JP2006345259A - Receiving part termination system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To adopt a transmission line equalization system capable of preventing the generation of inter-line skews caused by the dispersion of elements in a differential mode on a receiving end of a differential transmission system and to constitute the transmission line equalization system so as not to operate as equalization (high frequency compensation) in an unnecessary common mode, to suppress the irradiation of the common mode and to perform matched termination in the common mode. <P>SOLUTION: A termination circuit for a differential mode signal is constituted between lines as a two-terminal network and a characteristic for compensating attenuation on a transmission line is provided to the network. A circuit for separating a common mode signal is used for the matching termination of the common mode signal. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an equalization method in which attenuation characteristics generated on a transmission line are compensated at a receiving end in an interface between substrates and apparatuses that transmit signals using a differential transmission line.

  In the case of transmitting a high-speed signal with a pattern or a cable on a printed wiring board, in order to suppress unnecessary radiation noise, a case where low voltage differential signal transmission (LVDS) technology is used is increasing. .

  In general, in low-voltage differential signal transmission, a differential signal transmission IC is designed so that only a differential phase current of opposite phase flows between two transmission lines through which a differential signal flows. FIG. 5 is an explanatory diagram showing an example of a transmission method of a general LVDS interface.

  In FIG. 5, between the transmission side driver IC 1 and the reception side receiver IC 2, the forward transmission line 3 and the backward transmission line 4 having a characteristic impedance of odd mode impedance Zoo (characteristic impedance with respect to the reference potential of each signal line in the differential state). It is tied by

  The forward transmission line 3 and the return transmission line 4 have the same electrical characteristics, and a so-called balanced transmission line is formed to be a differential signal line. The forward transmission line 3 and the backward transmission line 4 are terminated with an impedance substantially equal to the differential transmission line impedance Zdiff in the vicinity of the input end of the receiver-side receiver IC 2. In a general LVDS interface, the differential transmission line impedance is often set to a pure resistance of 100Ω, and is terminated by a termination resistor 5 of 100Ω.

  When the driver IC 1 drives a current of about 3.5 mA, a voltage of about 350 mV is generated in the termination resistor 5.

  Based on a signal input to the driver IC 1, a differential signal that generates a potential difference between the forward transmission line 3 and the backward transmission line 4 is generated.

  Due to the differential signal, the differential mode current flows in the reverse direction in the forward transmission line 3 and the backward transmission line 4 as indicated by arrows in the figure. On the other hand, the receiver-side receiver IC2 converts the differential signal into a CMOS level that operates at a logic amplitude based on a power supply potential such as 0-5V or 0-3.3V, and the receiver IC2 uses this as a single-ended signal output. Output.

  The principle of LVDS is that a signal current generated by the transmission side driver IC 1 is passed through a loop formed by a balanced transmission line formed by a combination of the forward transmission line 3 and the return transmission line 4 and a termination resistor 5 near the reception side receiver IC 2. Thus, a signal voltage is generated in the terminal resistor 5 to transmit the signal. The L logic / H logic of the signal is identified by switching the direction of current flow. At this time, since the currents flowing in the forward transmission line 3 and the backward transmission line 4 are ideally the same in magnitude and reverse in direction, the magnetic field generated by the current flowing in the forward transmission line 3 and the backward transmission line 4 Cancel each other, and as a result, generation of radiation noise and crosstalk noise can be suppressed. Further, even if external noise is affected in the same way by the forward transmission line 3 and the backward transmission line 4, the signal logic is not affected and the noise resistance is excellent.

  Such a differential transmission method is used not only for signal connection on a substrate but also for connection between substrates, connection between devices, and the like.

  In recent years, with the increase in speed and widespread use of various interfaces and networks, balanced cable transmission, which is advantageous in terms of cost compared to coaxial cables, has become the mainstream.

  High-frequency attenuation characteristics in cables on printed circuit boards, between boards, and between equipment are becoming more and more important with the increase in transmission signal speed, and an equalization circuit that compensates for attenuation in transmission lines at the receiving end or transmission On the side, the pre-emphasis operation in which the transmission line attenuation amount is emphasized in advance, or compensation is performed by both means so that intersymbol interference is prevented, and reception is normally performed.

  In general, the attenuation characteristics of lines and metal cables on printed circuit boards consist of conductor loss caused by the material of the conductor and physical configuration, and dielectric loss caused by the insulator (dielectric material) surrounding the conductor. Appear in

  Attenuation characteristics of lines and metal cables on a substrate: Lc (f) is generally expressed by the following approximate expression.

  Here, f is a frequency, a and b are a conductor loss coefficient and a dielectric loss coefficient, respectively, and have values inherent to the line or cable. Lc (f) is an attenuation (decibel value) per unit length, and the attenuation changes in proportion to the cable length.

  Since the dielectric loss coefficient b is much smaller than the conductor loss coefficient a, it can be considered that the transmission loss is proportional to the frequency route in a relatively low frequency region.

  In order to compensate the attenuation characteristics of such lines and cables, the flatness with respect to the frequency is obtained by giving the reverse amplitude characteristic of the above equation to the pre-emphasis circuit on the transmission side or the equalization circuit on the reception side, and intersymbol interference Is preventing.

  The pre-emphasis method on the transmission side increases the signal amplitude in the high band, so the transmission side predicts the problem of output capability of the transmission driver, the problem of unnecessary radiation from the transmission line, and the attenuation at the reception end. Is rarely used because it is difficult.

  FIG. 6 shows an example of an equalization circuit at the receiving end that is generally held in order to compensate for attenuation characteristics (frequency-amplitude characteristics) in the differential transmission path.

  FIG. 1 shows a transmission driver IC, and a differential signal is transmitted using a transmission path 3 and a transmission path 4.

  The signals transmitted through the transmission path 3 and the transmission path 4 are attenuated, and particularly the components in the high frequency region are further attenuated and reach the receiving end.

  At the receiving end, attenuation on the transmission line is compensated by the resistors 6 and 8 and the capacitors 7 and 9 arranged on the transmission line, and the load resistor 5 connected between the transmission lines so that a flat frequency-amplitude characteristic is obtained. A constant is set.

  FIG. 7 will be described with attention paid to the equalization circuit portion.

  FIG. 41 shows a signal source which is a 2 · V1 signal source as a differential signal voltage. This signal source impedance is 2 × Rs, and is represented by a resistor 42 and a resistor 43. A differential current flows from the signal source composed of these to the load, and a differential output voltage appears in the loads 48 and 49.

  When the load 48 and the load 49 have the same impedance: Rb, the junction point is regarded as a virtual ground, so the upper part of FIG.

  Focusing on the upper part of the figure, the signal source 41 is considered as a signal source that outputs a half voltage V1, and its signal source impedance is Rs.

  When a parallel circuit of the resistor 44 and the capacitor 45 is inserted in series in such a signal source and connected to the load resistor 48, the output voltage V2 (ω) has the characteristics as shown in FIG.

  The characteristic of the normal transmission path expressed by the first equation is opposite to the amplitude characteristic shown in FIG. 8, and a certain amount of correction can be performed by selecting a constant in the angular frequency range of ω <ω2.

  In actuality, since they are handled as differential signals, the load resistors 48 and 49 are constituted by one element (resistor), and the load voltage appearing at both ends thereof is 2 × V2.

  In the differential transmission method, when considering only the differential mode signal to be originally transmitted, the equalization method and termination processing described so far are performed, but the common mode signal generated due to the characteristics of the transmission driver IC and the transmission path In view of unwanted radiation caused by mismatched reflection at the receiving end of the excited common mode noise, it is desirable to provide a termination circuit for the common mode signal.

  In order to suppress this common mode current component, it is conceivable to increase the coupling of the differential signal line to the ground (GND) of the printed wiring board. Therefore, a component having a resistance value matched with the differential impedance (Zdiff) of the differential signal transmission line is arranged in series between the forward transmission line 3 and the backward transmission line 4 in the vicinity of the input end of the reception-side receiver IC 2. It is considered to solve the unnecessary radiation noise problem by using a termination method called a center tap termination instead of a normal termination method. (Reference Material: “Transistor Technology” July 1997 Special Issue p.280).

  An example of an equalization circuit using a center tap termination circuit is shown in FIG.

  In the figure, 1 is a transmission side driver IC, and 2 is a reception side receiver IC. A signal is transmitted from the transmission-side driver IC 1 to the reception-side receiver IC 2 through the forward transmission line 3 and the backward transmission line 4. The forward transmission line 3 and the return transmission line 4 have an odd-mode characteristic impedance: Zoo that has the same electrical characteristics and are equal to each other, and a balanced transmission line is formed to be a differential signal line.

  At the input end of the receiver-side receiver IC 2, termination resistors 11 and 12 each having a differential characteristic impedance of the differential signal transmission line: 1/2 of Zdiff are connected in series between the forward transmission line 3 and the backward transmission line 4. One end of the resistor 13 is connected between the resistors 11 and 12 and connected to the ground.

  In such a configuration, when the element values of the resistor 11 and the resistor 12 are equal, the resistor 13 is connected between the virtual ground and the ground with respect to the differential mode signal, and the load is a series composition of the resistor 11 and the resistor 12. It becomes.

  On the other hand, for the common mode signal, the respective lines 3 and 4 can be considered independently, and each is configured with the ground as a return path.

  Therefore, the termination circuit for the common mode signal is configured by an element value obtained by adding a value obtained by doubling the element values of the resistor 11 and the resistor 13 to the line 3. Similarly, the line 4 is composed of a resistance 12 and an element value obtained by adding a value obtained by doubling the element value of the resistance 13.

  With this configuration, it is possible to terminate both differential mode and common mode signals.

  The transmission line equalization method similar to that in FIG. 6 is also effective in the differential transmission method employing such a termination method. In FIG. 9, the combination of the resistor 6 and the capacitor 7, the combination of the resistor 8 and the capacitor 9, and the frequency-amplitude characteristic given by the circuit configuration including the terminating resistors 11 and 12 gives the reverse characteristic of the transmission line, and equalization Is realized.

  FIG. 10 shows another conventional example. Similar to FIG. 9, this is a system that enables termination of both differential mode and common mode signals, and the termination method is changed. In this case, the same equalization as described above is realized.

Moreover, as a prior art example, patent document 1 and patent document 2 can be mention | raise | lifted, for example.
JP 2004-096351 A JP 2004-153626 A

  In the above-described conventional equalization method, complete differential transmission is realized when the equalization circuit element values and load circuit element values arranged at the respective transmission lines and reception ends are in equilibrium.

  However, in the conventional equalization circuit method, equalization circuits are individually arranged in the respective transmission lines. Therefore, individual equalization characteristics vary depending on variations in element values, and a complete equilibrium state is not achieved. This is because the balanced state cannot be maintained due to variations in the termination resistance when the termination system as shown in FIGS.

  In addition, since the equalization circuit is arranged independently on each line, and each has an individual reactance element, there is a problem that variation in delay time occurs.

  Furthermore, since the equalization circuit is inserted into the balanced transmission line, the high frequency is emphasized not only for the original purpose differential mode signal but also for the unnecessary common mode signal. There was a problem that was disadvantageous.

  In the present invention, in view of the above problems, an equalization circuit that is always balanced with respect to the differential mode signal, which is the main transmission purpose in the differential transmission method, is used, and unnecessary high-frequency emphasis is not performed on the common mode signal. An object is to provide an equalization termination circuit for reducing radiation.

  In the present invention, the termination circuit for the differential mode signal and the termination circuit for the common mode signal are configured independently, the termination circuit for the differential mode and the equalization circuit are configured by a two-terminal network, and the respective transmission lines are connected. In addition, a common mode signal termination circuit is separately configured.

  In order to configure an independent termination circuit for each mode, it is necessary to separate the signals of the respective modes that have propagated through the common transmission line, and a transmission mode separation circuit for that purpose is used.

  This will be specifically described with reference to FIG.

  3 and 4 are balanced transmission lines coupled to each other. The right end of the figure is a receiving unit, and the differential mode signal is terminated at a differential mode equalization termination 41. This termination 41 is a two-terminal network, and becomes an equalizer and termination that maintain a balance between the lines.

  On the other hand, the common mode signal is led to the mode separation circuit 42 because the previous equalization termination 41 behaves as a high-impedance element, and is matched and terminated at the common mode termination 43.

  The mode separation circuit 42 is a circuit network that has a high impedance with respect to a signal propagated as a differential mode and transmits only the signal propagated as a common mode.

  The equalization termination system at the receiving end of the differential transmission system of the present invention uses a mode separation circuit 42 as shown in the figure, and the termination circuits for the respective modes are arranged in parallel.

  Since the termination circuit and the equalization circuit configuration can be made independent of each other as described above, a balanced two-terminal network can be used for the termination of the differential mode, which is a problem in the conventional equalization method. A differential equalization termination circuit can be realized in which an incomplete equilibrium state caused by variations in element values and the like, a delay time difference between lines can be prevented, and unnecessary radiation of unnecessary modes can be reduced.

  As described above, according to the present invention, at the receiving end of the differential transmission system, equalization characteristics are given only to the differential mode signal, unnecessary high frequency emphasis is not applied to the common mode signal, and matching termination is enabled. In addition, unnecessary radiation can be reduced, a delay time difference between transmission lines can be reduced, and common mode noise excited on the transmission line can be effectively canceled out.

  An embodiment of the present invention will be described with reference to FIG.

  FIGS. 21 and 22 are transmission-side differential signal input ports, respectively, to which differential signals whose polarities are inverted are input.

  The positive and negative signals are respectively transmitted through the differential transmission lines 25 and 26 toward the right end receiving unit.

  The differential mode signal, which is originally intended for transmission, is terminated by a differential mode signal termination circuit including resistors 27 and 28 and an inductor 29, and is differentially output to the reception output ports 23 and 24.

  On the other hand, the common mode signal is not terminated in the differential mode signal termination circuit, but is transmitted through the transformer 30 and matched and terminated by the resistor 31 matched to the common mode impedance.

  When a differential mode signal (a phase opposite to each other) is input, the transformer 30 operates as a large inductance and does not transmit the differential mode signal because the directions of magnetic fluxes generated by the mutual currents are the same.

  Therefore, the transmitted differential mode signal is terminated only by the differential mode signal termination circuit (comprising elements 27, 28, and 29), and is differentially output to the reception output ports 23 and 24.

  On the other hand, with respect to common mode signals that are in phase with each other, the previous transformer 30 operates to cancel each other's magnetic flux and behaves as a low impedance element. Therefore, the common mode signal passes through the transformer 30 and is matched and terminated by the resistor 31. Further, since the common mode signal gives the same potential to both ends of the differential mode signal termination circuit, the common mode current does not flow and no common mode voltage is given between the reception output ports 23 and 24.

  The operation of the termination portion with respect to the differential mode signal will be described with reference to FIG.

  FIG. 41 shows a signal source which is a 2 · V1 signal source as a differential signal voltage. This signal source impedance is 2 × Rs, and is represented by a resistor 42 and a resistor 43. A differential current flows from the signal source composed of these to the load.

In consideration of the virtual ground, the load portion for the differential mode signal shown in FIG. 1 can be rewritten into a circuit in which the element values are equally distributed as shown in FIG. 3 and the elements are arranged symmetrically with respect to the virtual ground. . Where each element value is
R27 '= R27 / 2 (resistors 51 and 52)
R28 '= R28 / 2 (resistors 53, 54)
L29 '= L29 / 2 (Inductors 55, 56)
It expresses.

  Focusing on the upper part of FIG. 3, the signal source 41 is considered as a signal source that outputs half the voltage V1, and the signal source impedance is Rs.

  A resistor 51 and a parallel circuit of an inductance 55 and a resistor 53 are connected to such a signal source as a load. At this time, the load voltage V2 (ω) considered with reference to the virtual ground has characteristics as shown in FIG. Thus, it is possible to give the differential mode load the same equalization characteristics as when the equalization circuit described in the prior art is arranged on the transmission line.

  On the other hand, the circuit described above does not affect the common mode signal, and the common mode signal is terminated by the resistor 31 shown in FIG. 1 and is not emphasized (equalization operation).

The figure explaining the Example of this invention The figure explaining the structure of the Example of this invention The figure explaining operation | movement of the Example of this invention The figure explaining the operating characteristic of the Example of this invention An explanatory diagram showing an example of a general differential transmission system An explanatory view showing an example of a conventional differential transmission type equalization circuit The figure explaining operation of the equivalent circuit of the conventional differential transmission system The figure explaining the operation characteristic of the equivalent circuit of the conventional differential transmission system A diagram showing an example of a conventional differential transmission equivalent termination circuit A diagram showing an example of a conventional differential transmission equivalent termination circuit

Explanation of symbols

1 Transmission driver IC
2 Receiver IC
3, 4, 25, 26 Differential transmission line
5 Terminating resistor
6, 8, 11, 12, 13, 14, 15, 16, 44, 46, 48, 49, 51, 52, 53, 54 Resistors
7, 9, 45, 47 capacitors
29, 55, 56 inductor
30 transformer
31 Common mode termination resistor
42, 43 Signal source impedance

Claims (3)

With a communication system that transmits differential signals between a transmission device and a reception device, using a balanced transmission path as a communication medium,
A receiver termination system characterized in that, at the receiving end, a two-terminal network is arranged as a signal terminator between each differential transmission line and an arbitrary frequency-amplitude characteristic is given to the two-terminal network.   The receiving unit termination system according to claim 1, wherein a mode separation circuit for separating the differential mode signal and the common mode signal is used, and a termination circuit for the common mode is separately configured.   4. The receiving unit termination system according to claim 3, wherein a transformer having a winding method that blocks the differential mode and transmits the common mode is used for the mode separation circuit for separating the differential mode signal and the common mode signal.

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