JP2009004402A - Communication circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a communication circuit which achieves high-frequency and low-voltage operations and miniaturization, and reduces radiation noises while ensuring resistance against static electricity. <P>SOLUTION: A GND connection circuit 6 is connected between a first GND 2 and a second GND 3, and its impedance is controlled by a GND control signal input from a GND control signal line 5. In a period T1, no communication signal travels through a communication signal line 1 and the impedance Z23 of the GND connection circuit 6 is high, so that the first GND 2 is isolated from the second GND 3. When insertion of a cable is detected in a period T2 and transmission of a communication signal V12 is started in a period T3, a GND control signal V52 makes the first and second GNDs 2 and 3 common in impedance to ensure a return path for the communication signal V12. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a communication circuit mounted on an image forming apparatus or the like and used for communication with a network.

  Due to the high frequency, low voltage, and miniaturization of communication circuits, the deterioration of the electromagnetic environment due to electrostatic current has become a problem. For example, when an external cable such as a USB is inserted into a device, if static electricity is generated between the device and the connector, the static current reaches the communication signal generation circuit via GND, resulting in malfunction or destruction. -There is

  In such a case, conventionally, a method has been used in which the GND of the communication circuit is separated between the communication signal generation unit side and the side to which the external cable is connected to block the propagation of electrostatic current.

  For example, as illustrated in FIG. 7, the communication signal wiring 101 includes a first GND 102 and a second GND 103 that are separated from each other, and a communication signal is provided between the communication signal wiring 101 and the first GND 102. The generation circuit 104 is connected. According to this configuration, even when an electrostatic current flows into the second GND 103, propagation to the first GND 102 is interrupted, so that arrival at the communication signal generation circuit 104 can be suppressed, and electrostatic resistance can be ensured. It becomes.

  However, in this method, radiation noise generated during communication becomes a big problem. That is, during communication, a high-speed communication signal flows through the communication signal wiring. At that time, GND acts as a return path for the communication signal. Here, when this GND is separated for the purpose of blocking propagation of electrostatic current, the return path is also blocked. A voltage oscillation is generated between the first and second GNDs separated by the interrupted current and is radiated as an electromagnetic wave, which causes a big problem.

  When the first and second GNDs are used in common, the return path of the communication signal is not interrupted, so that radiation noise can be reduced. However, an electrostatic current is generated via the second GND. Since it flows into the circuit, static electricity resistance cannot be secured.

  Thus, reduction of radiation noise and securing of static electricity resistance may be in a trade-off relationship, and it is becoming difficult year by year to satisfy both at the same time.

  Therefore, conventionally, the communication circuit is electromagnetically shielded by adding a metal member to the casing of the device. However, in recent years, there are increasing cases in which metal members cannot be added due to downsizing of devices.

  Patent Document 1 discloses a configuration in which a voltage stabilization circuit is provided between separated GNDs. That is, a voltage sensor is provided between the separated GNDs, and when the read voltage exceeds a set value, the transistor is operated to suppress voltage fluctuation between the GNDs.

Japanese Patent Laid-Open No. 2002-134988

  However, in the above configuration, since the vibration in the negative direction is not removed, it is difficult to reduce high frequency noise. For this reason, it is difficult to obtain sufficient effects in the communication circuits with higher frequencies.

  In addition to these, the increase in radiation noise due to the higher frequency of communication circuits and the deterioration of static electricity resistance due to the lower voltage of the circuit drive voltage have become obvious. With the conventional technology, both reduction of radiation noise and securing of static electricity resistance are compatible. It is becoming difficult.

  The present invention has been made in view of the above-described problems, and provides a communication circuit capable of reducing both radiation noise and ensuring electrostatic resistance in response to higher frequency, lower voltage, and smaller size of the communication circuit. It is intended to provide.

  In order to achieve the above object, a communication circuit of the present invention includes a communication signal generating circuit that generates a communication signal, a communication signal wiring connected to the communication signal generating circuit, and a communication signal generating circuit connected to the communication signal generating circuit. A first GND; a second GND connected to an external GND; and a GND connection circuit that connects between the first GND and the second GND. The GND connection circuit includes: The impedance between the first GND and the second GND is controlled by a GND control signal.

  By controlling the impedance between the first and second GNDs by the GND control signal, the first and second GNDs are separated depending on the situation, and high-frequency noise is reduced.

  The best mode for carrying out the present invention will be described with reference to the drawings.

  FIG. 1A is a schematic diagram illustrating a communication circuit according to an embodiment, which includes a communication signal line 1 and first and second GNDs 2 and 3 separated from each other. A communication signal generation circuit 4 that generates a communication signal is connected between the first GND 2 and the first GND 2.

  The second GND 3 is connected to the external GND. A GND connection circuit 6 connected to the GND control signal line 5 is connected between the first and second GNDs 2 and 3. A GND control signal for controlling the impedance between the first and second GNDs is input to the GND connection circuit 6 via the GND control signal wiring 5. The GND connection circuit 6 includes a terminal 7 connected to the first GND 2, a terminal 8 connected to the second GND 3, and a terminal 9 connected to the GND control signal wiring 5.

  The GND connection circuit 6 is a circuit that controls the impedance between the first and second GNDs and can freely change the electrical connection and separation between the two.

  FIG. 1B is a graph showing the operation sequence of the communication circuit, and the horizontal axis represents time. V52 on the vertical axis represents the voltage of the GND control signal, Z23 represents the impedance between the first and second GNDs 2 and 3, and V12 represents the voltage of the communication signal wiring 1.

  The period T1 is a period in which the cable is inserted into the device, and no communication signal flows through the communication signal wiring 1. Further, the impedance between the terminals 7 and 8 of the GND connection circuit 6 is in a high state, which is equivalent to a state in which the first and second GNDs 2 and 3 are separated. For this reason, the inflow path of the electrostatic current generated with the insertion of the cable is blocked, and the electrostatic resistance can be ensured. Since no communication signal is flowing, no radiation noise is generated due to the return path being divided.

  In the period T2, the insertion of the cable is detected, and the transmitted GND control signal flows to the terminal 9. As a result, the impedance Z23 between the terminals 7 and 8 of the GND connection circuit 6 transitions to a low state, which is equivalent to a state where the first and second GNDs are shared.

  In the period T3, transmission of a communication signal is started. In this case, since the first and second GNDs are equivalent to a common state, a return path for the communication signal is secured, and radiation noise can be reduced.

  As described above, according to the present embodiment, the communication circuit can be increased in frequency, voltage and size, and radiation noise can be reduced and electrostatic resistance can be ensured.

  A simulation was performed using the communication circuit having the configuration shown in FIG.

  The communication circuit of this embodiment includes a communication signal line 1, first and second GNDs 2 and 3 separated from each other, a communication signal generation circuit 4 connected between the communication signal line 1 and the first GND 2, and GND control. A signal wiring 5 and a GND connection circuit 6 are provided. The GND connection circuit 6 includes a terminal 7 connected to the first GND 2, a terminal 8 connected to the second GND 2, and a terminal 9 connected to the GND control signal wiring 5.

  The GND connection circuit 6 includes contacts 10 and 11, a first capacitor 12 connected to the terminal 7, a second capacitor 13 connected between the terminal 8 and the contact B11, and the contact 10 toward the contact 11. And a PIN diode 14 connected in the forward direction. A resistor 15 is connected between the terminal 9 and the contact 10, and an inductor 16 is connected between the input sides of the first and second capacitors 12, 13, that is, between the terminal 7 and the contact 11.

  FIG. 2B is a graph showing the forward current dependency of the high-frequency impedance characteristic in the PIN diode 14.

  3A and 3B show the results of the simulation. FIG. 3A shows the frequency characteristics of Z23 in the period T1, and FIG. 3B shows the frequency characteristics of Z23 in the period T3.

  The drive frequency of the communication signal is 100 MHz, the voltage amplitude of the GND control signal is 5 V, the capacitance value of the first and second capacitors is 1 nF, the resistance is 5 kΩ, the inductor is 3 μH, and the PIN diode is HVU133 (manufactured by Renesas Technology) To do.

  In the period T1, when electrostatic discharge occurs and an electrostatic current flows from the second GND to the first GND, the low frequency component of the electrostatic current is blocked by the first and second capacitors. The GND control signal is 0 V, and no direct current flows through the PIN diode. Therefore, as shown in FIG. 2B, the high frequency impedance of the PIN diode is 100Ω or more. This cuts off the high frequency component of the electrostatic current. Z23 has a high value up to the high frequency band as shown in FIG. 3A, and can prevent the inflow of electrostatic current.

  In the period T2, when the GND control signal transits from 0V to 5V, the direct current Id flows in the order of the GND control signal wiring, the resistor, the PIN diode, the inductor, and the first GND. Since the direct current Id is Id = V52 / 5 kΩ, a direct current of 1 mA flows through the PIN diode. The impedance of the PIN diode is 0.6Ω as shown in FIG. As a result, Z23 transitions to the characteristics shown in FIG.

  In the period T3, when a communication signal flows, Z23 shown in (b) of FIG. 3 becomes a sufficiently low value to act as a return path, and reduction of radiation noise can be realized.

  According to this embodiment, it is possible to simultaneously reduce radiation noise and ensure electrostatic resistance.

  For example, when the communication circuit is a USB system, the power supply wiring is 0 V before the cable connection, and when it is recognized that the cable is connected, a predetermined voltage is applied to the power supply wiring. In the communication circuit shown in FIG. 2A, by connecting the terminal 9 to the power supply, it is not necessary to design an operation sequence, and it is possible to easily reduce radiation noise and ensure electrostatic resistance.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram illustrating an embodiment, in which (a) illustrates a circuit configuration of a communication circuit, and (b) illustrates an operation sequence of the communication circuit. The GND connection circuit will be described, in which (a) is a diagram showing the circuit configuration, and (b) is a graph showing the forward current dependency of the high-frequency impedance characteristic in the PIN diode. It is a graph which shows the change of the frequency characteristic of the impedance of a GND connection circuit. It is a figure which shows the communication circuit by one prior art example.

Explanation of symbols

1 Communication signal wiring 2 First GND
3 Second GND
4 Communication Signal Generation Circuit 5 GND Control Signal Wiring 6 GND Connection Circuit 12 First Capacitor 13 Second Capacitor 14 PIN Diode 15 Resistor 16 Inductor

Claims (3)

A communication signal generation circuit for generating a communication signal;
A communication signal wiring connected to the communication signal generation circuit;
A first GND connected to the communication signal generation circuit;
A second GND connected to the external GND;
A GND connection circuit for connecting between the first GND and the second GND,
The GND connection circuit controls an impedance between the first GND and the second GND by a GND control signal.   2. The GND connection circuit can freely change electrical connection and separation between the first GND and the second GND according to the GND control signal. Communication circuit. The GND connection circuit is:
A first capacitor is connected from the first GND to the second GND, a PIN diode that is forward from the first GND to the second GND, and a second capacitor is connected. Circuit portion
The first GND and an inductor connected between the PIN diode and the second capacitor;
The communication circuit according to claim 1, wherein the GND control signal is input between the first capacitor and the PIN diode.

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