Differential LC high-pass filter device with programmable cut-off frequency

Abstract

The invention discloses a differential LC high-pass filter device with a programmable cut-off frequency, relates to the technical field of power line carrier communication interference resistance, and solves the technical problems that the signal-to-noise ratio of a signal is seriously compressed due to pulse ringing of an existing fixed HPF high-pass filter, an analog front-end low-noise amplifier is saturated, and a receiver is blocked. According to the technical scheme, a bipolar triode is adopted as an electronic switch, a fixed L / C high-pass filter is configured to be a programmable filter, the cut-off frequency of the filter is adjusted as required, and the anti-interference performance of the high-frequency carrier receiver is improved; a bypass capacitor is connected in parallel to a base electrode and an emitting electrode, high-frequency feedback current leaked from a Miller capacitor of a triode to the base electrode is shunted, the influence on output impedance is reduced, and impedance balance at the two differential ends is kept; and meanwhile, the ringing duration of the L / C high-pass filter is reduced, and the blocking recovery time of the receiver is shortened under strong pulse interference.

Description

Technical Field

[0001] The present application relates to the technical field of power line carrier communication anti-interference, and in particular to a differential LC high-pass filter device with programmable cut-off frequency. Background Art

[0002] As society enters a highly information-based era, automatic meter reading is increasingly being used. The power sector hopes to complete electricity metering and fee management without leaving home or even requiring manual labor. At the same time, the development of science and technology and industry has promoted the improvement of human living standards. There are more and more types of electrical equipment and the power load is getting bigger and bigger. In order to improve the power efficiency of electrical equipment and enhance the intelligence and precise controllability of equipment, many devices use switching power supplies. Among the electrical equipment in residential areas, energy-saving lamps, LED lighting, televisions, induction cookers, network equipment, computers, lithium battery chargers, etc. all use switching power supplies.

[0003] The switching power supply first rectifies the industrial frequency AC power into DC power, and then converts the DC power into other power forms using the PWM (Pulsewidth modulation) method. The switching frequency of PWM is generally tens of kHz, and the switching pulse current generated on the power conversion side will leak into the power line through the rectifier bridge. Because the power line has parasitic inductance and parasitic capacitance, after the current switching pulse flows through the power line loop, a noise voltage is formed between the live wire and the neutral wire. The high end of the spectrum of the noise voltage extends to the PLC (Power Line Communication) carrier communication frequency band.

[0004] The transmission medium of PLC is power line. The transmission distance depends on the channel attenuation and channel noise. The channel attenuation is related to the power wiring and the load impedance of the power consumption end. The channel noise mainly comes from the power consumption equipment. Therefore, the coverage of PLC is mainly affected by the transmission distance, the type of power consumption equipment and the size of the power load. When the power grid is in the peak period of power consumption, the power grid noise is large, the PLC network has many levels, and some nodes are disconnected from the network, causing meter reading failure. What's more, the power load of the power grid is special, and the noise remains high for a long time, making the PLC network fragile, causing many edge nodes to be unable to access the network, seriously affecting the real-time power meter reading.

[0005] At present, PLC meter reading often encounters the problem of strong interference in power lines, which seriously hinders PLC networking. There are three common practices: one is to increase the average power output of PLC, the second is to select a working frequency band with relatively low interference intensity, and the third is to increase relays.

[0006] Increasing the average output power of the PLC transmitter can slightly increase the SNR (Signal to Noise Ratio) of the PLC signal and extend the transmission distance, but the improvement effect is relatively limited. Power line interference is mainly caused by pulse leakage generated by the switching power supply. The interference generated by pure resistive loads and pure electric loads is mainly in the low frequency band, which has a slight impact on the PLC frequency band. The switching frequency of the switching power supply generally exceeds the upper limit of human hearing, mostly concentrated in 20-60kHz, with a large amplitude and rich harmonics, which seriously interferes with the PLC frequency band. However, the higher the frequency, the lower the power spectrum amplitude of the interference. The PLC communication frequency range is usually 0.78-5.6MHz, spanning several octaves. If a higher frequency band is selected, the impact of switching interference can be reduced and the transmission distance can be increased. The role of the relay is to increase the signal power to the initial value when the SNR drops to a certain level, so as to fill the signal coverage gap.

[0007] The front-end block diagram of the power line high-frequency carrier communication receiver is as follows: Figure 1 As shown in Figure 1, the interface between the receiver unit and the power line consists of two parts: a coupler and an external L / C filter. The external L / C filter is composed of a cascade of HPF (High Pass Filter) and LPF (Low Pass Filter) to form a BPF (Band Pass Filter), which limits the communication spectrum to the passband of the band pass filter. Power line interference is mainly concentrated in the lower frequency band, so the HPF high pass filter almost determines the anti-interference performance of the receiver.

[0008] There are several standard frequency bands for PLC communication, and each frequency band may be used. From the perspective of anti-interference, in principle, the passband of the external filter should adapt to the spectrum range of the signal. However, whether in terms of technical complexity or cost, it is simpler and cheaper to use a fixed large set of external filters as the cutoff frequency of the external filter. Most existing technologies use fixed large set external filters, which limits the further development of the anti-interference performance of the receiver.

[0009] The fixed HPF high-pass filter uses L / C as the basic device. When the L / C loop is underdamped, its unit step response waveform is an attenuated ringing, such as Figure 2 , Figure 3 shown.

[0010] When a strong interference pulse on the power line excites the external L / C filter of the PLC receiver, the ringing of the HPF will be superimposed on the input of the receiver AFE (Analog Front End) as the strong pulse appears. Since the ringing amplitude is much higher than the signal amplitude, on the one hand, the SNR of the signal is severely compressed, and on the other hand, the AFE low-noise amplifier is saturated, blocking the receiver. Summary of the invention

[0011] The present application provides a differential L / C high-pass filter device with programmable cutoff frequency, the technical purpose of which is to adjust the cutoff frequency of the differential L / C high-pass filter according to the working frequency band to improve the anti-interference performance of the high-frequency carrier receiver; maintain the impedance balance at both ends of the differential L / C high-pass filter and maintain a sufficiently low noise level.

[0012] The above technical objectives of this application are achieved through the following technical solutions:

[0013] A differential LC high-pass filter device with programmable cut-off frequency comprises a DC isolation bias unit, a high-pass filter, a first inductance switching unit and a second inductance switching unit; the DC isolation bias unit comprises a first capacitor and a second capacitor, the high-pass filter comprises a first inductance, a third capacitor and a fourth capacitor, the first inductance switching unit comprises a first switch and a third inductance connected in series, and the second inductance switching unit comprises a second switch and a second inductance connected in series;

[0014] The first inductor, the second inductor and the third inductor are all connected in parallel; one end of the first capacitor is connected to the positive electrode of the analog front end, the other end of the first capacitor, the first inductor, the second switch, the third inductor and one end of the third capacitor are all connected, and the other end of the third capacitor is connected to the positive electrode of the receiver; one end of the second capacitor is connected to the negative electrode of the analog front end, the other end of the second capacitor, the first inductor, the second inductor, the first switch and one end of the fourth capacitor are all connected, and the other end of the fourth capacitor is connected to the negative electrode of the receiver.

[0015] Further, the DC blocking bias unit also includes a first resistor and a second resistor; the first switch is a first triode, and the first inductance switching unit also includes a third triode, a third resistor, a fifth resistor, a seventh resistor, a ninth resistor, an eleventh resistor, a fifth capacitor and a seventh capacitor; the second switch is a second triode, and the second inductance switching unit also includes a fourth triode, a fourth resistor, a sixth resistor, an eighth resistor, a tenth resistor, a twelfth resistor, a sixth capacitor and an eighth capacitor;

[0016] One end of the first resistor is connected to the power supply voltage, and the other end of the first resistor, the other end of the first capacitor, one end of the third capacitor, the first inductor, the emitter of the first transistor, the second inductor and one end of the fourth resistor are all connected;

[0017] One end of the second resistor is connected to the power supply voltage, and the other end of the second resistor, the other end of the second capacitor, one end of the fourth capacitor, the first inductor, the emitter of the second transistor, the third inductor and one end of the third resistor are all connected;

[0018] The GPIO output is inputted to the first inductor switching unit and the second inductor switching unit respectively from the seventh resistor and the eighth resistor;

[0019] The seventh resistor, the ninth resistor, the fifth capacitor and the base of the third transistor are all connected, one end of the ninth resistor and the fifth capacitor are all grounded, the emitter of the third transistor, the eleventh resistor and the seventh capacitor are all connected, and one end of the eleventh resistor and the seventh capacitor are all grounded; one end of the fifth resistor is connected to the power supply voltage, the other end of the fifth resistor, the collector of the third transistor and the base of the first transistor are all connected, and the collector of the first transistor, the third inductor and the third resistor are all connected;

[0020] The eighth resistor, the tenth resistor, the sixth capacitor and the base of the fourth transistor are all connected, one end of the tenth resistor and the sixth capacitor are grounded, the emitter of the fourth transistor, the twelfth resistor and the eighth capacitor are all connected, and one end of the twelfth resistor and the eighth capacitor are grounded; one end of the sixth resistor is connected to the power supply voltage, the other end of the sixth resistor, the collector of the fourth transistor and the base of the second transistor are all connected, and the collector of the second transistor, the second inductor and the fourth resistor are all connected.

[0021] The beneficial effects of the present application are as follows: the differential L / C high-pass filter device with programmable cutoff frequency described in the present application adopts a bipolar transistor as an electronic switch, configures a fixed L / C high-pass filter as a programmable filter, and adjusts the cutoff frequency of the filter as needed, thereby improving the anti-interference performance of a high-frequency carrier receiver; by connecting bypass capacitors in parallel to the base and emitter, the high-frequency feedback current leaking from the Miller capacitor of the transistor to the base is shunted, thereby reducing the impact on the output impedance and maintaining the impedance balance at both ends of the differential; at the same time, the ringing duration of the L / C high-pass filter is reduced, and the blocking recovery time of the receiver is shortened under strong pulse interference. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is the front-end block diagram of the power line high frequency carrier receiver;

[0023] Figure 2 It is a schematic diagram of a differential second-order L / C high-pass filter circuit;

[0024] Figure 3 It is a schematic diagram of the step response of a second-order RLC high-pass filter;

[0025] Figure 4It is a simplified circuit diagram of a differential L / C high-pass filter device with programmable cut-off frequency in an embodiment of the present application;

[0026] Figure 5 Detailed circuit diagram of a differential L / C high-pass filter device with programmable cut-off frequency in an embodiment of the present application;

[0027] Figure 6 This is a schematic diagram of the frequency characteristic curve of the transistor collector output impedance in the embodiment of the present application;

[0028] Figure 7 This is a schematic diagram of a high-frequency model of a triode in an embodiment of the present application;

[0029] Figure 8 Schematic diagram of the AC equivalent circuit of the transistor collector output impedance in the embodiment of the present application;

[0030] Fig. 9 Schematic diagram of a high-frequency noise equivalent circuit of a current source in an embodiment of the present application;

[0031] Fig.10 It is a schematic diagram of a specific embodiment of the present application. DETAILED DESCRIPTION

[0032] The technical solution of the present application will be described in detail below with reference to the accompanying drawings.

[0033] Figure 4 A simplified circuit of a differential L / C high-pass filter device with programmable cutoff frequency in this application is shown in FIG. Figure 4 As shown, the circuit is based on the second-order L / C HPF high-pass filter. The fixed capacitor C remains unchanged, and the cutoff frequency of the HPF is adjusted by switching the inductor L to adapt to the change of the working frequency band.

[0034] Figure 5 The specific circuit of the differential L / C high-pass filter device with programmable cut-off frequency in this application is: Figure 5 In the embodiment, the electronic switch used to switch the inductor is realized by a high-frequency transistor. Two switching switches can realize four states, so that GPIO (General Purpose Input / Output) can be used to switch four cut-off frequencies.

[0035] Figure 5In the invention, the differential L / C high-pass filter device includes a DC isolation bias unit, a high-pass filter, a first inductance switching unit and a second inductance switching unit. The DC isolation bias unit includes a first capacitor C1, a second capacitor C2, a first resistor R1 and a second resistor R2; the high-pass filter includes a first inductance L1, a third capacitor C3 and a fourth capacitor C4; the first inductance switching unit includes a first transistor Q1, a third inductance L3, a third transistor Q3, a third resistor R3, a fifth resistor R5, a seventh resistor R7, a ninth resistor R9, an eleventh resistor R11, a fifth capacitor C5 and a seventh capacitor C7; the second inductance switching unit includes a second transistor Q2, a second inductance L2, a fourth transistor Q4, a fourth resistor R4, a sixth resistor R6, an eighth resistor R8, a tenth resistor R10, a twelfth resistor R12, a sixth capacitor C6 and an eighth capacitor C8.

[0036] The working principle of the differential L / C high-pass filter device is as follows:

[0037] (1) Inductor switching

[0038] by Figure 5 For example, when GPIO_a outputs a high level (V H ≥3V), after voltage division by R7 and R9, it provides bias for the base of Q3, R11 provides current negative feedback for Q3, R7, R9, R11, Q3 together form a constant current source, providing base bias current for the conduction of transistor Q1. After Q1 is turned on, L3 is connected to both ends of the differential and connected in parallel with L1.

[0039] When GPIO_a outputs a low level (V L ≤0.4V), Q3 is cut off, the base voltage of Q1 is pulled up to 6V by R5, Q1 is cut off, as long as the instantaneous value of the signal voltage at the Rx_in+ end is <6-3.3=2.7V, Q1 can still remain cut off and will not cause signal distortion.

[0040] Under the influence of the two control quantities GPIO_a and GPIO_b, the inductance value of the differential loop presents four states: L1, L1 / / L2, L1 / / L3, L1 / / L2 / / L3, corresponding to four cutoff frequencies respectively. The smaller the inductance value, the higher the HPF cutoff frequency.

[0041] (2) Impedance balance

[0042] After Q1 is turned on, Q3 becomes the AC load of the Rx_in+ terminal and is connected in parallel with the bias resistor R1, which reduces the load of the filter. Since Q3 is a current source and works in the amplification region, its output impedance is relatively high. However, due to the existence of the transistor inter-electrode capacitance, the collector output impedance Z c As the frequency increases, it decreases, such as Figure 6 shown.

[0043] Figure 7 is a schematic diagram of the high-frequency model of the triode. Figure 7 It can be seen that the high-frequency current I flowing into the collector c c Through the Miller capacitance C b’c Leaks to the base b and generates a feedback voltage U on the base b’e ≈I b’c *r b’e , this feedback voltage excites the voltage-controlled current source g m *U b’e . Collector current I c =I b’c +I cce +I rce +g m *U b’e , g m *U b’e =β*I b ≈β*I b’c , I b’c The collector current I c The collector output impedance Z c =U c / I c It decreases rapidly with increasing frequency.

[0044] If bypass capacitors are connected in parallel to the base and emitter of the transistor (C5 at the base of Q3, C7 at the emitter; C6 at the base of Q4, C8 at the emitter), such as Figure 8 As shown, the b and e electrodes are grounded in AC equivalent. b’e , base resistance r bb’ Very small, i.e. r bb’ < <r b’e , which makes the leakage voltage U b’e ≈I b’c *r bb’ < b’c *r b’e , I c The output impedance Z of the transistor collector is significantly reduced, thereby increasing c .

[0045] Equivalent load impedance Zo of differential end Rx_in+ + =R1 / / Zc_Q3, when Zc_Q3>>R1, the equivalent load impedance is Zo + ≈R1=R2, the load impedance at both ends of the differential is balanced.

[0046] (3) Thermal noise suppression

[0047] ​Taking the first inductor switching module as an example, when the transistor Q3 is turned on, Q3 works in the linear amplification region, its base has a bias resistor, and its emitter has a feedback resistor, such as Fig. 9 As shown. Resistors will generate thermal noise. The larger the resistance, the larger the thermal noise voltage. The resistor is equivalent to a noise voltage source and a noise-free source resistor, ignoring the decoupling capacitor C. B and C E , then for the base noise source U nB Q3 is a common emitter amplifier with a voltage gain of A. VB ≈R L / R E ; For emitter noise source U nE Q3 is a common base amplifier with a voltage gain of A. VE ≈R L / R E It can be seen that at the collector, the two noise source voltages are amplified and superimposed on the differential end of the HPF, raising the noise floor of the filter.

[0048] In order to reduce the output noise of the transistor, bypass capacitors are connected in parallel between the base and emitter of the current source transistor, and the base bias resistor R B and emitter feedback resistor R E The generated thermal noise is bypassed, minimizing the transistor collector output noise.

[0049] Bypass capacitor C B and C E It has two functions: increasing the collector output impedance and reducing the collector output noise.

[0050] Fig.10 This is a specific embodiment of a differential L / C high-pass filter device with programmable cutoff frequency of the present application, wherein:

[0051] Current source current: 2.1mA, transistor collector output impedance: 21kΩ@1MHz, 5.6kΩ@6MHz;

[0052] HPF cutoff frequency: 1MHz@GPIO_b=L,GPIO_a=L;

[0053] 1.5MHz@GPIO_b=L, GPIO_a=H;

[0054] 2.2MHz@GPIO_b=H, GPIO_a=L;

[0055] 2.6MHz@GPIO_b=H, GPIO_a=H.

[0056] The above are exemplary embodiments of the present application, and the protection scope of the present application is defined by the claims and their equivalents.

Claims

1. A differential LC high-pass filter device with programmable cut-off frequency, characterized in that: It includes a DC isolation bias unit, a high-pass filter, a first inductor switching unit and a second inductor switching unit; the DC isolation bias unit includes a first capacitor and a second capacitor, the high-pass filter includes a first inductor, a third capacitor and a fourth capacitor, the first inductor switching unit includes a first switch and a third inductor connected in series, and the second inductor switching unit includes a second switch and a second inductor connected in series; The first inductor, the second inductor and the third inductor are all connected in parallel; one end of the first capacitor is connected to the positive electrode of the analog front end, the other end of the first capacitor, the first inductor, the second switch, the third inductor and one end of the third capacitor are all connected, and the other end of the third capacitor is connected to the positive electrode of the receiver; one end of the second capacitor is connected to the negative electrode of the analog front end, the other end of the second capacitor, the first inductor, the second inductor, the first switch and one end of the fourth capacitor are all connected, and the other end of the fourth capacitor is connected to the negative electrode of the receiver.

2. The differential LC high-pass filter device according to claim 1, characterized in that The DC blocking bias unit further includes a first resistor and a second resistor; the first switch is a first triode, and the first inductance switching unit further includes a third triode, a third resistor, a fifth resistor, a seventh resistor, a ninth resistor, an eleventh resistor, a fifth capacitor, and a seventh capacitor; the second switch is a second triode, and the second inductance switching unit further includes a fourth triode, a fourth resistor, a sixth resistor, an eighth resistor, a tenth resistor, a twelfth resistor, a sixth capacitor, and an eighth capacitor; One end of the first resistor is connected to the power supply voltage, and the other end of the first resistor, the other end of the first capacitor, one end of the third capacitor, the first inductor, the emitter of the first transistor, the second inductor and one end of the fourth resistor are all connected; One end of the second resistor is connected to the power supply voltage, and the other end of the second resistor, the other end of the second capacitor, one end of the fourth capacitor, the first inductor, the emitter of the second transistor, the third inductor and one end of the third resistor are all connected; The GPIO output is inputted to the first inductor switching unit and the second inductor switching unit respectively from the seventh resistor and the eighth resistor; The seventh resistor, the ninth resistor, the fifth capacitor and the base of the third transistor are all connected, one end of the ninth resistor and the fifth capacitor are all grounded, the emitter of the third transistor, the eleventh resistor and the seventh capacitor are all connected, and one end of the eleventh resistor and the seventh capacitor are all grounded; one end of the fifth resistor is connected to the power supply voltage, the other end of the fifth resistor, the collector of the third transistor and the base of the first transistor are all connected, and the collector of the first transistor, the third inductor and the third resistor are all connected; The eighth resistor, the tenth resistor, the sixth capacitor and the base of the fourth transistor are all connected, one end of the tenth resistor and the sixth capacitor are grounded, the emitter of the fourth transistor, the twelfth resistor and the eighth capacitor are all connected, and one end of the twelfth resistor and the eighth capacitor are grounded; one end of the sixth resistor is connected to the power supply voltage, the other end of the sixth resistor, the collector of the fourth transistor and the base of the second transistor are all connected, and the collector of the second transistor, the second inductor and the fourth resistor are all connected.

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