GB2280808A

GB2280808A - A filter and a radio communications device comprising the same.

Info

Publication number: GB2280808A
Application number: GB9316456A
Authority: GB
Inventors: Matthias Koenig
Original assignee: Motorola GmbH
Current assignee: Motorola Solutions Germany GmbH
Priority date: 1993-08-07
Filing date: 1993-08-07
Publication date: 1995-02-08
Anticipated expiration: 2013-08-07
Also published as: GB9316456D0; GB2280808B

Abstract

To provide a low pass filter (10) with good harmonic rejection and minimal insertion loss, a relatively low impedance (L6) is selectively switched in parallel with a shunt impedance (L3) located in a network of the filter (10), whereby the series resonant frequency provided by the shunt impedance in combination with a relatively high impedance point is altered and the filter characteristic is changed; whereby the passband is selectively widened and narrowed whilst stopband rejection is kept below a specific level thereby filtering out harmonics in the stop band. In a radio communications transceiver, Fig 4, not shown, a microprocessor (48) is used to control the switching voltage Ux in response to a transmission frequency. <IMAGE>

Description

A FILTER AND A RADIO COMMUNICATIONS DEVICE COMPRISING THE SAME Backnound to the Invention This invention relates, in general, to filters and is particularly, but not exclusively, applicable to filters for radio communications devices.

Summarv of the Prior Art In existing filters, and especially small sized radio frequency low pass harmonic filters in transmitters of modern radio communications devices, there is a requirement to provide good harmonic rejection.

However, such filters suffer from a high insertion loss. Therefore, in order to achieve good harmonic rejection, a microprocessor is often used to selectively control the characteristic of the filer in response to a current operating frequency. More particularly, it is known to adjust the characteristic of the filter by switching in and out a whole filter network, such as a T- or s- network. It is also known to switch between entire filters having entirely different filter characteristics. As will be appreciated, with the limited availability of space in modern radio communications equipment, the provision of two filters is both expensive in terms of monetary costs and in the consumption of available space.

Similarly, the provision of an additional filter network is relatively expensive because inductors located in such a network are usually realised as discrete coils.

Clearly, it can be appreciated that there is a requirement in the arts to provide an inexpensive filter with a low insertion loss and high harmonic rejection.

Summarv of the Invention In accordance with the present invention there is provided a low pass filter comprising an input and an output and a series of inductors between the input and the output having a series of shunt elements coupled between common points between adjacent inductors and ground, such that the common points have a high impedance relative to ground, characterised in that a switching circuit is provided for switching a relatively low impedance between a relatively low impedance point of one of the shunt elements and ground, whereby the passband is selectively widened and narrowed whilst the stopband rejection, provided by the series of shunt elements, is kept below a specific level thereby filtering out harmonics in the stopband.

Typically, the filter has a harmonic rejection of better than 80dB and is suitable for use in a radio communications device, in which case the switching element switches in response to a transmission frequency for the radio communications device.

An exemplary embodiment of the present invention will now be described with reference to the accompanying drawings.

Brief Descrintion of the Drawings Fig. 1 illustrates a filter constructed in accordance with a preferred embodiment of the present invention.

Figs. 2 and 3 illustrate filter characteristics of the filter of Fig. 1.

Fig. 4 shows a radio communications device comprising the filter ofFig. 1.

Detailed Descrintion of a Preferred Embodiment Referring to Fig. 1, there is shown a filter 10 constructed in accordance with a preferred embodiment of the present invention. The filter 10 comprises an input 12 and an output 14. Coupled between the input 12 and output 14 are a series of inductors L7-L10. A number of circuit nodes (common points) 16-26 exist between the input 12, the inductors L7-b10 and the output 14. These circuit nodes have a high impedance relative to ground. A capacitor C6 is located between circuit nodes 24 and 26. A capacitor C1 is coupled in series between circuit node 16 and a shunt element (inductor) L1. The shunt element L1 is also coupled to ground.Similarly, inductor-capacitor combinations (C2, L2), (C:3, L3), (C4, L4) and (C5, L5) are coupled between ground and circuit nodes 18-24 respectively. An inductor L11, coupled between ground and circuit node 26, provides protection against electro-static discharge (ESD). The common points have a relatively high impedance relative to that of ground.

A switching circuit comprises an input 27, typically coupled to a microprocessor (not shown), serially coupled through a resistor Rx, a relatively low impedance inductor L6 and a diode D1 to a circuit node 28 located between capacitor C3 and shunt element L3. The resistor Rx and the inductor L6 are coupled through a capacitor C7 to ground. Circuit node 20 has a relatively high impedance value with respect to circuit node 28. The diode D1, which may be a PIN diode, is controlled by a control voltage Ux applied to the input 27 by the microprocessor, for example.

Typically, inductors L1-L6 are realised as small size printed circuit board (PCB) inductors and, as such, do not add significantly to the cost to the filter 10. Inductors L7-L11 may be discrete coils. As will be understood, component values will vary according to the exact design requirements for the filter, but by way of example the values shown in Table 1 would be suitable for a filter having a desired frequency range of between 136MHz and 174MHz.

Table 1

Component No. Value L1 4.5nH L2 4.0nH L3 8.0nH L4 2.5nH 2.0nH L6 4.5nH L7 42.5nH L8 45.0nH L9 49.5nH Llo 45.0nH L11 100nH C1 22pF C2 39pF C3 33pF C4 27pF 22pF 470pF C7 3300pF As will be understood by one of ordinary skill in the art, a diode will usually introduce harmonics into a filter because of the non-lineality of the diode at high radio frequency (RF) power. However, in the present invention, the diode is in a low impedance path which is selectively switched into the filter 10 in response to the application of a control voltage Ux applied thereto. This low impedance path does not introduce the non-linearity.

An operational description of the filter will now be undertaken by way of example. If the transmitter frequency is below, say, 155MHz, then the control voltage Ux, driven by the microprocessor, is low, e.g.

0 volts. (For high RF power applications, a negative voltage might be used instead). If the transmitter frequency is above 155MHz, the control voltage U; provided to input 27 is set high, e.g. +5V, whereby the inductance LG is switched in parallel with inductor L3. Therefore, the series resonant frequency of pole X (as indicated on Fig. 2) is changed, with its notch moved to the position of pole Xa in Fig. 3. In this way, the pass band can be selectively widened or narrowed, whilst the stop band is kept below a specific level thus ensuring that undesirable harmonics are rejected in the stop band. As will be appreciated, there is a requirement to select the component values to ensure that the notches (poles) in the filter characteristic are kept close to the harmonics in the stop band.

With regard to Fig. 2, there is shown a filter characteristic for filter 10 when no control voltage is supplied. Referring to Fig. 3, the inductance L6 has been switched in parallel with inductor L3 through the application of the control voltage Ux. Furthermore, by direct comparison of these figures, it can be seen that the pass band of Fig. 3 is wider than that of Fig. 2 and that the stop band rejection of the characteristic of Fig. 3 has been improved. More specifically, pole X in Fig. 2 has a frequency 270MHz and a rejection of 90 decibels (dB) whereas corresponding pole Xa has a frequency 430MHz and a rejection of - 1GOdB.

It will be understood by one skilled in the art that the aforementioned filter design may be further simplified through the reduction of component count, but at the expense of decreased stop band rejection. Specifically, components C1, L1 and L7 and/or C5, L5 and L1 may be eliminated if lower harmonic rejection is acceptable.

Furthermore, it is recommended that, in order to achieve best performance for the filter, the filter 10 is notched such that poles X and Y substantially correspond to the second frequency harmonics, e.g.

2x136MHz and 2x155MHz, respectively, in the exemplary embodiment.

In summary, a relatively low impedance (relative to the relatively high impedance provided by the series combination of inductors L7-L10) is switched between a relatively low impedance point of one of the shunt elements and ground, whereby the passband is selectively widened and narrowed whilst stopband rejection, provided by the series of shunt elements, is kept below a specific level thereby filtering out harmonics in the passband.

Fig. 4 shows a radio communications device 40. The radio communications devices comprises an antenna 42 for transmitting and receiving radio frequency signals 44. The antenna 42 is coupled to a transceiver 46, responsive to a microprocessor 48. The microprocessor 48 provides operational control of the radio communications device 40.

A filter 10, constructed in accordance with the aforementioned description, is located within the transceiver 46. As will be appreciated, it is advantageous for the filter 10 to be responsive to the microprocessor, whereby the microprocessor 48 provides the required control signal Ux to the filter 10 since the microprocessor is already present in modern radio communications devices. Signals received or transmitted by the communications device 40 are filtered by the filter 10 of the transceiver 46. Inputloutput circuitry 50, responsive to the microprocessor 48, is coupled to the transceiver to allow data or voice messages to be broadcast or transmitted from the radio communications device 40. As will be appreciated, the transceiver 46 may be alternatively realised by separate receiver and transmitter sections.

Implementation of the filter 10 in the radio communications device 40 allows a transmitter thereof to operate in a very wide frequency band whilst providing low insertion loss and very high harmonic rejection.

A filter so designed and described produces the novel advantages of a relatively inexpensive filter with a low insertion loss and high harmonic rejection, and which allows the bandwidth of the passband of the filter to be varied according to operational requirements for the filter. More particularly, the passband may be selectively widened or narrowed whilst stop band rejection is kept below a specified level to filter out harmonics in the passband.

Claims

1. A low pass filter comprising an input and an output and a series of inductors between the input and the output having a series of shunt elements coupled between common points between adjacent inductors and ground, such that the common points have a high impedance relative to ground, characterised in that: a switching circuit is provided for switching a relatively low impedance between a relatively low impedance point of one of the shunt elements and ground, whereby the passband is selectively widened and narrowed whilst stopband rejection, provided by the series of shunt elements, is kept below a specific level thereby filtering out harmonics in the stop band.

2. A low pass filter as claimed in claim 1, wherein the filter has a harmonic rejection of at least 80dB.

3. A radio communication device having a filter in accordance with any preceding claim.

4. A radio communication device as claimed in claim 3, wherein the switching element switches in response to a transmission frequency for the radio communications device.

5. A low pass filter substantially as described herein with reference to Figs 1-3 of the accompanying drawings.

6. A radio communication device substantially as described herein with reference to the accompanying drawings.