Circuit structure of 108MHz-678MHz high-suppression large-bandwidth electrically tunable filter

Abstract

The utility model discloses a circuit structure of a 108MHz-678MHz high-suppression large-bandwidth electrically tunable filter, which belongs to the technical field of electrically tunable filters and comprises a circuit structure of a 108MHz-225MHz tuned filter, a circuit structure of a 225MHz-400MHz tuned filter, a circuit structure of a 108MHz-225MHz tuned filter and a circuit structure of a 108MHz-678MHz straight-through path. One end of each of the circuit structures of the 108MHz-225MHz tuned filter, the 225MHz-400MHz tuned filter and the 108MHz-225MHz tuned filter and the circuit structure of the 108MHz-678MHz straight-through path is connected with the radio frequency switch U1, the other end of each of the circuit structures of the 108MHz-225MHz tuned filter and the circuit structure of the 108MHz-678MHz straight-through path is connected with the three low-pass filters and the radio frequency switch U2, and the other ends of the three low-pass filters are connected with the radio frequency switch U2. The main magnet ring inductor of the electrically tunable filter is a hollow coil wound by an enameled wire. According to the utility model, the size of the electrically tunable filter is reduced by reducing the number of inductors; through the testing tool, the pins are prevented from being damaged, and the testing efficiency and accuracy are improved.

Description

Technical Field

[0001] This utility model relates to the circuit structure of a high-suppression, wide-bandwidth electrically tunable filter for 108MHz to 678MHz, and belongs to the field of electrically tunable filter technology. Background Technology

[0002] like Figure 8 The electrically tunable filter shown includes a housing, a coil encapsulated within the housing, and pins located at the bottom of the housing. The pin definitions are shown in Table 1 below.

[0003] Table 1 Pin Definitions of Electrically Adjustable Filters

[0004] Pin number Corresponding definition 1 GND 2 VCC 3 VT 4 GND 5 GND 6 RF 7 GND 8 GND 9 GND 10 GND 11 RF 12 GND 13 GND 14 GND 15 D1 16 D0

[0005] An excellent filter should have the lowest possible insertion loss, the highest possible suppression of ±10% at any center frequency, and the highest possible far-end suppression at twice the center frequency.

[0006] Since insertion loss is inversely proportional to the ±10% suppression at any center frequency, a good result in one metric will result in a poor result in the other.

[0007] At the same time, the suppression of ±10% at any center frequency is inversely proportional to the far-end suppression at twice the center frequency; if one indicator is good, the other indicator will be bad.

[0008] Therefore, the aforementioned electrically tunable filters, for the same size, cannot meet the requirements of insertion loss less than or equal to 4dB, suppression of ±10% at any center frequency from 225MHz to 678MHz greater than or equal to 13dBc, and far-end suppression at twice the center frequency can only reach 22dB to 26dB, which cannot meet the needs of users. Utility Model Content

[0009] The technical problem to be solved by this utility model is to provide a circuit structure for a high-suppression, wide-bandwidth electrically tunable filter with a frequency range of 108MHz to 678MHz, so as to solve the above-mentioned technical problems.

[0010] The technical solution adopted by this utility model is as follows: a circuit structure for a 108MHz~678MHz high-suppression, large-bandwidth electrically tunable filter, including a circuit structure for a 108MHz~225MHz tuning filter, a circuit structure for a 225MHz~400MHz tuning filter, a circuit structure for a 108MHz~225MHz tuning filter, and a circuit structure for a 108MHz~678MHz direct path. One end of each of the circuit structures for the 108MHz~225MHz tuning filter, the 225MHz~400MHz tuning filter, the 108MHz~225MHz tuning filter, and the 108MHz~678MHz direct path is connected to an RF switch U1. The circuit structures of the device, the 225MHz to 400MHz tuning filter, and the 108MHz to 225MHz tuning filter are respectively connected in series with LTCC low-pass filters LFCN3216-190A01, LTCC low-pass filters LFCN3216-420A01, and LTCC low-pass filters LFCN3216-575A01. The other end of the circuit structure of LTCC low-pass filters LFCN3216-190A01, LTCC low-pass filters LFCN3216-420A01, LTCC low-pass filters LFCN3216-575A01 and the 108MHz to 678MHz direct path is connected to the RF switch U2. The main magnetic ring inductor of the electrically tunable filter uses an air coil wound with enameled wire.

[0011] Furthermore, the circuit structures of the 108MHz–225MHz tuning filter, the 225MHz–400MHz tuning filter, the 108MHz–225MHz tuning filter, and the 108MHz–678MHz direct path all include inductors L1, L2, L3, L4, L5, L6, capacitors C1 and C2; inductors L5, L1, L2, and L6 are connected in series, and inductor L... 5. The other end is connected to RF switch U1. Inductor L6 is connected directly or in series with an LTCC low-pass filter and then connected to RF switch U2. One end of the series connection between inductor L5 and inductor L1 is connected to one end of inductor L3. The other end of inductor L3 is connected to one end of capacitor C1. One end of the series connection between inductor L2 and inductor L6 is connected to one end of inductor L4. The other end of inductor L4 is connected to one end of capacitor C2. The other ends of capacitor C1, capacitor C2, and the end of the series connection between inductor L1 and inductor L2 are grounded.

[0012] The filter consists of two parts: capacitors and inductors. When the inductance remains constant, capacitors C1 and C2 are varactor diodes. Based on the characteristics of varactor diodes, an adjustable reverse bias voltage VT is provided to the varactor diodes to change the capacitance value, thereby changing the resonant frequency and achieving the purpose of frequency conversion.

[0013] Furthermore, the aforementioned inductors L4, L1, L2, and L3 all use the enameled wire of a single hollow coil, and are soldered to different positions on the pads to obtain four inductors.

[0014] The beneficial effects of this utility model are as follows: Compared with the prior art, the effects of this utility model are as follows:

[0015] 1) The main magnetic ring inductor of the electrically tunable filter in this utility model uses an air coil wound with enameled wire, which makes the Q value higher than that of conventional inductors. At the same time, an LTCC low-pass filter is connected in series after the filter, which improves the relative bandwidth by 1.5dB, the relative bandwidth by 0.5dB, the suppression of ±6% and ±10% of the center frequency, and the suppression at twice the center frequency while ensuring the insertion loss of the filter.

[0016] Compared to the previous arrangement of 7 magnetic core wire-wound inductors, by reducing the number of inductors to 2 magnetic core wire-wound inductors paired with one air-core coil inductor, the size of the electrically tunable filter is reduced to 25×25×5(±0.2)mm (the conventional size is 30×30×7.5mm). Conventional electrically tunable filters, at the same size, cannot meet the requirements of insertion loss less than or equal to 8dB in the 108MHz–225MHz range, insertion loss less than or equal to 5.5dB in the 225MHz–678MHz range, and a relative bandwidth of 1.5dB in the 225MHz–678MHz range. When the relative bandwidth of 0.5dB is greater than or equal to 5MHz and the range of 400MHz to 678MHz is greater than or equal to 8MHz, the suppression of ±10% at any center frequency from 108MHz to 225MHz is greater than or equal to 20dBc, the suppression of ±6% at any center frequency from 225MHz to 678MHz is greater than or equal to 20dBc, the suppression of ±10% is greater than or equal to 29dBc, and the far-end suppression at twice the center frequency is greater than or equal to 30dBc; this is significantly better than conventional filters, whose specifications are shown in the table below:

[0017] Table 2. Specifications of Conventional Filters

[0018]

[0019] 2) Using enameled wire to make air-core coils to replace conventional inductors, the Q value can reach 250-300 and is adjustable. While ensuring the insertion loss of the filter, it also improves the relative bandwidth by 1.5dB, the relative bandwidth by 0.5dB, the center frequency ±6%, 10%, and the suppression of the far-end double harmonic. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the first layer layout of a PCB circuit board;

[0021] Figure 2 This is a schematic diagram of the second layer layout of a PCB circuit board;

[0022] Figure 3 This is a schematic diagram of the third layer layout of a PCB circuit board;

[0023] Figure 4 This is a schematic diagram of the fourth layer layout of a PCB circuit board;

[0024] Figure 5 This is a schematic diagram of the circuit principle of an electrically tunable filter;

[0025] Figure 6 This is a schematic diagram of the circuit structure of an electrically tunable filter;

[0026] Figure 7 This is the PCB layout diagram of the test fixture;

[0027] Figure 8 This is a top view of the electrically tunable filter.

[0028] Figure 9 This is a schematic diagram of the front view structure of an electrically tunable filter;

[0029] Figure 10 This is a schematic diagram of the electrically tunable filter from below.

[0030] Figure 11 This is a diagram of the varactor diode drive circuit for a 108MHz to 225MHz tuned filter circuit;

[0031] Figure 12 This is a diagram of the varactor diode drive circuit for a 225MHz to 400MHz tuned filter circuit;

[0032] Figure 13 This is a diagram of a varactor diode drive circuit for a 400MHz to 678MHz tuned filter circuit. Detailed Implementation

[0033] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0034] Example 1: As Figure 1-13 As shown, a method for manufacturing a high-suppression, wide-bandwidth electrically tunable filter with a frequency range of 108MHz to 678MHz is disclosed. The method includes the following steps:

[0035] Step 1: Product Solution Determination: The system consists of a 108MHz–225MHz tuning filter, a 225MHz–400MHz tuning filter, a 400MHz–678MHz tuning filter, a 108MHz–678MHz pass-through path, and two RF switches. An LTCC low-pass filter (LFCN3216-190A01) is added in series to the 108MHz–225MHz tuning filter; an LTCC low-pass filter (LFCN3216-420A01) is added in series to the 225MHz–400MHz tuning filter; and an LTCC low-pass filter (LFCN3216-575A01) is added in series to the 108MHz–225MHz tuning filter.

[0036] Step 2, PCB layout: The PCB has four layers. The first layer holds the components of the electronically tunable filter, the second layer is a ground plane, the third layer contains network connections and 50-ohm impedance striplines, and the fourth layer is a ground plane with exposed copper. The layout of each filter segment on the PCB is symmetrical.

[0037] Step 3, the specific manufacturing method includes the following steps:

[0038] 3.1) After preparing the surface mount components for the electrically tunable filter, mount them according to the circuit diagram of the electrically tunable filter. Use leaded solder paste Sn40Pb60 for SMT machine mounting and reflow soldering to obtain the surface mount semi-finished product.

[0039] 3.2) Prepare the corresponding inductor according to the following channel requirements;

[0040] 108MHz~225MHz channel: Inductor name: main inductor, inductor specifications: 1.8mm winding bar, copper wire diameter: 0.38mm, number of turns: 18, tap positions: 3, quantity: 1, winding direction: counterclockwise;

[0041] Input / output channels: Inductor specification T10-0, copper wire diameter 0.17mm, number of turns 5, quantity 2, winding direction counterclockwise;

[0042] 225MHz~400MHz channel: Inductor name: main inductor, inductor specifications: 2.4mm winding bar, copper wire diameter: 0.45mm, number of turns: 10, tap positions: 2, quantity: 1, winding direction: counterclockwise;

[0043] Input / output channels: Inductor specification T10-0, copper wire diameter 0.17mm, number of turns 2, quantity 2, winding direction counterclockwise;

[0044] 400MHz~678MHz channel: Inductor name: main inductor, inductor specifications: 2.4mm winding bar, copper wire diameter: 0.45mm, number of turns: 6, tap positions: 2, quantity: 1, winding direction: counterclockwise;

[0045] Input / output channels: Inductor specification T10-0, copper wire diameter 0.17mm, number of turns 1, quantity 2, winding direction counterclockwise;

[0046] 3.3) Using leaded solder wire Sn40 / Pb60, 0.5mm, solder the inductors of different channels in step 3.2) to the corresponding positions on the PCB board according to the pad diagram;

[0047] 3.4) Assemble the semi-finished product that has been welded in step 3.3) onto the test fixture, adjust the filter inductor to achieve the set performance indicators, and then fix it with Nanda 704 silicone.

[0048] 3.5) Use a metal casing and leaded tin wire (Sn40 / Pb60), 0.5mm, for sealing;

[0049] 3.6) After sealing, conduct tests and electrical performance index tests to generate data reports. Then, remove the prepared electrically tunable filter products from the test fixture, clean them, perform external inspections, and package them to form finished products.

[0050] Furthermore, the circuit of the 108MHz-678MHz high-suppression, low-loss electrically tunable filter prepared above includes circuit structures for a 108MHz-225MHz tuning filter, a 225MHz-400MHz tuning filter, a 108MHz-225MHz tuning filter, and a 108MHz-678MHz through-path. One end of the circuit structures for the 108MHz-225MHz tuning filter, the 225MHz-400MHz tuning filter, the 108MHz-225MHz tuning filter, and the 108MHz-678MHz through-path is connected to an RF switch U1. The circuit structures of the MHz tuned filter, the 225MHz to 400MHz tuned filter, and the 108MHz to 225MHz tuned filter are respectively connected in series with LTCC low-pass filters LFCN3216-190A01, LFCN3216-420A01, and LFCN3216-575A01. The other end of the circuit structure of the LTCC low-pass filters LFCN3216-190A01, LFCN3216-420A01, and LFCN3216-575A01 and the 108MHz to 678MHz straight-through path is connected to the RF switch U2.

[0051] Furthermore, the circuit structures of the 108MHz–225MHz tuning filter, the 225MHz–400MHz tuning filter, the 108MHz–225MHz tuning filter, and the 108MHz–678MHz direct path all include inductors L1, L2, L3, L4, L5, L6, capacitors C1 and C2; inductors L5, L1, L2, and L6 are connected in series, and inductor L... 5. The other end is connected to RF switch U1. Inductor L6 is connected directly or in series with an LTCC low-pass filter and then connected to RF switch U2. One end of the series connection between inductor L5 and inductor L1 is connected to one end of inductor L3. The other end of inductor L3 is connected to one end of capacitor C1. One end of the series connection between inductor L2 and inductor L6 is connected to one end of inductor L4. The other end of inductor L4 is connected to one end of capacitor C2. The other ends of capacitor C1, capacitor C2, and the end of the series connection between inductor L1 and inductor L2 are grounded.

[0052] The components in an electrically tunable filter are divided into two parts: capacitors and inductors. When the inductance remains constant, capacitors C1 and C2 are varactor diodes. Based on the characteristics of varactor diodes, a tunable reverse bias voltage VT is provided to the varactor diodes to change the capacitance value, thereby changing the resonant frequency and achieving the purpose of frequency conversion. The details are as follows:

[0053] 1.1 In the 108MHz~225MHz tuned filter circuit:

[0054] 1) L5: Uses magnetic core T10-0, wound 5 turns clockwise with 0.17mm enameled wire;

[0055] 2) L6: Uses magnetic core T10-0, wound 5 turns clockwise with 0.17mm enameled wire;

[0056] 3) L3, L1, L2, and L4 are actually implemented as a single hollow coil: with a winding diameter of 1.8mm, and 18 turns counterclockwise using 0.35mm enameled wire.

[0057] From left to right, the hollow coils are arranged as follows: L3 takes 6 turns, L1 takes 3 turns, L2 takes 3 turns, and L4 takes 6 turns.

[0058] 4) C1, C2 (detailed circuit diagram as follows) Figure 11 The output voltage VT is a varactor diode, which controls each group of varactor diodes by outputting a corresponding tuning voltage VT, thereby changing the capacitance of each group and thus changing the resonant frequency to achieve the purpose of frequency conversion.

[0059] 1.2 225MHz~400MHz Tuned Filter:

[0060] 1) L5: Uses magnetic core T10-0, wound 2 turns clockwise with 0.17mm enameled wire;

[0061] 2) L6: Uses magnetic core T10-0, with 0.17mm enameled wire wound clockwise twice;

[0062] 3) L3, L1, L2, and L4 are actually implemented as a single hollow coil: with a winding diameter of 2.4mm, and 10 turns counterclockwise using 0.4mm enameled wire.

[0063] From left to right, the hollow coils are arranged as follows: L3 takes 4 turns, L1 takes 1 turn, L2 takes 1 turn, and L4 takes 4 turns.

[0064] 4) C1, C2 (detailed circuit diagram as follows) Figure 12 The output voltage VT is a varactor diode, which controls each group of varactor diodes by outputting a corresponding tuning voltage VT, thereby changing the capacitance of each group and thus changing the resonant frequency to achieve the purpose of frequency conversion.

[0065] 1.3 400MHz~678MHz Tuned Filter:

[0066] 1) L5: Uses magnetic core T10-0, with 0.17mm enameled wire wound clockwise once;

[0067] 2) L6: Use magnetic core T10-0, and wind 0.17mm enameled wire clockwise once;

[0068] 3) L3, L1, L2, and L4 are actually implemented as a single hollow coil: with a winding diameter of 2.4mm, and 6 turns counterclockwise using 0.4mm enameled wire.

[0069] From left to right, the hollow coils are arranged as follows: L3 takes 2 turns, L1 takes 1 turn, L2 takes 1 turn, and L4 takes 2 turns.

[0070] 4) C1, C2 (detailed circuit diagram as follows) Figure 13 The output voltage VT is a varactor diode, which controls each group of varactor diodes by outputting a corresponding tuning voltage VT, thereby changing the capacitance of each group and thus changing the resonant frequency to achieve the purpose of frequency conversion.

[0071] 1.4 108MHz~678MHz direct path

[0072] The 108MHz to 678MHz pass-through path is achieved by a single trace on a PCB. The product PCB is a four-layer board, and an 8.3mil trace width is used on the third layer to achieve a 50-ohm impedance trace, thus achieving a 108MHz to 678MHz pass-through path.

[0073] The LTCC low-pass filters LFCN3216-190A01, LFCN3216-420A01, and LFCN3216-575A01 are all manufactured by Guiyang Shunluo Xunda Electronics Co., Ltd., and belong to known technology. They are used in the post-stage of each filter to improve the far-end suppression performance. The specific parameters are shown in Table 3 below.

[0074]

[0075] Furthermore, the aforementioned inductors L4, L1, L2, and L3 all use the enameled wire of a single hollow coil, which is soldered to different positions on the pads to obtain four inductors, resulting in a compact structure and low cost.

[0076] Table 3 Circuit Performance Parameters

[0077]

[0078] Conventional electrically tunable filters, for the same size, cannot meet the following requirements: insertion loss less than or equal to 8dB for 108MHz to 225MHz, insertion loss less than or equal to 5.5dB for 225MHz to 678MHz, 1.5dB relative bandwidth greater than or equal to 5MHz for 225MHz to 678MHz, 0.5dB relative bandwidth greater than or equal to 8MHz for 400MHz to 678MHz, while simultaneously achieving ±10% suppression of ≥20dBc for any center frequency in 108MHz to 225MHz, ±6% suppression of ≥20dBc for any center frequency in 225MHz to 678MHz, ±10% suppression of ≥29dBc, and far-end suppression at twice the center frequency greater than or equal to 30dBc.

[0079] The main magnetic ring inductor of the electrically tunable filter prepared in this invention uses an air-core coil wound with enameled wire, which makes the Q value higher than that of conventional inductors. At the same time, an LTCC low-pass filter is connected in series after the filter, which improves the relative bandwidth by 1.5dB, the relative bandwidth by 0.5dB, the suppression of ±6% and ±10% of the center frequency, and the suppression at twice the center frequency while ensuring the filter insertion loss.

[0080] like Figure 7 As shown, the test fixture includes a PCB board with sixteen solder pads, which correspond to a total of sixteen terminals on the left and right sides of the electronically tunable filter.

[0081] The test fixture solders the electrically tunable filter onto the pads on the PCB board, enabling reliable and stable testing of the new customized electrically tunable filter. Data testing is accurate, and the soldered pins are fixed for testing. After testing, the solder joints can be loosened using a soldering gun or similar method without damaging the pins. The test fixture directly solders the grounding pin to the pad, eliminating the need for the product's bottom surface, thus avoiding any impact on the product's functionality caused by soldering grounding pins during testing. This effectively solves the following problems associated with existing technologies that use connecting cables to connect the power supply and RF terminals for testing:

[0082] 1) Because there are too many connecting wires, it is easy to make mistakes and also easy to lead to poor test accuracy;

[0083] 2) There is a risk of accidentally tearing off the product's pins;

[0084] 3) The RF cable connecting RFin and RFout needs to be grounded. If the test fixture is not used, the bottom of the product will be grounded, which will affect the appearance of the product and most of the solder, thus affecting the functionality of the product.

[0085] To quickly locate the electrically tunable filter, a limit indicator frame is drawn on the PCB board. The limit indicator frame is larger than the electrically tunable filter, and six solder pads are arranged on the left, top, and right sides of the limit indicator frame. The limit indicator frame can quickly limit the electrically tunable filter, thereby quickly soldering the terminals and improving soldering and testing efficiency.

[0086] To prevent the electrically tunable filter from shaking during soldering, two rows of limiting rolling pins (tangent to the outer side of the limiting indicator frame) are symmetrically arranged on the front and back sides of the upper surface of the PCB board. There are four limiting rolling pins in total, symmetrical in front, back, left and right. The two rows of limiting rolling pins can limit the front and back ends of the electrically tunable filter. When in use, the electrically tunable filter is quickly placed into the two rows of limiting rolling pins and moved left and right to the corresponding soldering pad of the terminal. Before soldering, it can quickly position the filter and prevent shaking during soldering, making the soldering more stable and reliable.

[0087] The working principle of the test fixture:

[0088] 1. Solder and fix the two RF cables to the respective positions. Figure 7 At the "RFin" and "RFout" pins in the PCB;

[0089] 2. Align the pins 1 to 7 of the electronically tunable filter under test with the solder pads on the test fixture, and then fix the pins 1 to 7 with leaded solder wire Sn40 / Pb60, 0.5mm.

[0090] 3. Connect the “RFin” RF cable to the output port of the network analyzer, and the “RFout” RF cable to the input port of the network analyzer;

[0091] 4. Figure 7 The 22V power supply terminal connected to the "VT" pin is used for voltage tuning from 1 to 22V to achieve the required frequency.

[0092] By soldering the electrically tunable filter onto the test fixture pads, reliable and stable testing of the new customized electrically tunable filter can be performed. The data test is accurate, and the soldered pins are fixed for testing. After the test is completed, the solder joints can be loosened by means of a soldering gun, without damaging the pins. The test fixture can be used to directly solder the grounding pin to the pad, eliminating the need to use the bottom of the product, thus avoiding any impact on the product's functionality caused by soldering grounding pins during testing.

[0093] The above description is merely a specific embodiment of this utility model, but the protection scope of this utility model is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this utility model should be included within the protection scope of this utility model. Therefore, the protection scope of this utility model should be determined by the protection scope of the claims.

Claims

1. A circuit structure for a high-suppression, wide-bandwidth electrically tunable filter with a frequency range of 108MHz to 678MHz, characterized in that, This includes circuit structures for 108MHz–225MHz tuning filters, 225MHz–400MHz tuning filters, and a 108MHz–678MHz direct path. One end of each of these circuit structures is connected to an RF switch U1. The circuit structure of the oscillator and the circuit structure of the 108MHz~225MHz tuned filter are respectively connected in series with LTCC low-pass filters LFCN3216-190A01, LTCC low-pass filters LFCN3216-420A01 and LTCC low-pass filters LFCN3216-575A01. The other end of the circuit structure of LTCC low-pass filters LFCN3216-190A01, LTCC low-pass filters LFCN3216-420A01 and LTCC low-pass filters LFCN3216-575A01 and the 108MHz~678MHz direct path is connected to the RF switch U2. The main magnetic ring inductor of the electrically tunable filter uses an air coil wound with enameled wire.

2. The circuit structure of a 108MHz~678MHz high-suppression, wide-bandwidth electrically tunable filter according to claim 1, characterized in that: The circuit structures for the 108MHz–225MHz tuned filter, the 225MHz–400MHz tuned filter, the 108MHz–225MHz tuned filter, and the 108MHz–678MHz direct-through path all include inductors L1, L2, L3, L4, L5, L6, capacitors C1 and C2; inductors L5, L1, L2, and L6 are connected in series, with the other end of inductor L5... Connected to RF switch U1, inductor L6 is directly connected or connected in series with an LTCC low-pass filter and then connected to RF switch U2. One end of the series connection between inductor L5 and inductor L1 is connected to one end of inductor L3, and the other end of inductor L3 is connected to one end of capacitor C1. One end of the series connection between inductor L2 and inductor L6 is connected to one end of inductor L4, and the other end of inductor L4 is connected to one end of capacitor C2. The other ends of capacitor C1, capacitor C2, and the series connection between inductor L1 and inductor L2 are grounded.

3. The circuit structure of a 108MHz~678MHz high-suppression, wide-bandwidth electrically tunable filter according to claim 2, characterized in that, Inductors L4, L1, L2, and L3 all use the enameled wire of a single air-core coil, and are soldered to different positions on the pads to obtain four inductors.

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