Safety detector and safety detection method

Abstract

The invention provides a safety detector and a safety detection method, the safety detector comprises a magnetic detection subsystem, and the magnetic detection subsystem comprises a magnetic detection front-end circuit, an instrument amplification circuit, a filter amplification circuit and a threshold comparison circuit; the magnetic detection front-end circuit is used for sensing a first voltage corresponding to the to-be-detected object when the to-be-detected object passes through the safety detector and outputting the first voltage; the instrument amplification circuit is used for performing differential amplification on a second voltage corresponding to the first voltage to obtain a third voltage and outputting the third voltage; the filtering and amplifying circuit is used for filtering the third voltage to obtain a fourth voltage and outputting the fourth voltage; the threshold comparison circuit is used for detecting whether the fourth voltage is in a threshold interval or not; if not, a prompt signal is output, and the prompt signal indicates that the to-be-detected object carries the detection target. According to the technical scheme, the safety detector is high in signal-to-noise ratio and long in detection distance.

Description

Technical Field

[0001] This application relates to the field of security inspection technology, and in particular to a security testing instrument and a security testing method. Background Technology

[0002] A security door is a detection device used to check whether people are carrying metal objects. Security doors are also known as metal detection doors. They are used in high-traffic public places such as airports, train stations, and large conference centers to detect whether people are carrying metal items.

[0003] When a person passes through the security gate, if the person is carrying a metal object, the security gate will immediately sound an alarm and display the location of the metal object, so that the manager can promptly discover the metal object carried by the person. Summary of the Invention

[0004] This application provides a security detector, which includes a magnetic detection subsystem, and the magnetic detection subsystem includes a magnetic detection front-end circuit, an instrument amplification circuit, a filter amplification circuit, and a threshold comparison circuit.

[0005] The magnetic detection front-end circuit is used to sense the first voltage corresponding to the object to be detected when the object to be detected passes through the safety detector, and output the first voltage.

[0006] The instrument amplification circuit is used to differentially amplify the second voltage corresponding to the first voltage to obtain a third voltage, and output the third voltage; wherein, the differential amplification is used to amplify the differential signal in the second voltage, but not to amplify the common-mode signal in the second voltage;

[0007] The filtering and amplification circuit is used to filter the third voltage to obtain a fourth voltage and output the fourth voltage; wherein, the filtering operation is used to filter out voltage signals with frequencies lower than the first frequency and voltage signals with frequencies higher than the second frequency in the third voltage;

[0008] The threshold comparison circuit is used to detect whether the fourth voltage is within the threshold range; if not, it outputs a prompt signal, which indicates that the object to be detected carries the detection target.

[0009] For example, the magnetic detection front-end circuit employs a tunneling magnetoresistive magnetic sensor; or,

[0010] The magnetic detection front-end circuit uses a fluxgate sensor; or...

[0011] The magnetic detection front-end circuit uses a magnetoresistive sensor; or...

[0012] The magnetic field front-end circuit uses a Hall sensor; or...

[0013] The magnetic field front-end circuit uses an inductive sensor.

[0014] For example, the first voltage can be determined as the second voltage; or,

[0015] The magnetic field detection subsystem further includes: a differential filter circuit; the differential filter circuit is used to perform differential filtering on the first voltage to obtain the second voltage, and output the second voltage; wherein, the differential filtering operation is used to filter out voltage signals in the first voltage that are greater than a third frequency;

[0016] The third frequency is greater than the second frequency, and the second frequency is greater than the first frequency.

[0017] For example, the instrument amplifier circuit includes a first input terminal, a second input terminal, and a voltage output terminal; wherein the first input terminal and the second input terminal are the input terminals of the second voltage;

[0018] The instrument amplification circuit is used to amplify the voltage difference between the first input terminal and the second input terminal to obtain the third voltage, and output the third voltage through the voltage output terminal.

[0019] For example, the filtering and amplification circuit includes a low-pass filter circuit and a high-pass filter circuit;

[0020] The low-pass filter circuit is used to perform a low-pass filter operation on the third voltage to obtain a low-pass filtered voltage, and output the low-pass filtered voltage. The low-pass filter operation is used to filter out voltage signals with frequencies greater than the second frequency in the third voltage. The high-pass filter circuit is used to perform a high-pass filter operation on the low-pass filtered voltage to obtain a fourth voltage, and output the fourth voltage. The high-pass filter operation is used to filter out voltage signals with frequencies less than the first frequency in the low-pass filtered voltage.

[0021] Alternatively, the high-pass filter circuit is used to perform a high-pass filter operation on the third voltage to obtain a high-pass filtered voltage, and output the high-pass filtered voltage. The high-pass filter operation is used to filter out voltage signals with frequencies lower than the first frequency in the third voltage. The low-pass filter circuit is used to perform a low-pass filter operation on the high-pass filtered voltage to obtain a fourth voltage, and output the fourth voltage. The low-pass filter operation is used to filter out voltage signals with frequencies higher than the second frequency in the high-pass filtered voltage.

[0022] For example, the magnetic detection subsystem also includes a sliding rheostat, and the voltage values ​​at the first and second ends of the sliding rheostat are adjusted by controlling the resistance value of the sliding rheostat.

[0023] Wherein, the voltage value at the first end of the sliding rheostat is used as the minimum boundary voltage of the threshold interval, and the voltage value at the second end of the sliding rheostat is used as the maximum boundary voltage of the threshold interval;

[0024] If the fourth voltage is greater than the maximum boundary voltage or less than the minimum boundary voltage, the threshold comparison circuit determines that the fourth voltage is not within the threshold range.

[0025] For example, the magnetic detection subsystem also includes an alarm circuit;

[0026] The alarm circuit is used to output an alarm signal to the user after receiving the prompt signal; wherein the alarm signal includes at least one of the following: an alarm signal based on a high-brightness LED; an alarm signal based on a vibration motor; an alarm signal based on a buzzer; or an alarm signal based on voice.

[0027] For example, the safety detector further includes a power supply subsystem for supplying power to the magnetic detection subsystem; wherein the power supply subsystem includes a battery and a battery charging and discharging circuit, and the power supply subsystem has at least one of the following functions: power level indication, charging indication, power bank function, and flashlight function.

[0028] This application provides a security detection method applied to a security detector, the security detector including a magnetic field detection subsystem; wherein, the magnetic field detection subsystem includes a magnetic field detection front-end circuit, an instrument amplification circuit, a filter amplification circuit, and a threshold comparison circuit; the method includes:

[0029] When the object to be tested passes through the security detector, the magnetic detection front-end circuit senses the first voltage corresponding to the object to be tested and outputs the first voltage;

[0030] The instrument amplifier circuit differentially amplifies the second voltage corresponding to the first voltage to obtain a third voltage, and outputs the third voltage; wherein, the differential amplification is used to amplify the differential signal in the second voltage, but not to amplify the common-mode signal in the second voltage;

[0031] The filtering and amplification circuit performs a filtering operation on the third voltage to obtain a fourth voltage and outputs the fourth voltage; wherein, the filtering operation is used to filter out voltage signals with frequencies lower than the first frequency and voltage signals with frequencies higher than the second frequency in the third voltage;

[0032] The threshold comparison circuit detects whether the fourth voltage is within the threshold range; if not, it outputs a prompt signal, which indicates that the object to be detected carries the detection target.

[0033] For example, the magnetic field detection subsystem further includes: a differential filter circuit; before the instrumentation amplifier circuit differentially amplifies the second voltage corresponding to the first voltage to obtain the third voltage, the method further includes: after receiving the first voltage, the differential filter circuit performs a differential filter operation on the first voltage to obtain the second voltage, and outputs the second voltage; wherein, the differential filter operation is used to filter out voltage signals with frequencies greater than the third frequency in the first voltage;

[0034] The third frequency is greater than the second frequency, and the second frequency is greater than the first frequency.

[0035] As can be seen from the above technical solutions, in this embodiment, the safety detector includes a magnetic field detection subsystem, and the magnetic field detection subsystem includes a magnetic field detection front-end circuit. The magnetic field detection front-end circuit is a lightweight device, rather than a heavy device like a coil, thus making the safety detector lightweight and enabling a portable structure, reducing the difficulty of handling and transferring, and facilitating carrying the safety detector. For example, the safety detector can be placed in a fixed location or used handheld, employing a passive magnetic field detection method, eliminating the need for a transmitting coil, preventing interference from the Earth's magnetic field, and resulting in more accurate detection results. The safety detector uses an instrumentation amplifier circuit to suppress common-mode noise (i.e., it does not amplify the common-mode signal in the second voltage), and a filtering amplifier circuit to suppress out-of-band noise (i.e., it filters out voltage signals with frequencies lower than the first frequency and higher than the second frequency in the third voltage), thereby improving the signal-to-noise ratio and extending the detection range of the safety detector. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the structure of a safety detector according to one embodiment of this application;

[0037] Figure 2 This is a schematic diagram of the instrumentation amplifier circuit in one embodiment of this application;

[0038] Figure 3 This is a schematic diagram of the structure of a filter amplifier circuit in one embodiment of this application;

[0039] Figure 4 This is a schematic diagram of the threshold comparison circuit in one embodiment of this application;

[0040] Figure 5 This is a schematic diagram of the structure of a safety detector according to one embodiment of this application;

[0041] Figure 6 This is a flowchart illustrating a security detection method in one embodiment of this application;

[0042] Figure 7 This is a flowchart illustrating a security detection method in one embodiment of this application. Detailed Implementation

[0043] A security gate is a detection device used to check whether people are carrying metal objects. When a person passes through the security gate, if they are carrying metal objects, the gate will immediately sound an alarm and display the location of the metal objects, allowing administrators to promptly detect them. For example, the gate's transmitting coil has a 10kHz alternating current, creating a 10kHz alternating magnetic field. This magnetic field is then received by the receiving coil, which is used to detect whether a person is carrying metal objects.

[0044] Security gates can be used in scenarios such as exam cheating prevention. When a person passes through the gate, if they are not carrying a mobile phone, the fluctuation of the AC magnetic field is small; if they are carrying a mobile phone, the fluctuation is large. The characteristics of the received signal can then be used to determine whether a person is carrying a mobile phone or other similar device. However, this detection method cannot detect targeted devices (such as Bluetooth headsets, cheating devices, and other small cheating devices). For example, if a person carries a targeted device through the security gate, the fluctuation of the AC magnetic field will be small, leading to a false detection result that the person is not carrying the targeted device.

[0045] In addition, security gates use an alternating magnetic field constructed by transmitting and receiving coils to detect whether people are carrying metal objects. Due to the large area, volume, and weight of the coils, the security gates are quite heavy, making it impossible to adopt a portable structure. This also makes the security gates difficult to move and transfer.

[0046] In response to the above findings, this application proposes a security detector (security inspection device). The security detector includes a magnetic detection subsystem, and the magnetic detection subsystem includes a magnetic detection front-end circuit. The magnetic detection front-end circuit is a lightweight device, rather than a heavy device such as a coil, which makes the security detector lighter and enables it to adopt a portable structure, reducing the difficulty of handling and transferring, and making it convenient to carry the security detector.

[0047] Furthermore, if a person carries a target type of equipment through a safety detector, the safety detector can amplify the weak voltage signal generated by the target type of equipment using an instrument amplification circuit, and suppress common-mode noise using an instrument amplification circuit and suppress out-of-band noise using a filter amplification circuit, thereby being able to detect the target type of equipment and obtain the detection result of the person carrying the target type of equipment, that is, the detection result is correct.

[0048] This application provides a security detector, see [link to relevant documentation]. Figure 1 The diagram shown is a structural schematic of a safety detector. The safety detector may include a magnetic detection subsystem and a power supply subsystem, with the power supply subsystem providing power to the magnetic detection subsystem. Alternatively, the safety detector may only include the magnetic detection subsystem, omitting the power supply subsystem; in this case, AC power can be directly used to power the magnetic detection subsystem.

[0049] See Figure 1 As shown, the magnetic field detection subsystem includes a magnetic field detection front-end circuit, a differential filter circuit, an instrument amplifier circuit, a filter amplifier circuit, a threshold comparison circuit, and an alarm circuit. The following describes the magnetic field detection front-end circuit, the differential filter circuit, the instrument amplifier circuit, the filter amplifier circuit, the threshold comparison circuit, and the alarm circuit.

[0050] First, the magnetic field detection front-end circuit. The magnetic field detection front-end circuit is used to sense the first voltage corresponding to the object to be detected when the object to be detected passes through the safety detector, and output the first voltage to the differential filter circuit.

[0051] For example, when security detectors are used in scenarios such as exam room anti-cheating, the objects to be detected are people passing through the security detector (such as examinees). When the object to be detected passes through the security detector, the magnetic detection front-end circuit can sense a voltage signal, which is called the first voltage corresponding to the object to be detected.

[0052] For example, the magnetic field detection front-end circuit can use a tunneling magnetoresistive (TMR) magnetic sensor; or, it can use a fluxgate sensor; or, it can use a magnetoresistive sensor; or, it can use a Hall sensor; or, it can use an inductive sensor. Of course, these are just examples of magnetic field detection front-end circuits, and there are no limitations on the specific circuit, as long as it can detect passive magnetic fields.

[0053] For ease of description, we will take the magnetic field front-end circuit using an inductive sensor as an example. When the magnetic field front-end circuit uses other sensors such as tunneling magnetoresistive effect magnetic sensors, fluxgate sensors, magnetoresistive sensors, and Hall sensors, their implementation principle is similar to that of inductive sensors, and will not be repeated hereafter.

[0054] For example, an inductive sensor is made by winding enameled wire on a magnetic core. When the number of magnetic field lines passing through the magnetic core changes, a voltage signal is generated at both ends of the inductive sensor. This voltage signal is the first voltage, that is, the inductive sensor can sense the first voltage and output the first voltage to the differential filter circuit.

[0055] For example, in scenarios such as preventing cheating in examination rooms, the following situations can cause a change in the number of magnetic field lines passing through the magnetic core, thereby generating a voltage signal (first voltage) across the inductive sensor:

[0056] In the working environment of a safety detector, electrical equipment (such as household electrical equipment using 220V, 50Hz electricity) radiates an alternating magnetic field (such as a 50Hz alternating magnetic field) into space, causing the inductive sensor to detect a sine wave and its harmonics (such as a 50Hz sine wave). Clearly, the initial voltage will include the voltage signal generated by this sine wave, and this voltage signal is interference and needs to be filtered out later.

[0057] The Earth has a strong geomagnetic field. Even a small displacement of the inductive sensor relative to the ground will generate a voltage signal across its terminals. Clearly, this initial voltage signal includes the voltage signal generated by the geomagnetic field, which is interference and needs to be filtered out later.

[0058] Target devices (such as small cheating devices like Bluetooth headsets or cheating rings, or large cheating devices like mobile phones) contain permanent magnets. When the target device and the inductive sensor move relative to each other, the number of magnetic field lines passing through the magnetic core changes, generating a voltage signal (e.g., a weak voltage signal) across the inductive sensor. Clearly, when the object to be detected carries the target device through the security detector, the first voltage will include the voltage signal generated by the target device (for example, a Bluetooth headset contains a sound unit, which contains a permanent magnet; when the permanent magnet moves, it generates a voltage signal on the inductive sensor). That is, the voltage signal generated by the target device can be sensed, and since this voltage signal is a valid signal, it is necessary to detect whether the voltage signal generated by the target device is present.

[0059] In summary, when the object to be tested carries the target type device through the security detector, the inductive sensor (i.e., the magnetic field front-end circuit) can sense the first voltage corresponding to the object, and the first voltage includes the voltage signal generated by the target type device and interference signals (such as the voltage signal generated by electrical equipment and / or the voltage signal generated by the geomagnetic field). Alternatively, when the object to be tested does not carry the target type device through the security detector, the inductive sensor can also sense the first voltage corresponding to the object, and the first voltage includes interference signals (such as the voltage signal generated by electrical equipment and / or the voltage signal generated by the geomagnetic field).

[0060] Second, the differential filter circuit. The differential filter circuit receives the first voltage, performs a differential filter operation on the first voltage to obtain the second voltage, and outputs the second voltage to the instrumentation amplifier circuit. Specifically, the differential filter operation is used to filter out voltage signals in the first voltage that have a frequency greater than the third frequency.

[0061] For example, since the first voltage includes interference voltage signals generated by electrical equipment, and the interference voltage signals are high-frequency signals (such as high-frequency signals greater than 50Hz), while the voltage signals generated by the target type of equipment are low-frequency signals, the high-frequency signals in the first voltage can be initially filtered out.

[0062] Based on this, a differential filter circuit can be used to perform differential filtering on the first voltage. The differential filtering operation is used to filter out voltage signals in the first voltage that are greater than the third frequency (the third frequency can be arbitrarily configured, such as 50Hz). The voltage after differential filtering is called the second voltage.

[0063] Differential filtering circuits can be classified as differential filters. Differential filters are used to reduce noise in signals or remove redundant information from signals. Differential filters use differential operations on discrete signals to eliminate high-frequency noise or redundant information. The reason for using differential filters is that noise or redundant information usually manifests as rapid changes in the signal, and differential operations can capture the rate of change of the signal. Therefore, differential filtering operations do not impose any restrictions on this.

[0064] Third, the instrumentation amplifier circuit. This circuit receives the second voltage, differentially amplifies it to obtain a third voltage, and outputs the third voltage to the filter amplifier circuit. The differential amplification amplifies the differential signal in the second voltage but not the common-mode signal. In this embodiment, the differential signal can be understood as the effective signal during target detection, and the common-mode signal can be understood as the invalid signal or noise signal during target detection.

[0065] For example, see Figure 2 The diagram shows the structure of an instrumentation amplifier circuit. The amplifier circuit may include a first input terminal, a second input terminal, and a voltage output terminal. The first and second input terminals are input terminals for a second voltage; that is, the first input terminal is connected to a differential filter circuit, and the second input terminal is also connected to a differential filter circuit. Furthermore, the voltage output terminal can be connected to a filter amplifier circuit.

[0066] Based on this, the instrument amplifier circuit can amplify the voltage difference between the first input terminal and the second input terminal to obtain a third voltage, and output the third voltage to the filter amplifier circuit through the voltage output terminal.

[0067] For example, when the instrumentation amplifier circuit amplifies the voltage difference (i.e., potential difference) between the first input terminal and the second input terminal, the instrumentation amplifier circuit only amplifies the differential signal (i.e., the effective voltage difference signal between the first input terminal and the second input terminal) and does not amplify the common-mode signal (i.e., the voltage signal that exists at both the first input terminal and the second input terminal, i.e., the noise signal), thereby increasing the common-mode rejection ratio and suppressing common-mode interference.

[0068] For example, an instrumentation amplifier circuit can amplify differential signals but not common-mode signals. The reason is as follows: Assuming the input voltage at the first input terminal of the instrumentation amplifier circuit is V1+V2, and the input voltage at the second input terminal is V3+V4, where V1 and V3 represent the voltages of the differential signals, and V2 and V4 represent the voltages of the common-mode signals, that is, V1, V2, V3, and V4 are the second voltages input to the differential filter circuit, then after the instrumentation amplifier circuit differentially amplifies the second voltage, the differentially amplified third voltage can be: A*[(V3+V4)-(V1+V2)], where A represents the amplification factor. This amplification factor is an inherent parameter of the instrumentation amplifier circuit and is a fixed value, such as 10000, 8000, etc., and is not limited thereto.

[0069] Regarding the differential signal voltages (V1 and V3), if the object under test carries the target type equipment through the security detector, the differential signal voltage is the voltage generated by the target type equipment, and V1 and V3 are different, with a voltage difference between them. If the object under test does not carry the target type equipment through the security detector, then both V1 and V3 are not 0. Regarding the common-mode signal voltages (V2 and V4), the common-mode signal voltage is the voltage generated by interference signals (such as voltage signals generated by electrical equipment and / or voltage signals generated by the Earth's magnetic field), and V2 and V4 are the same (or approximately the same), with no voltage difference (or a very small voltage difference) between them.

[0070] In summary, if the object under test carries the target type device through the security detector, the third voltage after differential amplification of the second voltage by the instrument's amplifier circuit can be A*(V3-V1), meaning the value of (V4-V2) is approximately 0. Thus, only the voltage difference of the differential signal (V3-V1) is amplified, while the voltage difference of the common-mode signal (V4-V2) is not amplified. Similarly, if the object under test does not carry the target type device through the security detector, the third voltage after differential amplification of the second voltage by the instrument's amplifier circuit can be approximately 0, meaning that the third voltage is 0 because there is no differential signal voltage.

[0071] In summary, by deploying an instrumentation amplifier circuit, differential signals can be amplified without amplifying common-mode signals, thereby obtaining observable voltage signals for the target type of equipment.

[0072] Because the input impedance of the instrument amplifier circuit is very high, that is, the first input terminal and the second input terminal of the instrument amplifier circuit are disconnected, it will not interfere with the front-end magnetic detection circuit.

[0073] For example, an instrumentation amplifier circuit can be composed of multiple operational amplifiers, multiple instrumentation amplifiers, or multiple differential amplifiers. There are no restrictions on the structure of the instrumentation amplifier circuit, as long as the instrumentation amplifier circuit can increase the common-mode rejection ratio (i.e., the instrumentation amplifier circuit can amplify the differential signal, but the instrumentation amplifier circuit does not amplify the common-mode signal).

[0074] Fourth, the filtering and amplification circuit. This circuit receives the third voltage, filters it to obtain the fourth voltage, and outputs the fourth voltage to the threshold comparison circuit. The filtering operation removes voltage signals in the third voltage that are lower than the first frequency, and also removes voltage signals in the third voltage that are higher than the second frequency. The first, second, and third frequencies can all be configured empirically; there are no restrictions on their values. The third frequency can be higher than the second frequency, and the second frequency can be higher than the first frequency.

[0075] For example, the frequency range of the voltage generated by the target type device is usually between the first frequency and the second frequency. That is, the frequency of the voltage generated by the target type device will not be lower than the first frequency, and the frequency of the voltage generated by the target type device will not be higher than the second frequency. The first and second frequencies are empirical values. Voltage signals lower than the first frequency and voltage signals higher than the second frequency can be considered as interference voltage signals, and interference voltage signals in the third voltage need to be filtered out.

[0076] Based on this, a filter amplifier circuit can be used to filter the third voltage. The filtering operation is used to filter out voltage signals with frequencies lower than the first frequency and to filter out voltage signals with frequencies higher than the second frequency. The voltage after the filtering operation can be called the fourth voltage.

[0077] For example, the filtering amplifier circuit may include a low-pass filter circuit and a high-pass filter circuit. The low-pass filter circuit is used to filter out voltage signals with frequencies greater than the second frequency, and the high-pass filter circuit is used to filter out voltage signals with frequencies less than the first frequency, thereby obtaining a fourth voltage after filtering.

[0078] In one possible implementation, the instrumentation amplifier circuit can input a third voltage to a low-pass filter circuit, which performs a low-pass filter operation on the third voltage to obtain a low-pass filtered voltage. The low-pass filter operation is used to filter out voltage signals in the third voltage that are higher than the second frequency.

[0079] The low-pass filter circuit can input the low-pass filtered voltage to the high-pass filter circuit, which can then perform a high-pass filtering operation on the low-pass filtered voltage to obtain a fourth voltage. The high-pass filtering operation is used to filter out voltage signals with frequencies lower than the first frequency in the low-pass filtered voltage. After obtaining the fourth voltage, the high-pass filter circuit can also input it to the threshold comparison circuit.

[0080] In one possible implementation, the instrumentation amplifier circuit can input a third voltage to a high-pass filter circuit, which performs a high-pass filter operation on the third voltage to obtain a high-pass filtered voltage. The high-pass filter operation is used to filter out voltage signals in the third voltage that are less than a first frequency.

[0081] The high-pass filter circuit can input the high-pass filtered voltage to the low-pass filter circuit, which can then perform a low-pass filtering operation on the high-pass filtered voltage to obtain a fourth voltage. This low-pass filtering operation is used to filter out voltage signals with frequencies higher than the second frequency in the high-pass filtered voltage. After obtaining the fourth voltage, the low-pass filter circuit can also input it to the threshold comparison circuit.

[0082] For example, see Figure 3 The diagram shows the structure of a filter amplifier circuit, which can include a low-pass filter circuit and a high-pass filter circuit. The input to the low-pass filter circuit is the INA_OUT (third voltage) output from the instrumentation amplifier circuit. The low-pass filter circuit performs a low-pass filtering operation on INA_OUT at a certain cutoff frequency (such as the second frequency) to obtain FLT_OUT1, which is then output to the high-pass filter circuit. The high-pass filter circuit performs a high-pass filtering operation on FLT_OUT1 at a certain cutoff frequency (such as the first frequency) to obtain FLT_OUT2, which is the fourth voltage that needs to be input to the threshold comparison circuit.

[0083] A low-pass filter circuit can be a low-pass filter, which is a filter that allows signals below the cutoff frequency (such as the second frequency) to pass through, but signals above the cutoff frequency cannot pass through.

[0084] A high-pass filter circuit can be a high-pass filter, while a low-pass filter is a filter that allows signals above the cutoff frequency (such as the first frequency) to pass through, but signals below the cutoff frequency cannot pass through.

[0085] In summary, it can be seen that the filtering and amplification circuit can amplify the voltage signal in the low-frequency band (i.e., the voltage signal from the first frequency to the second frequency) and attenuate the signal outside the frequency band (i.e., attenuate the voltage signal greater than the second frequency and attenuate the voltage signal less than the first frequency), thereby improving the signal-to-noise ratio.

[0086] Fifth, the threshold comparison circuit. This circuit receives the fourth voltage and detects whether it falls within the threshold range. If not, it outputs a warning signal indicating that the object being detected carries the target. For example, the threshold comparison circuit might output a warning signal to an alarm circuit.

[0087] For example, the threshold range includes the minimum boundary voltage and the maximum boundary voltage. If the fourth voltage is not less than the minimum boundary voltage and not greater than the maximum boundary voltage, the threshold comparison circuit determines that the fourth voltage is within the threshold range, that is, the object to be detected does not carry the detection target (i.e., the target type device).

[0088] If the fourth voltage is greater than the maximum boundary voltage, or if the fourth voltage is less than the minimum boundary voltage, the threshold comparison circuit determines that the fourth voltage is not within the threshold range, meaning that the object to be detected carries the target.

[0089] For example, after processing by differential filtering circuits, instrumentation amplification circuits, and filter amplification circuits, the fourth voltage only includes the voltage generated by the target type device (other voltages are approximately 0). Therefore, if the fourth voltage is greater than the maximum boundary voltage and is positive (i.e., its absolute value is large), it indicates that the object under test carries the target type device, and the voltage generated by the target type device makes the fourth voltage greater than the maximum boundary voltage. Alternatively, if the fourth voltage is less than the minimum boundary voltage and is negative (i.e., its absolute value is large), it indicates that the object under test carries the target type device, and the voltage generated by the target type device makes the fourth voltage less than the minimum boundary voltage.

[0090] For example, see Figure 4 The diagram shows the structure of the threshold comparison circuit. FLT_OUT2 represents the fourth voltage, BB_THD_P represents the maximum boundary voltage (upper threshold), and BB_THD_N represents the minimum boundary voltage (lower threshold). Based on this, if FLT_OUT2 is greater than BB_THD_P, the threshold comparison circuit can output a warning signal to the alarm circuit. If FLT_OUT2 is less than BB_THD_N, the threshold comparison circuit can output a warning signal to the alarm circuit.

[0091] For example, when the threshold comparison circuit outputs a prompt signal to the alarm circuit, it can use a high or low level to output the prompt signal (i.e., a digital prompt signal, such as a high level indicating a prompt signal or a low level indicating a prompt signal). For instance, when the fourth voltage is within the threshold range, the threshold comparison circuit outputs a low level to the alarm circuit. When the alarm circuit receives the low level, it knows that the object to be detected does not carry the detection target. When the fourth voltage is not within the threshold range, the threshold comparison circuit switches the low level to a high level, that is, the threshold comparison circuit outputs a high level to the alarm circuit. When the alarm circuit receives the high level, it determines that it has received a prompt signal and, based on the prompt signal, knows that the object to be detected carries the detection target.

[0092] Alternatively, when the fourth voltage is within the threshold range, the threshold comparison circuit outputs a high level to the alarm circuit. When the alarm circuit receives the high level, it knows that the object to be detected does not carry the target. When the fourth voltage is not within the threshold range, the threshold comparison circuit outputs a low level to the alarm circuit. When the alarm circuit receives the low level, it determines that it has received a prompt signal and, based on the prompt signal, knows that the object to be detected carries the target.

[0093] For example, the minimum boundary voltage can be configured empirically, or it can be input by the user to the threshold comparison circuit; the maximum boundary voltage can be configured empirically, or it can be input by the user to the threshold comparison circuit.

[0094] In one possible implementation, the magnetic field detection subsystem may further include a sliding rheostat. By controlling the resistance value of the sliding rheostat, the voltage values ​​at its first and second terminals can be adjusted. The voltage value at the first terminal of the sliding rheostat can serve as the minimum boundary voltage of the threshold range, and the voltage value at the second terminal can serve as the maximum boundary voltage of the threshold range.

[0095] For example, the working principle of a sliding rheostat is to change the resistance by altering the length of the resistance wire connected to the circuit, thereby gradually changing the magnitude and direction of the current in the circuit. By changing the magnitude and direction of the current in the circuit, the voltage across the sliding rheostat is changed. Based on the working principle of the sliding rheostat, it can be known that by controlling the resistance value of the sliding rheostat (i.e., changing the length of the resistance wire connected to the circuit), the voltage values ​​at the first and second terminals of the sliding rheostat can be adjusted.

[0096] For example, a sliding rheostat has a sensitivity adjustment knob. Users can adjust the resistance value of the sliding rheostat by adjusting the sensitivity adjustment knob, and then adjust the voltage value at the first end and the second end of the sliding rheostat so that the voltage value at the first end and the voltage value at the second end are the threshold voltages desired by the user.

[0097] Based on this, the voltage value at the first end of the sliding rheostat can be used as the minimum boundary voltage of the threshold interval, and the voltage value at the second end of the sliding rheostat can be used as the maximum boundary voltage of the threshold interval.

[0098] For example, the threshold comparison circuit can be implemented by a voltage comparator, or it can be implemented by a microcontroller. There are no restrictions on this, as long as the voltage comparison function can be achieved.

[0099] Sixth, alarm circuit. The alarm circuit is used to output an alarm signal to the user after receiving a prompt signal. This alarm signal can be an audible alarm signal or a visual alarm signal. For example, the alarm signal can include, but is not limited to, at least one of the following: an alarm signal based on a high-brightness LED; an alarm signal based on a vibration motor; an alarm signal based on a buzzer; or an alarm signal based on voice.

[0100] For example, the alarm circuit may include a high-brightness LED, which can output an alarm signal based on the high-brightness LED, i.e., the alarm signal is represented by the high-brightness LED. Alternatively, the alarm circuit may include a vibration motor, which can output an alarm signal based on the vibration motor, i.e., the alarm signal is represented by vibration. Alternatively, the alarm circuit may include a buzzer, which can output an alarm signal based on the buzzer, i.e., the alarm signal is represented by sound. Alternatively, the alarm circuit may include a voice component, which can output an alarm signal based on voice, i.e., the alarm signal is represented by voice. Of course, the above are just a few examples of alarm patterns and are not limited thereto, as long as the alarm circuit can output an alarm signal to the user, the alarm signal is used to prompt security personnel to check whether the object to be inspected is carrying a detection target.

[0101] In one possible implementation, see Figure 1 As shown, the safety detector may also include a power supply subsystem, which may include, but is not limited to, a battery (such as a rechargeable lithium battery) and a battery charging and discharging circuit (i.e., a circuit with battery charging function and a battery discharging function). The charging function of the battery charging and discharging circuit can charge the battery, thereby enabling the battery to power the magnetic field detection subsystem. The discharging function of the battery charging and discharging circuit enables the battery to power the magnetic field detection subsystem.

[0102] Based on the discharge function of the battery charging and discharging circuit, the battery can also power other devices (such as mobile phones), meaning the power subsystem functions as a power bank. Furthermore, the power subsystem can also incorporate a flashlight, giving it flashlight functionality. Additionally, the power subsystem can have both a power level indicator and a charging indicator. The power level indicator shows the remaining battery power, while the charging indicator shows the current charging status (e.g., charging in progress).

[0103] Based on the buttons in the power subsystem, related functions of the power subsystem can be implemented. For example, a short press of the button turns the power subsystem on, and a double short press turns it off. A long press turns the flashlight function on, and a double long press turns it off. Of course, the above are just examples.

[0104] As can be seen from the above technical solutions, in this embodiment, the safety detector includes a magnetic field detection subsystem, which includes a magnetic field detection front-end circuit. The magnetic field detection front-end circuit is a lightweight device, not a heavy device like a coil, thus making the safety detector lightweight and enabling a portable structure. It does not rely on a 220V power supply, reducing the difficulty of handling and transferring, and facilitating portability. The safety detector can be placed in a fixed location or used handheld. It employs a passive magnetic field detection method, eliminating the need for a transmitting coil, preventing interference from the Earth's magnetic field, and resulting in higher detection accuracy. The safety detector uses an instrumentation amplifier circuit to suppress common-mode noise (i.e., it does not amplify the common-mode signal in the second voltage) and a filter amplifier circuit to suppress out-of-band noise (i.e., it filters out voltage signals with frequencies lower than the first frequency and higher than the second frequency in the third voltage), thereby improving the signal-to-noise ratio. The safety detector has a long detection distance, such as up to 70cm.

[0105] This application provides a security detector, see [link to relevant documentation]. Figure 5 The diagram shown is a structural schematic of a safety detector. The safety detector may include a magnetic field detection subsystem and a power supply subsystem, with the power supply subsystem providing power to the magnetic field detection subsystem. The magnetic field detection subsystem includes a magnetic field detection front-end circuit, an instrumentation amplifier circuit, a filter amplifier circuit, a threshold comparison circuit, and an alarm circuit. The functions of these circuits are described below.

[0106] Magnetic detection front-end circuit. The magnetic detection front-end circuit is used to sense the first voltage corresponding to the object to be detected when it passes through the safety detector, and output the first voltage to the instrument amplification circuit.

[0107] Instrumentation amplifier circuit. This circuit receives a first voltage, differentially amplifies it to obtain a third voltage, and outputs the third voltage to the filtering amplifier circuit. The differential amplification amplifies the differential signal in the first voltage but not the common-mode signal.

[0108] and Figure 1 Compared to the safety testing equipment shown, in Figure 5 In this system, the safety detector does not include a differential filter circuit. The magnetic detection front-end circuit directly inputs the first voltage to the instrument amplification circuit. This can be understood as using the first voltage as the second voltage, and the instrument amplification circuit differentially amplifies the second voltage to obtain the third voltage.

[0109] A filtering and amplification circuit is used to receive a third voltage, filter it to obtain a fourth voltage, and output the fourth voltage to a threshold comparison circuit. The filtering operation removes voltage signals in the third voltage that are lower than a first frequency, and also removes voltage signals in the third voltage that are higher than a second frequency. Both the first and second frequencies can be configured empirically without limitation, and the second frequency can be higher than the first frequency.

[0110] Threshold comparison circuit. This circuit receives a fourth voltage and detects whether it falls within a threshold range. If not, it outputs a warning signal indicating that the object being detected carries the target. For example, the threshold comparison circuit can output a warning signal to an alarm circuit.

[0111] Alarm circuit. The alarm circuit is used to output an alarm signal to the user after receiving a prompt signal. This alarm signal can be an audible alarm signal or a visual alarm signal. For example, the alarm signal can include, but is not limited to, at least one of the following: an alarm signal based on a high-brightness LED; an alarm signal based on a vibration motor; an alarm signal based on a buzzer; or an alarm signal based on voice.

[0112] This application proposes a security detection method that can be applied to a security detector. The security detector may include a magnetic field detection subsystem, which may include a magnetic field detection front-end circuit, a differential filter circuit, an instrumentation amplifier circuit, a filter amplifier circuit, and a threshold comparison circuit. See also Figure 6 The diagram shown is a flowchart of the security detection method, which may include:

[0113] Step 601: When the object to be tested (such as a test taker) passes through the security detector, the magnetic detection front-end circuit senses the first voltage corresponding to the object to be tested and outputs the first voltage.

[0114] Step 602: After receiving the first voltage, the differential filter circuit performs a differential filter operation on the first voltage to obtain a second voltage, and then outputs the second voltage. The differential filter operation can be used to filter out voltage signals with frequencies higher than the third frequency in the first voltage.

[0115] Step 603: After receiving the second voltage, the instrumentation amplifier circuit differentially amplifies the second voltage to obtain a third voltage and outputs the third voltage. The differential amplification amplifies the differential signal in the second voltage but does not amplify the common-mode signal in the second voltage.

[0116] Step 604: After receiving the third voltage, the filtering and amplification circuit performs a filtering operation on the third voltage to obtain a fourth voltage and outputs the fourth voltage. The filtering operation is used to remove voltage signals with frequencies lower than the first frequency and voltage signals with frequencies higher than the second frequency from the third voltage. The third frequency can be higher than the second frequency, and the second frequency can be higher than the first frequency.

[0117] Step 605: After receiving the fourth voltage, the threshold comparison circuit detects whether the fourth voltage is within the threshold range; if not, it outputs a prompt signal, indicating that the object to be detected carries the detection target.

[0118] For example, the magnetic detection front-end circuit uses a tunneling magnetoresistive effect magnetic sensor; or, the magnetic detection front-end circuit uses a fluxgate sensor; or, the magnetic detection front-end circuit uses a magnetoresistive sensor; or, the magnetic detection front-end circuit uses a Hall sensor; or, the magnetic detection front-end circuit uses an inductive sensor.

[0119] For example, the instrumentation amplifier circuit includes a first input terminal, a second input terminal, and a voltage output terminal, where the first and second input terminals are input terminals for a second voltage. The instrumentation amplifier circuit differentially amplifies the second voltage to obtain a third voltage, which may include, but is not limited to, amplifying the voltage difference between the first and second input terminals to obtain the third voltage, and outputting the third voltage through the voltage output terminal.

[0120] For example, the filtering amplifier circuit includes a low-pass filter circuit and a high-pass filter circuit. The filtering amplifier circuit performs a filtering operation on the third voltage to obtain a fourth voltage and outputs the fourth voltage. This may include, but is not limited to: the low-pass filter circuit performing a low-pass filtering operation on the third voltage to obtain a low-pass filtered voltage and outputting the low-pass filtered voltage, wherein the low-pass filtering operation is used to filter out voltage signals with frequencies higher than the second frequency in the third voltage. The high-pass filter circuit performing a high-pass filtering operation on the low-pass filtered voltage to obtain a fourth voltage and outputting the fourth voltage, wherein the high-pass filtering operation is used to filter out voltage signals with frequencies lower than the first frequency in the low-pass filtered voltage. Alternatively, the high-pass filter circuit performing a high-pass filtering operation on the third voltage to obtain a high-pass filtered voltage and outputting the high-pass filtered voltage, wherein the high-pass filtering operation is used to filter out voltage signals with frequencies lower than the first frequency in the third voltage. The low-pass filter circuit performing a low-pass filtering operation on the high-pass filtered voltage to obtain a fourth voltage and outputting the fourth voltage, wherein the low-pass filtering operation is used to filter out voltage signals with frequencies higher than the second frequency in the high-pass filtered voltage.

[0121] For example, the magnetic field detection subsystem may also include a sliding rheostat, and the voltage values ​​at the first and second terminals of the rheostat can be adjusted by controlling the resistance value of the rheostat. The voltage value at the first terminal of the rheostat serves as the minimum boundary voltage of the threshold interval, and the voltage value at the second terminal serves as the maximum boundary voltage of the threshold interval. If the fourth voltage is greater than the maximum boundary voltage or less than the minimum boundary voltage, the threshold comparison circuit determines that the fourth voltage is not within the threshold interval.

[0122] For example, the magnetic detection subsystem also includes an alarm circuit; after the threshold comparison circuit outputs a prompt signal, the alarm circuit receives the prompt signal and outputs an alarm signal to the user; wherein the alarm signal includes at least one of the following: an alarm signal based on a high-brightness LED; an alarm signal based on a vibration motor; an alarm signal based on a buzzer; and an alarm signal based on voice.

[0123] For example, the safety detector also includes a power supply subsystem that can power the magnetic detection subsystem; wherein, the power supply subsystem includes a battery and a battery charging and discharging circuit, and the power supply subsystem has at least one of the following functions: power level indication function, charging indication function, power bank function, and flashlight function.

[0124] As can be seen from the above technical solutions, in this embodiment, the safety detector includes a magnetic field detection subsystem, which includes a magnetic field detection front-end circuit. The magnetic field detection front-end circuit is a lightweight device, not a heavy device like a coil, thus making the safety detector lightweight and enabling a portable structure. It does not rely on a 220V power supply, reducing the difficulty of handling and transferring, and facilitating portability. The safety detector can be placed in a fixed location or used handheld. It employs a passive magnetic field detection method, eliminating the need for a transmitting coil, preventing interference from the Earth's magnetic field, and resulting in higher detection accuracy. The safety detector uses an instrumentation amplifier circuit to suppress common-mode noise (i.e., it does not amplify the common-mode signal in the second voltage) and a filter amplifier circuit to suppress out-of-band noise (i.e., it filters out voltage signals with frequencies lower than the first frequency and higher than the second frequency in the third voltage), thereby improving the signal-to-noise ratio. The safety detector has a long detection distance, such as up to 70cm.

[0125] This application proposes a security detection method applicable to a security detector. The security detector includes a magnetic detection subsystem, which comprises a magnetic detection front-end circuit, an instrument amplification circuit, a filter amplification circuit, and a threshold comparison circuit. See also... Figure 7 The diagram shown is a flowchart of the method, which includes:

[0126] Step 701: When the object to be tested (such as a test taker) passes through the security detector, the magnetic detection front-end circuit senses the first voltage corresponding to the object to be tested and outputs the first voltage.

[0127] Step 702: After receiving the first voltage, the instrumentation amplifier circuit differentially amplifies the first voltage to obtain a third voltage and outputs the third voltage. The differential amplification amplifies the differential signal in the first voltage but does not amplify the common-mode signal in the first voltage.

[0128] Step 703: After receiving the third voltage, the filtering and amplification circuit performs a filtering operation on the third voltage to obtain a fourth voltage and outputs the fourth voltage; wherein, the filtering operation is used to filter out voltage signals with frequencies lower than the first frequency and voltage signals with frequencies higher than the second frequency in the third voltage.

[0129] Step 704: After receiving the fourth voltage, the threshold comparison circuit detects whether the fourth voltage is within the threshold range; if not, it outputs a prompt signal, indicating that the object to be detected carries the detection target.

[0130] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, embodiments of this application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0131] The above description is merely an embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principle of this application should be included within the scope of the claims of this application.

Claims

1. A safety detector, characterized in that, The safety detector includes a magnetic detection subsystem, which includes a magnetic detection front-end circuit, an instrument amplification circuit, a filter amplification circuit, and a threshold comparison circuit. The magnetic detection front-end circuit is used to sense the first voltage corresponding to the object to be detected when the object to be detected passes through the safety detector, and output the first voltage. The instrument amplification circuit is used to differentially amplify the second voltage corresponding to the first voltage to obtain a third voltage, and output the third voltage; wherein, the differential amplification is used to amplify the differential signal in the second voltage, but not to amplify the common-mode signal in the second voltage; The filtering and amplification circuit is used to filter the third voltage to obtain a fourth voltage and output the fourth voltage; wherein, the filtering operation is used to filter out voltage signals with frequencies lower than the first frequency and voltage signals with frequencies higher than the second frequency in the third voltage; The threshold comparison circuit is used to detect whether the fourth voltage is within the threshold range; if not, it outputs a prompt signal, which indicates that the object to be detected carries the detection target.

2. The safety detector according to claim 1, characterized in that, The magnetic detection front-end circuit employs a tunneling magnetoresistive magnetic sensor; or... The magnetic detection front-end circuit uses a fluxgate sensor; or... The magnetic detection front-end circuit uses a magnetoresistive sensor; or... The magnetic field front-end circuit uses a Hall sensor; or... The magnetic field front-end circuit uses an inductive sensor.

3. The safety detector according to claim 1, characterized in that, The first voltage is determined as the second voltage; or... The magnetic field detection subsystem further includes: a differential filter circuit; the differential filter circuit is used to perform differential filtering on the first voltage to obtain the second voltage, and output the second voltage; wherein, the differential filtering operation is used to filter out voltage signals in the first voltage that are greater than a third frequency; The third frequency is greater than the second frequency, and the second frequency is greater than the first frequency.

4. The safety detector according to claim 1, characterized in that, The instrument amplifier circuit includes a first input terminal, a second input terminal, and a voltage output terminal; wherein the first input terminal and the second input terminal are the input terminals of the second voltage; The instrument amplification circuit is used to amplify the voltage difference between the first input terminal and the second input terminal to obtain the third voltage, and output the third voltage through the voltage output terminal.

5. The safety detector according to claim 1, characterized in that, The filtering and amplification circuit includes a low-pass filter circuit and a high-pass filter circuit; The low-pass filter circuit is used to perform a low-pass filter operation on the third voltage to obtain a low-pass filtered voltage, and output the low-pass filtered voltage. The low-pass filter operation is used to filter out voltage signals with frequencies greater than the second frequency in the third voltage. The high-pass filter circuit is used to perform a high-pass filter operation on the low-pass filtered voltage to obtain a fourth voltage, and output the fourth voltage. The high-pass filter operation is used to filter out voltage signals with frequencies less than the first frequency in the low-pass filtered voltage. Alternatively, the high-pass filter circuit is used to perform a high-pass filter operation on the third voltage to obtain a high-pass filtered voltage, and output the high-pass filtered voltage. The high-pass filter operation is used to filter out voltage signals with frequencies lower than the first frequency in the third voltage. The low-pass filter circuit is used to perform a low-pass filter operation on the high-pass filtered voltage to obtain a fourth voltage, and output the fourth voltage. The low-pass filter operation is used to filter out voltage signals with frequencies higher than the second frequency in the high-pass filtered voltage.

6. The safety detector according to claim 1, characterized in that, The magnetic detection subsystem also includes a sliding rheostat, and the voltage values ​​at the first and second ends of the sliding rheostat are adjusted by controlling the resistance value of the sliding rheostat. Wherein, the voltage value at the first end of the sliding rheostat is used as the minimum boundary voltage of the threshold interval, and the voltage value at the second end of the sliding rheostat is used as the maximum boundary voltage of the threshold interval; If the fourth voltage is greater than the maximum boundary voltage or less than the minimum boundary voltage, the threshold comparison circuit determines that the fourth voltage is not within the threshold range.

7. The safety detector according to claim 1, characterized in that, The magnetic field detection subsystem further includes an alarm circuit; the alarm circuit is used to output an alarm signal to the user after receiving the prompt signal; wherein the alarm signal includes at least one of the following: Alarm signals based on high-brightness LEDs; Alarm signals based on vibration motors; Alarm signals based on buzzers; Voice-based alarm signals.

8. The safety detector according to any one of claims 1-7, characterized in that, The safety detector also includes a power supply subsystem, which is used to supply power to the magnetic detection subsystem; The power subsystem includes a battery and a battery charging and discharging circuit, and the power subsystem has at least one of the following functions: power level indication, charging indication, power bank function, and flashlight function.

9. A security detection method, characterized in that, The method is applied to a security detector, which includes a magnetic field detection subsystem; wherein the magnetic field detection subsystem includes a magnetic field detection front-end circuit, an instrument amplification circuit, a filter amplification circuit, and a threshold comparison circuit; the method includes: When the object to be tested passes through the security detector, the magnetic detection front-end circuit senses the first voltage corresponding to the object to be tested and outputs the first voltage; The instrument amplifier circuit differentially amplifies the second voltage corresponding to the first voltage to obtain a third voltage, and outputs the third voltage; wherein, the differential amplification is used to amplify the differential signal in the second voltage, but not to amplify the common-mode signal in the second voltage; The filtering and amplification circuit performs a filtering operation on the third voltage to obtain a fourth voltage and outputs the fourth voltage; wherein, the filtering operation is used to filter out voltage signals with frequencies lower than the first frequency and voltage signals with frequencies higher than the second frequency in the third voltage; The threshold comparison circuit detects whether the fourth voltage is within the threshold range; if not, it outputs a prompt signal, which indicates that the object to be detected carries the detection target.

10. The method according to claim 9, characterized in that, The magnetic field detection subsystem further includes: a differential filter circuit; before the instrumentation amplifier circuit differentially amplifies the second voltage corresponding to the first voltage to obtain the third voltage, the method further includes: After receiving the first voltage, the differential filter circuit performs a differential filter operation on the first voltage to obtain the second voltage and outputs the second voltage; wherein, the differential filter operation is used to filter out voltage signals in the first voltage that are greater than a third frequency; The third frequency is greater than the second frequency, and the second frequency is greater than the first frequency.

Images