JP7704077B2 - Signal Transmission Device - Google Patents

Description

The present invention relates to a signal transmission device.

A controller IC such as a USB-IC may be implemented in a signal transmission device. For example, a USB-IC outputs a USB (Universal Serial Bus) signal. The USB signal is a differential signal that complies with the USB standard. The USB signal is input to a device external to the signal transmission device via a connector.

Here, a common mode choke coil (CMCC) may be provided to reduce noise generated in the controller IC. The common mode choke coil (hereinafter sometimes referred to as CMCC) is provided, for example, between the controller IC and the connector (for example, Patent Document 1).

JP 2009-135760 A

In signal transmission devices, it is necessary to reduce noise and also to consider signal reflections in the signal lines between the signal transmission device and other devices.

The present invention has been made in consideration of the above, and aims to provide a signal transmission device that can reduce noise and signal reflection in the signal line between other devices.

A signal transmission device according to one aspect of the present disclosure includes a circuit board, a connector provided on the circuit board to which a cable for transmitting differential signals to an external device can be attached and detached, an integrated circuit mounted on the circuit board, a signal line provided on the circuit board for transmitting and receiving the differential signal between the integrated circuit and the connector, a common mode choke coil provided on the signal line for suppressing noise output from the integrated circuit, and a retimer IC provided on the signal line between the common mode choke coil and the connector, wherein the retimer IC is connected in a DC manner on the signal line, and the common mode choke coil is provided in the immediate vicinity of a terminal that outputs the differential signal from the integrated circuit to the signal line, where the immediate vicinity is within a distance of half a wavelength corresponding to the band of the differential signal.

The signal transmission device disclosed herein can reduce noise and signal reflections in the signal line between the device and other devices.

FIG. 1 is a diagram showing a configuration of a main part of a signal transmission device of a comparative example. FIG. 2 is a diagram showing an example of a common mode signal reflected toward the cable side in the configuration of FIG. FIG. 3 is a diagram showing a main part of a signal transmission device according to an embodiment. FIG. 4 is a diagram showing a configuration example of a retimer IC. FIG. 5 is an external view showing an example of a CMCC. FIG. 6 is a diagram showing frequency characteristics when a differential signal is input to the CMCC of FIG. FIG. 7 is a diagram showing frequencies when a common mode signal is input to the CMCC of FIG. FIG. 8 is a diagram for explaining measurement of noise in a signal transmission device. FIG. 9 is a diagram for explaining measurement of noise in a signal transmission device. FIG. 10 is a diagram showing an example of evaluation regarding noise. FIG. 11 is a diagram for explaining measurement of reflection characteristics of a signal transmission device as viewed from the connector side. FIG. 12 is a diagram for explaining measurement of reflection characteristics of a signal transmission device as viewed from the connector side. FIG. 13 is a diagram showing an example of a common mode signal reflected toward the cable side in the configuration of FIG.

Below, an embodiment of the signal transmission device of the present disclosure will be described in detail with reference to the drawings. Note that the present invention is not limited to these embodiments. Furthermore, the components of each embodiment include those that are replaceable and easy for a person skilled in the art, or those that are substantially the same. Each embodiment is an example, and partial replacement or combination of the configurations shown in different embodiments is possible. In the description of each embodiment below, components that are the same or equivalent to those in other embodiments are given the same reference numerals, and their description is simplified or omitted. The configurations described below can be combined as appropriate. Furthermore, the configurations can be omitted, replaced, or modified within the scope of the gist of this disclosure.

Comparative Example
For the understanding of the present disclosure, a comparative example will be described first.

Figure 1 is a diagram showing the configuration of the main parts of a signal transmission device 10a of a comparative example. In Figure 1, the signal transmission device 10a has a circuit board 1a, a controller IC 11, a connector 20, a CMCC 12, and a signal line 15. The controller IC 11, the connector 20, the CMCC 12, and the signal line 15 are mounted on the circuit board 1a.

The connector 20 is provided on the circuit board 1a. The connector 20 allows the cable 2 to be attached and detached. By connecting the cable 2 to the connector 20, an external device (not shown) can be electrically connected to the signal transmission device 10a. The cable 2 is, for example, a cable that transmits a differential signal to the external device.

The controller IC11 is an integrated circuit that functions as a controller and can input and output signals that comply with a specified standard. The controller IC11 is mounted on the circuit board 1a. When an external device (not shown) is connected to the connector 20, the controller IC11 is an integrated circuit that transmits and receives differential signals to and from the external device via the connector 20. The controller IC11 is, for example, a USB-IC that can input and output USB signals. The controller IC11 may be a CPU (Central Processing Unit) that functions as a controller.

The signal line 15 is provided on the circuit board 1a. The signal line 15 is a line for transmitting differential signals between the controller IC 11 and the connector 20. The CMCC 12 is provided on the signal line 15. The CMCC 12 suppresses common mode noise in the signal output from the controller IC 11 and transmitted through the signal line 15.

In the signal transmission device 10a shown in FIG. 1, a signal output from the controller IC 11 is transmitted through the signal line 15 and transmitted to an external device (not shown) via the connector 20 and the cable 2. A signal output from an external device (not shown) is input to the signal transmission device 10a via the cable 2 and the connector 20. A signal input to the signal transmission device 10a is input to the controller IC 11 by transmitting through the signal line 15. Common mode noise of the signal transmitted through the signal line 15 is reduced by the CMCC 12. Therefore, common mode noise flowing from the signal transmission device 10a to the cable 2 via the connector 20 is reduced by the CMCC 12.

Here, the CMCC 12 is provided between the controller IC 11 and the connector 20. For this reason, the signal output from the controller IC 11 is reflected by the CMCC 12 as shown by the arrow Y1 depending on the frequency band. It is not preferable for the signal reflected as shown by the arrow Y1 to be input to each part of the signal transmission device 10a.

In addition, after passing through connector 20, signals output from external devices are reflected by CMCC 12 as shown by arrow Y2 depending on the frequency band. The signal reflected as shown by arrow Y2 passes through connector 20 and is input to cable 2. It is not desirable for the signal reflected as shown by arrow Y2 to pass through cable 2 and be input to an external device.

Figure 2 is a diagram showing an example of a common mode signal reflected toward the cable 2 side in the configuration of Figure 1. In Figure 2, the horizontal axis shows frequency [GHz], and the vertical axis shows S parameter Scc11 [dB]. Scc11 shows the reflection characteristic of the common mode signal.

In FIG. 2, the signal range 100 is a range defined by a specific standard as an index for signal quality. For example, the signal range 100 is a range that specifies the common mode signal reflected to the USB connector side in the USB standard. Here, the CMCC 12 for noise removal has a characteristic of reflecting the input common mode signal to the input side without passing it. Therefore, due to the reflection characteristic of the CMCC 12, the common mode signal is reflected to the connector 20 side as shown by the arrow Y2. Therefore, in the configuration of the comparative example shown in FIG. 1, there are parts that fall outside the signal range 100 in FIG. 2, and the index for common mode return loss cannot be satisfied.

(Embodiment)
Next, a signal transmission device according to an embodiment will be described.

Figure 3 is a diagram showing the main parts of a signal transmission device 10 according to an embodiment. As shown in Figure 3, the signal transmission device 10 of this example has a circuit board 1, a controller IC 11, a connector 20, a CMCC 12, a signal line 15, and a retimer IC 13. What is different from the comparative example described with reference to Figure 1 is that a retimer IC 13 is added to the signal line 15. The retimer IC 13 is provided on the signal line 15 between the CMCC 12 and the connector 20. The signal transmission device 10 is, for example, a notebook personal computer, a desktop personal computer, a game machine, etc.

(Retimer IC)
The retimer IC 13 converts the input differential signal into a single-ended signal, then converts it into a differential signal, and outputs it. Therefore, the common mode component of the input signal does not flow to the output signal side. At this time, the retimer IC 13 outputs a differential signal corresponding to the input differential signal in synchronization with another clock. In other words, the retimer IC 13 regenerates the input differential signal in synchronization with another clock. The common mode reflected signal by the CMCC 12 is canceled by the retimer IC 13. Therefore, the common mode reflected signal is not transmitted to the connector 20 side.

The retimer IC 13 does not amplify the input signal, but rather has the function of creating a completely new signal. Therefore, the signal output from the retimer IC 13 is not affected by the input signal.

Figure 4 is a diagram showing an example of the configuration of the retimer IC 13. In Figure 4, the retimer IC 13 has a receiving unit (Rx) 121, a clock data recovery unit (CDR) 122, an elastic buffer 123, a clock generating unit (Local Clock) 124, and a transmitting unit (Tx) 125.

The receiver 121 converts the differential signal 110 into a single-ended signal 111. That is, the receiver 121 receives the differential signal 110 and outputs the single-ended signal 111.

The clock data recovery unit 122 receives the single-ended signal 111 and outputs data 112 and a clock 113. That is, the clock data recovery unit 122 generates data 112 and a clock 113 from the single-ended signal 111 output from the receiving unit 121.

The elastic buffer 123 inputs the data 112 and the clock 113 generated by the clock data recovery unit 122. The elastic buffer 123 stores the data 112 at the timing of the clock 113.

The clock generating unit 124 generates a clock CK for reading the data 114 from the elastic buffer 123. The clock generating unit 124 generates a clock CK that is different from the clock 113.

The transmitting unit 125 converts the data 114 into a differential signal 115. That is, the transmitting unit 125 inputs the data 114 read from the elastic buffer 123 and outputs the differential signal 115.

(Example of CMCC)
Fig. 5 is an external view showing an example of a CMCC. In Fig. 5, for example, the vertical length A of the CMCC 12 is 0.65 [mm], the horizontal length B is 0.5 [mm], and the height C is 0.3 [mm].

CMCC12 has four terminals T11, T12, T21, and T22. Terminals T11 and T12 are connected to the controller IC11 side of CMCC12, for example, on signal line 15. Terminals T21 and T22 are connected to the connector 20 side of CMCC12, for example, on signal line 15 in FIG. 1.

By providing the CMCC 12 on the signal line 15, it is possible to suppress common mode noise that occurs during transmission of a differential signal. Generally, the source of common mode noise is the controller IC 11 that generates the differential signal. In addition, regarding noise radiation, it is common that common mode noise generated from the controller IC 11 is transmitted to the signal line 15, causing noise to radiate from the signal line 15. By reflecting the common mode noise transmitted from the controller IC 11 to the CMCC 12 to the controller IC 11 side, it is possible to suppress the passing noise. Therefore, as shown in FIG. 3, by mounting the CMCC 12 between the controller IC 11 and the retimer IC 13 on the signal line 15, it is possible to suppress the common mode noise N radiated from the signal line 15 (X mark in FIG. 3).

In the signal line 15 of FIG. 3, the CMCC 12 is preferably provided in close proximity to the terminals T1 and T2 that output a differential signal from the controller IC 11 to the signal line 15. Providing the CMCC 12 in close proximity to the terminals T1 and T2 has a large effect of suppressing noise. Here, to suppress the effect of noise on the 5 GHz communication band of the wireless communication standard, for example, the terminals T1 and T2 are provided within 3 cm, which is half of one wavelength corresponding to 5 GHz, that is, about 6 cm. Note that this "3 cm" is the result of wavelength shortening taking into account the relative dielectric constant of 4.5 of FR-4 (Flame Retardant Type 4), a common board material. In other words, it is preferable to mount the CMCC 12 with a wiring length of 3 cm or less from the terminals T1 and T2 on the board on which the controller IC 11 and the CMCC 12 are mounted.

(CMCC frequency characteristics)
6 and 7 are diagrams showing frequency characteristics of the CMCC 12 of FIG. 5. FIG. 6 is a diagram showing frequency characteristics when a differential signal is input to the CMCC 12 of FIG. 5. In FIG. 6, the horizontal axis shows frequency [Hz], and the vertical axis shows Sdd11 [dB] of the S parameter. FIG. 7 is a diagram showing frequency when a common mode signal is input to the CMCC 12 of FIG. 5. In FIG. 7, the horizontal axis shows frequency [Hz], and the vertical axis shows Scc11 [dB] of the S parameter. The frequency f1 in FIG. 7 corresponds to 2.4 [GHz] of the wireless communication standard. The frequency f2 corresponds to 5 [GHz] of the wireless communication standard. In the frequency bands near these frequencies f1 and f2, the CMCC 12 of this example has good reflection characteristics.

Returning to FIG. 3, if a CMCC 12 is mounted between the retimer IC 13 and the connector 20, the common mode signal will be reflected to the connector 20 as shown by the arrow Y3. The retimer IC 13 is an IC that corrects signals by regenerating a new differential signal. Therefore, the signal output from the retimer IC 13 is not affected by the common mode reflected signal that is reflected as shown by the arrow Y3. Therefore, the CMCC 12 is mounted between the controller IC 11 and the retimer IC 13 as shown in FIG. 3. The common mode reflected signal by the CMCC 12 is canceled by the retimer IC 13. Therefore, the common mode reflected signal is not transmitted to the connector 20.

(Noise measurement)
8 and 9 are diagrams for explaining the measurement of noise in the signal transmission device 10. In Fig. 8 and Fig. 9, noise radiated from the signal line 15 between the controller IC and the retimer IC is measured when the signal transmission device 10 and the external device 16 transmit and receive a USB signal 200 via the cable 2. In this example, the radiation level of noise interfering with the 2.4 GHz band of the wireless communication standard is measured using a spectrum analyzer 18.

Figure 8 shows the measurement situation when no CMCC is installed on the signal line 15 between the controller IC 11 and the retimer IC 13. Figure 9 shows the measurement situation when a CMCC 12 is installed on the signal line 15 between the controller IC 11 and the retimer IC 13.

8 and 9, the signal transmission device 10 is placed in the radio wave anechoic box 300, and an antenna 14 is provided inside the signal transmission device 10. The antenna 14 is provided at a position 10 cm away from the board on which the controller IC is mounted. The antenna 14 is an antenna that can receive the radiation level of noise that interferes with the 2.4 GHz band of the wireless communication standard. A cable C1 connected to the antenna 14 is connected to a preamplifier 17, and the signal amplified by the preamplifier 17 is input to a spectrum analyzer 18 via a cable C2. An external device 16 is connected to the connector 20 via a cable 2. In this example, the external device 16 is a storage device such as an SSD (Solid State Drive).

Figure 10 shows an example of noise evaluation. In Figure 10, the horizontal axis is frequency [MHz], and the vertical axis is noise level [dBm].

The solid line in FIG. 10 indicates a state in which no signal is output (No communication). The dashed line in FIG. 10 indicates a state in which a USB4 signal is output without a CMCC (No CMCC) as in FIG. 8. The dashed line in FIG. 10 indicates a state in which a USB4 signal is output with a CMCC (with CMCC) as in FIG. 9. As shown by the dashed ellipse D in FIG. 10, an improvement of approximately 4.5 dBm was confirmed for the 2.4 GHz band of the wireless communication standard. Therefore, as shown in FIG. 3, the noise N in the signal transmission device 10 can be reduced.

(Measurement of Reflection Characteristics)
11 and 12 are diagrams for explaining the measurement of the reflection characteristics of the signal transmission device 10 as viewed from the connector 20 side. As shown in Fig. 11 and 12, a jig 40 was connected to the connector 20, and the reflection characteristics were measured by a network analyzer 19. That is, the common mode return loss, i.e., Scc11, was measured for a signal reflected in the connector 20 as indicated by the arrow Y4.

Figure 11 shows the measurement situation when a CMCC is not installed on the signal line 15 between the controller IC 11 and the retimer IC 13. Figure 12 shows the measurement situation when a CMCC 12 is installed on the signal line 15 between the controller IC 11 and the retimer IC 13.

The jig 40 converts the connector 20 of the signal line 15 into a 50 Ω coaxial line. In both cases of FIG. 11 and FIG. 12, the common mode signals Tx+ and Tx- converted in the jig 40 by the cables C3 and C4 were input to channels Ch1 and Ch3 of the network analyzer 19. The network analyzer 19 measured the common mode return loss as seen from the connector 20 side, i.e., Scc11.

Figure 13 is a diagram showing an example of a common mode signal reflected toward the cable 2 side in the configuration of Figure 3. In Figure 13, the horizontal axis shows frequency [GHz], and the vertical axis shows S parameter Scc11 [dB]. Unlike the common mode signal 101 described with reference to Figure 2, the common mode signal 102 is a signal that changes within the signal range 100. Therefore, the common mode signal 102 meets a predetermined standard.

As described above, in the signal transmission device 10, a CMCC 12 that suppresses noise that interferes with, for example, the 2.4 GHz communication band of the wireless communication standard is implemented on the signal line 15 between the controller IC and the retimer IC. This makes it possible to suppress the emission of noise that interferes with the communication band of the wireless communication standard while satisfying the common mode return loss, i.e., Scc11, standard.

1, 1a Circuit board 2 Cable 10, 10a Signal transmission device 11 Controller IC
13 Retimer IC
14 Antenna 15 Signal line 20 Connector 121 Receiving unit 122 Clock data recovery unit 123 Elastic buffer 124 Clock generating unit 125 Transmitting unit

Claims (2)

A circuit board;
a connector provided on the circuit board to which a cable for transmitting a differential signal can be detachably connected to an external device;
an integrated circuit mounted on the circuit board;
a signal line provided on the circuit board for transmitting and receiving the differential signal between the integrated circuit and the connector;
a common mode choke coil provided on the signal line for suppressing noise output from the integrated circuit;
a retimer IC provided in the signal line between the common mode choke coil and the connector,
The retimer IC is connected in a DC manner to the signal line,
the common mode choke coil is provided immediately adjacent to a terminal that outputs the differential signal from the integrated circuit to the signal line,
The term "close vicinity" refers to a signal transmission device that is within a distance of half the wavelength corresponding to the band of the differential signal. The retimer IC includes:
2. The signal transmission device according to claim 1, comprising: a receiving unit that receives the differential signal and outputs a single-ended signal; a clock data recovery unit that generates data and a clock from the single-ended signal output from the receiving unit; an elastic buffer that stores the data generated by the clock data recovery unit at the timing of the clock; a clock generating unit that generates a clock for reading data from the elastic buffer; and a transmitting unit that inputs the data read from the elastic buffer and outputs a differential signal.

Images