GB2381956A

GB2381956A - Suppression of rf interference on audio circuits

Info

Publication number: GB2381956A
Application number: GB0214196A
Authority: GB
Inventors: Xavier Denechaud
Original assignee: Sendo International Ltd
Current assignee: Sendo International Ltd
Priority date: 2002-05-27
Filing date: 2002-06-19
Publication date: 2003-05-14
Also published as: GB0212141D0; GB0214196D0

Abstract

A wireless communication device 200 such as a mobile telephone, comprises RF circuitry 210 and audio circuitry 230, each connected to respective portions RF_GND and AUD_GND of a common ground plane. The power supply may also be coupled to the common ground plane and all portions may be merged near the power supply. The radio and audio circuits 210, 230 may be coupled by a circuit resonating at radio frequency, and the audio circuit input may be electrically connected to another circuit resonating at radio frequency. A radio frequency shield 240 may also be coupled to RF_GND.

Description

INTERFERENCE SUPPRESSION Field of the Invention The present invention relates to an arrangement for suppressing radio interference in an audio circuit, in particular to an arrangement of suppressing radio interference in an audio circuit of a radio communication device.

Background of the Invention FIG. 1 illustrates a typical arrangement 100 for the audio ground plane of a radio communications device such as a mobile cellular telephone or the like. The radio communications device comprises radio (RF) circuitry 110 coupled to an antenna 120. The RF circuitry 110 is grounded to a common ground plane (GND) of the device.

Located proximate to the RF circuitry 110 is audio circuitry 130, comprising a speaker etc. (not shown). The audio circuitry 130 is also grounded to the common ground plane (GND) of the device, and is shielded from the RF circuitry 110 and antenna 120 by way of an RF shield 140.

The RF shield 140, which in general consists of a conductive material, is also grounded to the common ground plane (GND), thus providing a grounded screen between the RF circuitry 110 and the audio circuitry 130.

Although shielded from the RF circuitry 110 and antenna 120, the audio circuitry 130 is affected by RF interference. The RF interference is then audible through the speaker of the audio circuitry 130.

The interference is caused to a large extent by the RF circuitry 110, antenna 120 and RF shielding 140 being connected to the same ground plane as the audio circuitry 130.

The ground plane is connected to the ground terminal of the power source for the device, which in general is a battery (not shown) or the like. The ground terminal is generally able to withstand interference on the ground plane caused by, for example, the RF circuitry 110 and antenna 120. This problem of suppressing interference is particularly required in radio communications devices where an antenna is located within a main housing of the device. In this configuration increased RF signal levels within the radio communication device housing clearly increases any RF interference level imparted onto the ground plane.

This is in contrast to an external antenna configuration that extends from the main housing, where the antenna is substantially removed from, for example, audio circuitry and the like. In such a configuration, the RF shielding etc. is less susceptible to RF interference, and therefore there is less RF interference experienced on the audio ground plane. However, as radio frequency power amplifier devices remain internal to the radio

housing, a substantial amount of RF interference is often still imparted onto the ground plane. Suppression of this interference is therefore still required.

With the consumer demand for high quality devices, there is a need for overcoming the above mentioned problem of RF interference being audible through a speaker of the audio circuitry of a device such as a radio communication device. This need is increased by the additional consumer demand for smaller devices, which makes the use of an internal antenna a desirable option.

Statement of Invention According to the present invention there is provided a wireless communication device comprising a radio frequency circuit operably coupled to a first radio frequency portion of a common ground plane; and an audio circuit for enunciating audio signals. The audio circuit is located proximate to the radio frequency circuit and is operably coupled to a second audio portion of the common ground plane. The first radio frequency portion and the second audio portion of the common ground plane are sufficiently distal to substantially remove RF interference on audio signals enunciated by the audio circuit.

By providing, effectively, a separate ground plane for the audio circuitry, the RF interference it experiences through the common ground plane is significantly reduced.

Preferably, a first resonator may also be provided between the RF ground plane and audio ground plane, providing a damping coupling there between. Furthermore, a second resonator may, in addition or as an alternative to the first resonator, be operably coupled to an input of the audio circuitry.

Brief Description of the Drawings FIG. 1 illustrates a block diagram of a known arrangement for providing a ground plane for audio circuitry.

Exemplary embodiments of the present invention will now be described, with reference to the accompanying drawings, in which: FIG. 2 illustrates a block diagram of a radio communication device including an arrangement for providing a ground plane for audio circuitry according to a first embodiment of the present invention.

FIG. 3 illustrates a block diagram of a radio communication device including an arrangement for providing a ground plane for audio circuitry according to a second embodiment of the present invention.

FIG. 4 illustrates a block diagram of a radio communication device including an arrangement for providing a ground plane for audio circuitry according to a third embodiment of the present invention.

FIG. 5 illustrates a graph showing the properties of a resonator used in the second or third embodiments of the present invention illustrated in FIG. 3 and/or FIG. 4.

Description of Preferred Embodiments In summary, the inventor of the present invention has appreciated the fact that when the RF circuitry and/or internal antenna in a wireless communication device are located at a relatively large distance from the battery terminal, the RF interference at the battery's ground terminal, assuming a common ground plane is used, is lower than expected. Hence, a typical distal nature between the RF ground plane and the battery terminal ground ensures a substantially RF-interference free ground plane near to the ground terminal of the battery.

In this regard, the inventor of the present invention has also appreciated the fact that the inherent capacitance and impedance within the metallic tracks of, for example, a PCB (not shown) providing the ground plane reduces the ability of the battery terminal to maintain a steady ground plane in areas distant there from.

In the same manner, since audio circuitry 130 is often arranged proximate to RF circuitry 110 in a radio communication device, a significant level of RF interference appears on the audio circuit ground plane, when a common ground plane is used. Extending this concept, any proximal region of the ground plane, to which the RF circuitry 110 and RF shielding 140 (which is

substantially adjacent the antenna 120) are coupled, is susceptible to RF interference.

Referring to FIG. 2 there is illustrated a wireless communication device 200 according to a first embodiment of the present invention, comprising a novel ground plane arrangement. The wireless communication device may be any radio communications device, such as a mobile cellular telephone or the like. The wireless communications device comprises radio (RF) circuitry 210 coupled to an antenna 220. The RF circuitry 210 is grounded to an RF ground plane (RFGND).

The RF circuitry 210 may comprise a front-end switch coupled to the antenna, a transmit chain and a receive chain. Such RF circuitry is well known in the art, and so will not be described further herein.

Located proximate to the RF circuitry 210 is audio circuitry 230. Audio circuitry of devices such as radio communications devices in general comprise a microphone, a speaker, analogue to digital and digital to analogue converters, etc. For the present invention, the analogue circuitry 230 located proximate to the RF circuitry 210 comprises at least a speaker (not shown). The analogue circuitry 230 located proximate to the RF circuitry 210 may further comprise a digital to analogue converter. A microphone and analogue to digital converter are preferably located distal to the RF circuitry 210 of the radio communications device. Hereinafter, the term audio circuitry 230 refers to only those audio components

located proximate to the RF circuitry 210, such as the speaker and possibly the digital to analogue converter.

In accordance with the preferred embodiment of the present invention, the audio circuitry 230 is grounded to an audio ground plane (AUDGND). In particular, the audio ground plane is configured to be distal from the RF ground plane of the RF circuitry 210. Furthermore, the audio circuitry is shielded from the RF circuitry 210 and antenna 220 by way of an RF shield 240.

Notably, the RF shield 240, which in general consists of a conductive material, is also grounded to the RF ground plane (RF~GND), thus providing a grounded screen between the RF circuitry 210 and the audio circuitry 230.

Advantageously, by providing the audio circuitry 230 with, effectively, a separate ground plane to that used by the RF circuitry 210 and RF shield 240, the RF interference experienced by the audio circuitry 230 is significantly reduced. Indeed, it has been determined by the inventor of the present invention that this reduces the interference in the audio circuitry by approximately -lOdBs.

The individual ground planes RF~GND and AUD~GND are preferably each separately connected to the ground terminal of the power source of the device, such as a battery (not shown) or the like. Alternatively, the individual ground planes RF~GND and AUD~GND are merged into a single ground plane at a point near to the ground

terminal of the power source. In particular, the merging point should not be proximal to both the RF circuitry 210 and audio circuitry 230. In this manner, RF interference appearing on the RF ground plane RFGND is effectively operably de-coupled from the audio ground plane AUDGND.

Referring now to FIG. 3, there is illustrated a wireless communication device 300 according to a further enhanced embodiment of the present invention. This enhanced embodiment differs from the embodiment illustrated in FIG. 2, and described above, in that a resonator 310 is connected between the audio ground plane (AUDGND) and the RF ground plane (RFGND).

The resonator 310 is preferably arranged such that it provides a high impedance to electrical signals having a frequency substantially the same as, or close to, the frequency of the RF interference. In this regard, the resonator 310 provides a lower impedance at other frequencies. For example, for a mobile communications device adapted to operate with a GSM (Global System for Mobile Communications) network, the radio frequencies used will be in the region of 900MHz. Therefore, the resonator 310 provides a high impedance to electrical signals having a frequency around 900MHz. In the same way for networks operating over different frequency bands, such as PCS (Personal Communication System) or DCS (Digital Cellular System) networks, the resonator 310 provides a high impedance to electrical signals at those different frequencies i. e. in these examples around 1900MHz or 1800MHz respectively.

The resonator 310 effectively provides a damping coupling between the two ground planes, and further improves the reduction in RF interference to the audio circuitry 230.

Referring to FIG. 5, the preferred characteristics of the resonator 310 are shown. The preferred application for the enhanced embodiment using the resonator 310 is in a radio communications device connected to a GSM network operating at a frequency range around 900MHz. Since primarily the RF circuitry 310 and the antenna 320 generate the RF interference, the RF interference will also have a frequency of around 900MHz.

As can be seen for this example, the resonance of the resonator 310 falls off at around 200MHz, well below the frequency range of the RF interference. Furthermore, the resistance and impedance are highest in the region of approximately 300-lOOOMHz. Thus the resonator 310 provides a damping effect to the RF interference.

Referring now to FIG. 4, a wireless communication device 400 according to a further enhanced embodiment of the present invention is illustrated. This embodiment differs from the embodiment of FIG. 3 in that a second resonator 410 is provided through which input signals to the audio circuitry 230 are provided. This second resonator 410 is also configured to provide a high impedance at the relevant radio frequency, and acts to filter out any RF interference that is coupled to the audio circuitry input, thereby affecting the audio circuitry input

signals. The resonator 410 operates in substantially the same manner as the resonator 310 described with respect to FIG. 3 and FIG. 5 Thus, it has been found that the use of the resonators 310,410 significantly reduces the RF interference by around-40dB. This reduction in interference is clearly advantageous in any wireless communication device, particularly those that are portable and small in size.

FIG. 5 shows that the impedance and resistance characteristics of the resonators 310,410, in the audio frequency range of human speech of around lOOHz to around 7MHz, drops off considerably, being measured in the tens of ohms as opposed to the hundreds of ohms. Thus, audio frequency (speech) signals are attenuated much less than higher frequency signals, for example RF interference.

In summary, the use of two effectively distinct ground planes, as taught by the present invention, significantly reduces the opportunity for (RF) interference appearing on one ground plane, to be coupled to the second (audio

circuitry) ground plane. In the context of the present invention, the two ground planes-RFGND and AUDGNDare viewed as distinct (separate) if the interference generated on the RF~GND is sufficiently attenuated to not affect the audible generation of speech signals by an audio circuit using an AUD~GND. Hence, although the two grounds may be physically coupled together, the location of the physical coupling is sufficiently distal from one or both of the RF circuitry 210 or audio circuitry 230 to

substantially suppress any RF interference, irrespective of the proximal relationship between the respective circuits.

In this way, the audible effect of the interference that a user experiences is also reduced, improving the user perception and speech quality of the radio communications device. Furthermore, the use of resonators, as illustrated in FIG. 3 and FIG. 4 has been shown to provide further improvements in the reduction of RF interference.

Although in the illustrated embodiments the RF shield 240 has been shown to be provided around the audio circuitry 230, it is within the scope of the present invention for the RF shielding 240 to be provided around the RF circuitry 210, instead, or in addition to, a grounded screen still being provided between the RF circuitry 210 and the audio circuitry 230.

The audio circuitry 230 located proximate the RF circuitry 210 may comprise such components as a speaker, a digital to analogue converter, etc. The present invention is not limited to reducing RF interference in all audio circuitry of the device being located proximate to the RF circuitry 210 and antenna 220. It is envisaged that a part of the audio circuitry may be located at another part of the device. In this case, the part of the audio circuitry not located proximate to the RF circuitry and/or the antenna may not require the separate ground plane, due to it being located apart (distal) from the RF

circuitry 210 and the antenna 220, and thereby not suffering from RF interference on a common ground plane to the same extent.

Indeed, since it is likely that a user would only notice such RF interference via the speaker, the second resonator 410 would substantially filter out RF interference. In this manner, it is not as critical for RF interference to be prevented from affecting other distal audio components.

The RF ground plane (RFGND) may be used as the ground plane for other components of the device. However, preferably the audio ground plane (AUDGND) is solely used by the audio circuitry 230 when the audio circuitry is located proximate to the RF circuitry 210 and antenna 220. In this way, the effect of RF interference is minimised.

Whilst the specific and preferred implementations of the embodiments of the present invention are described above, it is clear that one skilled in the art could readily apply variations and modifications of such inventive concepts.

Thus, a wireless communication device that suppresses interference coupling between an RF circuit and a proximal audio circuit within the wireless communication device has been described, where the aforementioned disadvantages with prior art arrangements have been substantially alleviated.

Claims

Claims 1. A wireless communication device comprises: a radio frequency circuit operably coupled to a first radio frequency portion of a common ground plane; and an audio circuit for enunciating audio signals, located proximate said radio frequency circuit and operably coupled to a second audio portion of said common ground plane, wherein said first radio frequency portion and said second audio portion of said common ground plane are sufficiently distal to substantially remove RF interference on audio signals enunciated by said audio circuit.
2. The wireless communication device according to Claim 1, wherein said first radio frequency portion and said second audio portion of said common ground plane are operably coupled together by a resonator that resonates substantially at a frequency radiated by said radio frequency circuit.
3. The wireless communication device according to Claim 1 or Claim 2, wherein said audio circuit includes an input for receiving audio signals to be enunciated, said input being operably coupled to a resonator that resonates substantially at a frequency radiated by said radio frequency circuit.
4. The wireless communication device according to any preceding Claim, wherein said radio frequency circuit

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and said audio circuit are operably coupled to a power supply that, in turn, is coupled to said common ground, wherein said first radio frequency portion and said second audio portion of said common ground plane are merged at, or proximate to, said power supply.
5. The wireless communication device according to any preceding Claim, wherein audio circuitry is shielded from the radio frequency circuitry by way of a radio frequency shield that is operably coupled to said first radio frequency portion of said common ground.
6. A wireless communication device substantially as hereinbefore described with reference to, and/or as illustrated by, FIG. 2 or FIG. 3 or FIG. 4 of the accompanying drawings.