Analog relay circuit for cancelling audio transformer

Abstract

The application discloses an analog relay circuit without audio transformer, and relates to the technical fields of communication and analog audio signal processing. The circuit comprises a PWM modulation unit, a high-voltage isolation coupling unit, a demodulation unit, an analog on-off hook control unit, a 2 / 4 line conversion unit, a ring current detection unit and an auxiliary power supply unit. The low-frequency analog audio signal is modulated into a high-frequency PWM signal by the PWM modulation unit, the high-frequency signal is transmitted in isolation by the high-voltage isolation coupling unit formed by a high-voltage patch capacitor, the original audio signal is obtained by the demodulation unit, and the on-off hook control, 2 / 4 line mute conversion and ring current detection are realized synchronously. The application cancels the audio transformer in the traditional analog relay circuit, realizes the high-voltage isolation of more than 1000V and the common-mode interference suppression of more than 300V, greatly reduces the circuit volume and manufacturing cost, and solves the industry pain point that the traditional audio transformer cannot be miniaturized.

Description

Technical Field

[0001] This invention relates to the fields of communication technology and analog audio signal processing technology, and in particular to an analog relay circuit that eliminates the need for an audio transformer, which can be applied to audio signal transmission scenarios requiring high-voltage isolation, such as audio business, magneto telephone, and analog relay interfaces. Background Technology

[0002] Currently, there are three main methods for transmitting audio signals: direct coupling, RC coupling, and transformer coupling. Each of these methods has its own applicable scenarios and technical limitations.

[0003] In scenarios where electrical isolation is not required, direct coupling is usually chosen because it offers the best frequency transmission characteristics and low signal distortion. In scenarios with lower isolation requirements and weaker electromagnetic interference, RC coupling is usually chosen because it results in a simpler circuit structure and lower component costs. In industrial and communication scenarios with high isolation withstand voltage requirements and strong power line common-mode interference, transformer coupling is usually chosen because it offers good isolation and strong anti-interference capabilities and is currently the mainstream solution for analog relay circuits.

[0004] For transformer coupling schemes, the core component is the audio transformer. The analog relay circuit needs to transmit audio signals in the frequency range of 300Hz~3400Hz. To meet the transmission requirements of low-frequency signals, the transformer winding inductance needs to reach the Henry level. At the same time, to meet the high-voltage isolation requirements, the transformer core skeleton and winding insulation spacing need to meet safety regulations. The number of coil turns cannot be reduced, which directly leads to the inability to miniaturize the audio transformer.

[0005] The core material of audio transformers has undergone three generations of iteration: in the initial stage, silicon steel sheet cores were used, and the minimum size to meet the requirements of audio transmission was EE20 specification; in the transition stage, ferrite cores were used, and the minimum size could be reduced to EE13 specification, but there was a problem of high frequency modulation interference; in the current practical stage, nickel steel sheet cores are used, and the minimum size still needs to reach ED-8 specification (14mm*8mm*4.4mm), which is still the largest device in analog relay circuits.

[0006] In addition, audio transformers suffer from high manufacturing costs and low production efficiency: silicon steel sheets and nickel steel sheets are both precious metals, resulting in high raw material costs; nickel steel sheet cores cannot be assembled using automated equipment and must be manually inserted piece by piece, limiting production efficiency and preventing further cost reduction. Furthermore, due to the limited size of audio transformers, existing analog repeater products cannot achieve pin compatibility with other communication products, severely hindering the miniaturization and integration of these products.

[0007] In existing technologies, although the RC coupling scheme can avoid the size defects of transformers, it requires the use of large-capacity coupling capacitors to meet the transmission requirements of low-frequency audio signals. However, the large-capacity capacitors that also meet the high-voltage isolation requirements are also large in size and cannot be miniaturized. Furthermore, the audio signal frequency of 300Hz~3400Hz is on the same order of magnitude as the 50Hz power frequency interference frequency, and RC coupling cannot effectively distinguish between them. It has extremely poor common-mode interference resistance and cannot be applied to strong interference scenarios. Summary of the Invention

[0008] To address the shortcomings of existing transformer-coupled solutions, such as large size, high cost, and inability to miniaturize, as well as the poor anti-interference capability and inability to meet high-voltage isolation requirements of RC coupling solutions, the present invention aims to provide an analog repeater circuit that eliminates the audio transformer. While achieving high-voltage isolation above 1000V and common-mode interference suppression above 300V power lines, the audio transformer is completely eliminated, significantly reducing circuit size and manufacturing costs. Simultaneously, it fully realizes all the functions required for analog repeater circuits, including on / off control, 2 / 4-wire side tone cancellation, and ringing current detection, thus promoting the miniaturization of audio repeater products.

[0009] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0010] An analog relay circuit that eliminates the audio transformer includes a PWM modulation unit, a high-voltage isolation coupling unit, a demodulation unit, an analog on / off control unit, a 2 / 4-wire conversion unit, a ringing current detection unit, and an auxiliary power supply unit.

[0011] The PWM modulation unit includes inverters U4B, U5C, and U5A, which are used to superimpose the input analog audio signal with a high-frequency carrier triangular wave signal to generate a high-frequency PWM signal whose pulse width varies with the amplitude of the analog audio signal.

[0012] The high-voltage isolation coupling unit includes a high-voltage surface-mount capacitor C14, a high-voltage surface-mount capacitor C20, and a high-voltage surface-mount capacitor C10. The input terminal of the high-voltage surface-mount capacitor C14 is connected to the output terminal of the inverter U4B, the input terminal of the high-voltage surface-mount capacitor C20 is connected to the output terminal of the inverter U5C, and the input terminal of the high-voltage surface-mount capacitor C10 is connected to the output terminal of the inverter U5A. It is used to isolate the transmission of the high-frequency PWM signal and simultaneously achieve high-voltage isolation between the line side and the equipment side and suppression of power frequency common-mode interference.

[0013] The demodulation unit includes a resistor R26, a capacitor C12, a resistor R35, and a capacitor C21. The resistor R26 and the capacitor C12 are connected in series and grounded to form a first RC low-pass filter circuit, the input of which is connected to the output of the high-voltage surface-mount capacitor C14. The resistor R35 and the capacitor C21 are connected in series and grounded to form a second RC low-pass filter circuit, the input of which is connected to the output of the high-voltage surface-mount capacitor C20. The demodulation unit is used to demodulate the high-frequency PWM signal after isolation transmission and restore it to obtain an analog audio signal.

[0014] The 2 / 4-line conversion unit includes operational amplifiers U6A, U6B, U7A, and U7B. The output of operational amplifier U6A is connected to the input of inverter U4B of the PWM modulation unit, the input of operational amplifier U7A is connected to the output of the first RC low-pass filter circuit of the demodulation unit, the output of operational amplifier U7B is connected to the input of inverter U5C of the PWM modulation unit, and the input of operational amplifier U6B is connected to the output of the second RC low-pass filter circuit of the demodulation unit. This unit is used to realize signal conversion and sidetone cancellation processing between the 2-line audio line and the 4-line transceiver channel.

[0015] The simulated hook-up / hook-up control unit includes a high-voltage switch chip U8, a rectifier diode D6, a capacitor C11, a resistor R18, a logic control circuit, a diode D7, a transistor Q3, and a rectifier D5. The anode of the rectifier diode D6 is connected to the capacitor C11, the resistor R38, and the logic control circuit. The cathode of the rectifier diode D6 is connected to the output terminal of the inverter U4. The output terminal of the logic control circuit is connected to the control terminal of the high-voltage switch chip U8. The high-voltage switch chip U8 is connected in series to the negative terminal of the output terminal of the rectifier D5. The input terminal of the rectifier D5 is connected to the TIP / RING interface on the line side. The collector of the transistor Q3 is connected to the positive terminal of the output terminal of the rectifier D5, forming a simulated load circuit with low DC impedance and high AC impedance. The simulated hook-up / hook-up control unit is used to realize the on / off control of the DC circuit on the line side according to the hook-up / hook-up control signal.

[0016] The ringing current detection unit includes an amplitude detection circuit. The input terminal of the amplitude detection circuit is connected to the output terminal of the first RC low-pass filter circuit of the demodulation unit. It is used to detect the ringing current signal on the line side based on the restored signal and output the corresponding detection logic level SHK.

[0017] The auxiliary power supply unit includes a switching transistor Q2, a pulse transformer B1, a rectifier diode D4, capacitors C7 and C8, and a voltage regulator chip U9. The gate of the switching transistor Q2 is connected to a high-frequency square wave signal. The drain of the switching transistor Q2 is connected to one end of the primary winding of the pulse transformer B1. The other end of the primary winding of the pulse transformer B1 is connected to the input power supply VCC. One end of the secondary winding of the pulse transformer B1 is connected to the anode of the rectifier diode D4. The cathode of the rectifier diode D4 is connected to one end of the capacitor C8 and the ground terminal. The anode of the diode D4 is connected to the voltage regulator chip U9. The auxiliary power supply unit is used to provide isolated DC power supply for the PWM modulation unit, demodulation unit, and each operational amplifier unit.

[0018] As a further technical solution of the present invention: the PWM modulation unit includes at least one inverter modulation circuit; the input terminal of the inverter modulation circuit is simultaneously superimposed with the DC midpoint potential VREF, the analog audio signal to be transmitted, and the high-frequency carrier triangular wave signal, and the output terminal outputs a high-frequency PWM signal whose pulse width varies with the amplitude of the analog audio signal.

[0019] As a further technical solution of the present invention: the frequency of the high-frequency carrier triangular wave signal is not less than 340kHz, and the frequency of the high-frequency carrier triangular wave signal is more than 10 times the highest frequency of the analog audio signal to be transmitted.

[0020] As a further technical solution of the present invention: the inverter modulation circuit includes an inverter U4B, a resistor R25, and a capacitor C13; the DC midpoint potential VREFA and the analog audio signal to be transmitted are connected to the input terminal of the inverter U4B through the resistor R25, and the high-frequency carrier triangular wave Fs_A is connected to the input terminal of the inverter U4B through the capacitor C13.

[0021] As a further technical solution of the present invention: the high-voltage chip capacitors C14, C20 and C10 of the high-voltage isolation coupling unit are all picofarad-level high-voltage capacitors with a withstand voltage of not less than 1000V and a package size of not more than 0.805.

[0022] As a further technical solution of the present invention: the RC low-pass filter circuit of the demodulation unit has a cutoff frequency higher than the highest frequency of the analog audio signal to be transmitted, and lower than the frequency of the high-frequency carrier triangular wave signal.

[0023] As a further technical solution of the present invention: the high-voltage switch chip U8 of the simulated hook-and-go control unit adopts a high-voltage optocoupler with integrated transient overcurrent protection function to replace the chip. The chip integrates an overcurrent detection and self-recovery unit. When the circuit current exceeds 100mA, it can disconnect within 1μs. In the disconnected state, it can withstand a voltage of more than 600V.

[0024] As a further technical solution of the present invention: the 2 / 4-line conversion unit includes a receiving channel and a transmitting channel; the receiving channel is a 2-to-4-line channel, which is sequentially connected to the line-side TIP / RING interface, an operational amplifier U6A, a first PWM modulation unit composed of an inverter U4B, a first high-voltage isolation coupling unit composed of a high-voltage surface-mount capacitor C14, a first demodulation unit composed of a resistor R26 and a capacitor C12, and an operational amplifier U7A, and finally outputs a 4-line receiving signal Vout; the transmitting channel is a 4-to-2-line channel, which is sequentially connected to the 4-line transmitting signal Vin input terminal, an operational amplifier U7B, a second PWM modulation unit composed of an inverter U5C, a second high-voltage isolation coupling unit composed of a high-voltage surface-mount capacitor C20, a second demodulation unit composed of a resistor R35 and a capacitor C21, and an operational amplifier U6B, and finally connected to the line-side TIP / RING interface; the 2 / 4-line conversion unit achieves signal cancellation from the transmitting channel to the receiving channel by matching impedance parameters, thus completing the sidetone cancellation processing.

[0025] As a further technical solution of the present invention: the amplitude detection circuit of the ringing current detection unit outputs a low-level detection signal when there is a ringing current signal on the line side, and outputs a high-level detection signal when there is no ringing current signal on the line side.

[0026] As a further technical solution of the present invention: the pulse transformer B1 of the auxiliary power supply unit adopts SMD-6P surface mount package.

[0027] One or more technical solutions provided in the embodiments of this application have at least the following technical effects or advantages:

[0028] 1. Completely eliminates the audio transformer in traditional analog repeater circuits. By using the technical path of "low-frequency audio signal PWM high-frequency modulation - high-voltage capacitor isolation transmission - demodulation and restoration", the isolation transmission function of the audio transformer is replaced. The largest component in the circuit is a miniature pulse transformer of 3.5mm*3.2mm*3mm. The overall size is reduced by more than 80% compared with the traditional solution. This solves the long-standing pain point in the industry that analog repeater circuits cannot be miniaturized and can achieve pin compatibility with other communication products.

[0029] 2. Significantly reduces manufacturing costs by eliminating the need for precious metal magnetic cores and audio transformers that require manual assembly. All circuits use general-purpose surface-mount devices that are automatically mounted, resulting in a significant reduction in raw material and manufacturing costs and a substantial increase in production efficiency.

[0030] 3. Achieve excellent high-voltage isolation and anti-interference performance. The picofarad-level high-voltage coupling capacitor can easily meet the isolation withstand voltage requirements of over 1000V, while presenting a high impedance of over 145MΩ for 50Hz power frequency common-mode interference. Its anti-interference capability is superior to traditional transformer coupling schemes, and it can be applied to harsh scenarios with strong power line interference.

[0031] 4. Strong functional integrity: While eliminating the audio transformer, it fully realizes all the functions required by analog repeater circuits, including analog on / off control, 2 / 4-line side tone cancellation conversion, ring current detection, and caller ID transmission. It can directly replace existing traditional analog repeater circuits without changing the peripheral application scheme.

[0032] 5. The circuit has high reliability. It adopts a high-voltage switching chip with integrated transient overcurrent protection function to replace the high-voltage optocoupler and external protection circuit in the traditional solution. It has built-in 1μs-level overcurrent disconnection protection and can withstand 600V disconnection voltage, which greatly improves the circuit's protection against lightning strikes and power line contact. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of a traditional analog relay circuit with an audio transformer in the existing technology;

[0034] Figure 2 This is a schematic diagram of an analog relay circuit that eliminates the need for an audio transformer in this invention.

[0035] Figure 3 This is a schematic diagram of the PWM modulation principle of the present invention. Detailed Implementation

[0036] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0037] like Figure 1 As shown in the figure, this embodiment of the invention discloses an analog relay circuit that eliminates the audio transformer, and the complete schematic diagram is as follows. Figure 2 As shown, it can directly replace the traditional analog relay circuit with audio transformer and can be applied to scenarios such as audio business, magneto telephone, and analog relay interface. The specific implementation and working principle of each part of the circuit are explained in detail below.

[0038] I. Core Modulation, Demodulation, and Isolation Transmission Circuit

[0039] The core innovation of this embodiment lies in replacing the audio transformer with PWM modulation and high-voltage capacitor coupling. The specific circuit and parameter settings are as follows:

[0040] PWM modulation circuit

[0041] In this embodiment, the PWM modulation unit is implemented using a general-purpose CMOS inverter, including two independent modulation circuits, corresponding to the receiving channel and the transmitting channel respectively. Taking the inverter U4B of the receiving channel as an example, the DC midpoint potential VREFA and the operationally amplified audio signal Fx_A are connected to the input terminal of U4B through resistor R25, and the high-frequency carrier triangular wave Fs_A is connected to the input terminal of U4B through capacitor C13.

[0042] In this embodiment, the inverter is powered by 5V (or 3.3V), the input threshold voltage corresponding to the polarity reversal of the inverter output is 2.5V (or 1.65V), and the DC midpoint potential VREFA is set to 2.5V (or 1.65V). The peak-to-peak value of the triangular wave Fs_A is set to 2.8V, and the maximum peak-to-peak value of the audio signal Fx_A is set to 1.4V. This ensures that the input potential of the inverter fluctuates between 1.4V and 4.2V, neither exceeding the power supply range nor entering the uncertain region of the inverter, thus ensuring a stable and distortion-free modulation process.

[0043] Modulation principle such as Figure 3 As shown, by simultaneously superimposing a DC midpoint, an audio signal, and a triangular wave carrier at the input of the inverter, the output of the inverter can generate a high-frequency PWM signal whose pulse width changes in real time with the amplitude of the audio signal, thus completing the modulation process.

[0044] In this embodiment, the frequency range of the audio signal to be transmitted is 300Hz~3400Hz. According to the principle of more than 10 times the frequency difference, the lowest frequency of the carrier signal Fs is selected to be above 340kHz. In this embodiment, 1MHz is preferred to ensure that the signal distortion in the modulation and demodulation process meets the requirements of audio transmission.

[0045] High-voltage isolation coupling unit

[0046] In this embodiment, the high-voltage isolation coupling unit uses picofarad-level high-voltage surface-mount capacitors, corresponding to the receiving channel C14, the transmitting channel C20, and the on / off control channel C10. All of them use 0805 packaged high-voltage surface-mount capacitors with a voltage rating of 22pF and a withstand voltage of 1000V or higher.

[0047] For a 1MHz high-frequency PWM carrier signal, the capacitive reactance of the 22pF capacitor is only 7.2kΩ, enabling low-loss signal transmission. For a 50Hz power frequency common-mode interference signal, the capacitive reactance of the 22pF capacitor is as high as 145MΩ, achieving extremely strong interference suppression. At the same time, the withstand voltage of over 1000V meets the high-voltage isolation requirements between the line side and the equipment side, perfectly solving the technical contradiction that traditional RC coupling cannot simultaneously achieve low-frequency transmission, high-voltage isolation, and anti-interference.

[0048] Demodulation circuit

[0049] In this embodiment, the demodulation unit adopts an RC low-pass filter circuit. The corresponding receiving channel is a low-pass filter circuit composed of R26 and C12, and the transmitting channel is a low-pass filter circuit composed of R35 and C21.

[0050] In this embodiment, the cutoff frequency of the RC low-pass filter circuit is set to 10kHz, which is higher than the highest frequency of the audio signal 3400Hz and much lower than the carrier frequency 1MHz. This effectively filters out high-frequency carrier components, completely restores the original analog audio signal, and achieves distortion-free isolated transmission of the audio signal.

[0051] Regarding the core component parameters of the modulation and demodulation circuits, this embodiment provides the following preferred value principles:

[0052] Taking the modulation coupling capacitor C22 and feedback resistor R42 as examples, C22 is a coupling capacitor for a 1MHz carrier signal, and the smaller the capacitive reactance, the better; for a 1kHz audio signal, it is a bypass capacitor, and the larger the capacitive reactance, the better. When the carrier frequency is 1MHz and the audio frequency is 1kHz, the capacitive reactance of the same capacitor for the two signals differs by a factor of 1000; the preferred value for C22 is 100pF, with a capacitive reactance Xs = 1.6kΩ for a 1MHz carrier and Xx = 1.6MΩ for a 1kHz audio signal; the preferred value for R42 is 51kΩ, satisfying the principle of being much greater than Xs and much less than Xx, ensuring optimal modulation effect.

[0053] II. Simulated hook-and-loop control unit

[0054] In this embodiment, the simulated hook-on / hook control unit includes a rectifier diode D6, a resistor R38, a capacitor C11, a logic control circuit, a high-voltage switching chip U8, and resistors R18, R19, R20, and a transistor Q3.

[0055] The working process is as follows: When the on / off control signal RC=1, the 1MHz high-frequency square wave Fs is transmitted to the rectifier and filter circuit through R22, inverter U5A, and high-voltage coupling capacitor C10. After rectification and filtering, the output RC_K=0. The logic control circuit drives the high-voltage switch chip U8 to conduct, and the DC channel between TIP and RING is connected, realizing the simulation of on / off. When RC=0, the high-frequency square wave Fs is shorted to ground by D7 through R22. The rectifier and filter circuit has no output, RC_K=1. The logic control circuit drives U8 to cut off, and the DC channel between TIP and RING is disconnected, realizing the simulation of on / off.

[0056] Among them, rectifier D5 is used to adapt to the different DC voltage polarities between TIP and RING; the analog load circuit is composed of R18, R19, R20 and Q3, which presents low DC impedance and high AC impedance characteristics, meets the line impedance matching requirements, and does not affect the transmission of audio signals.

[0057] In this embodiment, the high-voltage switch chip U8 uses the X1651 chip developed by Shenzhen Qianxing Electronics Co., Ltd. This chip replaces the high-voltage optocoupler in the traditional circuit and integrates transient overcurrent protection function. When the circuit current exceeds 100mA, it can quickly disconnect within 1μs. In the disconnected state, it can withstand 600V voltage. After the current returns to normal for 1 minute, it can recover on its own without the need for additional external protection circuit, which greatly improves the reliability of the circuit.

[0058] III. 2 / 4-line conversion and audio transmission unit

[0059] In this embodiment, the 2 / 4 line conversion unit includes a receiving channel (2-line to 4-line) and a transmitting channel (4-line to 2-line), which can realize the conversion between 2-line audio lines and 4-line transceiver channels, and at the same time complete the sidetone cancellation processing.

[0060] Receive channel (2 to 4 lines)

[0061] The signal is input differentially between TIP and RING, and outputs a 4-wire signal from Vout. The signal transmission path is: TIP / RING interface → C16, R27 → operational amplifier U6A → R25 → inverter U4B (PWM modulation) → high voltage coupling capacitor C14 → inverter U5B → R26, C12 (demodulation) → C15, R24 → operational amplifier U7A → Vout output.

[0062] When the handset is off, the high-voltage switch chip U8 is turned on, and the matching resistor RL is connected between TIP and RING to achieve line impedance matching, and the 2-wire interface presents low impedance. When the handset is on, U8 is turned off, RL is left floating, the 2-wire interface presents high impedance, and the signal transmission path is the same when the handset is on and off, so the caller ID signal can be transmitted normally. The signal gain of the receiving channel can be configured by adjusting the parameters of R23 and R24.

[0063] Sending channel (4 lines to 2 lines)

[0064] The signal is input from the Vin terminal and transmitted via 4 wires. It is output differentially between TIP and RING. The signal transmission path is as follows: Vin input terminal → R31 → Operational amplifier U7B → R33 → Inverter U5C (PWM modulation) → High voltage coupling capacitor C20 → Inverter U4C → R35, C21 (demodulation) → C19, R34 → Operational amplifier U6B → C17, Q3, D5 → TIP / RING interface.

[0065] When the handset is off, the signal transmission path is normally open, and normal calls can be made; when the handset is on, Q3 and D5 are cut off, the signal transmission path is interrupted, and calls cannot be made, which meets the functional requirements of an analog repeater circuit; the signal gain of the transmitting channel can be configured by adjusting the parameters of R32 and R34.

[0066] Sidetone cancellation processing: In this embodiment, the sidetone cancellation function is achieved by matching impedance parameters. By adjusting the parameters of R28 according to the different impedances of the TIP and RING lines, it can be ensured that when the Vin end inputs a transmitting signal, the Vout end has no receiving signal output, so that the signals from the transmitting channel to the receiving channel are completely canceled, thus completing the sidetone cancellation processing and avoiding the problem of spontaneous transmission and reception of sidetone during the call.

[0067] IV. Ringing Current Detection Unit

[0068] In this embodiment, the signal transmission path of the ringing current detection unit is as follows: TIP / RING interface → C16, R27 → operational amplifier U6A → R25 → inverter U4B (PWM modulation) → high voltage coupling capacitor C14 → inverter U5B → R26, C12 (demodulation) → C15, R24 → operational amplifier U7A → Vout terminal → amplitude detection circuit → SHK terminal outputs detection logic level.

[0069] The working process is as follows: When there is a ringing current signal input between TIP and RING, after modulation, isolation transmission, and demodulation, the Vout terminal outputs the corresponding ringing current signal. After the amplitude detection circuit detects the signal, the SHK terminal outputs a low level (SHK=0); when there is no ringing current signal between TIP and RING, the SHK terminal outputs a high level (SHK=1). By adjusting the parameters of the amplitude detection circuit, the required ringing current detection threshold can be configured to meet the requirements of different application scenarios.

[0070] V. Auxiliary Power Supply Unit

[0071] In this embodiment, the auxiliary power supply unit consists of a switching transistor Q2, a miniature pulse transformer B1, a rectifier diode D4, filter capacitors C7 and C8, a voltage regulator chip U9, and voltage divider resistors R40 and R41. The basic topology is a flyback isolated power supply.

[0072] The working process is as follows: A 1MHz high-frequency square wave signal controls the switching transistor Q2 to turn on and off, and the energy of the input power supply VCC is coupled to the secondary side through the pulse transformer B1. After being rectified and filtered by D4 and C7, the voltage is regulated by the voltage regulator chip U9 to output the isolated DC power supply VDD, which powers the inverter, operational amplifier and other devices in the circuit.

[0073] In this embodiment, the load power of VDD is only 5V / 2mA, the load is stable and the power is extremely small. The voltage regulator circuit can use an LDO chip to replace the voltage regulator chip, further optimizing the power supply ripple and efficiency. The pulse transformer B1 adopts an SMD-6P package and has a size of only 3.5mm*3.2mm*3mm, which is much smaller than the traditional ED-8 specification (14mm*8mm*4.4mm) audio transformer. It is the largest device in the circuit, realizing the miniaturization of the overall circuit.

[0074] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

[0075] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment have been appropriately combined to form other embodiments that are easy for those skilled in the art to understand.

Claims

1. An analog relay circuit that eliminates the need for an audio transformer, characterized in that, It includes a PWM modulation unit, a high-voltage isolation coupling unit, a demodulation unit, an analog on / off switch control unit, a 2 / 4-line conversion unit, a ringing current detection unit, and an auxiliary power supply unit; The PWM modulation unit includes inverters U4B, U5C, and U5A, which are used to superimpose the input analog audio signal with a high-frequency carrier triangular wave signal to generate a high-frequency PWM signal whose pulse width varies with the amplitude of the analog audio signal. The high-voltage isolation coupling unit includes a high-voltage surface-mount capacitor C14, a high-voltage surface-mount capacitor C20, and a high-voltage surface-mount capacitor C10. The input terminal of the high-voltage surface-mount capacitor C14 is connected to the output terminal of the inverter U4B, the input terminal of the high-voltage surface-mount capacitor C20 is connected to the output terminal of the inverter U5C, and the input terminal of the high-voltage surface-mount capacitor C10 is connected to the output terminal of the inverter U5A. It is used to isolate the transmission of the high-frequency PWM signal and simultaneously achieve high-voltage isolation between the line side and the equipment side and suppression of power frequency common-mode interference. The demodulation unit includes a resistor R26, a capacitor C12, a resistor R35, and a capacitor C21. The resistor R26 and the capacitor C12 are connected in series and grounded to form a first RC low-pass filter circuit, the input of which is connected to the output of the high-voltage surface-mount capacitor C14. The resistor R35 and the capacitor C21 are connected in series and grounded to form a second RC low-pass filter circuit, the input of which is connected to the output of the high-voltage surface-mount capacitor C20. The demodulation unit is used to demodulate the high-frequency PWM signal after isolation transmission and restore it to obtain an analog audio signal. The 2 / 4-line conversion unit includes operational amplifiers U6A, U6B, U7A, and U7B. The output of operational amplifier U6A is connected to the input of inverter U4B of the PWM modulation unit, the input of operational amplifier U7A is connected to the output of the first RC low-pass filter circuit of the demodulation unit, the output of operational amplifier U7B is connected to the input of inverter U5C of the PWM modulation unit, and the input of operational amplifier U6B is connected to the output of the second RC low-pass filter circuit of the demodulation unit. This unit is used to realize signal conversion and sidetone cancellation processing between the 2-line audio line and the 4-line transceiver channel. The simulated hook-up / hook-up control unit includes a high-voltage switch chip U8, a rectifier diode D6, a capacitor C11, a resistor R18, a logic control circuit, a diode D7, a transistor Q3, and a rectifier D5. The anode of the rectifier diode D6 is connected to the capacitor C11, the resistor R38, and the logic control circuit. The cathode of the rectifier diode D6 is connected to the output terminal of the inverter U4A. The output terminal of the logic control circuit is connected to the control terminal of the high-voltage switch chip U8. The high-voltage switch chip U8 is connected in series to the negative terminal of the output terminal of the rectifier D5. The input terminal of the rectifier D5 is connected to the TIP / RING interface on the line side. The collector of the transistor Q3 is connected to the positive terminal of the output terminal of the rectifier D5, forming a simulated load circuit with low DC impedance and high AC impedance. The simulated hook-up / hook-up control unit is used to realize the on / off control of the DC circuit on the line side according to the hook-up / hook-up control signal. The ringing current detection unit includes an amplitude detection circuit. The input terminal of the amplitude detection circuit is connected to the output terminal of the first RC low-pass filter circuit of the demodulation unit. It is used to detect the ringing current signal on the line side based on the restored signal and output the corresponding detection logic level SHK. The auxiliary power supply unit includes a switching transistor Q2, a pulse transformer B1, a rectifier diode D4, capacitors C7 and C8, and a voltage regulator chip U9. The gate of the switching transistor Q2 is connected to a high-frequency square wave signal, and the drain of the switching transistor Q2 is connected to one end of the primary winding of the pulse transformer B1. The other end of the primary winding of the pulse transformer B1 is connected to the input power supply VCC. One end of the secondary winding of the pulse transformer B1 is connected to the anode of the rectifier diode D4. The cathode of the rectifier diode D4 is connected to one end of the capacitor C8 and the ground terminal. The anode of the diode D4 is connected to the voltage regulator chip U9. The auxiliary power supply unit is used to provide isolated DC power supply for the PWM modulation unit, demodulation unit, and each operational amplifier unit.

2. The analog relay circuit for eliminating the audio transformer according to claim 1, characterized in that, The PWM modulation unit includes at least one inverter modulation circuit; the input of the inverter modulation circuit is simultaneously superimposed with the DC midpoint potential VREF, the analog audio signal to be transmitted, and the high-frequency carrier triangular wave signal, and the output output is a high-frequency PWM signal whose pulse width varies with the amplitude of the analog audio signal.

3. The analog relay circuit for eliminating the audio transformer according to claim 1, characterized in that, The frequency of the high-frequency carrier triangular wave signal is not less than 340kHz, and the frequency of the high-frequency carrier triangular wave signal is more than 10 times the highest frequency of the analog audio signal to be transmitted.

4. The analog relay circuit for eliminating the audio transformer according to claim 1, characterized in that, The inverter modulation circuit includes an inverter U4B, a resistor R25, and a capacitor C13; the DC midpoint potential VREFA and the analog audio signal to be transmitted are connected to the input terminal of the inverter U4B through the resistor R25, and the high-frequency carrier triangular wave Fs_A is connected to the input terminal of the inverter U4B through the capacitor C13.

5. The analog relay circuit for eliminating the audio transformer according to claim 1, characterized in that, The high-voltage surface-mount capacitors C14, C20, and C10 of the high-voltage isolation coupling unit are all picofarad-level high-voltage capacitors with a withstand voltage of not less than 1000V and a package size of not more than 0.

805.

6. The analog relay circuit for eliminating the audio transformer according to claim 1, characterized in that, The RC low-pass filter circuit of the demodulation unit has a cutoff frequency higher than the highest frequency of the analog audio signal to be transmitted, but lower than the frequency of the high-frequency carrier triangular wave signal.

7. The analog relay circuit for eliminating the audio transformer according to claim 1, characterized in that, The high-voltage switch chip U8 of the simulated hook-and-go control unit is replaced by a high-voltage optocoupler with integrated transient overcurrent protection. The chip integrates an overcurrent detection and self-recovery unit, which can disconnect within 1μs when the circuit current exceeds 100mA and can withstand voltages of over 600V when disconnected.

8. The analog relay circuit for eliminating the audio transformer according to claim 1, characterized in that, The 2 / 4-line conversion unit includes a receiving channel and a transmitting channel. The receiving channel is a 2-to-4-line channel, sequentially connected to the line-side TIP / RING interface, an operational amplifier U6A, a first PWM modulation unit composed of an inverter U4B, a first high-voltage isolation coupling unit composed of a high-voltage surface-mount capacitor C14, a first demodulation unit composed of a resistor R26 and a capacitor C12, and an operational amplifier U7A, ultimately outputting a 4-line received signal Vout. The transmitting channel is a 4-to-2-line channel, sequentially connected to the 4-line transmitted signal Vin input terminal, an operational amplifier U7B, a second PWM modulation unit composed of an inverter U5C, a second high-voltage isolation coupling unit composed of a high-voltage surface-mount capacitor C20, a second demodulation unit composed of a resistor R35 and a capacitor C21, and an operational amplifier U6B, ultimately connected to the line-side TIP / RING interface. The 2 / 4-line conversion unit achieves signal cancellation from the transmitting channel to the receiving channel by matching impedance parameters, completing the sidetone cancellation process.

9. The analog relay circuit for eliminating the audio transformer according to claim 1, characterized in that, The amplitude detection circuit of the ringing current detection unit outputs a low-level detection signal when there is a ringing current signal on the line side, and outputs a high-level detection signal when there is no ringing current signal on the line side.

10. The analog relay circuit for eliminating the audio transformer according to claim 1, characterized in that, The pulse transformer B1 of the auxiliary power supply unit is packaged in an SMD-6P surface mount package.

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