band 23a directly through. I:rom the figure can 20 be farther seen that the filter 40 filters the frequencies that are smaller than the frequency f40L away from the lowennost band i.e. band 21a. The filter 40 also filters the frequencies that are higher than the frequency POOH away Mom the uppermost band i.e. band 22a. The passband of the filter 40 is set so as to be such that it passes through at least half of each bared, e.g. of passband 22a.

As was earlier stated, the filter block 20 may comprise more than two passband filters, each of which forms at least one passband. Even in this situation,:hc passband filter 40 filters and attenuates, by means of earlier presented methods, no more than two passbands at a time, which have been formed by the filters disposed In the filter block 30 20. More specifically, the passband filter 40 filters the lowermost Or uppermost passband formed by filter block 20 or both. If there are three or more passbands formed by the filter block 20, the middlemost bands go through the passband filter 40 as such.

While the invention has been described above with reference to the examples according to accompanying drawings, it must be understood that the invention is not limited to them, instead a person skilled in the art may vary the presented solutions 5 without departing Mom the idea of the invention.

Claims

1. A filtering method for a frequency band received by a multiband receiver of a 5 terminal device utilizing wireless data transfer, characterized in that the filtering method comprises: the received frequency band is passband filtered to form at least two passbands, which passbands are farther passband filtered, and in which - the lowermost frequencies of the lowest received passband are f Itered out in order to narrow the aforementioned passband, and the other frequencies of the band are passed through, and/or - the uppermost frequencies of the uppermost received passband are filtered out in 15 order to narrow the aforementioned passband, and the other frequencies of the band are passed through.

2. A method according to claim 1, characterized in that no more than two different passbands are narrowed.

3. A method according to claim I or 2, characterized in that the passband is narrowed no more than up to middle of the bandwidth of a non-narrowed passband. 25

4. A method according to any one of the preceding claims, characterized in that the passbands are amplified prior to passing them to be passband filtered.

5. A method according to any one of the preceding claims, characterized in that the passbands are narrowed, at least a part of which are a little overlapping with 30 one another.

6. A method according to any one of the preceding claims, characterized in that a multiband receiver is used to receive signal from the DOS and PCS systems.

f'

7. A multi-band receiver of a terminal device utilizing wireless data transfer, characterized in that the receiver comprises 5 at least two passband filters (21, 22), the passband of the first passband filter (21) of which extends to a lower frequency than the passband of the second passband filter (22); for narrowing the passband of the first or second passband filter or both of the 10 passbands, a third passband filter (40), for the passband of which, one or the other or both of the following conditions are valid: - the lowermost pass frequency of the Bird passband filter (40) is disposed in the passband ofthe first passband filter (21), 15 - the uppermost pass frequency of the third passband filter (40) is disposed in the passband of the second passband filter (22).

8. A terminal device according to claim 7, characterized in that the first and second passband filter are interconnected in parallel.

9. A terminal device according to claim 7 or 8, characterized in that the third passband filter (40) is adapted to narrow the passband no more than up to the middle of the bandwidth of the passband.

25

10. A tennmal device according to any one of claims 7-9, characterized in that each passband filter is adapted to adenuate the frequencies disposed outside of its own passband.

11. A terminal device according to any one of claims 7-10, characterized in that 30 the terminal device further comprises an amplifier (31) between the first and third passband filter for amplifying the sisals disposed in the passband of the first passband filter (21)

12. A terminal device according to any one of claims 7-11, characterized in that the terminal device further comprises art amplifier (32) between the second and third passband filter for amplifying the sisals disposed in the passband of the 5 second passband filter (22).

13. A tenninal device according to any one of claims 7-12, characterized In that in the passbands of the first and second passband filter there are signals of the DCS and PCS system so that in both passbands there are signals of different 10 systems at the same time.

14. A terminal device according to any one of claims 7-13, characterized in that the third passband filter (40) has been implemented using a ceramic filter.

15 15. A terminal device according to any one of claims 7-14, characterized in that the first and second passband filter are SAW filters.

16. A terminal device according to any one of claims 7-15, characterized in that the terminal device is a mobile station such as a mobile phone

17. A passband filter arrangement, characterized in that the arrangement compnses: at least two passband filters (21, 22), the passband of the first passband filter (21) of 25 which extends to a lower Frequency than the passband of the second passband filter (22); for narrowing the passband of the first or second passband filter or both passbands, a third passband filter (40), for the passband of which the one or the other or both of the 30 following conditions are valid: the lowermost pass frequency of the third passband filter (40) is disposed in the passband of the first passband filter (21), passband of the second passband filter (22).