WO2002043257A1

WO2002043257A1 - Apparatus and method for suppressing conducted radio-frequency signals

Info

Publication number: WO2002043257A1
Application number: PCT/AU2001/000797
Authority: WO
Inventors: Laird Alan Varzaly; Mikhail Yuri Loukine
Original assignee: Simply The Best.Com.Au Pty. Ltd.
Priority date: 2000-11-22
Filing date: 2001-07-03
Publication date: 2002-05-30
Also published as: AUPR161300A0

Abstract

An apparatus and a method is disclosed for suppressing or reducing radio frequency signals induced in a cable from a source such as a mobile phone antenna. The cable may be for an accessory such as a headset intended for 'hands free' operation of the mobile phone. The apparatus includes means for generating at least one impedance discontinuity in the cable at a point intermediate the source of the signals on the cable and a location on the cable such as its termination at which it is desired to suppress the signals. The means for generating includes at least one transmission line operably connected to the cable at said point such that the discontinuity is generated substantially at a frequency at which it is desired to suppress the signals.

Description

APPARATUS AND METHOD FOR SUPPRESSING CONDUCTED RADIO-FREQUENCY SIGNALS

FIELD OF THE INVENTION

The present invention relates to apparatus and a method for suppressing or reducing radio frequency (RF) signals such as may be induced in a conductor from a source such as radio transmitter. In particular the invention relates to apparatus and a method for suppressing RF signals that are induced into and/or conducted along the conductor. The conductor may be a cable to an accessory such as a headset intended for "hands free" operation of a wireless transmitter or transceiver, such as a mobile phone.

BACKGROUND OF THE INVENTION

Figure 1A shows a simplified diagram of a generic system in which a reduction of induced RF signals may be required. In this system a radio-frequency transmitter 1 radiates electromagnetic energy by means of antenna 2. The electromagnetic radiation results in induced RF currents 3 in a conductor 4, which may be represented by a cable with two devices 5 & 6 connected to the ends of the conductor 4 as terminations. The induced currents 3 in conductor 4 will flow into termination devices 5 <°x 6, if they are not suppressed appropriately prior to entry into the devices. The flow of induced RF currents 3 into the termination devices or on a particular section of conductor 4 may be unwanted for various reasons. Hence, measures are desirable to suppress the currents 3 on conductor 4.

Figure 1 B depicts a diagram of a particular arrangement that may require suppression of RF currents 3 propagating on a conductor such as headset cable 4 which is exposed to RF radiation from mobile phone 1. A mobile phone accessory device, such as a "hands free" headset may be connected to mobile phone 1 to allow a user to be able to use the mobile phone without a need for close proximity of the phone handset to the user's ear. Close proximity may be undesirable for various reasons including convenience, legal requirement while driving, and perceived protection from direct RF radiation from the phone's antenna, etc. In the configuration shown in Fig. 1B headset cable 4 is connected to the mobile phone 1 , while headset speaker 6 associated with the headset is mounted on or inside the user's ear 7. In order to maintain wireless communications, mobile phone 1 radiates a strong electromagnetic field via its antenna 2. This field may induce RF currents in headset cable 4. The induced currents may propagate along cable 4 to the speaker end. The headset speaker 6, and its cable 4 may be mounted in direct contact with the body of the user 7. This may cause RF currents to be conducted directly into the user's tissue and/or re-radiated as secondary RF emissions within the user's ear.

Some medical researchers have suggested that exposure to RF currents and radiation generated by mobile phones and associated mobile phone "hands free" headsets may pose a health risk and thus may be harmful to the user. Accordingly, it is desired that the level of tissue exposure to mobile phone RF radiation be reduced. The present invention may provide a means to reduce such current and radiation.

Although the present invention will be described herein with reference to mobile phone "hands free" headsets, it is to be appreciated that it is not thereby limited to such applications. Thus the invention may be equally applied to conductors, such as wires and cables generally, wherein a need arises for suppressing RF currents as a source of interference or unwanted conducted emissions for any purpose. It may have particular advantages in applications in which low weight, ease of manufacture, small physical volume, and/or low cost may be an issue.

One traditional method of suppressing RF currents in a conductor is the practice of mounting a ferrite (or other high permeability material) sleeve on the conductor. This method reduces the level of RF currents in the conductor due to increased impedance of the section of the conductor covered by the ferrite sleeve. Due to weight and size this method is not appropriate, for a miniature, lightweight headset or other applications where size and weight are limiting factors. Another traditional method of suppressing RF currents in a conductor is the practice of "grounding" by connecting it to a conductive screen or to a large ground conductor. This causes RF currents to be bypassed to ground or to the screen, and not to propagate further along the cable. This method is also not appropriate for a portable application, such as a "hands free" headset.

An object of the present invention is to provide a method and apparatus for suppressing or reducing RF signals along a conductor such as a cable. The method/apparatus may be capable of reducing RF signals not only at terminations of the cable, but also in sections of the cable intermediate the terminations.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided an apparatus for suppressing radio frequency signals carried along a main conductor including means for generating at least one impedance discontinuity in said conductor at a point intermediate a source of said signals on said conductor and a location on said conductor at which it is desired to suppress said signals, said means for generating including at least one transmission line operably connected to said conductor at said point such that said discontinuity is generated substantially at a frequency at which it is desired to suppress said signals.

According to a further aspect of the present invention there is provided a method for suppressing radio frequency signals carried along a main conductor including generating at least one impedance discontinuity in said conductor at a point intermediate a source of said signals on said conductor and a location on said conductor at which it is desired to suppress said signals, said generating including operably connecting at least one transmission line to said conductor at said point such that said discontinuity is generated substantially at a frequency at which it is desired to suppress said signals. PREFERRED EMBODIMENTS OF THE INVENTION

Preferred embodiments of the invention will now be described with reference to the accompanying drawings wherein:

Figures 1A and 1 B show prior art arrangements that may require suppression of RF signals;

Figure 2 shows a basic form of a filter utilizing a single quarter-wavelength stub;

Figure 3 shows a cable impedance curve at a point of contact with the quarter- wavelength stub;

Figure 4 is a plot of a transfer function for RF currents propagating from the source of RF signals to the cable termination, when the quarter-wavelength stub is used;

Figure 5 is a modification of the embodiment shown in Figure 2 utilizing two quarter-wavelength stubs in parallel;

Figure 6 shows a filter wherein the quarter-wavelength stub of the embodiment shown in Figure 2 is replaced with a circular conductive plate;

Figure 7A shows a filter utilizing a folded filtering stub;

Figure 7B shows a filter utilizing a conductive sleeve mounted over the cable;

Figure 8 shows a generalised form of an embodiment wherein more than one filter is used on the cable;

Figure 9 is a plot of a transfer function for RF currents propagating along the cable wherein two filters tuned for adjacent frequencies are used; Figure 10 shows a representative embodiment, wherein two folded stubs tuned for adjacent frequencies, are used to suppress RF currents on a cable within a specific frequency band;

Figure 11 shows an embodiment wherein two pairs of folded stubs are used for suppressing two frequency bands; and

Figure 12 is a plot of a transfer function for RF currents propagating along the cable wherein the filter shown in Figure 11 is used.

A simplified diagram of a preferred embodiment, which may be used to filter RF currents along a headset cable such as a cable associated with the headset of a mobile phone, is shown in Figure 2. In this embodiment headset cable 8 is exposed to RF emissions, shown herein as a source 9 of conducted emissions, and induced RF currents 10 propagate along cable 8 towards a cable termination device 11 , such as a headset speaker. A linear portion of a conductor (stub) 12 is connected to cable 8 at a point A. This embodiment utilises a property of a quarter-wavelength transmission line to transform RF impedance. Herein stub 12 has an approximate length of L=λ/4, wherein, λ is a wavelength corresponding to the radio frequency to be filtered. Stub 12 transforms an open circuit (OC), i.e. high impedance, at its free end (Point B) into a short circuit (SC), i.e. very low impedance, at the opposite end (Point A). This transformation is valid in a vicinity of a particular frequency F_r, being the tuning frequency of stub 12.

Since stub 12 is connected to cable 8 at Point A as shown in Figure 2, point A also exhibits a low-impedance condition around the frequency F_r. The effect of quarter-wavelength conductor or stub 12 on the cable impedance at Point A in Figure 2 is shown in Figure 3 wherein the magnitude of the cable impedance Z(f) at Point A is plotted against frequency. The impedance curve has a minimum at the tuning frequency (F_r) of stub 12. Since, in general, a headset cable impedance relative to ground is high, a low-impedance condition at Point A in Figure 2 will cause incident RF currents on cable 8 to be reflected at Point A back to the source 9 of the RF emissions, due to an impedance discontinuity at Point A. Thus residual RF current leakage towards headset speaker 9 will be significantly reduced.

Figure 4 shows the transfer function G for RF currents propagating, as shown in Figure 2, from RF emissions source 9 to cable termination 11. The curve shown in Figure 4 is plotted against frequency. This plot represents a characteristic of a notch filter, i.e. least propagation of RF currents from source 9 to termination 11 in Figure 2 occurs at frequency F_r. If a reduced level of RF currents suppression shown as Si in Figure 4 can be accepted, then this embodiment may be considered to be a band-reject filter with frequency range from F to FHI and a minimum suppression value S-i.

In another preferred embodiment shown in Figure 5, there are two or more quarter-wavelength stubs 12, 13 connected to cable 8. These stubs, connected in parallel to a single Point A on cable 8, will further reduce cable impedance at Point A. Hence, this may result in a higher mismatch of impedances and a higher level of isolation (lower value of transfer function G) between RF emissions source 9 and cable termination 11.

In the third preferred embodiment shown in Figure 6, the linear quarter- wavelength stubs are replaced with a circular conductive plate 14. The radius of plate 14 equals approximately a quarter-wavelength at the frequency of the desired RF suppression. The centre of plate 11 is connected to cable 7 at a single Point A.

In some applications the embodiments described thus far may be challenging to implement due to physical constraints imposed by the quarter-wavelength stubs 12, 13 arranged as shown in Figures 2 & 5, or by the circular plate 14, shown in Figure 6. These constraints may be avoided by the alternative embodiments, shown in Figures 7A & 7B.

In the fourth preferred embodiment, shown in Figure 7A, a linear conductor 15, serving as a filtering stub, is folded and positioned parallel to cable 8. Conductor 15 has a length L_Sι that is approximately a quarter-wavelength at the suppression frequency F_r to produce a short circuit condition at a single point of connection A to cable 8. Using the effect of cross coupling between the closely spaced stub and the cable, the stub length may be reduced down to one-eighth of the wavelength. The stub length may be further reduced by the effect of a dielectric material surrounding the stub 15 and the cable 8 due to permittivity of the material being higher than free air permittivity. As in the embodiment of Figure 5, the embodiment of Figure 7A may contain two or more folded equal- length stubs positioned in parallel with the cable and connected to a single point A. The contribution of each filter stub maximises the effect of the composite filter by further lowering RF impedance of the cable at point A.

In the fifth embodiment, shown in Figure 7B, the folded stub of the Figure 7A embodiment is replaced with a conductive sleeve 16 mounted over cable 7. Sleeve 16 has a length which is approximately a quarter-wavelength, and it makes electrical contact with cable 8 at a single Point A. The length of sleeve 16 is designed to produce minimum cable impedance at Point A for specific cable and sleeve diameters and materials implemented.

The embodiments shown in Figures 7A &. 7B are relatively straightforward to implement in practice, wherein filtering stub 15 or sleeve 16 may become a conformal part of cable 8, and may be as lightweight and as flexible as cable 8 itself. However, some applications may require suppression of RF conducted emissions on a cable over a wider frequency band. These applications are addressed by the embodiments described below.

Figure 8 shows the sixth preferred embodiment, which suppresses RF conducted emissions 10 propagating from RF emissions source 9 along cable 8 towards cable termination device 11. The filtering in this embodiment is accomplished by implementing two or more filters in tandem. These filters are connected in tandem along cable 8 and may be tuned to different frequencies to cover different bands of the RF spectrum wherein suppression of RF emissions is required. In the case of two filters 17 & 18 employed for RF current suppression in Figure 8, one filter may be tuned to a first frequency F_rι, and the other may be tuned to a second frequency F_r2. The frequencies F_rι & F_r2 may be carefully selected to produce a band-stop response as shown in Figure 9. The frequency band Fι_₂ to FH2 covered by this filter is wider than the band F_Lι to Fm covered by a single filter as shown in Figure 4.

Filters, such as filters 17 8c 18, may be implemented as any of the herein described embodiments shown in Figures 2, 5, 6, 7A & 7B. For an application such as a mobile phone "hands free" headset cable, band-stop filters as shown in Figures 7A & 7B may be appropriate.

Figure 10 shows a representative embodiment of a wider frequency band filter accomplished by two folded stubs 19 & 20 connected in tandem to cable 8. The length of stub 19 is Li, and is tuned to frequency Fn; the length of stub 20 is L₂₎ and is tuned to frequency F_r2. The combination of the filters may be designed to produce a band-stop transfer function as shown in Figure 9. This embodiment may be utilised for suppression of RF currents on a headset cable, which originate from a mobile phone transmitting RF energy within the frequency range from FL2 to FH2- When multiple stubs are used the distance between electrical connection points of sequential stubs to the cable should approximately equal a quarter wavelength to achieve maximum RF suppression of the complete filter.

Another embodiment, which may be applied to a wireless headset, such as a mobile phone, and its headset cable, is shown in Figure 11. In this embodiment cable 8 carries two pairs of filters, represented herein as pairs of folded stubs. One pair of stubs 19, 20 with lengths Li & L₂ is tuned to adjacent frequencies F_ri δ F_r2, and another pair of stubs 21 , 22 with lengths L₃ δ L is tuned to adjacent frequencies F_r3 & F_r4 respectively. The latter pair of frequencies is located further up in the frequency spectrum as shown in Figure 12. This embodiment may be used for suppression of RF currents in applications where RF radiation may be utilised or RF conducted emissions may occur within two distinct frequency bands F 2 to FH2 and Fι_₃ to FH3- A representative example of this application may be a dual-band mobile phone. It should be noted that in applications that require a greater number of frequency bands to be covered by suppression filters, a combination of a larger number of filters tuned to appropriate frequencies may be employed.

It will be appreciated that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambient of the present invention.

Claims

1. Apparatus for suppressing radio frequency signals carried along a main conductor including means for generating at least one impedance discontinuity in said conductor at a point intermediate a source of said signals on said conductor and a location on said conductor at which it is desired to suppress said signals, said means for generating including at least one transmission line operably connected to said conductor at said point such that said discontinuity is generated substantially at a frequency at which it is desired to suppress said signals.

2. Apparatus according to claim 1 wherein said source includes signals induced in said main conductor by a transmitting antenna remote from said conductor.

3. Apparatus according to claim 1 or 2 wherein said at least one transmission line exhibits a low-impedance condition at said point of connection.

4. Apparatus according to claim 1 , 2 or 3 wherein said at least one transmission line includes a tuning stub which is open circuit at its free end.

5. Apparatus according to claim 4 wherein said tuning stub is substantially a quarter wavelength long at said frequency.

6. Apparatus according to any one of the preceding claims wherein said at least one transmission line includes a flexible conductor.

7. Apparatus according to any one of claims 1 to 5 wherein said at least one transmission line includes a conductive sleeve mounted over said main conductor, said sleeve being substantially a quarter wavelength long at said frequency.

8. Apparatus according to any one of the preceding claims including a plurality of transmission lines operably connected to said conductor at said point.

9. Apparatus according to any one of the preceding claims including at least one further transmission line operably connected to said conductor intermediate said point and said location at which it is desired to suppress said signals.

10. Apparatus according to any one of the preceding claims wherein said at least one impedance discontinuity is adapted to produce a band-stop filter over a relatively narrow band of frequencies.

11. Apparatus according to any one of claims 1 to 9 wherein said at least one impedance discontinuity is adapted to produce a band-stop filter over a relatively broad band of frequencies.

12. Apparatus according to any one of the preceding claims wherein said at least one impedance discontinuity is adapted to produce plural band-stop filters over two or more distinct frequency bands.

13. Apparatus according to any one of the preceding claims wherein said main conductor includes a cable for an accessory such as a headset intended for "hands free" operation of a wireless transmitter or transceiver such as a mobile phone.

14. A method for suppressing radio frequency signals carried along a main conductor including generating at least one impedance discontinuity in said conductor at a point intermediate a source of said signals on said conductor and a location on said conductor at which it is desired to suppress said signals, said generating including operably connecting at least one transmission line to said conductor at said point such that said discontinuity is generated substantially at a frequency at which it is desired to suppress said signals.

15. Method according to claim 14 wherein said source includes signals induced in said main conductor by a transmitting antenna remote from said conductor.

16. Method according to claim 15 or 16 wherein said at east one transmission line exhibits a low-impedance condition at said point of connection.

17. Method according to claim 15, 16 or 17 wherein said at least one transmission line includes a tuning stub which is open circuit at its free end.

18. Method according to claim 17 wherein said tuning stub is substantially a quarter wavelength long at said frequency.

19. Method according to any one of claims 14 to 18 wherein said at least one transmission line includes a flexible conductor.

20. Method according to any one of claims 14 to 18 wherein said at least one transmission line includes a conductive sleeve mounted over said main conductor, said sleeve being substantially a quarter wavelength long at said frequency.

21. Method according to any one of claims 14 to 20 including operably connecting a plurality of transmission lines to said conductor at said point.

22. Method according to any one of claims 14 to 21 including operably connecting at least one further transmission line to said conductor intermediate said point and said location at which it is desired to suppress said signals.

23. Method according to any one of claims 14 to 22 wherein said at least one impedance discontinuity is adapted to produce a band-stop filter over a relatively narrow band of frequencies.

24. Method according to any one of claims 14 to 22 wherein said at least one impedance discontinuity is adapted to produce a band-stop filter over a relatively broad band of frequencies.

25. Method according to any one of claims 14 to 24 wherein said at least one impedance discontinuity is adapted to produce plural band-stop filters over two or more distinct frequency bands.

26. Method according to any one of claims 14 to 25 wherein said main conductor includes a cable for an accessory such as a headset intended for

"hands free" operation of a wireless transmitter or transceiver such as a mobile phone.

27. Apparatus for suppressing radio frequency signals carried along a main conductor substantially as herein described with reference to figures 2 to 12 of the accompanying drawings.

28. A method for suppressing radio frequency signals carried along a main conductor substantially as herein described with reference to figures 2 to 12 of the accompanying drawings.