GB2508154A

GB2508154A - A differential signal data cable

Info

Publication number: GB2508154A
Application number: GB1220920.1A
Authority: GB
Inventors: Scott Berry
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-11-21
Filing date: 2012-11-21
Publication date: 2014-05-28
Anticipated expiration: 2032-11-21
Also published as: GB201220920D0; GB2508154B

Abstract

differential signal conductors 56, 57 configured to conduct digital data and a plug connector 60. The plug connector comprises a first common mode rejection choke 84 conductively connected to the first pair of differential signal conductors. It may also contain an inductor 82 and/or a resistor in power and/or ground lines 54, 55. The cable 50 may have a conductive sheath 52 which is not connected at this end but is connected to the skirt of plug 40 to ground it. The data may be USB, and may be digital audio driving a digital audio signal receiver 20 and a speaker 30.

Description

tM:;: INTELLECTUAL .*.. PROPERTY OFFICE Application No. 0B1220920.1 R'T'M Date:21 March 2013 The following terms are registered trademarks and should be read as such wherever they occur in this document: Firewire

HDMI

Intellectual Properly Office is an operaling name of Ihe Patent Office www.ipo.gov.uk

A DIFFERENTIAL SIGNAL DATA CABLE

DESCRIPTION

The present invention relates to a differential signal data cable, and in particular to a differential signal data cable comprising at least a first pair of differential signal conductors.

Differential signalling is widely used to transfer digital data between devices, and relies upon the voltage difference between two conductors of a pair of conductors to carry the data. Differential signalling commonly comprises the use of balanced transmitters and receivers, which use opposing positive and negative voltages on the conductors of the pair to give no net direct current between the transmitters and receivers.

Digital data between computers is typically sent in the form of data blocks which may allow for error checking and correction, and which do not rely upon specific timing at the transmitting and receiving ends of a differential signal data cable. However, a digital audio signal is typically sent as a continuous stream of data, and the receiver must remain properly synchronized to the incoming data stream in order to avoid losses of data.

Sources of interference can result in the sharp edged waveforms sent by the transmitter being rounded off and/or corrupted by the time that they have traversed a cable and reached the receiver, causing data errors.

Designers of high quality audio components go to great lengths to help avoid audio signals from being contaminated by voltage noise, and even low levels of noise can have a significant effect upon the sound perceived by a person listening to the audio. A cable supplying an audio signal from a transmitting device to a receiving device, will transmit unwanted noise from the transmitting device to the receiving device. This noise reduces the sound quality at the receiving device.

In particular, data signals sent from computers may be modulated with noise which is outside of the bandwidth of the data signal, but which couples into the audio device and results in unwanted distortion. Such noise can result from many sources such as switched mode power supplies within computer systems.

It is therefore an aim of the invention to provide an improved differential signal data cable.

According to a first aspect of the invention, there is provided a differential signal data cable, comprising at least a first pair of differential signal conductors for carrying digital data; and a plug connector having contacts configured to electrically connect the first pair of differential signal conductors to a mating connector. The plug connector comprises a first common mode rejection choke conductively connected to the first pair of differential signal conductors.

The noise in a differential digital signal may manifest itself as a common mode signal, which shifts the d.c. bias of the differential digital signal away from zero and results in data errors, and/or coupling of unwanted noise frequencies into the receiver, for example via parasitic capacitances/inductances at the receiver.

Common mode signals are signals which are common to both conductors of a pair of conductors, as is known to those skilled in the art.

Whilst it is known to incorporate common mode rejection chokes inside the actual receiver or the receivers mating connector, to reduce common-mode noise, this does not entirely prevent the common mode noise from being coupled into the receiver via parasitic components at the receiver. For example, coupling may take place via unintended capacitive couplings that are present between various parts of the receiver circuitry and electrical components that are connected at the input stage of the receiver before the common mode rejection choke.

Accordingly, the use of a common mode rejection choke within the cable itself according to the present invention can help prevent common mode signals from reaching any parasitic components of the receiver and coupling noise into the receiver. The common mode rejection choke is conductively connected to both conductors of the first pair of differential signal conductors, and is therefore not constituted by a Ferrite applied to the external of the cable and having no electrically conductive connection to the pair of differential signal conductors.

Ferrites are commonly designed to present an impedance to specified frequency(s) and to attenuate all signals of these frequencies, rather than just attenuating common mode signals.

A ferrite could be applied to the differential signal data cable of the invention in addition to the common mode rejection choke, although the blocking frequency range of the ferrite should be well away from the frequency range of the differential signal to avoid negatively affecting the differential signal.

Common mode rejection chokes are often specified in terms of the impedance that they present to common mode signals at 100MHz. According to established practice in the art, common mode rejection chokes should present an impedance to common mode signals that is similar to the characteristic impedance of the differential signal conductor pair, to prevent signal reflections. For example, the common mode rejection choke series DLP11TB available from Murata Manufacturing Co., Ltd and designed for LJSB 3.0, provides a provides a common mode impedance of 80 at 100 MHz, to substantially correspond to the 90 characteristic impedance of USB cables.

However, the inventor has determined that greater sound quality can actually be achieved using common mode rejection chokes that have much higher common mode impedance values, and postulates that signal reflections do not present a significant problem since the common mode impedance is realised with respect to the common mode signals, not with respect to the differential signals which carry the audio signal. Advantageously, the common mode impedance of the first common mode rejection choke at 100MHz may be greater than 1.5 times the characteristic impedance of the differential signal conductors.

If the characteristic impedance of the first pair of differential signal conductors is 90 ohms, then the common mode impedance of the first common mode rejection choke at 100MHz may be between 150 ohms and 10,000 ohms.

More preferably, the common mode impedance of the common mode rejection choke at 100MHz is between 2000 ohms and 8,000 ohms, for example 2200 ohms, or 5000 ohms. Such large impedances have been found to be particularly advantageous when transmitting audio signals along the cable, as judged by listening to the resulting sound output.

The plug connector may comprise a printed circuit board upon which the first common mode rejection choke is mounted, the printed circuit board electrically connecting the first pair of differential signal conductors to the contacts that are configured to electrically connect the first pair of differential signal conductors to a mating connector. The printed circuit board provides a convenient method of manufacturing and subsequently electrically connecting and mounting the first common mode rejection choke within the plug connector. The printed circuit board may be encapsulated within a solid plastics material to help provide structural integrity, for example a solid plastics material such as epoxy.

The differential signal data cable may further comprise power supply conductors, and the plug connector may comprise contacts configured to electrically connect the power supply conductors to a mating connector.

Advantageously, the printed circuit board may further comprise an inductor andlor resistor, and the inductor and/or resistor may be connected in series between one of the power supply conductors and one of the contacts configured to electrically connect the power supply conductors to a mating connector. The inductor may help prevent interference from being carried along the power supply conductor into the mating connector.

The first common mode rejection choke may comprise a first coil connected in series with one of the first pair of differential signal conductors and a second coil connected in series with the other one of the first pair of differential signal conductors, wherein the first and second coils share a common core and are wound around the core in opposite directions to one another. Winding the coils in opposite directions to one another means that the flux generated by one of the conductors opposes the flux generated by the other of the conductors when the conductor pair is carrying a common mode signal, thereby attenuating the common mode signal.

The differential signal data cable may comprise an electrically conductive sheaf surrounding at least the first differential signal conductors, to help shield the conductors from external EM radiators. The electrically conductive sheaf may be formed as a matrix of multiple conductors connected together.

Advantageously, the electrically conductive sheaf may be electrically insulated from all contacts of the plug connector that are configured to mate with a mating connector, to help ensure that any radiated emissions picked up by the sheaf are not coupled into a receiver via the mating connector. The differential signal data cable typically comprises a further plug connector connected at an opposite end of the differential signal conductors from the plug connector, and the further plug connector may have contacts connected to the electrically conductive sheaf for grounding the electrically conductive sheaf. The plug connector and/or further plug connector may be marked to distinguish them from one another, to inform a user that the plug connector should be connected to the receiver and the further plug connector to the transmitter.

The differential signal data cable may further comprise second and third pairs of differential signal conductors, the second and third pairs of differential signal conductors each being substantially the same as the first pair of differential signal conductors. The plug connector may have contacts configured to electrically connect the second and third pairs of differential signal conductors to the mating connector; and second and third common mode rejection chokes comprised within the plug connector. The second and third common mode rejection chokes may each be the same as the first common mode rejection choke, the second and third pairs of differential signal conductors conductively connected to the second and third common mode rejection chokes respectively. Then the differential signal data cable may carry more than just one pair of differential signals. The electrically conductive sheaf may also surround the second and third pairs of differential signal conductors.

The second and third common mode rejection chokes may each be the same as the first common mode rejection choke, except for having a lower impedance value at 1 00MHz than the first common mode choke, for example if the second and third common mode rejection chokes handle higher signal frequencies than the first common mode rejection choke, as may be desirable for USB 3.0 compliant signals.

The differential signal data cable is preferably a digital data cable.

Advantageously, the differential signal data cable may be configured to carry digital audio signals that are compliant with the USB 2.0 specification or the USB 3.0 specification. The differential signal data cable may be a digital audio signal cable.

According to a second aspect of the invention, there is provided a system comprising an audio signal transmitter, an audio signal receiver, and the differential signal data cable of the first aspect of the invention. The differential signal data cable is connected between the audio signal transmitter and the audio signal receiver for transferring an audio signal from the audio signal transmitter to the audio signal receiver, and the plug connector of the differential signal data cable is mated with a mating connector of the audio signal receiver. The audio signal transmitter may be a digital audio signal transmitter, and the audio signal receiver may be a digital audio signal receiver.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 shows a schematic diagram of a system comprising a digital audio signal transmitter, a digital audio signal receiver, and a differential signal data cable according to an embodiment of the invention; Fig. 2 shows a schematic diagram of a plug connector of the differential signal data cable of Fig. 1; Fig. 3 shows a schematic diagram of a plug connector of a differential signal data cable according to an alternate embodiment of the invention; and Fig. 4 shows a schematic diagram of a common mode rejection choke suitable for use in the plug connector of Fig. 2 and Fig. 3.

The schematic diagram of Fig. 1 shows a digital audio signal transmitter in the form of a personal computer 10, a digital audio signal receiver 20, and a speaker 30. The personal computer 10 is connected to the digital audio signal receiver 20 by a differential signal data cable that comprises a plug connector 60, a wiring portion 50, and a further plug connector 40. The further plug connector 40 is received in a socket 12 of the personal computer 10, and the plug connector 60 is received within a mating connector 22 of the audio signal amplifier 20.

In use, the personal computer 10 sends a digital audio signal to the digital audio signal receiver 20 via the differential signal data cable 40, 50, 60, and the audio signal receiver 20 decodes the digital signal and outputs an analog line level output. This analog output is then amplified within the speaker 30, producing sound 35.

The plug connector 60 is a USB type B plug and the further plug connector is a USB Type A plug. The plugs connectors are shown in Fig. 1 as being both Male, although one or both of the plugs could alternatively be Female. The conductors within the wiring portion 50 comprise a differential signal pair.

The schematic diagram of Fig. 2 shows the plug connector 60 in greater detail, as well as a part of the wiring portion 50 that is connected to the plug connector 60. The plug connector 60 comprises a body portion 62 and a skirt portion 65. The body portion 62 comprises a plastic housing that grips the wiring portion 50, and the skirt portion 65 is metallic and houses an insulative tongue 66 upon which contacts 68 for connecting to the mating connector 22 are formed.

The body portion 62 comprises a printed circuit board 80 which receives power conductors 54 and 55, and a pair of differential signal conductors 56 and 57, from the wiring portion 50. The printed circuit board 80 comprises a common mode rejection choke 84 that is connected in series with the pair of differential signal conductors 56 and 57, and further comprises an inductor 82 that is connected in series with the ground conductor 54 of the power conductors. As an alternative to this arrangement, a resistor may also be placed in series with the ground conductor 54 in addition to the inductor 82. There could also be an inductor and optionally a resistor placed in series with the power conductor 55.

The common mode rejection choke 84 presents an impedance of 2200 ohms to common mode signals at a frequency of 100MHz, to help prevent common mode signals on the pair of differential signal conductors 56 and 57 from reaching the contacts 68 and entering the audio signal receiver 20. The inductor 82 helps to reduce any noise passing along the ground conductor and to the audio amplifier 20.

The common mode rejection choke 84 and the inductor 82 are both surface-mount components that are soldered upon the printed circuit board 80.

Once the conductors 54 57 and contacts 68 have been connected to the printed circuit board 80, the printed circuit board 80 is encapsulated within an epoxy resin inside of the plastic housing of the body portion 62.

The wiring portion 50 also comprises a metallic sheaf 52 which surrounds the power and differential conductors. However, contrary to normal practice, no electrical connection is made between the metallic sheaf 52 and the metal skirt portion 66 to prevent electrical noise from being coupled into the audio amplifier 20. The metal sheaf 52 is connected to the metal skid of the further plug connector 40, to ground the metal sheaf 52.

The above embodiment describes a system that sends an audio signal to an audio signal amplifier in compliance with the USB 2.0 standard, although the general principles could equally be applied to cables configured to carry non-USB compliant audio signals, for example audio signals compliant with IEEE 1394, Ethernet, Firewire or HDMI. An additional resistor may be added on the printed circuit board in series with the inductor 82 to improve the filtering of the power supply offered by the inductor 82, which may prevent the differential signal data cable from being fully USB-compliant, but which may still be acceptable in audio applications where significant power is not being drawn from the power supply conductors of the differential signal data cable into the audio signal receiver The same circuitry as in the plug connector 60 could also be placed on the other end of the cable in the further plug connector 40. The same process of using a printed circuit board with encapsulation by epoxy could be used at the further plug connector 40.

An alternate embodiment will now be described with reference to Fig.3, which shows a schematic diagram of a plug connector 160 compliant with the USB 3.0 standard. The plug connector 160 is the same as the plug connector 60 of Fig.2, except for that the plug connector 160 additionally receives two higher-speed pairs of differential signal conductors from a wiring portion 150 similar to the wiring portion 50.

The two higher-speed pairs of differential signal conductors are connected in series with common mode rejection chokes 183 and 185 respectively. The common mode rejection choke 184 is the same as the common mode rejection choke 84 of Fig.2, and is connected to a first pair of differential signal conductors the same as the differential signal conductors 56 and 57. This is provided for backward-compatibility with devices that only support USB 2.0 rather than USB 3.0, as will be apparent to those skilled in the art. The plug connector 160 could be implemented in place of the plug connector 60.

The common mode rejection chokes 183, 184, 185 are all provided on a printed circuit board 180 within a body portion of the plug connector 160. The common-mode rejection chokes 183 and 185 have lower impedances as measured at 100MHz than the common mode rejection choke 184, since the common-mode rejection chokes 183 and 185 are intended for operation at higher frequencies. The plug connector 160 also comprises a metal skirt portion 165 similar to the metal skirt portion 65 of Fig. 2. The connections between the wiring portion 150, the printed circuit board 180, and the contacts of the plug connector for connecting to a mating connector, are not shown in Fig. 3 for the sake of clarity.

An example common mode rejection choke suitable for use in the plug connectors of Figs. 2 and 3 will now be described with reference to Fig. 4. As can be seen in Fig. 4, two coils are wound in opposite senses around a common iron core 300. The input connections V-f_IN and V-_IN may respectively connect to the conductors of a pair of differential signal conductors, such as the conductors 56 and 57, and the output connections V÷_OUT and V-_OUT may respectively connect to contacts of the plug connector, such as two of the contacts 68.

Further alternate embodiments falling within the scope of the appended claims will also be apparent to the skilled person.

Claims

CLAIMS1. A differential signal data cable, comprising: -at least a first pair of differential signal conductors for carrying digital data; and -a plug connector having contacts configured to electrically connect the first pair of differential signal conductors to a mating connector, wherein the plug connector comprises a first common mode rejection choke conductively connected to the first pair of differential signal conductors.
2. The differential signal data cable of claim 1, wherein a common mode impedance of the first common mode rejection choke at 100MHz is greater than 1.5 times a characteristic impedance of the first pair of differential signal conductors.
3. The differential signal data cable of claim 2, wherein the characteristic impedance of the first pair of differential signal conductors is 90 ohms, and wherein the common mode impedance of the first common mode rejection choke at 100MHz is between 150 ohms and 10,000 ohms.
4. The differential signal data cable of claim 3, wherein the common mode impedance of the common mode rejection choke at 100MHz is between 2000 ohms and 8,000 ohms.
5. The differential signal data cable of any preceding claim, wherein the plug connector comprises a printed circuit board upon which the first common mode rejection choke is mounted, the printed circuit board electrically connecting the first pair of differential signal conductors to the contacts that are configured to electrically connect the first pair of differential signal conductors to a mating connector.
6. The differential signal data cable of any preceding claim, wherein the differential signal data cable further comprises power supply conductors and wherein the plug connector comprises contacts configured to electrically connect the power supply conductors to a mating connector.
7. The differential signal data cable of claim 6 when appended to claim 5, wherein the printed circuit board further comprises an inductor, and wherein the inductor is connected in series between one of the power supply conductors and one of the contacts configured to electrically connect the power supply conductors to a mating connector.
8. The differential signal data cable of claim 7, wherein the printed circuit board further comprises a resistor, and wherein the resistor is connected in series between one of the power supply conductors and one of the contacts configured to electrically connect the power supply conductors to a mating connector.
9. The differential signal data cable of any preceding claim, wherein the first common mode rejection choke comprises a first coil connected in series with one of the first pair of differential signal conductors and a second coil connected in series with the other one of the first pair of differential signal conductors, wherein the first and second coils share a common core and are wound around the core in opposite directions to one another.
10. The differential signal data cable of any preceding claim, wherein the differential signal data cable comprises an electrically conductive sheaf surrounding at least the first differential signal conductors, and wherein the electrically conductive sheaf is electrically insulated from all contacts of the plug connector that are configured to mate with a mating connector.
11. The differential signal data cable of any preceding claim, further comprising: -second and third pairs of differential signal conductors, the second and third pairs of differential signal conductors each being similar to the first pair of differential signal conductors, the plug connector having contacts configured to electrically connect the second and third pairs of differential signal conductors to the mating connector; and -second and third common mode rejection chokes comprised within the plug connector, the second and third common mode rejection chokes each being similar to the first common mode rejection choke, the second and third pairs of differential signal conductors conductively connected to the second and third common mode rejection chokes respectively.
12. The differential signal data cable of claim 11 when appended to claim 10, wherein the electrically conductive sheaf also surrounds the second and third pairs of differential signal conductors.
13. The differential signal data cable of any preceding claim, wherein the differential signal data cable is configured to carry digital audio signals that arecompliant with the USB 2.0 specification.
14. The differential signal data cable of any preceding claim, wherein the differential signal data cable is configured to carry digital audio signals that arecompliant with the USB 3.0 specification.
15. The differential signal data cable of any preceding claim, wherein the differential signal data cable is an audio signal cable.
16. A system comprising an audio signal transmitter, an audio signal receiver, and the differential signal data cable of any preceding claim, wherein the differential signal data cable is connected between the audio signal transmitter and the audio signal receiver for transferring an audio signal from the audio signal transmitter to the audio signal receiver, and wherein the plug connector of the differential signal data cable is mated with a mating connector of the audio signal receiver.
17. A differential signal data cable substantially as described herein with reference to the accompanying drawings.