Broadband oscilloscope input overvoltage protection circuit

Abstract

The invention discloses a broadband oscilloscope input overvoltage protection circuit which is characterized in that an input signal V1 after voltage division attenuation is sent to a shift circuit, so that the input signal V1 is shifted to be within two comparison voltages, when the voltage of an input signal V2 is within the two comparison voltages VH and VL, a low level is output to a D trigger circuit, and when the voltage of the input signal V2 is outside the two comparison voltages VH and VL, a high level is output to the D trigger circuit. When input signal overvoltage occurs, the D flip-flop circuit outputs a high level to the enable end of the switch control circuit, so that output switch switching control signals of the switch control circuit are all low levels, and the switch circuit disconnects the input signal from a post-stage circuit. The shift circuit is adopted to shift the input signal V1 after voltage division attenuation, then comparison is carried out, and the input signal V1 exceeding the range of two comparison voltages VH and VL is overvoltage, so that waveform amplitude does not need to be converted into a waveform amplitude effective value signal, integral time delay does not exist, and the response speed is high; the problem that a single-pulse overvoltage signal cannot be detected due to the fact that a voltage measurement result is small is solved, and the problem that a broadband oscilloscope is prone to being damaged due to the fact that large voltage is input into a low-resistance channel is further solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of wideband oscilloscopes, and more specifically, relates to an overvoltage protection circuit for the input of a wideband oscilloscope. Background Art

[0002] A wideband oscilloscope is one of the most commonly used instruments in the field of electronic measurement, and is used to observe, measure, and record various high-frequency electrical signals or instantaneous electrical signals, and display the distribution of their characteristics over time in the form of waveforms. The low-impedance input impedance of a wideband oscilloscope is generally 50Ω. When the input voltage is too large, the generated current is too large, which will cause irreversible damage to devices such as amplifiers and RF switches in the path. Moreover, the damaged wideband oscilloscope channel can only be repaired by returning it to the factory, which affects the stability and reliability of the wideband oscilloscope.

[0003] This requires an overvoltage protection circuit to be set up in the low-impedance channel to promptly respond to overvoltage signals input in the forms of DC, AC, single pulses, and periodic pulses, and cut off the connection between the input port and the subsequent circuit.

[0004] In the prior art, a clamping circuit is generally built using diodes to protect a wideband oscilloscope: the clamping circuit is connected in series in the signal link, and by virtue of the conduction characteristics of the two diodes, the amplitude of the signal input to the subsequent circuit is limited to achieve the protection function. Since diodes have junction capacitance, connecting them in series in the signal link will reduce the bandwidth of the oscilloscope, so they are mostly used in oscilloscopes with lower bandwidths.

[0005] In the Chinese invention patent application published on July 6, 2021, with the publication number CN113075439A, "An Input Impedance Protection Circuit, Method and Digital Oscilloscope for Digital Oscilloscopes" is disclosed, which includes a signal connection terminal, a voltage dividing circuit, a voltage clamping circuit, a true RMS conversion circuit, a preset value setting circuit, a comparison circuit, a switch control circuit, and a matching resistor switch circuit. The input impedance protection circuit is used to convert the waveform amplitude after amplitude voltage division of the signal to be processed into a waveform amplitude RMS signal, and compare the waveform amplitude RMS signal with a preset signal to be compared. When the voltage value of the waveform amplitude RMS signal is greater than the voltage value of the signal to be compared, the connection between the first input matching resistor of the preset digital oscilloscope and the signal to be processed is disconnected. Since the signal input matching resistor of the digital oscilloscope is controlled based on the comparison result between the signal to be processed and the preset signal to be compared, the low-resistance signal input matching resistor is protected, the failure rate of the digital oscilloscope is reduced, and the stability of the product is further improved. In this invention patent application, the input signal is voltage-divided and attenuated, and the DC voltage is output through an amplitude-to-RMS chip, which is compared with a preset voltage in a comparator. The output of the comparator controls the power-on and power-off of a relay to achieve the protection function. It can respond to both AC overvoltage signals and DC overvoltage signals in a timely manner. However, on the one hand, due to the integration time of the amplitude-to-RMS chip, the response speed is slow. On the other hand, the effective value detection device often has a small measurement result for high-voltage single-pulse signals, resulting in the inability to detect single-pulse overvoltage signals, which may cause overvoltage damage to the subsequent amplification device. Summary of the Invention

[0006] The object of the present invention is to overcome the deficiencies of the prior art and provide an input overvoltage protection circuit for a broadband oscilloscope to quickly respond to (detect) overvoltage signals and be applicable to single-pulse overvoltage signals, and further solve the problem that the low-impedance channel of the broadband oscilloscope is easily damaged by large voltages.

[0007] To achieve the above object of the invention, the input overvoltage protection circuit for a broadband oscilloscope of the present invention includes:

[0008] A voltage dividing and attenuating circuit: used to perform amplitude voltage division and attenuation on the input signal Vin to obtain the input signal V1 after voltage division and attenuation, and input it to the shifting circuit;

[0009] Characterized in that it further includes:

[0010] A shifting circuit, used to shift the input signal V1 after voltage division and attenuation to shift the input signal V1 within two comparison voltages, namely the high comparison voltage VH and the low comparison voltage VL, to obtain the input signal V2 after voltage division and shifting, and input it to the dual-threshold comparison circuit;

[0011] A dual - threshold comparator circuit is used to compare the divided - voltage and shifted input signal V2 with two comparison voltages, namely the high - comparison voltage VH and the low - comparison voltage VL. When the voltage of the input signal V2 is within the two comparison voltages VH and VL, a low - level signal is output to the D - flip - flop circuit; when it is outside the two comparison voltages VH and VL, a high - level signal is output to the D - flip - flop circuit.

[0012] A D - flip - flop circuit, whose output serves as an over - voltage flag signal, is connected to the output of the dual - threshold comparator circuit. When a rising edge occurs at the output of the dual - threshold comparator circuit (i.e., when the low - level signal turns into a high - level signal), the D - flip - flop circuit outputs a high - level signal and holds it.

[0013] A clearing determination circuit is used to receive the reset signal from the microprocessor and detect the output level of the dual - threshold comparator circuit. If it is a low - level signal (i.e., the input signal over - voltage has been removed), a clearing signal is output to the clear input terminal of the D - flip - flop circuit, making the output of the D - flip - flop circuit a low - level signal.

[0014] The output of the D - flip - flop circuit is connected to the enable terminal of the switch control circuit. When the over - voltage flag signal is at a low level, the switch control circuit outputs corresponding switch - switching control signals according to the switch - switching instruction output by the microprocessor. When the over - voltage flag signal is at a high level, all the switch - switching control signals output by the switch control circuit are at a low level. The switch circuit connects or disconnects the input signal from the subsequent circuit according to the high - level or low - level of the switch - switching control signal.

[0015] The object of the present invention is achieved as follows.

[0016] The input over - voltage protection circuit of the broadband oscilloscope of the present invention sends the input signal V1 after voltage division and attenuation into a shift circuit, shifting the input signal V1 within the two comparison voltages, namely the high - comparison voltage VH and the low - comparison voltage VL. Then, in the dual - threshold comparator circuit, it is compared with the high - comparison voltage VH and the low - comparison voltage VL. When the voltage of the input signal V2 is within the two comparison voltages VH and VL, a low - level signal is output to the D - flip - flop circuit; when it is outside the two comparison voltages VH and VL, a high - level signal is output to the D - flip - flop circuit. When an over - voltage of the input signal occurs, the D - flip - flop circuit outputs a high - level signal to the enable terminal of the switch control circuit, making all the switch - switching control signals it outputs at a low level, and the switch circuit disconnects the input signal from the subsequent circuit. The present invention uses a shift circuit to shift the input signal V1 after voltage division and attenuation, and then makes a comparison. Those beyond the range of the two comparison voltages VH and VL are over - voltages. In this way, there is no need to convert the waveform amplitude into a waveform amplitude effective - value signal, there is no integral time delay, and the response speed is fast. On the other hand, for single - pulse over - voltage signals, there is no problem that the voltage measurement result is too small to detect the single - pulse over - voltage signal, further solving the problem that the low - impedance channel of the broadband oscilloscope is easily damaged by large input voltages.

[0017] In addition, the present invention also adds a reset judgment circuit to cooperate with the D trigger circuit to prevent erroneous operation when releasing the overvoltage state. The function of the reset judgment circuit is to judge whether the output of the double threshold comparator circuit is a low level when the microprocessor sends a command to release the protection state, that is, a reset signal, that is, to judge whether the overvoltage signal is removed. If it is a low level, the protection state is allowed to be released, otherwise it is not allowed to be released. In this way, erroneous operation when releasing the overvoltage state is prevented, that is, when the protection state is released, the input overvoltage signal has not been removed, and the overvoltage signal will enter the signal path to cause irreversible damage. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a principle block diagram of a specific implementation mode of the broadband oscilloscope input overvoltage protection circuit of the present invention;

[0019] Figure 2 It is a control flow chart of a specific implementation of the broadband oscilloscope input overvoltage protection circuit of the present invention;

[0020] Figure 3 It is a circuit schematic diagram of a specific implementation of the broadband oscilloscope input overvoltage protection circuit of the present invention;

[0021] Figure 4 It is a timing diagram of a specific implementation of the broadband oscilloscope input overvoltage protection circuit of the present invention, wherein (a) is the situation when the input overvoltage signal is removed, and (b) is the situation when the input non-overvoltage signal is removed. DETAILED DESCRIPTION

[0022] The specific implementation of the present invention is described below in conjunction with the accompanying drawings so that those skilled in the art can better understand the present invention. It should be noted that in the following description, when the detailed description of known functions and designs may dilute the main content of the present invention, these descriptions will be omitted here.

[0023] Figure 1 The invention discloses a principle block diagram of a specific implementation mode of a broadband oscilloscope input overvoltage protection circuit.

[0024] In this embodiment, if Figure 1 As shown, the broadband oscilloscope input overvoltage protection circuit of the present invention includes a voltage division attenuation circuit 1, a shift circuit 2, a dual threshold comparator circuit 3, a comparison voltage setting circuit 4, a D trigger circuit 5, and a clearing determination circuit 6.

[0025] The voltage division attenuation circuit 1 is used to perform amplitude voltage division attenuation on the input signal Vin to obtain the input signal V1 after voltage division attenuation, and input it into the shift circuit 2. The shift circuit 2 is used to shift the input signal V1 after voltage division attenuation to shift the input signal V1 within two comparison voltages, namely the high comparison voltage VH and the low comparison voltage VL, to obtain the input signal V2 after voltage division and shift, and input it into the dual-threshold comparator circuit 3. The dual-threshold comparator circuit 3 is used to compare the input signal V2 after voltage division and shift with the two comparison voltages, namely the high comparison voltage VH and the low comparison voltage VL. When the voltage of the input signal V2 is within the two comparison voltages VH and VL, a low level is output to the D flip-flop circuit 5, and outside the two comparison voltages VH and VL, a high level is output to the D flip-flop circuit 5. In this embodiment, the high comparison voltage VH and the low comparison voltage VL are given by the comparison voltage setting circuit 4. The output of the D flip-flop circuit 5 is used as an overvoltage flag signal, which is input and connected to the output of the dual-threshold comparator circuit 3. When a rising edge occurs at the output of the dual-threshold comparator circuit 3, that is, when the low level turns to a high level, the D flip-flop circuit 5 outputs a high level and holds it. The clear determination circuit 6 is used to receive the reset signal of the microprocessor and detect the output level of the dual-threshold comparator circuit 3. If it is a low level, that is, the input signal overvoltage has been removed, a clear signal is output to the clear input terminal of the D flip-flop circuit 5 to make the output of the D flip-flop circuit 5 a low level. The output of the D flip-flop circuit 5 is connected to the enable terminal of the switch control circuit 7. When the overvoltage flag signal is at a low level, the switch control circuit 7 outputs corresponding switch switching control signals according to the switch switching instruction output by the microprocessor. When the overvoltage flag signal is at a high level, all the switch switching control signals output by the switch control circuit 7 are at a low level. The switch circuit 8 connects or disconnects the input signal from the subsequent circuit according to the high level or low level of the switch switching control signal.

[0026] Figure 2 is the control flow chart of the input overvoltage protection circuit of the broadband oscilloscope of the present invention.

[0027] In this embodiment, as Figure 2 shown, for a broadband oscilloscope, a high-frequency electrical signal or an instantaneous electrical signal is input, and then it is judged whether there is overvoltage. If not, the waveform signal is normally output. If there is overvoltage, the input signal is disconnected from the subsequent circuit and overvoltage is prompted. Then the user issues a reset signal, that is, a protection release command, through the microprocessor, and then it is judged whether the overvoltage is removed. If it is not removed, continue to wait until the overvoltage is removed, the protection is released, and the input signal is connected to the subsequent circuit.

[0028] Figure 3 is the circuit principle of a specific implementation of the input overvoltage protection circuit of the broadband oscilloscope of the present invention.

[0029] In this embodiment, as Figure 3As shown, the voltage division attenuation circuit 1 is composed of resistors R1 and R2, which realizes amplitude voltage division attenuation of the input signal Vin. Its resistance value is set in the order of kΩ to reduce its influence on the low-resistance main signal path. Its output relationship is:

[0030] V1 = R2 × Vin / (R1 + R2)

[0031] The attenuation ratio is generally set between 10 times and 20 times. If the attenuation ratio is too small, the output swing of the operational amplifier in the subsequent shift circuit 2 will be too large, reducing the bandwidth. If the attenuation ratio is too large, the threshold voltage accuracy will be affected.

[0032] In this embodiment, as Figure 2 shown, the shift circuit 2 is composed of an operational amplifier A1 and resistors R3, R4, and R5. The voltage V + is connected to the positive terminal of the operational amplifier A1 through the resistor R3, and the voltage-divided and attenuated input signal V1 is input to the negative terminal of the operational amplifier A1 through the resistor R4. The resistor R is connected between the positive and negative terminals of the operational amplifier A1, and the operational amplifier A1 outputs the voltage-divided and shifted input signal V2.

[0033] With the help of the voltage V + the voltage-divided and attenuated input signal V1 can be shifted, and its output relationship is:

[0034] V2 = (1 + R5 / R4) × V+ - R5 × V1 / R4

[0035] In this embodiment, when the resistors R4 = R5 are set, the output relationship is:

[0036] V2 = 2 × V+ - V1

[0037] Among them, the resistors R4 and R5 should be set to about 100 KΩ. Using resistors with larger resistance values will affect the bandwidth of the shift circuit. Using resistors with smaller resistance values, the voltage at the negative terminal of the operational amplifier A1 will form a current on the resistors R4 and R1, resulting in a non-zero static current at the input end of the channel.

[0038] The comparison voltage setting circuit 4 is built in the form of resistor voltage division. The expressions of the two comparison voltages, namely the high comparison voltage VH and the low comparison voltage VL, are respectively:

[0039] VH = R7 × VCC / (R6 + R7), VL = R9 × VCC / (R8 + R9)

[0040] The input signal V2 after voltage division attenuation and shifting is compared with two comparison voltages, namely, high comparison voltage VH and low comparison voltage VL, in the dual threshold comparator circuit 3. The internal structure of the dual threshold comparator circuit 3 consists of two comparators, an inverter and field effect transistors J1 and J2. Its output mode is "open drain". With diodes D1 and D2, it can realize the "wired OR" logic. The dual threshold comparator circuit 3 can select ADCMP395, and R10 and R11 are pull-up resistors, both set to 3kΩ. The output status of the dual threshold comparator circuit 3 is shown in Table 1.

[0041]

[0042]

[0043] Table 1

[0044] The D flip-flop circuit 5 includes a D flip-flop and a pull-up resistor R16. Since the overvoltage signal may be input in the form of AC or pulse, the output of the dual threshold comparator circuit 3 may be a periodic signal or a pulse signal, so the D flip-flop needs to maintain a high level.

[0045] The D terminal of the D flip-flop is set to a high level through the pull-up resistor R16, the CLK terminal is connected to the output of the dual threshold comparator circuit 3, and the Q terminal is the output of the D flip-flop, that is, the overvoltage flag signal. In the non-overvoltage state, the output of the dual threshold comparator circuit 3 is a low level, and the overvoltage flag signal output by the D flip-flop is a low level; when the overvoltage signal is input, the output of the dual threshold comparator circuit generates a rising edge, and the overvoltage flag signal output by the D flip-flop changes from a low level to a high level and is maintained.

[0046] The CLR terminal of the D flip-flop is a low-level effective clear terminal, which is connected to the output of the clear determination circuit 6 and receives a clear signal. When the clear signal is low, the output Q of the D flip-flop becomes low.

[0047] The function of the clearing judgment circuit 6 is to judge whether the comparator output is low level when the microprocessor sends a command to release the protection state, that is, to judge whether the overvoltage signal is removed. If it is low level, the protection state is allowed to be released, otherwise it is not allowed to be released. In this way, it prevents erroneous operation when releasing the overvoltage state, that is, when the protection state is released, the input overvoltage signal has not been removed, and the overvoltage signal will enter the signal path to cause irreversible damage.

[0048] In this embodiment, the clearing determination circuit 6 is composed of a transistor J3 and resistors R12 to R15, wherein the input terminals are respectively connected to the output of the comparator and the clearing signal MCU_CLR sent by the MCU. Set R12 = 1kΩ, R13 = 100Ω, R14 = 20kΩ, and R15 = 3kΩ, then the states of each part in the circuit are shown in Table 2. The transistor can also be replaced by a field effect transistor.

[0049]

[0050] Table 2

[0051] According to Table 2, if the overvoltage signal is not removed, the dual threshold comparator circuit 3 outputs a high level. Even if MCU_CLR is at a low level, the D_CLR terminal is still at a high level, and the protection state cannot be released. After the input overvoltage signal is removed, the dual threshold comparator circuit 3 outputs a low level. If MCU_CLR is at a low level, the D_CLR terminal becomes a low level, and the protection state can be released.

[0052] It is necessary to ensure that after the dual threshold comparator circuit 3 outputs a high level VCC through diodes D1, D2 and J3, the voltage at the CLR terminal of the D flip-flop can be recognized as a high level by the D flip-flop, that is, greater than the minimum high level input voltage of the D flip-flop.

[0053] In this embodiment, the switch control circuit 7 is composed of an encoder, whose input end receives the switch control instruction sent by the microprocessor, and the enable end is connected to the output of the D flip-flop, that is, the overvoltage flag signal. When the overvoltage flag signal is at a low level, it outputs the corresponding switch switching control signal according to the input microprocessor signal; when the overvoltage flag signal is at a high level, the output is all at a low level. The encoder can be a 2-4 encoder or a 3-8 encoder SN74HCS238.

[0054] The switch circuit 8 is composed of a MEMS switch, whose control end is connected to the switch control signal, RFC is connected to the input signal, and RF1 to RF4 represent four signal paths with different gains. When the switch switching control signals are all low level, the subsequent circuit is disconnected to protect the subsequent circuit. Among them, the MEMS switch can be ADGM1144.

[0055] The response of the broadband oscilloscope input overvoltage protection circuit of the present invention has the following four stages, and its timing is as follows: Figure 4 As shown in (a), Vin1 is less than the threshold voltage and Vin2 is greater than the threshold voltage.

[0056] Stage 1: The input signal does not exceed the threshold voltage, the dual threshold comparator circuit 3 outputs a low level, the D flip-flop circuit 5 outputs a low level, and the switch control circuit 7 outputs the corresponding switch switching control signal according to the switch switching instruction output by the microprocessor. The channel responds normally to the input signal, Vout1 = Vin1 × G, where G is the total gain of the channel;

[0057] Phase 2: The input signal exceeds the threshold voltage, and the outputs of the dual-threshold comparator circuit 3 and the D-flip-flop circuit 5 become high level. However, since the protection circuit takes a period of time Δt1 to respond, the channel responds normally to the input signal within Δt1, and Vout2 = Vin2 × G.

[0058] Stage 3: After time Δt1, the switch circuit 8 switches to the NC state, disconnecting the input signal from the subsequent circuit, the channel presents a high impedance state, and the output is 0;

[0059] Phase 4: After the input overvoltage signal is removed, the microprocessor controls the D flip-flop circuit 5 to reset, and the output of the D flip-flop circuit 5 becomes a low level. The protection state is released after time Δt2, and the channel responds normally, Vout1 = Vin1 × G.

[0060] Figure 4 In (b), stages 1 to 3 are consistent with the above. In stage 4, when the input overvoltage signal is not removed, the microprocessor sends a reset instruction to the D flip-flop circuit 5 (i.e., MCU_CLR becomes a low level). At this time, the reset terminal D_CLR of the D flip-flop cannot be pulled low, and the protection state cannot be released.

[0061] Summarize

[0062] In this embodiment, a broadband oscilloscope input overvoltage protection circuit is constructed using components such as a dual threshold comparator circuit, a triode, a D trigger, an encoder, and a MEMS switch. When an overvoltage signal is input, the subsequent circuit is disconnected for protection, and the protection state is locked when the overvoltage signal is not removed, so as to avoid damage to the devices in the path caused by user misoperation. The present invention has a fast response speed, and there is no problem that the voltage measurement result of a single pulse overvoltage signal is too small to cause the single pulse overvoltage signal to be unable to be detected, which further solves the problem that the low-impedance channel of the broadband oscilloscope is easily damaged by a large voltage input.

[0063] Although the above describes the illustrative specific embodiments of the present invention to facilitate those skilled in the art to understand the present invention, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations using the concept of the present invention are protected.

Claims

1. A broadband oscilloscope input overvoltage protection circuit, comprising: A voltage division and attenuation circuit: used to perform amplitude voltage division and attenuation on the input signal Vin to obtain the input signal V1 after voltage division and attenuation, and input it to the shift circuit; It is characterized in that it further comprises: A shift circuit, used to shift the input signal V1 after voltage division and attenuation, shift the input signal V1 within two comparison voltages, namely the high comparison voltage VH and the low comparison voltage VL, to obtain the input signal V2 after voltage division and shift, and input it to the dual-threshold comparator circuit; A dual-threshold comparator circuit, used to compare the input signal V2 after voltage division and shift with the two comparison voltages, namely the high comparison voltage VH and the low comparison voltage VL. When the voltage of the input signal V2 is within the two comparison voltages VH and VL, output a low level to the D flip-flop circuit, and when it is outside the two comparison voltages VH and VL, output a high level to the D flip-flop circuit; A D flip-flop circuit, whose output is used as an overvoltage flag signal, is connected to the output of the dual-threshold comparator circuit. When a rising edge occurs at the output of the dual-threshold comparator circuit, that is, when the low level changes to a high level, the D flip-flop circuit outputs a high level and holds it; A clear determination circuit, used to receive the reset signal of the microprocessor and detect the output level of the dual-threshold comparator circuit. If it is a low level, that is, the input overvoltage has been removed, output a clear signal to the clear input terminal of the D flip-flop circuit to make the output of the D flip-flop circuit a low level; The output of the D flip-flop circuit is connected to the enable terminal of the switch control circuit. When the overvoltage flag signal is at a low level, the switch control circuit outputs a corresponding switch switching control signal according to the switch switching instruction output by the microprocessor. When the overvoltage flag signal is at a high level, the switch switching control signals output by the switch control circuit are all at a low level. The switch circuit connects or disconnects the input signal and the subsequent circuit according to the high or low level of the switch switching control signal.

2. The overvoltage protection circuit for the input of a broadband oscilloscope according to claim 1, characterized in that, The shift circuit consists of an operational amplifier and resistors R3, R4, and R5. The bias voltage V + is connected to the positive terminal of the operational amplifier A1 through the resistor R3. The input signal V1 after voltage division and attenuation is input to the negative terminal of the operational amplifier A1 through the resistor R4. The resistor R is connected between the positive and negative terminals of the operational amplifier A1. The operational amplifier A1 outputs the voltage-divided and shifted input signal V2; With the help of the bias voltage V + Shift the input signal V1 after voltage division attenuation, and its output relationship is: V2 = (1 + R5 / R4) × V+ - R5 × V1 / R4.

3. The overvoltage protection circuit for the input of a broadband oscilloscope according to claim 2, characterized in that, When the resistors R4 = R5 are set, the output relationship is: V2 = 2 × V+ - V1 where the resistors R4 and R5 are set to 100 KΩ.

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