Signal transmission circuit

Abstract: A signal transmission circuit (70) is provided for transmitting signals having a wide frequency spectrum over a cable (30). The circuit includes an isolating transformer (L1-L2) having a primary winding referenced to a ground voltage of the transmitting end, and composing part of a high pass filter (61); a low-pass filter (63); and means (L2) for summing the output voltages of the low-pass filter and the high-pass filter. The input (62) of the low pass filter is common with the input of the high pass filter.

Description: The invention relates to circuits for carrying signals having a wide frequency spectrum. In particular, it relates to cables carrying such signals. A domain of particular relevance is that of cables used to connect computers to monitors, and for interconnecting circuit boards. Figure 1 shows a computer 10 connected to a monitor 20 by a cable 30. The cable typically contains several signal-carrying conductors 32, in particular a line synchronisation conductor 33, and a common ground return conductor 34 connecting a ground voltage G M of the monitor to a ground voltage G C of the computer. The ground return conductor 34 possesses a parasitic inductance 36. Although present in all conductors, this is only shown in figure 1 for ground return conductor 34, as this parasitic inductance is of particular interest in the present invention. The line synchronisation conductor 33 is connected to a first input terminal of a voltage comparator 38. A second input terminal of the voltage comparator 38 is connected to an output terminal of a reference voltage generator 40, which generates a threshold voltage Vref. The other conductors 32 communicate signals to a control circuit 42 within the monitor 20, which also receives an output signal from the comparator 38. The cable 30 is typically connected to the computer 10 with a plug and socket type connector 44, and to the monitor 20 with another connector 46. The signals transmitted through the cable 30 may include, for example, one or more video signals, line and frame synchronisation pulses and voltage supply signals. The line synchronisation signal carried by conductor 33 and provided at the output of the voltage comparator 38 to the control circuit 42 contains pulses, the leading edge of which indicates the beginning of a line sweep on the monitor. The polarity and length of the pulses carry other useful information. The timing of the leading edge must be accurate to within 1ns, otherwise an unpleasant jittering of the image on the monitor 20 will result. To avoid such jittering, the line synchronisation signal must be received by the control circuit 42 with accurate timing and without deformation. In the cables currently used, return signals are transmitted by a conductor 34 connected at one end to the ground G C of the computer 10, and at the other end to the ground G M of the monitor. This ground return conductor 34 may carry a number of currents. The parasitic inductance of the ground return conductor 34, in conjunction with these currents, causes voltage noise to occur between the ground of the computer 10 and the ground of the monitor 20. Often, this noise takes the form of sharp voltage spikes, caused by relatively large currents being switched in the computer 10 or in the monitor 20. For example, the line synchronisation conductor 33 transmits a signal referenced to the ground G C of the computer, to the first input of comparator 38, whereas the comparator 38 and its second input are referenced to the ground G M of the monitor 20. As a result, the above voltage spikes mentioned appear to be added to the signals present on the line synchronisation conductor 33. Figure 2 is a voltage-time plot showing the effect of these voltage spikes on the detection of a leading edge of a pulse in a line synchronisation signal. The leading edge has a finite rise time, and appears as a ramp when magnified in the time dimension. The leading edge changes from ground voltage G C to Vcc. Detection of this leading edge is performed by comparator 38, in comparison with the threshold voltage value Vref, such as 3V, provided by the generator 40. For example, the leading edge may go from 0V to 5V in 20ns. As shown in figure 2, this detection should normally be done at time t1, when the signal crosses the value Vref. If parasitic voltage peaks 50 are present on the signal, the crossing of Vref may be detected at a time t2, earlier than time t1. If the voltage peaks 50 are of 0.25V, the time t2 may be 1ns early as compared with time t1. This will cause jitter of the image on the monitor 20. A classical solution for similar problems consists in using an isolating circuit, such as a transformer, to transmit a sensitive signal between two distant circuits. It will, however, be shown that this approach is not acceptable for the current application. Figure 3 shows a modification to the circuit of figure 1, wherein the line synchronisation conductor 33 is connected to a terminal of a primary winding of a transformer T, which has its second terminal connected by a dedicated conductor to the ground G C of the computer 10. A first terminal of the secondary winding of the transformer T is connected to the first input of comparator 38. The other terminal of the secondary winding is connected to the ground G M of the monitor 20. Figure 4A shows exemplary line synchronisation signals, generated by computer 10. The line synchronisation signals have either a rising leading edge as shown by signal 402, or a falling leading edge as shown by signal 404. If the transformer T is of high quality (and therefore expensive), and with good low frequency transmission, the comparator 38 receives the signals 406 and 408 shown in figure 4B on its first input. Signals 406 and 408 correspond to signals 402 and 404 respectively. These signals 406, 408 from the secondary winding are averaged about zero, so the DC component of the signals is lost. For signals having a rising leading edge such as signal 402, this does not cause a problem, since a threshold voltage Vth may easily be defined to reliably detect the synchronisation pulses, and used as the threshold level Vref of the comparator 38. However, for signals with a falling leading edge such as signal 408, it would be very difficult to define a suitable threshold level Vth which would be unaffected by noise. So, this solution involves not only an expensive transformer, but also the use of complex circuitry to detect the timing and length of the pulses. If a small, low cost transformer T is used, which inherently has a poor low frequency transmission, signals such as signals 410 and 412 shown in figure 4C are received. Signals 410, 412 correspond to transmitted signals 402, 404, respectively. If these signals are used with a simple comparator 38, the information contained in the length of positive-going pulses is lost. In addition, a delay is introduced in detecting negative-going pulses, since the trailing edge of the pulse is detected instead of the leading edge. Thus, this solution is also unsuitable for solving the current problem unless complex and expensive comparison circuits are used. The document US-A-2,794,866 discloses a signal transmission circuit for transmitting signals over a cable according to the preamble of claim 1. An object of the present invention is to provide a circuit for transmitting signals with a wide frequency spectrum over a cable, whereby the DC component of the signals is maintained, no deformation of the signal occurs, and the transmission is free from voltage spikes. A further object of the present invention is to provide such a circuit which may be used without modifying the existing circuitry of the signal transmitter, nor the signal receiver. A further object of the invention is to provide a cable carrying many conductors, including at least one conductor carrying a pulse signal, whereby the pulse signal applied to one end of the cable may be retrieved at the other end of the cable without deformation, DC offset or induced noise. To achieve these objectives, the present invention provides a circuit for transmitting signals having a wide frequency spectrum over a cable. The signal transmission circuit includes an isolating transformer having a primary winding which is referenced to a ground voltage of the transmitting end and has a high-pass filtering function; a low-pass referenced to a ground voltage of the receiving end and having an input that is common with the input of the high pass filter; and means for summing the voltages at the output nodes of the high pass filter and the low pass filter. According to an embodiment of the current invention, the cut-off frequencies of the high pass filter and the low pass filter are the same. According to an embodiment of the current invention, a first conductor within the cable is connected, at a receiving end, by a first resistor to a terminal of the primary winding of the transformer. The other terminal of the primary winding is connected by a second conductor to a ground terminal of the transmitting end of the cable. The first conductor is connected by a second resistor and the capacitor to the ground terminal of the receiving end of the cable. A node between the second resistor and the capacitor is connected through a secondary winding of the transformer to an input of the receiving circuit. According to an embodiment of the current invention, a first conductor within the cable is connected, at a transmitting end, to a terminal of the secondary winding of the transformer. The other terminal of the secondary winding is connected to the output of the mow pass filter. A second conductor is connected between a ground terminal of the low pass filter and a ground terminal of the receiving end of the cable. An input to the cable is connected to an input of the low pass filter, and also through a first resistor to a first terminal of the primary winding. A second terminal of the primary winding is connected to the ground voltage of the transmitting end. According to an embodiment of the present invention, the circuitry of the invention is contained within at least one connector of the cable. These and other objects, characteristics and advantages of the present invention, will become apparent from the following description of embodiments of the present invention, given by way of example with reference to the accompanying drawings, wherein: figure 1, already discussed, shows a signal transmission circuit of the prior art; figure 2, already discussed, shows a voltage-time plot of a synchronisation pulse according to the prior art; figure 3, already discussed, represents a variant of the prior art signal transmission circuit figure 1; figure 4A shows exemplary line synchronisation signals having rising and falling leading edges; figures 4B and 4C, already discussed, show synchronisation pulse signals received from different types of transformer, implemented in the circuit of figure 3; and figure 5 shows a preferred embodiment of the signal transmission circuit according to the present invention. Figure 5 shows one preferred embodiment of the signal transmission circuit of the present invention. Features common to those illustrated in figure 1 carry identical reference labels. The conductor 33 carrying transmitted signals from computer 10 is connected through a resistor R1 to a first terminal A of a first inductor L1. Terminal D of the first inductor L1 is connected to the ground G C of the computer 10 through a conductor 60 within cable 30. The conductor 33 is also connected through a second resistor R2 to a node B between a capacitor C and a first terminal of a second inductor L2. The other terminal of the capacitor C is connected to the ground voltage G M of the monitor 20. The second terminal E of the second inductor L2 is connected to the first input terminal of the comparator 38. The two inductors L1 and L2 have a high coupling factor. Preferably, they are two windings of a transformer wound onto a common core. In such a case, the coupling factor is almost unity, and the efficiency of such a transformer approaches 100%. The ground voltage G C of the computer and the ground voltage G M of the monitor are still connected by the conductor 34 having parasitic inductance 36. According to a feature of the invention, the transmitted signal is split into a high frequency component and a low frequency component. Resistor R1 and inductor L1 may be considered to be a high pass filter, referenced to the ground voltage G C of the computer 10, by a conductor 60. This filter 61 has as an input node the first terminal 62 of resistor R1, a ground node D at the ground voltage G C of the computer 10 and an output node A located between the resistor R1 and the inductor L1. Resistor R2 and capacitor C may be considered to be a low pass filter 63 referenced to the ground voltage G M of the monitor 20. The filter 63 has an input node 62 at the first terminal of the resistor R2, a ground node at the ground voltage G M of the monitor and an output node B between the resistor R2 and the capacitor C. For the sake of explanation, we assume that the inductors L1 and L2 have equal inductance, and perfect coupling. This will mean that a voltage VL2 across the secondary winding L2 is always equal to a voltage VL1 across the primary winding L1. Assuming a voltage step is transmitted along the conductor 33, which corresponds quite well to the form of line synchronisation pulses, the voltage present at the output node A of the high pass filter 61 referenced to the ground voltage G C of the computer is VA = Vi.exp(-R1.t/L1), where Vi is the step voltage applied to conductor 33, R1 is the resistance of resistor R1, L1 is the inductance of inductor L1, and t is the time elapsed since the arrival of the voltage step. Similarly, the voltage at the output node B of the low pass filter 63 referenced to the ground voltage G M of the monitor is VB = Vi(1-exp(-t/R2.C)), where R2 is the resistance of resistor R2, and C is the capacitance of capacitor C. As the second winding L2 is perfectly coupled to the first winding L1 and of the same value, the voltages VL1 and VL2 across the two windings L1 L2 of the transformer are equal. As VL1 = VL2, it follows that: VE = VL2 + VB = VA + VB, where VE is the voltage at the node E, with reference to the ground voltage G M of the monitor 20. For this signal VE to be identical to the transmitted voltage step Vi, the step voltage Vi must be equal to the sum of the voltage VB at the output of the low-pass filter 63 and the voltage VA at the output of the high pass filter 61 independently of time. That is, Vi = VA + VB, giving: Vi = Vi.exp(-R1.t/L1) + Vi(1-exp(-t/(R2.C))). This condition is fulfilled if: R1/L1 = 1/(R2.C). That is, when the high pass filter 61 and the low pass filter 63 have the same cut-off frequency, the pulses transmitted by computer 10 over conductor 33 arrive without deformation at the terminal E of the comparator 38. Furthermore, the received pulses are free from noise. This is evident for high frequency components of the transmitted signal, which are transmitted by the isolating transformer L1-L2 without intervention of voltages induced across inductance 36. This is also true for the low frequency components, since they are transmitted by the low pass filter 63 which eliminates all parasitic voltage spikes. Furthermore, if the values of resistors R1 and R2 are equal, the impedance of the circuit 70 seen by the computer 10 is equal to this resistance value, and is independent of frequency. This feature may be used to obtain cables 30 of a particular impedance value, and to avoid signal reflections at the receiving end of the cable. Significantly, since the circuit 70 of the present invention requires a transformer L1-L2 only for carrying high frequency signals, this transformer may be small and inexpensive. The components of the circuit 70 of the present invention (the transformer containing inductors L1, L2; capacitor C; resistors R1, R2) are preferably positioned as close as possible to the comparator 38. More preferably, and according to a particular embodiment of the invention, these added components are incorporated within the connector 46. The connection between ground voltage G C of the computer 10 and conductor 60 may be made within connector 44. Suitable values for the components are: R1=R2=470Ω, C=1.3nF, and L1=L2=287nH. The present invention thus provides a circuit 70 for transmitting signals with a wide frequency spectrum over a cable 30, whereby the DC component of the signal is maintained, no deformation of the signal occurs, and the transmission is free from voltage spikes. The present invention also provides a cable 30 carrying many conductors 32, including at least one conductor 33 for carrying a pulse signal, wherein the pulse signal applied to one end of the cable 30 may be retrieved at the other end of the cable without deformation, DC offset or noise induced by signals in other conductors contained in the cable. Advantageously, this result is obtained without modification to the transmitting and receiving circuits connected by the cable, but only by modifying the cable (by adding a conductor 60), and modifying a connector at an end of the cable, possibly within the plug connected to the cable, or the socket which receives the plug. Although described with regard to a specific application to cables for connecting a computer to a monitor, the circuit of the invention may be used in any domain where it is necessary to transmit signals having a wide frequency spectrum over a cable with high accuracy, but where high frequency noise may be introduced in the cable. Such an application is the interconnection of circuit boards treating high frequency digital signals, such as circuit boards within a computer, a monitor, or other electronic equipment. Although discussed as a signal receiving circuit, the circuit 70 according to the present invention may be installed on the transmitting end of the cable. In such a case, the added conductor 60 in the cable 30 is connected between capacitor C and the ground voltage G M of the receiving circuit 20, and conductor 33 is connected between node E and the receiving circuit 20. Other equivalent circuits could be used to replace isolation transformers, having a simple high pass filter and a simple low pass filters, such as the resistor-capacitor filters described. Other equivalent isolation circuits, not necessarily in the form of inductors or transformers could also be used.

Claims: A circuit (70) for transmitting signals having high and low frequencies components over a cable (30) from a transmitting end (10) to a receiving end (20), comprising: an isolating transformer (L1, L2) having a primary winding (L1) which composes part of a high pass filter (61); a low-pass filter (63), the input (62) of the low-pass filter receiving (33) signals from the transmitting end and being common with the input (62) of the high pass filter; means (L2) for summing (E) the voltages at output nodes (A, B) of the high pass filter and the low pass filter, to produce a signal at the receiving end, characterized in that the high pass filter (61) is referenced to a ground voltage (G C ) of the transmitting end (10) of the cable and in that the low pass filter (63) is referenced to a ground voltage (G M ) of the receiving end of the cable. A circuit according to claim 1 wherein the cut-off frequencies of the high pass filter (61) and the low pass filter (63) are the same. A circuit according to claim 1 characterised in that a first conductor (33) of the cable has a first end at the transmitting end, and a second end connected by a first resistance (R1) to a first terminal (A) of the primary winding (L1), the other terminal of the primary winding being connected by a second conductor (60) to a ground terminal (G C ) of a transmitting end of the cable, said first conductor (33) being connected to the input (62) of the low pass filter (63), which comprises a second resistance (R2) and a capacitor (C), connected respectively in series to a ground terminal (G M ) of the receiving end of the cable; a node (B) between the second resistance (R2) and the capacitor (C) being connected to a first terminal of a secondary winding (L2) of the isolating transformer. A circuit according to any of claims 1 to 3 characterised in that the transformer, the capacitor and the resistances are contained within at least one connector (44, 46) of the cable. A cable (30) comprising: a sufficient number of signal carrying conductors (32, 33) for carrying a required number of signals; a ground return conductor (34) for connecting a ground (G C ) of the transmitting end to a ground (G M ) of the receiving end; a supplementary conductor (60) connected to the ground voltage (G C ) of the transmitting end of the cable; an isolating transformer (L1, L2) having a first terminal of a primary winding (L1) connected to the supplementary conductor (60), a second terminal of a secondary winding connected to a conductor (E) at the receiving end of the cable; at least one signal carrying conductor (33) being connected through a first resistance (R1) to a second terminal of the primary winding, and further connected to an input of a low pass filter (63) referenced to a ground voltage (G M ) of the receiving end of the cable; and an output (B) of the low-pass filter connected to a first terminal of the secondary winding (L2). A cable according to claim 5 wherein the low pass filter comprises a resistance (R2) and a capacitance (C) connected in series between the signal conductor (33) and the ground voltage (G M ) of the receiving end of said c able. A cable according to claim 5 wherein the low pass filter (63) comprises a second resistance (R2) connected between the input node (62) and the output node (B), and a capacitance (C) connected between the output node and the ground node (G M ) of the receiving end of said cable. A method for transmitting signals having high and low frequency components over a cable (30) from a transmitting end (10) to a receiving end (20), comprising the steps of: transmitting the signals referenced to a ground voltage (G C ) of the transmitting end of the cable; applying the signals concurrently to a high pass filter (61) and to a low pass filter (63) in order to divide the signals into a high frequency component and a low frequency component; isolating the high frequency component; and adding the isolated high frequency component to the low frequency component, in order to form a single transmitted signal (E), referenced to a ground voltage (G M ) of the receiving end of the cable, characterized in that the high pass filter (61) is referenced to the ground voltage of the transmitting end of the cable and in that the low pass filter (63) is referenced to the ground voltage (G C ) of the receiving end of the cable.

Inventors: MOREAU JEAN-MICHEL, MOREAU, JEAN-MICHEL
Applicants: ST MICROELECTRONICS SA, STMICROELECTRONICS S.A.
Application year: 1996
Main classification: H04B3
IPC codes: H04B3