A low cost voice protection circuit

Abstract

The utility model relates to interface EMC protection technical field, concretely is a low cost voice protection circuit, a low cost voice protection circuit, including speech interface RJ11, voice protection circuit, speech matching circuit and speech SLIC chip that electrically connect gradually, voice protection circuit includes the primary protection circuit and secondary protection circuit that set gradually along signal transmission direction, the input end of primary protection circuit connects speech interface RJ11's tip end and ring, and the input end of secondary protection circuit is connected to the output end of primary protection circuit. The present application sets up the hierarchical protection framework containing primary protection circuit and secondary protection circuit between speech interface and SLIC chip, and combines the hierarchical configuration of matching resistance group, and forms the low cost, high reliability speech signal protection system.

Description

Technical Field

[0001] This utility model relates to the field of interface EMC protection technology, specifically a low-cost voice protection circuit. Background Technology

[0002] In voice communication systems, voice interfaces are susceptible to external noise such as electrostatic discharge, surge voltage, and high-frequency electromagnetic interference, which can damage the backend voice SLIC chip (subscriber line interface circuit chip) or distort the voice signal. Traditional voice protection circuits typically employ a single-stage protection architecture, achieving overvoltage protection and interference suppression through integrated surge protection devices or simple RC filter circuits. However, this approach suffers from the following technical bottlenecks: First, single-stage protection circuits have limited suppression capabilities for interference signals across different frequency bands, particularly struggling to simultaneously filter out high-frequency differential-mode and common-mode interference. Second, impedance matching units often employ fixed resistor networks, failing to achieve graded impedance transformation, which can lead to residual interference voltages easily exceeding the tolerance threshold of the backend chip. Third, integrated protection devices are costly, making it difficult to meet the demands of large-scale applications in low-cost communication equipment.

[0003] While existing technologies offer multi-level protection solutions, they often rely on complex active circuits or customized protection modules, leading to redundant circuit layouts, limited component selection, and difficulty in achieving a balance between cost control and protection performance. Therefore, how to construct a voice protection circuit with graded interference suppression capabilities and adaptive impedance matching functions using discrete components while ensuring low cost has become a pressing technical problem. Utility Model Content

[0004] This invention provides a low-cost voice protection circuit, aiming to overcome at least one of the defects existing in the prior art.

[0005] To achieve the above objectives, the technical solution disclosed in this invention is as follows:

[0006] According to one aspect of this disclosure, a low-cost voice protection circuit is provided, comprising a voice interface RJ11, a voice protection circuit, a voice matching circuit, and a voice SLIC chip connected in sequence.

[0007] The voice protection circuit includes a primary protection circuit and a secondary protection circuit arranged sequentially along the signal transmission direction;

[0008] The input terminal of the primary protection circuit is connected to the Tip and Ring terminals of the RJ11 voice interface, and the output terminal is connected to the input terminal of the secondary protection circuit.

[0009] The output of the secondary protection circuit is connected to the input of the voice matching circuit;

[0010] The voice matching circuit includes a matching resistor group connected in series between the secondary protection circuit and the voice SLIC chip, which is used to match the voice link impedance and limit the residual voltage of interference signals;

[0011] Both the primary protection circuit and the secondary protection circuit include a differential mode interference suppression unit and a common mode interference suppression unit. The differential mode interference suppression unit is connected between the Tip terminal and the Ring terminal, and the common mode interference suppression unit is connected between the Tip terminal or the Ring terminal and ground.

[0012] Furthermore, the differential mode interference suppression unit of the first-level protection circuit includes a bidirectional TVS transistor TVS1 and a coupling capacitor C1 connected between the Tip terminal and the Ring terminal, with the bidirectional TVS transistor TVS1 and the coupling capacitor C1 connected in parallel.

[0013] The common-mode interference suppression unit of the primary protection circuit includes a common-mode capacitor C2 connected between the Tip terminal and ground, and a common-mode capacitor C3 connected between the Ring terminal and ground.

[0014] Furthermore, the primary protection circuit also includes a bridge protection circuit, which includes diodes D1, D2, D3, and D4.

[0015] The anode of diode D1 is connected to the Tip terminal, and the cathode is connected to the VBAT terminal of the voice boost voltage. The cathode of diode D2 is connected to the Tip terminal, and the anode is grounded. The anode of diode D3 is connected to the Ring terminal, and the cathode is connected to the VBAT terminal. The cathode of diode D4 is connected to the Ring terminal, and the anode is grounded. The input terminal of the bridge protection circuit is connected to the Tip terminal and the Ring terminal of the voice interface RJ11, and the output terminal is connected to the parallel node of the bidirectional TVS diode TVS1 and the coupling capacitor C1.

[0016] Furthermore, the differential mode interference suppression unit of the secondary protection circuit includes a bidirectional TVS transistor TVS2 connected between the Tip terminal and the Ring terminal of the input terminal of the secondary protection circuit;

[0017] The common-mode interference suppression unit of the secondary protection circuit includes a common-mode capacitor C4 connected in parallel with the bidirectional TVS transistor TVS2, and a common-mode capacitor C5 connected between the Ring terminal and the Tip terminal.

[0018] Furthermore, the matching resistor set includes:

[0019] The first matching resistor R1 and the third matching resistor R3 are connected in series in the Tip signal path. One end of the first matching resistor R1 is connected to the Tip output node of the first-level protection circuit, and the other end is connected to one end of the third matching resistor R3. The other end of the third matching resistor R3 is connected to the Tip terminal of the voice SLIC chip.

[0020] The second matching resistor R2 and the fourth matching resistor R4 are connected in series in the Ring signal path. One end of the second matching resistor R2 is connected to the Ring output node of the first-level protection circuit, and the other end is connected to one end of the fourth matching resistor R4. The other end of the fourth matching resistor R4 is connected to the Ring terminal of the voice SLIC chip.

[0021] Furthermore, the first matching resistor R1 and the second matching resistor R2 have the same resistance value, the third matching resistor R3 and the fourth matching resistor R4 have the same resistance value, and the resistance value of the first matching resistor R1 is greater than the resistance value of the third matching resistor R3, so as to form a graded impedance matching structure.

[0022] Furthermore, the device parameters of the primary protection circuit and the secondary protection circuit are configured independently. The breakdown voltage of bidirectional TVS transistors TVS1 and TVS2 is greater than the operating voltage of the voice SLIC chip. The capacitance values ​​of common-mode capacitors C2 and C3 range from 10pF to 100pF, and the capacitance values ​​of common-mode capacitors C4 and C5 range from 1pF to 10pF.

[0023] The beneficial effects of this utility model are:

[0024] This invention establishes a low-cost, high-reliability voice signal protection system by setting up a hierarchical protection architecture including primary and secondary protection circuits between the voice interface and the SLIC chip, combined with the hierarchical configuration of matching resistor groups.

[0025] Specifically, the primary protection circuit, through the cooperation of a bridge protection circuit and a bidirectional TVS diode, constructs a bidirectional overvoltage discharge path, which can effectively clamp abnormal voltages at the Tip and Ring terminals to the VBAT terminal or ground potential, preventing overvoltage surges from damaging the SLIC chip. The parallel coupling capacitor and the bidirectional TVS diode constitute a differential mode interference suppression unit, which can provide a low impedance path for high-frequency differential mode noise. Combined with the ground discharge path formed by the common mode capacitor, it achieves the initial filtering of common mode interference.

[0026] Furthermore, the secondary protection circuit uses a bidirectional TVS diode connected across the signal terminals and common-mode capacitors with decreasing capacitance to suppress residual high-frequency interference a second time, forming a frequency-selective hierarchical filtering effect. The matching resistor group adopts a two-stage series structure, achieving hierarchical impedance matching through resistance differences. This limits the residual voltage of interference signals while ensuring that the voice link impedance matches the input characteristics of the SLIC chip, reducing signal reflection and attenuation. This invention avoids the high cost problem of integrated protection devices through the combination design of discrete components. At the same time, it integrates overvoltage protection, high and low frequency interference suppression, and impedance matching functions using a hierarchical protection architecture, reducing production costs while ensuring circuit reliability. It is especially suitable for cost-sensitive voice communication terminal equipment. Attached Figure Description

[0027] Figure 1 This is a system architecture diagram of the present invention;

[0028] Figure 2 This is the circuit diagram of this utility model. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this utility model clearer, the present utility model will be further described in detail below with reference to the accompanying drawings and embodiments. Examples of embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present utility model, and should not be construed as limiting the present utility model. Furthermore, it should be understood that the specific embodiments described herein are merely for explaining the present utility model and are not intended to limit the present utility model.

[0030] refer to Figure 1 and Figure 2 As shown, the present invention provides the following preferred embodiments:

[0031] Example 1

[0032] To address the balance between low cost and high efficiency in existing voice protection circuits, this embodiment further refines the overall architecture design of the voice protection circuit. Through the coordinated configuration of hierarchical protection and impedance matching, a signal transmission path that is both reliable and economical is constructed.

[0033] The core architecture of the voice protection circuit includes a voice interface RJ11, a voice protection circuit, a voice matching circuit, and a voice SLIC chip, all electrically connected in sequence. The voice interface RJ11 serves as the external signal input port, with its Tip and Ring terminals corresponding to the positive and negative polarity transmission lines of the voice signal, respectively, for connection to telephone lines or other voice transmission media. The voice protection circuit, as the core protection unit, is divided into a primary protection circuit and a secondary protection circuit along the signal transmission direction. Both levels of protection circuits integrate differential-mode interference suppression units and common-mode interference suppression units, forming a hierarchical filtering mechanism for different types of interference signals.

[0034] The input of the primary protection circuit is directly connected to the Tip and Ring terminals of the RJ11. Its design aims to provide initial overvoltage protection and high-frequency interference suppression for the input signal. A differential-mode interference suppression unit is connected between the Tip and Ring terminals, using a combination of bidirectional conducting devices and energy storage elements to bypass differential-mode noise between the two lines to ground or the power supply. A common-mode interference suppression unit is connected between the Tip and ground, and between the Ring and ground, respectively, using a capacitive path to introduce common-mode noise into the ground plane, preventing common-mode signals from being converted into differential-mode interference that could affect downstream circuitry. The input of the secondary protection circuit is connected to the output of the primary protection circuit. Its function is to perform secondary suppression of residual interference after the primary protection circuit, especially for high-frequency common-mode and differential-mode noise, achieving frequency-selective filtering through differentiated configuration of device parameters.

[0035] The voice matching circuit is located between the secondary protection circuit and the voice SLIC chip. Its matching resistor group achieves progressive impedance matching of the link through a series structure. It's important to understand that voice signals are prone to signal reflection during transmission due to impedance mismatch, leading to waveform distortion and energy loss. The introduction of the matching resistor group adapts the output impedance of the front-end protection circuit to the input impedance of the back-end SLIC chip. Simultaneously, the voltage division effect of the resistors limits the residual voltage of interference signals, preventing overvoltage damage to the chip.

[0036] Regarding component selection, the RJ11 voice interface can use a standard 6P4C connector to meet the physical connection requirements of general voice lines. For differential mode suppression in the primary and secondary protection circuits, bidirectional TVS diodes (such as the SMBJ33A model, with a typical breakdown voltage of 33V, meeting the operating voltage range of most SLIC chips) can be used. Common mode suppression capacitors can be ceramic capacitors with excellent high-frequency characteristics, with capacitance values ​​configured according to the interference frequency range. The resistance design of the matching resistor group needs to be combined with the standard characteristic impedance of the voice link (such as 600Ω). The specific parameters of each stage of the resistors should be determined through simulation or actual measurement to ensure a balance between DC bias and AC signal transmission.

[0037] Furthermore, the symmetry of the signal path must be considered during circuit layout. The trace length and width of the Tip and Ring terminals should be consistent to reduce unbalanced noise introduced by parasitic parameter differences. The ground plane should adopt a complete copper-clad design to provide a low-impedance return path for the common-mode capacitor and improve the filtering efficiency of high-frequency interference. A decoupling capacitor should be configured between the power supply VBAT and ground to suppress the impact of power supply noise on the protection circuit and ensure the stability of the clamping voltage of the bridge protection circuit.

[0038] The advantage of this embodiment lies in that by dividing the protection circuit into two functional levels, Level 1 and Level 2, it processes interference signals with different energy levels and frequency characteristics respectively, avoiding the limitations of single-level protection. The matching resistor group not only achieves impedance matching but also provides voltage limiting. By using a combination of discrete components instead of expensive integrated modules, it effectively controls costs while ensuring performance. The entire architecture is modularly designed, with clearly defined functions for each unit, facilitating component selection and circuit debugging, and is suitable for mass-produced voice communication equipment.

[0039] Example 2

[0040] To improve the suppression effect of differential-mode and common-mode interference of the primary protection circuit, this embodiment further optimizes its internal component configuration. By using a parallel structure of bidirectional TVS diodes and coupling capacitors, combined with a symmetrical layout of common-mode capacitors to ground, a front-end protection unit with both overvoltage protection and frequency filtering functions is constructed.

[0041] The differential-mode interference suppression unit of the primary protection circuit consists of a bidirectional TVS diode (TVS1) connected in parallel between the Tip and Ring terminals and a coupling capacitor (C1). The bidirectional TVS diode (TVS1) is the core component for overvoltage protection; its breakdown voltage must be higher than the maximum operating voltage of the voice SLIC chip (e.g., if the SLIC chip's operating voltage is 24V, then the breakdown voltage of TVS1 can be selected as 30V±5%). This ensures that it operates in a high-impedance state during normal operation, and when electrostatic discharge (ESD) or surge voltage occurs at the input port, it quickly conducts to clamp the overvoltage to a safe range. The coupling capacitor (C1) targets high-frequency differential-mode noise. Its capacitance value is typically between 1nF and 10nF. Utilizing the low impedance characteristic of capacitors to high-frequency signals, it provides a bypass path for differential-mode noise, reducing noise transmission to subsequent circuits. It is important to understand that the parallel structure of TVS1 and C1 integrates overvoltage protection and high-frequency filtering functions, avoiding the stacking of discrete components and simplifying circuit design.

[0042] The common-mode interference suppression unit includes a common-mode capacitor C2 connected between the Tip terminal and ground, and a common-mode capacitor C3 connected between the Ring terminal and ground. Both are typically chosen with the same capacitance value (e.g., 47pF) to ensure circuit symmetry. The function of the common-mode capacitor is to couple common-mode noise from the Tip and Ring terminals to ground. Since the common-mode signal is in phase on both lines, symmetrical capacitance to ground effectively attenuates the common-mode component while having minimal impact on differentially transmitted voice signals. It is important to note that the capacitance value of the common-mode capacitor must balance high-frequency filtering effectiveness with DC bias stability. An excessively large capacitance value may cause low-frequency components of the voice signal to leak to ground, affecting call quality. Therefore, it is usually controlled within the range of 10pF to 100pF.

[0043] In the signal transmission path, the externally input voice signal first enters the primary protection circuit via the Tip and Ring terminals of the RJ11. The DC bias signal and low-frequency voice signal continue to be transmitted through the parallel node of TVS1 and C1, while high-frequency differential-mode noise is bypassed by C1, and overvoltage is clamped by TVS1. Common-mode noise is discharged to ground through C2 and C3, completing the initial noise filtering. This design utilizes the frequency selectivity of capacitors and the transient response characteristics of TVS diodes to achieve comprehensive suppression of various interferences without introducing active components.

[0044] Furthermore, the device can be packaged as a surface-mount (SMD) device, such as a 0603 or 0805 package, to reduce the board area and meet the needs of miniaturized devices. The response time of TVS1 must be less than 1ns to ensure rapid clamping of nanosecond-level pulse interference; C1, C2, and C3 should be NPO or X7R capacitors with low dielectric loss to avoid their own losses causing additional attenuation of the voice signal. During circuit layout, TVS1 should be placed as close as possible to the RJ11 interface to shorten the discharge path of interference signals and reduce the impact of parasitic inductance; the ground pin of the common-mode capacitor should be directly connected to the main ground plane to avoid grounding impedance introduced by excessively long traces.

[0045] The advantage of this embodiment lies in its clear functional division of labor among the components in the primary protection circuit. It utilizes a combination of bidirectional TVS diodes and capacitors to achieve layered suppression of overvoltage protection and high / low frequency interference. Symmetrically arranged common-mode capacitors ensure circuit balance and effectively reduce common-mode to differential-mode conversion noise. This structure requires no complex control logic; the predetermined function can be achieved simply by configuring the parameters of passive components, reducing circuit design difficulty and production costs. It is suitable for voice communication scenarios where both cost and reliability are critical.

[0046] Example 3

[0047] To enhance the clamping capability of the primary protection circuit against abnormal voltages, this embodiment introduces a bridge protection circuit into the primary protection circuit. Through the bridge configuration of four diodes, a bidirectional overvoltage discharge path is constructed, further improving the circuit's tolerance to positive and negative overvoltages.

[0048] The bridge protection circuit consists of diodes D1, D2, D3, and D4. The cathodes of D1 and D3 are connected to the VBAT terminal of the voice boost voltage, and their anodes are connected to the Tip and Ring terminals, respectively. The anodes of D2 and D4 are grounded, and their cathodes are connected to the Tip and Ring terminals, respectively. The input of this bridge structure is directly connected to the Tip and Ring terminals of the voice interface RJ11, and the output is connected to the parallel node of the bidirectional TVS diode TVS1 and the coupling capacitor C1. It is important to understand that when a positive overvoltage (above VBAT) occurs at the Tip or Ring terminal, D1 or D3 conducts, clamping the overvoltage to the VBAT level; when a negative overvoltage (below ground potential) occurs, D2 or D4 conducts, clamping the voltage to ground level, thus forming bidirectional limiting protection to prevent excessive positive and negative voltages from impacting downstream devices.

[0049] The diode selection must meet the requirements of reverse withstand voltage and forward conduction current. For example, the 1N4148 high-speed switching diode can be selected, with a reverse breakdown voltage greater than 100V and a forward conduction voltage drop of about 0.7V, which can respond to voltage surges within microseconds. The bridge protection circuit, together with the subsequent TVS1 and C1, forms a multi-stage limiting and filtering structure: the bridge circuit first clamps the abnormal voltage to the safe range of VBAT or ground, TVS1 further absorbs the residual transient energy, and C1 filters out high-frequency components. The three work together to achieve gradient suppression of overvoltage.

[0050] During signal transmission, the normal operating voice signal voltage (typically an alternating signal in the range of -48V to +12V) is lower than the VBAT level (e.g., 30V), so the bridge diodes are in the off state and do not affect signal transmission. When encountering transient overvoltages such as electrostatic discharge (ESD) or power line coupling interference, the corresponding diodes immediately conduct, limiting the overvoltage to a safe range. It is important to note that the VBAT terminal must be connected to a stable boost power supply to ensure the stability of the clamping voltage. Additionally, an energy storage capacitor (e.g., a 100μF electrolytic capacitor) should be configured between VBAT and ground to handle the discharge requirements of high-energy transient interference.

[0051] Furthermore, the layout of the bridge protection circuit must follow the principle of "placing the input port close to the circuit." Diodes D1-D4 should be soldered adjacent to the RJ11 interface to shorten the overvoltage discharge path and reduce clamping delay caused by trace inductance. The polarity of the four diodes must be strictly connected according to the design to avoid protection failure due to reverse connection. In the PCB design, the traces at the tip and ring ends must form an independent protection loop through the bridge circuit to avoid crossing with other signal layers and reduce electromagnetic coupling interference.

[0052] The advantage of this embodiment is that the introduction of the bridge protection circuit adds a bidirectional limiting mechanism to the first-level protection circuit, compensating for the shortcomings of a single TVS diode in negative overvoltage protection, and is particularly suitable for scenarios where reverse polarity voltage input may occur. This structure utilizes the mature diode bridge rectification principle and achieves overvoltage protection through a combination of passive components, eliminating the need for additional control circuitry, thus improving reliability while maintaining a low-cost advantage. Combined with the subsequent TVS diode and coupling capacitor, a three-stage processing flow of "limiting-clamping-filtering" is formed, effectively enhancing the circuit's adaptability to complex interference environments.

[0053] Example 4

[0054] To achieve deep suppression of residual high-frequency interference, this embodiment refines the structure of the secondary protection circuit. By combining bidirectional TVS diodes and common-mode capacitors, a secondary filtering unit for high-frequency interference is constructed, forming a frequency-segmented collaborative protection system with the primary protection circuit.

[0055] The differential-mode interference suppression unit of the secondary protection circuit consists of a bidirectional TVS diode (TVS2) connected between the input Tip and Ring terminals. Its main function is to perform secondary clamping on high-frequency differential-mode noise that may still exist after the first-stage protection. Simultaneously, it serves as a redundant overvoltage protection measure, ensuring that the signal voltage entering the matching circuit is always within the tolerance range of the SLIC chip. The breakdown voltage parameter of TVS2 is consistent with or slightly lower than that of TVS1 to create a graded clamping effect. For example, a device with a breakdown voltage of 28V is selected to ensure that subsequent energy can still be absorbed after the TVS1 response.

[0056] The common-mode interference suppression unit includes a common-mode capacitor C4 connected in parallel with the bidirectional TVS diode TVS2, and a common-mode capacitor C5 connected between the Ring and Tip terminals. It's important to understand that the capacitance values ​​of C4 and C5 are typically smaller than the common-mode capacitors in the first-level protection circuit (e.g., C4 and C5 are 1pF to 10pF), designed to filter out common-mode noise at higher frequencies (e.g., above 100MHz). C4 is connected between the Tip terminal and ground or the Ring terminal and ground (depending on circuit symmetry), while C5 is directly connected between the two signal terminals. Together, they form a high-frequency attenuation network for common-mode signals. Due to the phase consistency of the high-frequency common-mode signal on both lines, C5 can be equivalent to a parallel capacitor to ground, enhancing the bypass capability against common-mode noise while having minimal impact on differentially transmitted voice signals (low-frequency components).

[0057] After the signal is processed by the primary protection circuit, the residual high-frequency common-mode noise and differential-mode noise enter the secondary protection circuit: differential-mode noise is clamped by TVS2, while C5 capacitively attenuates the high-frequency differential-mode components; common-mode noise is discharged through the ground path of C4 and C5, where C4 addresses single-ended common-mode noise and C5 addresses the coupling leakage of common-mode noise at both ends, forming a dual filtering effect. It is important to note that the component parameters of the secondary protection circuit need to be configured according to the target interference frequency range. For example, in a GSM band interference scenario, C4 and C5 can be set to 5pF to suppress noise above 900MHz.

[0058] Furthermore, TVS2 can be a surface-mount packaged low-capacitance TVS device (such as P6SMB33CA) with a junction capacitance of less than 50pF to avoid additional loss to the high-frequency components of the voice signal; C4 and C5 should be NPO capacitors with good temperature stability to ensure stable filtering performance in a wide temperature range. During circuit layout, the secondary protection circuit should be located close to the output of the primary protection circuit to reduce parasitic parameters introduced by inter-stage routing, while maintaining the symmetry of the Tip and Ring signals to avoid noise coupling caused by layout differences.

[0059] The advantage of this embodiment lies in the fact that the independent design of the secondary protection circuit achieves frequency-selective suppression of interference signals, complementing the low-frequency filtering of the primary protection and constructing an interference suppression system covering the entire frequency band. The redundant configuration of the bidirectional TVS diodes improves the reliability of overvoltage protection, while the use of low-value common-mode capacitors effectively filters out high-frequency noise without affecting low-frequency signal transmission, ensuring that the signal received by the backend SLIC chip has a low noise floor. This structure achieves complex filtering and protection functions through a simple combination of passive components, meeting the requirements of low-cost design.

[0060] Example 5

[0061] To address the signal reflection problem caused by impedance mismatch in the voice link, this embodiment features a detailed design of the resistor group structure for the voice matching circuit. By setting series matching resistors in the Tip and Ring signal paths, a progressive impedance transformation network is constructed, which balances impedance matching and residual voltage limiting functions.

[0062] The matching resistor group includes a first matching resistor R1 and a third matching resistor R3 connected in series in the Tip signal path, and a second matching resistor R2 and a fourth matching resistor R4 connected in series in the Ring signal path. Specifically, one end of R1 is connected to the Tip output node of the first-stage protection circuit, and the other end is connected to one end of R3. The other end of R3 is connected to the Tip terminal of the voice SLIC chip. The connection of R2 and R4 is symmetrical to the Tip path. It is important to understand that this series structure divides the entire matching circuit into two stages: the first-stage resistors (R1, R2) are close to the output terminal of the protection circuit, and the second-stage resistors (R3, R4) are close to the input terminal of the SLIC chip. Through voltage division and impedance transformation of the two stages of resistors, the transition from the output impedance of the protection circuit to the input impedance of the chip is achieved.

[0063] In impedance matching design, the standard characteristic impedance of a voice link is typically 600Ω, while the input impedance of an SLIC chip may be high (e.g., tens of kΩ). Therefore, the values ​​of the pre-stage resistors R1 and R2 can be set to voltage divider resistors close to 600Ω (e.g., 300Ω), and the post-stage resistors R3 and R4 are configured according to the chip's input impedance (e.g., 150Ω), making the equivalent impedance of the entire matching network close to 600Ω, reducing reflection loss during signal transmission. Simultaneously, the voltage divider effect of the resistors can attenuate the residual voltage of interference signals proportional to their resistance values. For example, if the residual voltage in the pre-stage is 10V, after passing through a series resistor of 300Ω + 150Ω, the voltage input to the chip drops to (150 / 450) × 10V = 3.3V, which is lower than the withstand voltage threshold of most chips.

[0064] When selecting resistors, power dissipation and accuracy requirements must be considered. Since the power of voice signals is relatively low (typically in the mW range), 1 / 8W or 1 / 16W surface-mount resistors can be used, with accuracy controlled within ±5%. R1 and R2, and R3 and R4, must be resistors of the same value to ensure the symmetry of the Tip and Ring paths and reduce the introduction of unbalanced noise. For example, when R1 = R2 = 300Ω and R3 = R4 = 150Ω, the total resistance of each path is 450Ω. Combined with the 150Ω input resistance (equivalent) inside the SLIC chip, an ideal 600Ω matching impedance can be formed.

[0065] Furthermore, the placement of matching resistors should follow the principle of shortest signal path to avoid introducing additional inductance through excessively long traces, which could affect the matching effect of high-frequency signals. Resistors in the Tip and Ring paths should be placed symmetrically, with pad sizes consistent with trace widths to ensure that the parasitic parameters of the two channels are consistent. During circuit debugging, the VSWR of the link can be measured using a network analyzer, and the resistor values ​​can be adjusted until the VSWR is less than 1.5, meeting the loss requirements for voice signal transmission.

[0066] The advantage of this embodiment lies in its integration of impedance matching and voltage limiting functions through the design of a series matching resistor group, eliminating the need for an expensive integrated matching network and reducing material costs. The hierarchical series structure makes the impedance transformation process smoother, effectively reducing signal reflection and improving the transmission quality of voice signals. Simultaneously, the voltage division effect of the resistors further suppresses residual interference voltage, providing dual protection for the SLIC chip. This solution utilizes the mature principle of resistor voltage division, is simple and reliable in structure, and is easy to standardize parameters for mass production.

[0067] Example 6

[0068] To further optimize the impedance matching effect, this embodiment, based on embodiment five, constructs a hierarchical impedance matching structure by differentiating the resistance values ​​of the matching resistors, thereby achieving adaptive adjustment for signals in different frequency bands and enhancing the attenuation capability against high-frequency interference.

[0069] Specifically, the first matching resistor R1 and the second matching resistor R2 have the same resistance value (denoted as R_high), and the third matching resistor R3 and the fourth matching resistor R4 have the same resistance value (denoted as R_low), with R_high > R_low, forming a hierarchical matching network with high impedance in the front stage and low impedance in the rear stage. For example, selecting R_high = 470Ω and R_low = 100Ω makes the total resistance of the Tip and Ring paths 570Ω, close to the standard voice impedance of 600Ω. At the same time, the high impedance in the front stage has a stronger attenuation effect on high-frequency interference, while the low impedance in the rear stage better matches the input impedance of the SLIC chip.

[0070] It's important to understand that the design principle of graded impedance matching is as follows: the higher resistor in the first stage (R_high) presents high impedance to high-frequency interference signals, increasing their transmission loss. Combined with the filtering effect of the protection circuit, this further suppresses high-frequency noise. The lower resistor in the second stage (R_low) reduces the total impedance of the signal path, minimizing the difference with the chip's input impedance and improving the transmission efficiency of low-frequency voice signals. This impedance gradient, formed by the difference in resistance values, makes the matching circuit exhibit low-pass characteristics in the frequency domain. It attenuates high-frequency interference better than low-frequency signals, which aligns with the transmission characteristics of voice signals, which are predominantly low-frequency components.

[0071] When configuring parameters, the ratio of R_high to R_low needs to be adjusted according to the actual application scenario, and is usually controlled within the range of 2:1 to 5:1 to avoid excessive attenuation of DC bias voltage due to excessive resistance difference. For example, when the operating voltage of the SLIC chip depends on the external bias, an excessively high front-stage resistor may lead to insufficient bias current, affecting the normal operation of the chip. Therefore, the optimal resistance combination needs to be determined through circuit simulation or actual measurement.

[0072] Furthermore, the temperature coefficient of the resistors should be ≤100ppm / ℃ to ensure stable impedance matching characteristics under temperature changes. The package size of surface mount devices (such as 0603 or 0805) should be selected according to the board space and power requirements to avoid resistance drift due to overheating. In signal integrity design, a hierarchical matching structure can be used in conjunction with the filter capacitors of the protection circuit to form an RC low-pass filter, further suppressing out-of-band interference and improving the system's anti-interference capability.

[0073] The advantage of this embodiment lies in the fact that, through the design of a graded impedance matching structure, differentiated processing of voice signals and interference signals is achieved. While ensuring efficient transmission of low-frequency voice signals, the attenuation capability for high-frequency noise is enhanced. The differential resistance configuration strategy does not require the addition of extra components; it can be achieved simply by adjusting the parameters of existing resistors. This improves circuit performance without increasing costs, making it particularly suitable for communication environments with high electromagnetic compatibility (EMC) requirements.

[0074] Example 7

[0075] To ensure the adaptability of the protection circuit to different interference scenarios, this embodiment clarifies the principle of independent configuration of device parameters for the first-level and second-level protection circuits. By setting differentiated breakdown voltage and capacitance values, a flexibly adjustable hierarchical protection system is constructed to meet diverse application needs.

[0076] The independent configuration of component parameters in the primary and secondary protection circuits means that components such as bidirectional TVS diodes and common-mode capacitors in both circuits can be selected specifically based on the actual interference intensity and frequency characteristics. For example, for primary protection against strong interference directly from external interfaces, the breakdown voltage of TVS1 can be set to 35V (higher than the SLIC chip's operating voltage of 24V) to withstand larger transient energy; for secondary protection as back-end redundancy, the breakdown voltage of TVS2 can be set to 30V to ensure protection is still provided even if TVS1 partially fails. Regarding common-mode capacitors, the capacitance values ​​of C2 and C3 in primary protection are set to the upper limit of 100pF to enhance the filtering capability for low-frequency common-mode noise; the capacitance values ​​of C4 and C5 in secondary protection are set to the lower limit of 1pF, focusing on suppressing high-frequency common-mode noise, forming a frequency-segmented filtering characteristic.

[0077] The breakdown voltage of the bidirectional TVS diode must be strictly greater than the operating voltage of the voice SLIC chip, typically with a safety margin of over 20%. For example, if the chip's maximum operating voltage is 25V, the TVS diode's breakdown voltage should be ≥30V to avoid false triggering during normal operation. The common-mode capacitor's capacitance range (10pF~100pF for stage 1, 1pF~10pF for stage 2) is based on the division between interference frequency and signal frequency band: low-frequency common-mode noise (such as 50Hz power coupling) requires a larger capacitance value to provide a low-impedance path, while high-frequency noise (such as GHz-level RF interference) relies on the high-frequency characteristics of a smaller capacitance value for filtering. The parameter ranges of both cover the interference frequency range in most practical applications.

[0078] During the production and commissioning phase, the performance of protection circuits with different parameter configurations can be verified using EMC testing equipment. For example, a surge generator can be used to test the clamping effect of the TVS diode, and a spectrum analyzer can be used to measure the noise attenuation curves of capacitors with different capacitance values. Ultimately, the optimal parameter combination for a specific application scenario can be determined. It is important to note that the parasitic inductance and ESR (equivalent series resistance) of the capacitor will affect the high-frequency filtering effect. Therefore, the common-mode capacitor of the secondary protection circuit should be a high-frequency device with low parasitic parameters.

[0079] Furthermore, the principle of independent parameter configuration provides flexibility in circuit design. For example, in areas prone to lightning strikes, the withstand voltage rating of the TVS diodes for primary protection can be enhanced (e.g., using devices with a 50V breakdown voltage), and the capacitance values ​​of C2 and C3 can be increased to 100pF to improve the discharge capability for large energy surges. In contrast, in high-frequency electromagnetic environments, the capacitance values ​​of C4 and C5 for secondary protection can be reduced to 5pF to optimize the suppression of radio frequency noise. This modular parameter adjustment does not require changes to the circuit architecture; it can be achieved simply by replacing components, reducing the cost of customized designs.

[0080] The advantage of this embodiment lies in the fact that by clearly defining the independent configuration principle of device parameters, the hierarchical protection architecture can be flexibly adjusted according to actual application scenarios, taking into account both versatility and specificity. The breakdown voltage safety margin design of the bidirectional TVS diode and the frequency segmented configuration of the common-mode capacitor ensure reliable operation of the circuit under different interference conditions. At the same time, the standardized selection of discrete components maintains the low-cost advantage, providing a feasible technical solution for the large-scale application of voice protection circuits.

[0081] The beneficial effects of this utility model are specifically reflected in the fact that the above are only preferred embodiments of this utility model and are not intended to limit this utility model. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this utility model should be included within the protection scope of this utility model.

Claims

1. A low cost voice protection circuit, characterized by, This includes an RJ11 voice interface, a voice protection circuit, a voice matching circuit, and a voice SLIC chip, which are connected in sequence. The voice protection circuit includes a primary protection circuit and a secondary protection circuit arranged sequentially along the signal transmission direction; The input terminal of the primary protection circuit is connected to the Tip and Ring terminals of the voice interface RJ11, and the output terminal is connected to the input terminal of the secondary protection circuit. The output terminal of the secondary protection circuit is connected to the input terminal of the voice matching circuit; The voice matching circuit includes a matching resistor group connected in series between the secondary protection circuit and the voice SLIC chip, which is used to match the voice link impedance and limit the residual voltage of interference signals. Both the primary protection circuit and the secondary protection circuit include a differential mode interference suppression unit and a common mode interference suppression unit. The differential mode interference suppression unit is connected between the Tip terminal and the Ring terminal, and the common mode interference suppression unit is connected between the Tip terminal or the Ring terminal and ground.

2. A low cost voice protection circuit as claimed in claim 1, characterized in that, The differential mode interference suppression unit of the primary protection circuit includes a bidirectional TVS transistor TVS1 and a coupling capacitor C1 connected between the Tip terminal and the Ring terminal. The bidirectional TVS transistor TVS1 and the coupling capacitor C1 are connected in parallel. The common-mode interference suppression unit of the primary protection circuit includes a common-mode capacitor C2 connected between the Tip terminal and ground, and a common-mode capacitor C3 connected between the Ring terminal and ground.

3. The low-cost voice protection circuit as described in claim 2, characterized in that, The primary protection circuit also includes a bridge protection circuit, which includes diodes D1, D2, D3, and D4. The anode of diode D1 is connected to the Tip terminal and the cathode is connected to the VBAT terminal of the voice boost voltage. The cathode of diode D2 is connected to the Tip terminal and the anode is grounded. The anode of diode D3 is connected to the Ring terminal and the cathode is connected to the VBAT terminal. The cathode of diode D4 is connected to the Ring terminal and the anode is grounded. The input terminal of the bridge protection circuit is connected to the Tip terminal and the Ring terminal of the voice interface RJ11, and the output terminal is connected to the parallel node of the bidirectional TVS diode TVS1 and the coupling capacitor C1.

4. The low-cost voice protection circuit as described in claim 1, characterized in that, The differential mode interference suppression unit of the secondary protection circuit includes a bidirectional TVS transistor TVS2 connected between the Tip terminal and the Ring terminal of the input terminal of the secondary protection circuit. The common-mode interference suppression unit of the secondary protection circuit includes a common-mode capacitor C4 connected in parallel with the bidirectional TVS transistor TVS2, and a common-mode capacitor C5 connected between the Ring terminal and the Tip terminal.

5. The low-cost voice protection circuit as described in claim 1, characterized in that, The matching resistor group includes: A first matching resistor R1 and a third matching resistor R3 are connected in series in the Tip signal path. One end of the first matching resistor R1 is connected to the Tip output node of the first-level protection circuit, and the other end is connected to one end of the third matching resistor R3. The other end of the third matching resistor R3 is connected to the Tip terminal of the voice SLIC chip. A second matching resistor R2 and a fourth matching resistor R4 are connected in series in the Ring signal path. One end of the second matching resistor R2 is connected to the Ring output node of the first-level protection circuit, and the other end is connected to one end of the fourth matching resistor R4. The other end of the fourth matching resistor R4 is connected to the Ring terminal of the voice SLIC chip.

6. The low-cost voice protection circuit as described in claim 5, characterized in that, The first matching resistor R1 and the second matching resistor R2 have the same resistance value, the third matching resistor R3 and the fourth matching resistor R4 have the same resistance value, and the resistance value of the first matching resistor R1 is greater than the resistance value of the third matching resistor R3, so as to form a graded impedance matching structure.

7. The low-cost voice protection circuit as described in any one of claims 1-6, characterized in that, The device parameters of the primary protection circuit and the secondary protection circuit are configured independently. The breakdown voltage of the bidirectional TVS transistors TVS1 and TVS2 is greater than the operating voltage of the voice SLIC chip. The capacitance values ​​of the common-mode capacitors C2 and C3 range from 10pF to 100pF, and the capacitance values ​​of the common-mode capacitors C4 and C5 range from 1pF to 10pF.

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