GB2333905A

GB2333905A - Filter for electrical signals

Info

Publication number: GB2333905A
Application number: GB9801786A
Authority: GB
Inventors: Peter Readman
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1998-01-29
Filing date: 1998-01-29
Publication date: 1999-08-04
Also published as: GB9801786D0

Abstract

The notch filter 12 for radio signals comprises at least one strip line component which is coupled to a switch 50. The filter operates to attenuate radio frequency signals having a predetermined range of frequencies in dependence upon the state of the switch. The strip line may comprise first and second strip lines 40,42 connected in parallel, and a serially connected further strip line component 48' having a predetermined characteristic impedence. The serially connected strip line component may include a tuning capacitor. Matching circuits 52,54 may also be provided at the input and output ends of the filter.

Description

FIL TER FOR ELECTRICAL SIGNALS The present invention relates to filters for electrical signals which operate to attenuate the electrical signals in dependence upon the frequency of the signals. More particularly, but not exclusively, the present invention relates to filters for radio frequency signals.

In transmitters and receivers for radio signals there is often a requirement to discriminate between signals in accordance with the frequency of the signals. Filters for radio frequency signals are provided to effect this discrimination by attenuating signals within a predetermined range of frequencies whilst conveying signals outside this range substantially un-attenuated.

Known radio frequency filters are fabricated from strip line components which provide an impedance to the radio signals at a desired frequency to the effect that radio frequency signals are attenuated within a predetermined range of frequencies. However, a technical problem with filters fabricated from strip line is that once fabricated, such attenuate frequencies in a fixed frequency range cannot thereafter be altered. The technical problem of providing a radio frequency filter in which a range of frequencies in which signals are attenuated can be altered represents a technical problem which is addressed by the filter according to the present invention.

The technical solution is provided by combining at least one strip line component in combination with a switch.

According to the present invention there is provided a filter for radio frequency signals comprising at least one strip line component coupled in operative association with a switch, wherein said filter op4erates to attenuate radio frequency signals having a predeytermined range of frequencies in dependence upon whether said switch is in an open or a closed state.

According to the present invention there is provided a filter for radio frequency signals comprising a first strip line component coupled between an input of the filter and an output of the filter and a second strip line component coupled between the input of the filter and the output of the filter via a switch, wherein said filter operates to pass radio frequency signals of substantially all frequencies when said switch is closed and said filter operates to attenuate signals with a predetermined range of frequencies when said switch is open.

By providing a filter comprising first and second strip line components with a switch coupled into a second of the strip line components and arranged to pass all signals with all frequencies when the switch is in a closed position and when the switch is in an open position the first and second strip line components form an open circuit stub, a radio frequency strip line filter is provided which can act as an all pass filter when the switch is closed and a notch filter when the switch is open.

Advantageously, the first strip line components may further include a series section line having a predetermined characteristic impedance.

The filter may comprise a first signal matching means coupled between the input of the filter and the first and second strip line components.

The filter may further comprise a second signal matching means coupled between the output of the filter and the series section line.

In order to provide a matching for the radio frequency signals to other components in which the filter may be embodied, the first and second signal matching means are provided to effect a matching impedence at the input and output of the filter.

The components which comprise the signal matching means may be adapted in accordance with an impedence provided by the first and second strip line components.

The series section line may further include a tuning capacitor coupled in series with the series section line.

The switch may be a pin diode.

Using a switch to convert an all pass configuration provided by first and second strip line components of the filter when the switch is closed and an RF open circuit configuration when the radio frequency switch is open has an effect that a substantial RF potential difference is generated between an input and an output of the switch. However, by introducing a tuning capacitor within the series section strip line, an equalising phase shift is introduced into the signal in the first strip line to the effect that the signal at the output of the switch is substantially in phase with that of the input of the switch, therefore providing substantially zero potential difference between the input and the output of the switch.

One embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings wherein FIGURE 1 is a schematic block diagram of a dual mode transmitter forming part of a mobile radio telephone, FIGURE 2a is a representation of a power spectrum for a first mode of the handset shown in Figure 1, FIGURE 2b is a representation of a power spectrum for the second mode of the handset shown in Figure 1, FIGURE 3 is a schematic block diagram of a configurable notch filter, FIGURE 4 is schematic block diagram of a further configurable notch filter, and FIGURE 5 is a schematic representation of a signal matching means forming part of the configurable notch filter shown in Figure 4.

An illustrative application of an embodiment of the present invention will now be described for a mobile telephone which is adapted and arranged to operate in one of two modes. In the field of mobile radio telephony, there are two associated standards administered by the European Telecommunications Standards Institute. These are the global system for mobiles (GSM) and the digital cellular system 1800. These systems are substantially identical apart from a frequency band in which these systems operate. In the case of GSM, the frequency band is in the order of 900 MHz, whereas for DCS 1800, the frequency band is in the order of 1800 MHz. In order to provide a facility whereby a mobile radio telephone can be used in both GSM and DCS 1800 systems, a dual mode mobile radio telephone must be provided wherein the telephone can switch to one or the other standards in accordance with which of the systems it is desired to communicate. To this end Figure 1 provides a schematic block diagram of part of a transmitter of such a mobile radio telephone. In Figure 1 signals to be transmitted are represented by a first oscillator 1 and a second oscillator 2. The first oscillator 1 is representawtive of signals as would be produced by a GSM system. The frequency oscillator 2 is representative of signals that would be produced by a DCS 1800 system. Coupled to respective outputs of the first and second oscillators 1, 2, are the first and second inputs of a switch 4. A control input of switch 4 is coupled to a control processor 6 via a conductor 8. An output of the switch 4 feeds a power amplifier 10 which operates to amplify signals from either the GSN signal source 1 or the DCS 1800 signal source 2 in dependence upon a configuration of the switch 4. Signals at the output of the power amplifier 10 are fed to a notch filter 12 which operates to attenuate signals associated with a predetermined frequency. An output of the notch filter 12 is fed to an input of a low pass filter 14. The low pass filter 14 operates to attenuate signals having frequencies above a further predetermined frequency.

The filtered output signals are thereafter fed to terminal 16 for further processing within the dual mode mobile radio telephone.

The operation of the notch filter 12 is arranged to be configurable so as to either attenuate signals associated with the predetermined frequency or to allow all signals to pass to the output substantially unattenuated. Hence the notch filter 12 is configurable and the configuration is determined in dependence upon the mode in which the mobile radio telephone operates. This will be more easily explained with reference to the power spectral density of signals generated at the output of the power amplifier 10 in dependence upon whether the signal source is from the GSM signal source or from the DCS 1800 signal source. Figure 2a provides an exemplary illustration of a power spectral density of signals at the output of the power amplifier 10 when the switch 4 is configured to feed signals from the GSM signal source 1. As a result of non linearities within the power amplifier, in addition to the amplified GSM signal component designated 20 in Figure 2a, other harmonic signal components at multiples of the GSM signal component of 900 MHz appear at 1800 MHz, 2700 MHz and 3600 MHz which are designated 22, 24 and 26 respectively. If, however, switch 4 is configured to feed signals from the DCS 1800 signal source to the power amplifier 10, the power spectral density of signals at the output of the power amplifier 10 will appear substantially as illustrated in Figure 2b.

In Figure 2b there is a signal component represented by the line 28 which corresponds to the component provided from the DCS 1800 signal source 2. Correspondingly there are further harmonic components generated by non-linearities within the power amplifier 10 at integer multiples of the component at 1800 MHz. These are illustrated by the line 30 at 3600 MHz.

In order to provide a mobile radio telephone with a facility of operating with signals at either 900 MHz or 1800 MHz in an arrangement illustrated by Figure 1, the low pass filter 14 shown in Figure 1 is provided to attenuate harmonic components generated by the power amplifier 10 above a predetermined frequency. As illustrated in the power spectral diagrams shown in Figures 2a and 2b, a line 32 is representative of a frequency response of the low pass filter 14. The frequency response 32 is shown to attenuate signals above a frequency F1 which is slightly greater than the signal component at 1800 MHz.

Thereafter the low pass filter serves to attenuate signals with an increasing degree of attenuation until signals above a frequency of F2 are substantially completely attenuated. The effect of the low pass filter 14 is to remove all harmonic signal components above the harmonic components which appears at 1800 MHz for the case of the GSM mode and the DCS 1800 mode. Therefore, in the case when the dual mode handset is configured to operate with signals from the DCS 1800 signal source 2, the low pass filter is effective to remove all harmonic signal components. However, in the case of the mobile radio telephone being configured to operate with signals from a GSM signal source, although harmonic components above 1800 MHz will be substantially removed, there remains the component shown in Figure 2a as line 22 at around 1800 MHz, which will be substantially un-attenuated by the low pass filter 14. In order to remove the signal component 22 at 1800 MHz when the dual mode handset is operating in the GSM mode, the notch filter 12 is provided which is arranged to attenuate all signals associated with a frequency of 1800 MHz. This is shown in Figure 2a as the line 34. However, when the dual mode handset is operating with the configuration for DCS 1800, the notch filter 12 needs to be configured to allow all signals to pass within the 1800 MHz region to the effect that the notch filter is switched to an all pass mode wherein none of the signals are attenuated. To this end the control processor 6 provides signals on a conductor 36 in accordance with the change in configuration of the switch 2 to the effect of switching the notch filter between an all pass configuration and a notch filter configuration illustrated by the line 34 in Figure 2a.

An exemplary embodiment of the present invention is illustrated in Figure 3, where parts also appearing in Figure 1 bear identical numerical designations. In Figure 3 there is shown a configurable notch filter 12 comprising first and second strip line components 40, 42. The first of the strip line components 40 is connected in series between an input of the filter 44 and an output of the filter 46. The series section 48 of the strip line component 40 has a characteristic impedence Z 12. The second strip line component 42 has one end connected to the input of the filter 44, and another end connected to a first input of a switch 50. The other input to the switch 50 is connected to the series section line 48. A further input to the switch 50 is provided from a conductor 36 which is connected to the control processor 6 as described in Figure 1.

The configurable notch filter 12 is arranged to operate in a first all pass configuration with the switch 50 in a closed position. The configurable notch filter 12 is arranged to operate in a second notch configuration when the switch 50 is in an open state. When the switch 50 is in an open state, the first and second strip line components 40, 42 present a structure equivalent to an open circuit stub to signals presented on the input terminal 44 with a characteristic impedence Z1 followed by the series section line having the characteristic impedence Z 12. Hence, in the notch configuration, the characteristic impedence of the open circuit stub Z 1 is given by Equation 1: Z1 = ZOO/ZOO (ZOO+ Zoe)/2 with the characteristic impedence of the series section line Z12 given by Equation 2: Z12 = (Zoo+Zoe)/2 If there is no coupling between the first and section strip line components 40, 42, then Zoo = Zoe, Z1 = Zoo and Z12 = Zoo, where Zoo and Zoe are the odd and even mode impedence.

With the switch 50 in a closed state, the filter 12 presents an all pass configuration between the input terminal 44 and the output terminal 46 to the effect that the structure is equivalent to a series section of line with a characteristic impedence 212' which is equal to Zoe/2. These equations describe a case in which the first and second strip line components 40, 42 are symmetrically coupled. However, asymmetrically coupled lines could also be used and which may give another degree of freedom.

A further enhanced embodiment of the present invention is shown in Figure 4 where parts also appearing in Figures 3 and 1 bear identical numerical designations. The configurable notch filter 12 shown in Figure 1 is substantially the same as the configurable notch filter shown in Figure 3 and so the differences between Figures 4 and 3 will only be described. The configurable notch filter shown in Figure 4 further includes first and second signal matching circuits 52, 54 coupled in series with the input terminal 44 and the strip line components 40, 42, and the output terminal 46 and the series line section 48. An example of the signal matching circuits 50, 52 appears in Figure 5. The signal matching circuits 50, 52 serve to provide a matched characteristic impedence of the filter input and output to the components with which the filter is connected. In this case the input is coupled to the power amplifier 10 whereas the output is connected to the low pass filter 14.

By providing a circuit as shown in Figure 5, a matching of the values of the components shown in Figure 5 in accordance with the characteristic impedences provided by the strip line filter has an effect of matching the input and output impedences accordingly. To this end the circuit diagram in Figure 5 comprises first and second capacitors CPCN coupled to first and second inductors with inductance LGSM with a further inductance LPCN coupled between a junction connecting the capacitors to the inductors. The dual match allows the filter to be matched for the pass band in both states, i.e. 900MHz and 1800 MHz. As a result of the 900MHz matching, the Q of the notch is increased. The matching is interactive, the match at 900MHz effects the match at 1800MHz and vice versa, but can be achieved with the aid of computer aided design.

The form of the matching network shown in Figure 5 is provided in dependence upon the impedence level of the first and second coupled strip lines 40, 42. As such the input and output signal matching circuit 52, 54 will not be identical.

The switch 50 shown in Figures 3 and 4 which operates within the configural notch filter 12 may advantageously be a pin diode. However, when the switch is in an open mode a large radio frequency potential is generated across the diode at a fundamental frequency of the GSM signal. As such significant distortion and corrupting signal harmonics are generated. The deleterious effects of these components can be reduced by appropriately selecting the diode and by appropriately selecting a reverse DC voltage bias. Furthermore, a significant feature of the enhanced embodiment of the present invention shown in Figure 4, is the presence of the capacitor CPD shown coupled between the series section strip line components 48'. The radio frequency voltage developed across the switching diode is substantially reduced at the associated GSN fundamental frequency. This is achieved through the tuning capacitor CpD which has an effect of providing an equivalent face shift introduced by the series section of line 48'. This is k/8 in the case of a GSM band whereas it is /4 in the case of the DCS 1800 band and has an effect of equalising the RF potential on either side of the diode.

As a result, no potential difference is generated across the inputs of the diode with the corresponding effect that no distorting products are generated.

As will be appreciated by those skilled in the art, various modifications may be made to the embodiment of the invention hereinbefore described without departing from the scope of the present invention. In particular, the configural notch filter may find application in any number of radio frequency applications or indeed applications at other frequencies. Furthermore, the invention is not limited to operation within a dual mode radio telephone system adapted for operation with GSM and DCS 1800.

Claims

CLAIMS: 1. A filte for radio frequency signals comprising at least one strip line component coupled in operative association with a switch, wherein said filter operates to attenuate radio frequency signals having a predetermined range of frequencies in dependence upon whether said switch is in an open or a closed state.
2. A filter as claimed in Claim 1, wherein said at least one strip line component comprises first and second strip line components coupled in parallel between an input of the filter and an output of the filter, said second component being connected to said output via said switch, said filter operating to pass substantially all frequencies when said switch is in the closed state and attenuating signals with a predetermined range of frequencies when said switch is open.
3. A filter as claimed in Claim 2, wherein said first strip line component further includes a series section line having a predetermined characteristic impedence.
4. A filter as claimed in any preceding Claim, said filter further comprising a first signal matching means coupled between the input of the filter and the first and second strip line components, and a second signal matching means coupled between the output of the filter and the series section line.
5. A filter as claimed in any preceding Claim, wherein the first and second signal matching are adapted in accordance with an impedence provided by the first and second strip line components.
6. A filter as claimed in any preceding Claim, wherein the series section line further includes a tuning capacitor coupled in series with the series section line.
7. A filter as claimed in any preceding Claim, wherein said switch is a pin diode.
8. A radio signal transmitter for use in transmitting radio signals from a plurality of sources having different frequencies, said transmitter comprising a selector means to which said plurality of sources are coupled, a power amplifier coupled to an output of the selector means for amplifying signals fed thereto by the selector, a first filter as claimed in any preceding claim, coupled to an output of said power amplifier, for selectively attenuating signals of a predetermined frequency, a second filter coupled to an output of the first filter, for attenuating signals having frequencies below a predetermined frequency and a controller coupled to the selector means and to said first filter and arranged to control the selector means to determine which source of radio signals is to be passed to the power amplifier, and to control the operation of the first filter.