Patent Publication Number: US-2022224367-A1

Title: Communications system

Description:
FIELD 
     The disclosure relates to a wired communications system having a transmitter and a receiver connected via a transmission line. 
     BACKGROUND 
     Wired communication interfaces may be galvanically isolated to ensure that voltage levels at a receiver and transmitter side of a transmission line are isolated from each other. Galvanic isolation may commonly be achieved by use of a ferrite transformer, which allows transmission of the full bandwidth of an input signal. An example communications system  100  is illustrated in  FIG. 1 . The system  100  comprises a transmitter side  101  and receiver side  102 , connected via a transmission line  103 . The transmitter and receiver sides  101 ,  102  both comprise a ferrite transformer  104 ,  105  connected across the transmission line  103 , which may be a differential line comprising a twisted pair of wires. 
     Also shown in  FIG. 1  is an example set of signals relating to the communications system  100 . A transmission signal TX is provided at the input side of the transmitter transformer  104  in the form of a square wave pulse, which results in a signal  107  transmitted along the transmission line and a received signal RX at the output side of the receiver transformer  105 . The transmitted and received signals TX, RX are similar in form, with a delay  108  corresponding to a length of the transmission line  103 . 
     Each transformer  104 ,  105  in the system  100  adds a parasitic capacitance  106   a - d  across each pair of input and output terminals. In a typical application, each transformer  104 ,  105  may have an inductance of around 150 μH and may contribute a parasitic capacitance of around 80 pF between the primary and secondary coils. This relatively high capacitance has a low impedance at high frequencies, which can result in a high current when subjected to a bulk current injection (BCI) test unless a common mode choke is added. The common mode of the receiver may reach up to +/−40V, which may require the receiver to have high ESD protection, requiring additional silicon area. These factors all add to the bulk and cost of the overall system. 
     SUMMARY 
     In accordance with a first aspect there is provided a communications receiver comprising: 
     a pair of input connections for connecting to a transmission line; 
     a termination resistance equal to a characteristic impedance of the transmission line; 
     an air core transformer having an input coil connected to the pair of input connections via the termination resistance; and 
     a comparator circuit connected to an output coil of the air core transformer, the comparator circuit configured to provide an output signal responsive to detection of voltage pulses across the output coil. 
     The termination resistance may be alternatively or additionally defined to be greater than, and optionally greater than twice that of, the impedance of the input coil at a frequency of operation of the communications receiver. 
     The comparator circuit may be configured to switch the output signal between a first level and a second level upon reception of a voltage pulse across the output coil. 
     The comparator circuit may be configured to switch the output signal from a first voltage level to a second voltage level upon reception of a positive voltage pulse across the output coil and to switch the output signal from the second voltage level to the first voltage level upon reception of a negative voltage pulse across the output coil. 
     The air core transformer may have a coil ratio of n:1, where n is the number of turns of the input coil. 
     In accordance with a second aspect there is provided a communications transmitter comprising: 
     a pair of output connections for connecting to a transmission line; 
     a termination resistance equal to a characteristic impedance of the transmission line; 
     an air core transformer having an output coil connected to the pair of output connections via the termination resistance; and 
     a driver circuit connected to an input coil of the air core transformer, the driver circuit configured to drive a current through the input coil to provide a voltage pulse across the input coil in response to a change of state of an input signal provided to an input of the driver circuit. 
     The termination resistance may be alternatively or additionally defined to be greater than, and optionally greater than twice that of, the impedance of the output coil at a frequency of operation of the communications transmitter. 
     The driver circuit may be configured to provide the voltage pulse having a quadratic shaped rising edge. 
     The air core transformer may have a coil ratio of n:1, where n is the number of turns of the output coil. 
     In the communications receiver or the communications transmitter, n may be between around 1 and around 5, and optionally between around 2 and 3. 
     The air core transformer or the communications receiver or transmitter may be formed from parallel wire loops within a PCB. A parasitic capacitance of the air core transformer may be less than 0.5 pF. Each turn of the input or output coil may have an inductance of less than 50 nH. 
     In accordance with a third aspect there is provided a communications system comprising:
         a communications transmitter according to the first aspect;   a communications receiver according to the second aspect; and   a transmission line connected between the output connections of the transmitter and the input connections of the receiver.       

     These and other aspects of the invention will be apparent from, and elucidated with reference to, the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments will be described, by way of example only, with reference to the drawings, in which: 
         FIG. 1  is a schematic diagram of an example existing wired communications system; 
         FIG. 2  is a schematic diagram of an example wired communications system according to the present disclosure; 
         FIG. 3  is a schematic diagram of the receiver of the communications system of  FIG. 2 ; 
         FIG. 4  is an alternative schematic diagram of the receiver of the communications system of  FIG. 2 ; 
         FIG. 5  is a diagram of a series of voltage transitions at the output of the receiver of  FIG. 4  generated by a series of voltage pulses; 
         FIG. 6  is a schematic diagram of the transmitter of the communications system of  FIG. 2 ; 
         FIG. 7  is a series of signals relating to the communications system of  FIG. 2 ; and 
         FIG. 8  is an alternative schematic diagram of the transmitter of  FIG. 6 . 
     
    
    
     It should be noted that the Figures are diagrammatic and not drawn to scale. Relative dimensions and proportions of parts of these Figures have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar feature in modified and different embodiments. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 2  illustrates an example wired communications system  200  comprising a transmitter  201  and a receiver  202  connected via a transmission line  203 . The transmission line  203  is a twisted pair of wires and has a characteristic impedance Zc at a frequency of operation, which in this example is 100Ω. The frequency of operation may for example be in the region of 100 MHz. A current I LINE  passes along the transmission line  203 , generated by the transmitter  201  and received at the receiver  202 . 
     The receiver  202 , which is shown in more detail in  FIG. 3 , has a pair of input connections  211 ,  212  connected to the transmission line  203 . A termination resistance  213 , which in the illustrated example is divided between first and second termination resistances  213   a ,  213   b , is connected in series with the input connections  211 ,  212 . The termination resistance  213  has a total resistance equal to the characteristic impedance Zc of the transmission line at its frequency of operation. In the illustrated example the termination resistance comprises first and second resistances  213   a ,  213   b  each of 50Ω, together matching the 100Ω transmission line impedance Zc. In other examples a single resistance connected to one of the input terminals  211 ,  212  may be provided with a resistance equal to Zc or different resistances may be provided that together make up a total equal to Zc, the effect of which is the same. 
     The receiver  202  has an air core transformer  205  with an input coil  206  connected to the input connections  211 ,  212  via the termination resistance  213  and an output coil  207 . The term air core transformer as used herein may encompass a transformer have a core with a relative permeability of around 1, i.e. similar to that of air. Materials other than air may therefore form the core, such as FR epoxy commonly used for PCB insulating layers. Unlike conventional transformers, the termination resistance  213  is connected in series rather than in parallel with the input coil  206  because the coil  206  is in effect configured to measure current rather than voltage due to its low impedance. The output coil  207  is connected to a comparator circuit  208  that is configured to provide an output signal RX dependent on changes in the differential signal received at the input connections  211 ,  212 . The transformer  205  has parasitic capacitances C between the input and output coils  206 ,  207 . The comparator circuit  208  in the illustrated example is configured to provide the output signal RX responsive to detection of voltage pulses across the output coil  208  of the transformer  205 . The comparator circuit  208  in  FIG. 3  comprises a resistance  221  connected across the output coil  207  and first and second comparators  222   a ,  222   b  having inputs connected across the resistance  221 . A voltage pulse (comp) is generated across the resistor  221  when a pulse is received by the receiver  202 . The comparators  222   a ,  222   b  may be arranged to provide opposing output signals, i.e. when comparator  222   a  provides a high output signal comparator  222   b  provides a low output signal and vice versa. A pair of comparators are connected across the output coil  207  to ensure that the impedance seen by the output coil  207  is the same regardless of the sign of pulse. Only one of the comparators  222   a ,  222   b  may, however be required to generate an output signal. A signal from one or both of the comparators  222   a ,  222   b  drives an output latch module  223  that generates the output signal RX. 
       FIG. 4  illustrates an alternative diagram of the receiver  202 , showing one of the comparators  222   a  having its output connected to a latch module  223 , which in this case is a gated D latch having its Q output connected to its D input so that a signal provided at its clock input ck generates a change in state of the Q output. The comparator circuit is thereby configured to provide an output signal RX that changes state upon receiving a pulse across the output coil  207 . The resistor  221  in  FIG. 3  is shown in  FIG. 4  divided into first and second resistors  221   a ,  221   b  connected in series across the output coil  207 , with a ground connection connected to a mid-point between the resistors  221   a ,  221   b . Each comparator  222   a ,  222   b  is also connected to the ground connection, resulting in the comparator circuit transforming the differential signal input across the output coil  207  to a single-ended output signal RX. 
       FIG. 5  illustrates schematically operation of the receiver  202  of  FIG. 4 , showing a series of input pulses on an input signal IN_RX and a corresponding output signal in the form of a sequence of bits. Each pulse causes the output to switch between first and second voltage levels, thereby forming the output bit sequence. In a general aspect therefore, the comparator circuit  208  of the receiver  202  may be configured to switch the output signal between a first voltage level and a second voltage level upon reception of a voltage pulse across the output coil  207  of the transformer  205 . 
       FIG. 6  illustrates the example transmitter  201  of the communications system  200  in more detail. The transmitter  201  comprises a transmitter driver  401 , which provides an amplified differential version of a single-sided input signal TX to the input coil  402  of an air core transformer  404 . An output coil  403  of the transformer  404  is connected to a pair of output terminals  405 ,  406  via a terminal resistance  407   a ,  407   b . As with the terminal resistance of the receiver  202 , the terminal resistance  407   a ,  407   b  may be divided between the pair of output terminals  405 ,  406 . The terminal resistance  407   a ,  407   b  has a total equal to the characteristic impedance Zc of the transmission line  203  at the frequency of operation. The transformer  405  has parasitic capacitances C between the input and output coils  402 ,  403 . 
       FIG. 7  shows an example sequence of signals illustrating operation of the transmitter  201  and receiver  202  of the communications system  200 . A transmission signal TX  501  is generated at the transmitter  201 , which results in a transmission line signal (Line)  502 . The line signal  502  is received at the receiver  202  and generates a received signal (comp)  503 , which generates an output signal RX  504 . In this example, the transmission signal  501  is a pulse of around 100 ns duration. Due to the low impedance and low parasitic capacitance characteristics of the air core transformer  404 , the transmission pulse  501  results in a first positive pulse  505  after a rising edge  506  of the transmission pulse  501  and a second negative pulse  507  after a falling edge  508  of the transmission pulse  501 . The first and second pulses  505 ,  507  have a shorter duration of around 6 ns, i.e. much shorter than the length of the transmission pulse  501 . The first and second pulses  505 ,  507  result in corresponding respective positive and negative voltage pulses  511 ,  512  across the resistance  221  across the output coil  207  of the receiver transformer  207 . These pulses  511 ,  512  each result in a transition of the output signal RX  504  from the receiver  202 . The positive first pulse  511  results in a positive transition in the output signal  504 , followed by the negative second pulse  512  resulting in a negative transition in the output signal  504 . In a general aspect therefore, the comparator circuit  208  of the receiver  202  may be configured to switch the output signal RX  504  from a first voltage level  509  to a second voltage level  510  upon reception of a positive voltage pulse  505  across the output coil  207  of the transformer  205  and to switch the output signal RX  504  from the second voltage level  510  to the first voltage level  509  upon reception of a negative voltage pulse  507  across the output coil  207 . As a result, the receiver output signal  504  replicates the transmitter input signal  501  with a delay corresponding to the transmission line length. 
       FIG. 8  illustrates an example implementation of the transmitter  201 , in which the transmitter driver circuit  401  comprises first and second transistors M 1 , M 2 , with the first transistor M 1  in series with a current source  801  between a supply line Vdd and ground Gnd and the second transistor M 2  connected as a follower stage driven by the first transistor M 1 . A current through the input coil  402  of the transformer  404  is driven by the second transistor M 2 . The voltage across the input coil  402 , the inductance of which is low, is of the form shown in  FIG. 8 , where the shape of the voltage ramp in response to a step change at the input to the transmitter driver  401  is of the form at 2 , where a is proportional to the step change in the input and t is time, i.e. the shape of the voltage ramp is quadratic. The voltage signal across the coil Vcoil peaks and then drops below zero as the inductor discharges and returns to zero. The driving circuit  401  is thereby configured to drive a current through the input coil  402  to generate a voltage pulse  803  across the input coil  402  in response to a change of state of an input signal provided to an input  802  of the driver circuit  401 . 
     In a specific example, the transmission line  203  is a twisted pair cable having a length of up to around 20 m. This results in the transmission line acting as a low pass filter above around 50 MHz with a time constant of 3.2 ns. A suitable pulse duration for the transmission signal may therefore be around 6 ns. If the maximum coil current for a 30 nH coil is set to be 130 mA, the maximum magnetic flux will be nearly 4 nWb. To stay within this maximum magnetic flux, a 2V pulse on the transmission line may be generated by different forms of input signal, for example a square pulse, a linear ramp or a quadratic shaped ramp. A square pulse will generate a 2 ns pulse, a linear ramp a 4 ns pulse and a quadratic ramp a 6 ns pulse for the same magnetic flux. The input signal TX  506  provided to the transmitter driver  401  may therefore be of differing forms to that shown in  FIGS. 7 and 8 , with the leading and trailing edges  506 ,  508  in the form of rising and falling ramps, which may be linear or quadratic in form. An input pulse having a rising ramp of a quadratic form is generally preferred because this transforms to a linear voltage rise in current through the transmission line. 
     Because each transformer is connected to the transmission line via a terminal resistance which is equal to the characteristic impedance of the transmission line, the transmitter and receiver are both adapted closely to the transmission line. 
     For simplification each of the transformers  205 ,  404  may be considered to have a transformer ratio of 1:1, with the input and output coils both having only one turn. Increasing the number of turns of the transformer side connected to the twisted pair transmission line, i.e. the input coil  206  of the receiver transformer  205  and the output coil  403  of the transmitter transformer  404 , increases the receiver level. In an ideal case, the receiver level is proportional to n 2 , resulting in for example 100 mV for n=1 and 400 mV for n=2. By symmetry, the n turns of each transformer is always connected with the transmission line via the terminal resistance. The receiver transformer  205  therefore multiplies the input signal by n, while the transmitter transformer  404  by symmetry will multiply the signal by ½. The receiver amplitude should therefore be the same. This does not, however, apply in this case because the amplitude of ViR at the receiver is a function of the inductance value. If the input coil has n turns, the inductance will be multiplied by n 2 . The maximum receiver signal amplitude is therefore roughly proportional instead to n 2 . As a result, changing the value of n for the transformers  205 ,  404  can help to increase the receiver level for the same input current. In practice, due to the way the air core transformer may be implemented by layering conductors in a PCB, the number chosen for n may be a small integer, for example between 1 and 5. 
     An air core transformer may provide up to around a 50% leakage inductance, meaning that the secondary coil will ‘see’ only around 50% of the input signal for a 1:1 coil ratio. A higher value of n therefore assists in compensating to some extent for this lack of signal. With n=2 the line voltage is partly compensated for a 50% leakage transformer, while with n=3 the line voltage is over-compensated. In some examples therefore the value for n may be between around 2 and 3. 
     Because the transmitter output and receiver input both have a low impedance, the transmission line may be considered to be practically shorted at both ends due to the low inductance of the air core transformers. This has an advantage of reducing the magnitude of line reflections. In an example where the characteristic impedance is 100Ω and a line delay is 100 ns (corresponding to a line length of around 20 m), simulations show that a received signal reflected back and forth from the transmitter side is reduced by around 20 times compared to the initial received signal. In a general aspect, the impedance of the output coil of the transmitter air core transformer and the input coil of the receiver air core transformer, which will generally have the same number of turns, will have an impedance at a frequency of operation that is smaller than, and may for example be less than half that of, the terminal resistance and the impedance of the transmission line. In a specific example, given a frequency of operation of 100 MHz and a coil impedance of 30 nH, the impedance of the transmission line and the terminal resistance may be both  100  Ω while the impedance of the coil will be around 19Ω. 
     In a specific example, each coil of the air core transformer of the transmitter and/or receiver may be formed as a conductive trace within a PCB. A coil having a diameter of 10 mm from a wire 0.45 mm wide results in an inductance of around 20 nH with a parasitic capacitance of 0.2 pF, while a wire 0.1 mm wide results in an inductance of 30 nH and a parasitic capacitance of around 0.06 pF. A coil of such dimensions can therefore be provided in a PCB implementation of transmitter and receiver of small size, requiring few additional components, resulting in considerable savings in cost and complexity compared to existing systems. 
     Applications for the communications system disclosed herein may be for example in battery management systems, in which communication of voltage and charging levels is required between multiple battery units while ensuring galvanic isolation. The size and cost of the communication systems in such applications is of increased significance due to the restricted space and cost requirements involved. In one aspect therefore there may be provided a battery management system comprising first and second battery units and a communications system as disclosed herein providing communication between the first and second battery units. 
     From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art of wired communications systems, and which may be used instead of, or in addition to, features already described herein. 
     Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. 
     Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom. 
     For the sake of completeness it is also stated that the term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality, a single processor or other unit may fulfil the functions of several means recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims.