Electronic system and high-speed broadband signal sensing method

Abstract

To suppress noise over a wide bandwidth and improve a dynamic range of a high-speed broadband signal when sensing the high-speed broadband signal.SOLUTION: According to one preferred aspect of the present invention, an electronic system comprises: a preamplifier board which converts a physical quantity output from a sensing element into an electric signal, amplifies the electric signal by a preamplifier, and outputs a differential signal; an information processing board for subjecting the signal from the preamplifier board to information processing; and two coaxial cables for transmitting a differential signal between the preamplifier board and the information processing board. The electronic system is characterized in that the two coaxial cables are twisted and a twist pitch of the twist is smaller than one-quarter of a wavelength of a maximum frequency signal of the electric signal.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a high-speed broadband signal sensing method capable of suppressing the influence of external noise and constructing an electronic system with a high dynamic range.

Background Art

[0002] In recent years, in the fields of mobility and industrial equipment, it has become important to improve the reliability of electronic systems and operate them stably for long-term use and operation. An electronic system converts the output of physical quantities from various sensing elements essential for its application device into an electrical signal, amplifies it with low noise by a preamplifier, and collects desired data by an information processing device. In many cases, the information processing device is equipped with an analog-to-digital conversion circuit and calculates desired information by digital signal processing. The output from the preamplifier is connected to the information processing device by a cable. With the advancement of electronic systems, the need for long-distance differential transmission of high-speed broadband signals including direct current, such as optical sensors, by cable has been increasing.

[0003] Here, a high-speed broadband signal including direct current (for example, frequency 0 to 25 MHz) is simply referred to as a high-speed broadband signal. In order to transmit the high-speed broadband signal output at maximum power, impedance matching is required, and it is necessary to match the output impedance of the preamplifier, the characteristic impedance of the cable, and the input impedance of the processing device in all desired frequency bands. Generally, the characteristic impedance is 50 Ω or 75 Ω. Also, the measuring instrument for verifying the characteristics of the electronic system is connected with the same characteristic impedance.

[0004] A coaxial cable is used for the cable. In a coaxial cable, the outer conductor and the inner conductor are arranged concentrically, and there is a dielectric between the outer conductor and the inner conductor. The outer conductor is made of aluminum foil, braided wire, etc. There is an outer coating for the purpose of insulation, etc. on the outside of the outer conductor.

[0005] The shielding effect of the outer conductor of a coaxial cable is ineffective in the low-frequency band. Also, since the shielding by a coaxial cable is realized by a non-magnetic material, it is impossible to reduce the effect of external noise being superimposed on the signal by a magnetic field. Therefore, external noise is suppressed by transmitting as a differential signal.

[0006] For low-frequency signals such as audio signals and temperature sensors, shielded twisted pair cables etc. are used. However, since it is not a structure that guarantees a uniform positional relationship among a plurality of copper wires, including when the cable is bent, it is difficult to uniformize the characteristic impedance within the band necessary for transmitting high-speed broadband signals, and the insertion loss is large.

[0007] Triple coaxial cables are used for cable TV and low-current sensing, but special connectors are essential. Also, measuring instruments for measuring high-speed broadband signals during the verification of an electronic system board generally have a structure for inputting and outputting measurement signals via 50Ω SMA type or N type connectors. For this reason, a conversion connector is required for the connection, which has the influence of measurement error due to the occurrence of unnecessary conversion loss, and there are problems such as an increase in man-hours required for these countermeasures. Also, it is possible to add a shielding structure by using a metal duct etc. outside the coaxial cable, but this needs to be considered as part of the housing design from the beginning of the electronic system design, and also the manufacturing cost increases because the member cost also rises.

[0008] As a means of enhancing the external noise resistance of high-speed broadband signals, for example, Patent Document 1 discloses a system configuration that differentializes and utilizes a cable.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0010] When a high-speed broadband signal is differentially transmitted by two coaxial cables, external noise that cannot be canceled out is superimposed on the signal due to the distance difference between the noise source and the plus-side signal coaxial cable and the minus-side signal coaxial cable. In addition, due to the loop antenna formed by the plus-side signal coaxial cable and the minus-side signal coaxial cable, external noise is superimposed on the signal by the magnetic field. In particular, when the cable length becomes longer than about 0.25 times the wavelength λ of the maximum frequency of the high-speed broadband signal, a band highly sensitive to external noise occurs within the signal band. Conversely, when the cable length is short, the band highly sensitive to external noise is outside the signal band.

[0011] For example, a 100 MHz signal has a wavelength of about 2 m in a coaxial cable considering a wavelength shortening rate of 66%. Therefore, if the cable length is shorter than 2 × 0.25 = 50 cm, the band highly sensitive to external noise is outside the signal band. However, for example, when transmitting a signal acquired by a sensor, in recent years, due to the increase in size and complexity of the object to be sensed, the required signal transmission distance tends to be longer.

[0012] Due to the above situation, for example, when the cable length becomes longer than 0.5 m, the problem of external noise cannot be ignored in a given application. Then, when finding the maximum frequency at which the band highly sensitive to external noise is outside the signal band for a 5 m coaxial cable, Coaxial cable length = Wavelength in the coaxial cable × 0.25 = Speed of light / Maximum frequency / 0.66 × 0.25 Therefore, the maximum frequency not affected by noise is about 25 MHz. For this reason, when transmitting a high-speed broadband signal with a frequency of 0 to 100 MHz using a normal coaxial cable, it will be affected by noise.

[0013] Figure 1 is a layout diagram of an experimental apparatus for inputting a high-speed broadband signal into an amplifier (AMP) and measuring it with a measuring instrument. As shown in Figure 1, a coaxial cable with a characteristic impedance of 50 Ω is arranged in a certain laboratory, and comparisons are made between the case of one coaxial cable with a cable length of 5 m and the case of two coaxial cables (positive side CP, negative side CM). The output of the amplifier was measured using a spectrum analyzer as the measuring instrument. The results are shown below.

[0014] Figure 2A is a graph showing the level of external noise in the case of one coaxial cable. The horizontal axis represents frequency, and the vertical axis represents noise level.

[0015] Figure 2B shows the level of external noise when two coaxial cables (differential transmission) are used. The horizontal axis represents frequency, and the vertical axis represents noise level. Since the differential amplifier circuit used is SA-420F5 (bandwidth 1 kHz to 70 MHz, Gain 46 dB) of NF Circuit Design Block Co., Ltd., the horizontal axis is up to 70 MHz. Here, when the maximum frequency is 100 MHz, the wavelength λ is 300,000 (km / sec) / 100,000,000 (Hz) = 3 m.

[0016] Here, the configuration of Figure 2A is called single, and the configuration of Figure 2B is called straight pair. For the single, one input of the differential amplifier circuit is terminated with 50 Ω and not used, so 6 dB is added to the measurement result. For this reason, although the floor noise seems to increase, here we focus on the peak level.

[0017] The coaxial cable has a shield, but as described above, the shielding effect in the low-frequency band is low, and the magnetic field does not have a sufficient effect on suppressing external noise. The high noise level in the single as shown in Figure 2A reflects this.

[0018] On the one hand, as shown in Fig. 2B, it can be seen that due to the effect of differential transmission, including the low-frequency band, the magnetic field is effective in suppressing external noise. However, the external noise in the 20 MHz band and 40 MHz band indicated by the dotted circle is hardly suppressed. Considering the wavelength shortening rate (the cable used in this experiment is 66%), this corresponds to 0.25 times and 0.25×2 = 0.5 times the frequency of the signal transmitted in the coaxial cable.

[0019] In an electronic system where the size of the measurement object is large and it is necessary to sense multiple sites, the cable will be long. In order to ensure the desired signal-to-noise ratio, it is necessary to amplify the signal with a preamplifier to a level where the influence of these external noises is eliminated. However, since there is an upper limit to the maximum amplitude level that can be output by the preamplifier, the signal dynamic range of the electronic system is restricted.

[0020] Therefore, an object of the present invention is to suppress noise over a wide band and improve the signal dynamic range when sensing a high-speed broadband signal.

Means for Solving the Problem

[0021] A preferred aspect of the present invention is an electronic system comprising a preamplifier board that converts the physical quantity output from a sensing element into an electrical signal, amplifies it with a preamplifier, and outputs a differential output, an information processing board that processes the signal from the preamplifier board, and two coaxial cables that transmit differential signals between the preamplifier board and the information processing board, wherein the two coaxial cables are twisted, and the twist pitch of the twist is less than one-fourth of the wavelength of the signal of the maximum frequency of the electrical signal.

[0022] Another preferred aspect of the present invention is a high-speed broadband signal sensing method using an electronic system comprising a preamplifier board that converts the physical quantity output from a sensing element into an electrical signal, amplifies it by a preamplifier, and outputs a differential output, an information processing board that processes the signal from the preamplifier board, and two coaxial cables that transmit differential signals between the preamplifier board and the information processing board. The method is characterized in that the two coaxial cables are twisted, and the twist pitch of the twist is less than one-fourth of the wavelength of the signal having the maximum frequency of the electrical signal.

Advantages of the Invention

[0023] According to the present invention, when sensing a high-speed broadband signal, noise can be suppressed over a wide bandwidth, and the dynamic range of the signal can be improved.

Brief Description of the Drawings

[0024]

Figure 1

Figure 2A

Figure 2B

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Best Mode for Carrying Out the Invention

[0025] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In each drawing for explaining the embodiments, the same components are given the same names and reference numerals, and repeated explanations thereof are omitted.

[0026] Expressions such as "first", "second", and "third" in this specification and the like are attached to identify components, and do not necessarily limit numbers or orders. Also, numbers for identifying components are used for each context, and a number used in one context does not necessarily indicate the same configuration in another context. Further, it does not prevent a component identified by a certain number from also having the functions of a component identified by another number.

[0027] As an example of the configuration described in the embodiment, in an electronic system including a preamplifier board that converts the physical quantity output from a sensing element into a high-speed broadband electrical signal, amplifies it with low noise by a preamplifier, and outputs a differential output, an information processing board that processes the signal from the preamplifier board, and two coaxial cables that transmit differential signals between the preamplifier board and the information processing board, it is useful that the two coaxial cables are twisted, and the twist pitch of the twist is smaller than the signal wavelength of the maximum frequency of the high-speed broadband electrical signal × 0.25.

Embodiment

[0028] FIG. 3 is a block diagram of the electronic system in Embodiment 1. In FIG. 3, the sensor 100 measures a desired physical quantity from the measurement target 10 and converts it into a signal and transmits it to the preamplifier board 110. The preamplifier board 110 and the information processing board 140 are connected by the cable 120 and the coaxial cable 130. Examples of the measurement target 10 include the temperature and acceleration of each part of an automobile or a train, but it is not particularly limited.

[0029] The desired physical quantity of the measurement object 10 is sensed using the sensor 100 and converted into an electrical signal. This electrical signal is low-noise amplified by the preamplifier board 110 so that the signal-to-noise ratio of the output is maximized and converted into a differential signal. This differential signal becomes a high-speed broadband signal including DC.

[0030] For example, when light is detected by the sensor 100 and converted into an electrical signal by a photomultiplier tube, it has a bandwidth from DC to several tens to hundreds of MHz. Here, as described above, a high-speed broadband signal including DC is simply called a high-speed broadband signal. To transmit a high-speed broadband signal, a coaxial cable is used. To transmit the high-speed broadband signal output at the maximum power, impedance matching is required, and it is necessary to match the output impedance of the preamplifier, the characteristic impedance of the cable, and the input impedance of the processing device over all desired frequency bands. Generally, the characteristic impedance is 50 Ω or 75 Ω. Also, a measuring instrument for verifying the characteristics of an electronic system is connected with the same characteristic impedance.

[0031] The shielding effect of a coaxial cable has no effect in the low-frequency band. Also, since the shielding of a coaxial cable is realized by a non-magnetic material, the effect of suppressing the superposition of external noise on the signal by a magnetic field cannot be reduced. Therefore, external noise is suppressed by transmitting as a differential signal.

[0032] To transmit a differential signal, a shielded twisted pair cable or the like is used for low-frequency signals such as audio signals and temperature sensors. However, since it is not a structure that guarantees a uniform positional relationship among a plurality of copper wires even when the cable is bent, it is difficult to uniformize the characteristic impedance within the bandwidth required for transmitting a high-speed broadband signal, and the insertion loss is large.

[0033] When a high-speed broadband signal is differentially transmitted by two coaxial cables, external noise that cannot be canceled out is superimposed on the signal due to the difference in distance between the noise source and the plus-side signal coaxial cable and the minus-side signal coaxial cable. In addition, external noise is superimposed on the signal by a magnetic field through a loop antenna formed by the plus-side signal coaxial cable and the minus-side signal coaxial cable. In particular, when the cable length becomes longer than about 0.25 times the wavelength λ of the maximum frequency of the high-speed broadband signal, a band highly sensitive to external noise is generated within the signal band. If it is an electronic system where the size of the measurement target is large and it is necessary to sense multiple parts, the cable becomes even longer.

[0034] Using a 5-m coaxial cable with a characteristic impedance of 50 Ω at the cable end shown in FIG. 4, a simulation was performed. The structure of the coaxial cable has an inner conductor with a radius of 0.255 mm at the center, and the outside is successively coated with a dielectric, an outer conductor, and a sheath. Each radius is as shown in FIG. 4.

[0035] As shown in FIG. 5, three coaxial cables with the structure of FIG. 4 are arranged in air (each coaxial cable is terminated with 50 Ω) and an electromagnetic field simulation is performed. Let CP and CM be coaxial cables for differential signal transmission, and CA be a coaxial cable serving as a noise source. The distance between the coaxial cable CA and the coaxial cable CP is 5 mm, and the distance between the coaxial cable CP and the coaxial cable CM is 3.2 mm. Here, it is evaluated how much of the noise input from port 1 leaks to port 2.

[0036] By calculating S21, the coupling degree of external noise from the coaxial cable CA to the coaxial cables CP and CM can be evaluated. The physical meaning of S21 is the signal that passes through the output terminal when a signal is input to the input terminal. The simulator is Ansys HFSS, and both the inner conductor and the outer conductor are modeled as uniform metals, but only the outer conductor is calculated including the inside of the conductor. Here, the twisted pair is modeled to be 5 m after twisting.

[0037] Fig. 6 shows an example of the twist of a coaxial cable. The twist pitch is the distance that the coaxial cable on the positive side or the negative side makes one turn.

[0038] Fig. 7 shows the simulation results. The horizontal axis represents the frequency, and the vertical axis represents S21. There is a peak of S21 in a specific frequency band, which is a frequency band with a large coupling degree. The frequency band with a large coupling degree is around 20 MHz and its N - fold frequencies. Considering the wavelength shortening rate (0.66), this is a frequency band that is N times λ / 4. Thus, it is the wavelength λ that is involved in the physical dimension of the length of the coaxial cable, and the sensitivity to noise becomes high in the integer - multiple frequency bands of the frequency band calculated from λ / 4.

[0039] Fig. 8 shows a graph summarizing the S21 peak values in the frequency band that is N times λ / 4. T represents the number of twists. The number of twists is defined such that when the cable is twisted 360° and returns to its original state, it is 1T.

[0040] 4T means a cable with the coaxial cable twisted 4 times. That is, when the cable length is 5 m, the twist pitch is 1.25 m, and the twisted pairs cross once for each twist pitch.

[0041] That is, Twist pitch × 0.5 = Wavelength in the coaxial cable × 0.25 = Speed of light / Maximum frequency / 0.66 × 0.25 So, 1.25 × 0.5 ≈ 300,000 (km / sec) / 80,000,000 (Hz) / 0.66 × 0.25 Therefore, the frequency that is N times λ / 4 calculated from the twist pitch is 80 MHz or higher. As can be seen in Fig. 8, when comparing the straight pair and 4T, there is a suppression effect of about 4 dB.

[0042] 5T is a cable with the coaxial cable twisted 5 times. That is, when the cable length is 5 m, the twist pitch is 1 m. However, the frequency that is N times of λ / 4 calculated from the twist pitch is 100 MHz or more. As shown in Fig. 8, when comparing the straight pair with 5T, there is a suppression effect of about 20 dB. From the results of this simulation, if the twist pitch of two coaxial cables is smaller than one-fourth of the wavelength of the maximum frequency of the high-speed broadband signal, a sufficient suppression effect can be obtained.

[0043] Normally, the shield of a coaxial cable is a braid or an aluminum foil, and it is difficult to twist it with a twist pitch of several centimeters like a normal twisted pair cable. However, according to this embodiment, a sufficient degree of external noise suppression can be obtained even with a wider twist pitch. As a result, the electronic system of this embodiment can reduce the gain of the preamplifier by about 20 dB, so the dynamic range is improved by about 20 dB due to the increase in the maximum amplitude of the desired signal by about 20 dB.

[0044] Using this embodiment, for example, when a large number of preamplifier boards are installed at multiple sites to be measured and multi-point sensing information is collected, not only the external noise superimposed on the signal via the coaxial cable from each preamplifier board to the information processing board, but also the signal interference from other preamplifier boards can be reduced, and high-reliability data can be collected.

Embodiment

[0045] Fig. 9 is a block diagram of the electronic system in Embodiment 2. In Fig. 9, the sensor 100 measures a desired physical quantity from the measurement object 10 and transmits it to the preamplifier board 110 after converting it into a signal. The preamplifier board 110 and the information processing board 140 are connected by a coaxial cable 130.

[0046] The preamplifier board 110 includes an amplifier circuit A1, an amplifier circuit A2, a terminating resistor R1, and a terminating resistor R2 for the electrical signal received from the sensor 100. The information processing board 140 includes an amplifier circuit A3, a terminating resistor R3, and a terminating resistor R4.

[0047] The desired physical quantity of the measurement object 10 is sensed using the sensor 100 and converted into an electrical signal. This electrical signal is low-noise amplified by the preamplifier board 110 so that the signal-to-noise ratio of the output is maximized. Specifically, it is low-noise amplified by the amplifier circuit A1 and further amplified by the amplifier circuit A2.

[0048] In Example 1, it is converted into a differential signal by the amplifier circuit A2, while in Example 2, it is a single transmission. This single signal is a high-speed broadband signal including DC. For example, when detecting light and converting it into an electrical signal with a photomultiplier tube, it has a bandwidth from DC to several tens to hundreds of MHz. Here, as described above, a high-speed broadband signal including DC is simply called a high-speed broadband signal.

[0049] To transmit a high-speed broadband signal, a coaxial cable is used. In order to transmit the high-speed broadband signal output at maximum power, impedance matching is required, and it is necessary to match the output impedance of the preamplifier, the characteristic impedance of the cable, and the input impedance of the processing device in all desired frequency bands. Generally, the characteristic impedance is 50Ω or 75Ω. Also, the measuring instrument for verifying the characteristics of the electronic system is connected with the same characteristic impedance.

[0050] The shielding effect of the coaxial cable has no effect in the low-frequency band. Also, since the shielding of the coaxial cable is realized by a non-magnetic material, the effect of reducing the superposition of external noise on the signal due to the magnetic field cannot be achieved. In Example 1, external noise was suppressed by transmitting as a differential signal, while in Example 2, a coaxial cable with the preamplifier board 110 side and the information processing board 140 side terminated with termination resistors R2 and R4 respectively is provided and twisted with it to suppress external noise.

[0051] The twist pitch is made smaller than one-fourth of the wavelength of the maximum frequency of the high-speed broadband signal as described in Example 1. Although the signal is single, the external noise is superimposed in balance on the twisted coaxial cable, so it is canceled by the amplifier circuit A3 which is a differential amplifier circuit.

[0052] Generally, the shield of a coaxial cable is a braid or an aluminum foil, and it is difficult to twist at a twist pitch of several centimeters like a normal twisted pair cable. However, according to this embodiment, a sufficient degree of external noise suppression can be obtained even at a wider twist pitch. As a result, the electronic system of this embodiment can reduce the gain of the preamplifier by about 20 dB, so that the maximum amplitude of the desired signal increases by about 20 dB, improving the dynamic range by 20 dB.

[0053] Using this embodiment, for example, when installing a large number of preamplifier boards at multiple sites to be measured and collecting multi-point sensing information, not only the external noise superimposed on the signal via the coaxial cable from each preamplifier board to the information processing board, but also the signal interference from other preamplifier boards can be reduced, and it becomes possible to collect highly reliable data.

Embodiment

[0054] The present invention is not limited to the above-described embodiments, and can be implemented with appropriate modifications. For example, the outer covering (sheath) of the coaxial cable can be removed to form a twisted pair structure, the loop area can be made smaller, and then the outer covering can be attached. Modifications such as providing a shield with aluminum foil or the like on the coaxial cable twisted pair structure without the outer covering and attaching the outer covering to the outside can also be considered.

[0055] According to this embodiment, when the cable length becomes longer than about 0.25 times the wavelength λ of the maximum frequency of the high-speed broadband signal, external noise can be shielded at low cost, and high reliability of differential signal transmission of the high-speed broadband signal can be realized. Since the gain of the preamplifier can be reduced by this embodiment, it is possible to increase the upper limit of the signal level and increase the signal dynamic range of the electronic system.

[0056] According to the above embodiment, accurate sensing can be realized, so that efficient device control is possible, the energy consumption is low, the carbon emissions are reduced, global warming can be prevented, and it can contribute to the realization of a sustainable society.

Description of Reference Numerals

[0057] 10…Measurement object 100…Sensor 110…Preamplifier board 120…Cable 130…Coaxial cable 140…Information processing board A1~A3…Amplification circuit R1~R4…Termination resistor

Claims

1. A preamplifier board that converts the physical quantity output from a sensing element into an electrical signal, amplifies it by a preamplifier, and outputs a differential signal, An information processing board that processes the signal from the preamplifier board, In an electronic system composed of two coaxial cables that transmit differential signals between the preamplifier board and the information processing board, The two coaxial cables are twisted, and the twist pitch of the twist is smaller than one-fourth of the wavelength of the signal of the maximum frequency of the electrical signal. An electronic system characterized by this.

2. The electrical signal has a band from direct current to 25 MHz, The electronic system according to claim 1.

3. The electrical signal has a band from direct current to 100 MHz, The electronic system according to claim 1.

4. The coaxial cable has a length of 0.5 m or more, The electronic system according to claim 1.

5. The coaxial cable has a length of 5 m or more, The electronic system according to claim 1.

6. The coaxial cable has a laminated structure of an inner conductor, a dielectric covering the inner conductor, an outer conductor covering the dielectric, and a sheath covering the outer conductor, The electronic system according to claim 1.

7. The coaxial cable has a laminated structure of an inner conductor, a dielectric covering the inner conductor, and an outer conductor covering the dielectric, The twisted coaxial cable is covered with a sheath, The electronic system according to claim 1.

8. The coaxial cable has a laminated structure of an inner conductor, a dielectric covering the inner conductor, and an outer conductor covering the dielectric, Coat the twisted coaxial cable with a shield, and further coat the shield with a sheath. The electronic system according to claim 1.

9. The sensing element is a photomultiplier tube, characterized in that. The electronic system according to claim 1.

10. A high-speed broadband signal sensing method using an electronic system comprising a preamplifier board that converts the physical quantity output from a sensing element into an electrical signal, amplifies it with a preamplifier, and outputs a differential signal, an information processing board that processes the signal from the preamplifier board, and two coaxial cables that transmit differential signals between the preamplifier board and the information processing board, Twist the two coaxial cables, and the twist pitch of the twist is less than one-fourth of the wavelength of the signal of the maximum frequency of the electrical signal, characterized in that it is a high-speed broadband signal sensing method.

11. The electrical signal has a band from direct current to 10 MHz. The high-speed broadband signal sensing method according to claim 10.

12. The electrical signal has a band from direct current to 100 MHz. The high-speed broadband signal sensing method according to claim 10.

13. The coaxial cable has a length of 0.5 m or more. The high-speed broadband signal sensing method according to claim 10.

14. The coaxial cable has a length of 5 m or more. The high-speed broadband signal sensing method according to claim 10.

15. The sensing element is a photomultiplier tube, characterized in that. The high-speed broadband signal sensing method according to claim 10.

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