GB2359949A - A tunable two-path interference notch filter using a programmable delay - Google Patents

Description

1 TUNEABLE NOTCH FILTER 2359949

The present invention relates to the field of notch filters. More specifically, the present invention relates to a tuneable notch filter for use 5 in a wireless telecommunications system.

Presently, notch filters are used in wireless telecommunications systems to block potentially interfering signals from reaching the receiver. These interfering signals may originate from that system's own transmitter or from another wireless telecommunications system. These interfering signals have the effect of degrading the overall sensitivity of the receiver.

The massive growth in wireless telecommunications has caused an increase in the number of potentially interfering signals, thus increasing the demands on the performance of the notch filter.

In order to optimise the performance of the receiver, tuneable notch filters are currently being used. With a tuneable notch filter the frequency of maximum attenuation can be precisely tuned to the transmission frequency of the system, thereby reducing the amount of interfering signal reaching the receiver and enabling maximum sensitivity in the receiver.

The operation of tuneable notch filters is well know and will be briefly described with reference to figure 1.

2 Figure 1 is a diagram of a tuneable notch filter 10. A signal is input to the filter at input port 1 and then divided by splitter 12 into two paths 2 and 3. The signal in path 2 is left unaltered, whereas the signal along path 3 is delayed by a predefined period. The length (c) of the delay is determined by delay means 14 and dictates the period of the filter's notches. Path 3 also includes phase shifting means 16. Varying the phase of the signal along path 3 enables the filter's notch to be tuned over a frequency range. The signal from paths 2 and 3 are then recombined by combining means 18 and output from the filter as one signal via output port 4.

The rapid growth in wireless telecommunications has created an interest in multi-band and multi-mode mobile phones. To date, a mobile phone works in a predefined frequency range using a predefined mode. For example, GSM phones transmit in 880-915 MHz frequency range and receive in 925-960 MHz frequency range and operate in frequency division duplex (FDD) mode. Other types of mobile phones operate in different frequency ranges and different modes.

Currently, each type of mobile phone has a notch filter designed specifically for the predefined frequency range in which that phone operates. In order for a phone to operate in a different frequency range a new notch filter, amongst other things, would need to be installed in the phone's handset.

3 It is an object of the present invention to provide a tuneable notch filter which can operate over a wide frequency bandwidth.

According to the present invention there is provided a tuneable notch filter for use in a wireless telecommunications system comprising splitting means arranged to divide an input signal into a first and a second path, delay means arranged to delay the'signal in the first path by a fixed delay, phase shifting means comprising a series of delay elements arranged to phase shift the signal in the first path within a predefined range, recombining means arranged to recombine the signal in the first path and the signal in the second path to form an output signal, the output signal being capable of attenuating radiation over a range of frequencies, the range of frequencies being determined by the fixed delay and the amount of phase shift.

According to an aspect of the present invention, the series of delay elements are of phase shifting the signal in the first path from 0' to 360' The series of delay elements may be operated using PIN diode switches.

The series of delay elements may be surface acoustic wave devices.

The delay means may be a surface acoustic wave device.

At least one of the series of delay elements may be a printed line.

The series of delay elements may comprise seven delay elements.

4 According to the present invention there is provided a method of adjusting the frequency of maximum attenuation in a tuneable notch filter, the method comprising the steps of. inputting a signal, splitting the signal into a first and a second signal, delaying the first signal by a predefined a'mount,, phase shifting the first signal by a variable amount, recombining the first and second signals, and outputting the recombined signal such that the frequency of maximum attenuation is dependent on the predefined amount of delaying and the van able amount of phase shifting. While the principle advantages and features of the invention have been described above,, a greater understanding and appreciation of the invention may be obtained by referring to the drawings and detailed description of a preferred embodiment, presented by way of example only in which; Figure 2 is a diagram of the delay means used in the tuneable notch filter according to the present invention, and Figure 3 is a graph of frequency versus attenuation obtained with a tuneable notch filter using the delay means shown in figure 2.

In figure 2, delay circuit 20 is shown comprising delay means 22 and a series of delay elements 23-29. The operation of a notch filter using delay circuit 20 is similar to the operation of the notch filter shown in figure 1 and will now be described with reference to items appearing in figure 1. Delay means 22 delays the signal in path 3 by a fixed amount in the same manner as delay means 14. The amount of the fixed delay determines the initial notch to notch separation of the filter.

The series of delay elements 23-29 replace the single phase shifting means 16. The use of a series of delay elements that can be switched in and out, for example, by using PIN diode switches 33-39, allows the phase of the signal in path 3 to be varied from Tto 36T. By switching the series of delay elements 23-29 in a binary fashion using the PIN diode switches 33-39 a variety of phase switches can be obtained, thus enabling the notches to be shifted in short steps to cover the frequencies between the initial notch frequencies. This ability to vary the phase shift allows the notches to be tuned over a broad range of frequencies.

For example, by selecting the appropriate fixed delay and series of delay elements, a tuneable notch filter can be made according to the present invention which has notches having a bandwidth of greater than 5MHz at 20dB of attenuation and which can be tuned with steps of 5KHz from 800MHz to 2200MHz.

Figure 3 shows the frequency attenuation of a tuneable notch filter made according to the present invention. In figure 3, the X-axis represents frequency in gigahertz (Gliz) and the Y-axis represents attenuation in decibels (dB). The operational frequency range of this filter is 800MHz-2200MHz. A fixed delay (To) of 6250ps is introduced 6 to the signal in path 3 by delay means 22. This produces notches with a period of 160MHz. Seven further delay elements 23-29 are used, each introducing a different amount of delay as follows: T1 = 15.59ps, T2 = 31.17ps, T4= 62.34ps, T. 124.69ps, T,6= 249.38ps, T32= 498.76ps, and T64= 997.52ps. When all the delays 23-29 are switched in the total delay is 1979.45ps plus the fixed delay of 6250ps. This arrangement of delay elements allows the filter to be tuned over a 160MHz notch band in eighty nine 5KHz steps over a 800MHz to 2200MHz frequency range.

As will be appreciated, the number of delay elements and the amount of delay each element introduces could be varied to meet the specific requirements of the filter.

In the preferred embodiment, delay means 22 and series of delay elements 23-29 are surface acoustic wave (SAW) devices. SAW devices advantageously allow for large delays to be produced in small percentages. In an alternative embodiment, the small delays, for example, T1 and T2are implemented using printed lines.

It is not intended that the present invention be limited to the above embodiments and other modifications and variations are envisaged within the scope of the claims.

7

Claims (9)

1 A tuneable notch filter for use in a wireless telecommunications system comprising splitting means arranged to divide an input signal into a first and a second path, delay means arranged to delay the signal in the first path by a fixed delay, phase shiffing means comprising a series of delay elements arranged to phase shift the signal in the first path within a predefined range, recombining means arranged to recombine the signal in the first path and the signal in the second path to form an output signal, the output signal being capable of attenuating radiation over a range of frequencies, the range of frequencies being determined by the fixed delay and the amount of phase shift.

2 A filter as claimed in Claim 1, wherein the series of delay elements is capable of phase shifting the signal in the first path from 0' to 36T.

3 A filter as claimed in any preceding Claim, wherein the series of delay elements are operated using PIN diode switches.

4 A filter as claimed in any preceding Claim, wherein at least one of the series of delay elements is a surface acoustic wave device.

8 A filter as claimed in any preceding Claim, wherein the delay means is a surface acoustic wave device.

6 A filter as claimed in any preceding Claim, wherein the at least one 5 of the series of delay elements is a printed line.

7 A filter as claimed in any preceding Claim, wherein the series of delay elements comprises seven delay elements.

8 A method of adjusting the frequency of maximum attenuation in a tuneable notch filter, the method comprising the steps of. inputting a signal, splitting the signal into a first and a second signal, delaying the first signal by a predefined amount, phase shifting the first signal by a variable amount, recombining the first and second signals, and outputting the recombined signal such that the frequency of maximum attenuation is dependent on the predefined amount of delaying and the variable amount of phase shifting.

9 A tuneable notch filter as hereinbefore described with reference to the accompanying figures.