Circuit structure of electrically tunable filter with low insertion loss

Abstract

The utility model discloses a circuit structure of a low-insertion-loss electrically tunable filter, which comprises a first filter circuit, a second filter circuit, a third filter circuit, a radio frequency switch U1 and a radio frequency switch U2, the two ends of the first filter circuit, the two ends of the second filter circuit and the two ends of the third filter circuit are connected to the radio frequency switch U1 and the radio frequency switch U2 respectively, the first filter circuit is a circuit of a model of 30-90 MHz and 90-225 MHz, the second filter circuit is a circuit of a model of 225-650 MHz, 650-1200 MHz, 1200-1650 MHz and 1650-2100 MHz, the third filter circuit is a circuit of a model of 2100-2500 MHz, 225-650 MHz, 650-1200 MHz, 1200-1650 MHz and 1650-2100 MHz, and the radio frequency switch U2 is a circuit of a model of 2100-2500 MHz. A hollow coil is used for manufacturing an inductor at the 2100-2500MHz high frequency band, the Q value of the product is improved, and the larger the Q value is, the better the indexes of the product are; and the size and the cost are saved.

Description

Technical Field

[0001] The utility model relates to a circuit structure of a low insertion loss electrically tunable filter, belonging to the technical field of electrically tunable filters. Background Art

[0002] As Figures 11 - 13 The shown electrically tunable filter includes a housing, coils encapsulated in the housing, and pins provided at the bottom of the housing. The pin definitions are shown in the following table.

[0003] Pin Definition 1 GND 2 VT 3～10 GND 11 RFIN 12 RFOUT 13～18 GND 19 +3.3V 20 A0 21 A1 22 A2

[0004] For the above-mentioned electrically tunable filter prepared by conventional methods, it is difficult to improve the product Q value, and at the same time, the cost is high and the size is large. Summary of the Invention

[0005] The technical problem to be solved by the utility model is to provide a circuit structure of a low insertion loss electrically tunable filter to solve the problems existing in the above-mentioned prior art.

[0006] The technical solution adopted by the utility model is: a circuit structure of a low insertion loss electrically tunable filter, including a first path filter circuit, a second path filter circuit, a third path filter circuit, a radio frequency switch U1 and a radio frequency switch U2. The two ends of the first path filter circuit, the second path filter circuit and the third path filter circuit are respectively connected to the radio frequency switch U1 and the radio frequency switch U2. The first path filter circuit is a circuit of 30 - 90 MHz and 90 - 225 MHz models. The second path filter circuit is a circuit of 225 - 650 MHz, 650 - 1200 MHz, 1200 - 1650 MHz and 1650 - 2100 MHz models. The third path filter circuit is a circuit of 2100 - 2500 MHz model. For the high frequency bands of 225 - 650 MHz, 650 - 1200 MHz, 1200 - 1650 MHz, 1650 - 2100 MHz, and 2100 - 2500 MHz, air-core coils are used to make inductors.

[0007] Further, the radio frequency pin lead-out wires of the radio frequency switches U1 and U2 arranged on the above-mentioned PCB circuit board adopt 50-ohm impedance wiring on the first layer and the third layer.

[0008] Further, the filters arranged on the above-mentioned PCB circuit board adopt a left-right symmetric layout.

[0009] Further, the first filter circuit includes an inductor LIN, an inductor L19, an inductor L18, an inductor L17, an inductor L16, an inductor L15, an inductor L10, an inductor L11, an inductor LIN1, a capacitor C5, and a capacitor C3. The inductor LIN, the inductor L18, the inductor L17, the inductor L15, the inductor L16, the inductor L10, and the inductor LIN1 are connected in series in sequence. One end between the series connection of the inductor LIN and the inductor L18 is connected to one end of the inductor L19. One end between the series connection of the inductor L17 and the inductor L15 is connected to one end of the inductor L11. One end between the series connection of the inductor L18 and the inductor L17 is connected to one end of the capacitor C5. One end between the series connection of the inductor L16 and the inductor L10 is connected to one end of the capacitor C3. The other ends of the inductor L19, the inductor L11, the capacitor C5, and the capacitor C3 are grounded. The other ends of the inductor LIN and the inductor LIN1 are respectively connected to the RF switch U1 and the RF switch U2.

[0010] Further, the second filter circuit includes an inductor L5, an inductor L3, an inductor L1, an inductor L2, an inductor L4, an inductor L6, a capacitor C1, and a capacitor C2. The inductor L5, the inductor L1, the inductor L2, and the inductor L6 are connected in series in sequence. The other ends of the inductor L5 and the inductor L6 are respectively connected to the RF switch U1 and the RF switch U2. One end between the series connection of the inductor L5 and the inductor L1 is connected to one end of the inductor L3. The other end of the inductor L3 is connected to one end of the capacitor C1. One end between the series connection of the inductor L2 and the inductor L6 is connected to one end of the inductor L4. The other end of the inductor L4 is connected to one end of the capacitor C2. The other ends of the capacitor C1 and the capacitor C2 are grounded.

[0011] Further, the third filter circuit includes an inductor L5’, an inductor L3’, an inductor L1’, an inductor L2’, an inductor L4’, an inductor L6’, an inductor L7, an inductor L8, a capacitor C1’, and a capacitor C2’. The inductor L5’, the inductor L1’, the inductor L2’, and the inductor L6’ are connected in series in sequence. The other ends of the inductor L5’ and the inductor L6’ are respectively connected to the RF switch U1 and the RF switch U2. One end between the series connection of the inductor L5’ and the inductor L1’ is connected to one end of the inductor L3’. The other end of the inductor L3’ is connected to one end of the capacitor C1’. One end between the series connection of the inductor L2’ and the inductor L6’ is connected to one end of the inductor L4’. The other end of the inductor L4’ is connected to one end of the capacitor C2’. The other ends of the capacitor C1’ and the capacitor C2’ are grounded. One end of the inductor L7 is connected to one end between the connection of the inductor L3’ and the capacitor C1’. The other end of the inductor L7 is connected to one end of the inductor L8 and grounded. The other end of the inductor L8 is connected to one end between the connection of the inductor L4’ and the capacitor C2’.

[0012] Advantages of the present utility model: Compared with the prior art, the PCB circuit board of the present utility model adopts a large number of grounding vias to enhance the grounding effect. Moreover, the filters arranged on the PCB circuit board adopt a left-right symmetrical layout, making the parasitic capacitance and inductance generated by the left and right pads equal, which is convenient for the subsequent production of the product. The specific effects are as follows:

[0013] 1. The 30 - 2500 MHz frequency band is divided into 7 filters for implementation (30 - 90 MHz, 90 - 225 MHz, 225 - 650 MHz, 650 - 1200 MHz, 1200 - 1650 MHz, 1650 - 2100 MHz, 2100 - 2500 MHz);

[0014] 2. For the high - frequency bands of 225 - 650 MHz, 650 - 1200 MHz, 1200 - 1650 MHz, 1650 - 2100 MHz, and 2100 - 2500 MHz, air - core coils are used to make inductors to improve the Q - value of the product. The larger the Q - value, the better the various indicators of the product; at the same time, it saves size and cost;

[0015] 3. For 30 - 90 MHz and 90 - 225 MHz, a large inductance needs to be removed, and a magnetic core is required to achieve this;

[0016] 4. The product has a small size and is a 6 - layer board. The 50 - ohm microstrip line is routed on the first layer and the 50 - ohm strip line is routed on the third layer, reducing the connection loss between the RF ports of the RF switch and the input / output of the filter. Especially for the high - frequency part (1000 - 2500 MHz), a 50 - ohm RF line must be used for connection. Otherwise, it will have a great impact on the various indicators of the product and may cause the failure to achieve the indicators;

[0017] 5. For the filters in the 1650 - 2100 MHz and 2100 - 2500 MHz frequency bands, the air - core coil inductors are connected in parallel to reduce the inductance value, so that the filter frequencies reach 2100 MHz and 2500 MHz;

[0018] 6. The PCB base is docked with the metal shell to achieve the purpose of shielding external interference and at the same time reduce the size of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a schematic diagram of the layout of the first layer of the PCB circuit board;

[0020] Figure 2 is a schematic diagram of the layout of the second layer of the PCB circuit board;

[0021] Figure 3 is a schematic diagram of the layout of the third layer of the PCB circuit board;

[0022] Figure 4 is a schematic diagram of the layout of the fourth layer of the PCB circuit board;

[0023] Figure 5 It is a schematic diagram of the layout of the fifth layer of the PCB circuit board;

[0024] Figure 6 It is a schematic diagram of the circuit principle of the electrically tunable filter;

[0025] Figure 7 It is a schematic diagram of the circuit structure of the first filter of the electrically tunable filter;

[0026] Figure 8 It is a schematic diagram of the circuit structure of the second filter of the electrically tunable filter;

[0027] Figure 9 It is a schematic diagram of the circuit structure of the third filter of the electrically tunable filter;

[0028] Figure 10 It is a PCB layout diagram of the test fixture;

[0029] Figure 11 It is a schematic diagram of the top view structure of the electrically tunable filter;

[0030] Figure 12 It is a schematic diagram of the side view structure of the electrically tunable filter;

[0031] Figure 13 It is a schematic diagram of the bottom view structure of the electrically tunable filter. Specific implementation mode

[0032] The following further introduces the present utility model in conjunction with the accompanying drawings and specific embodiments.

[0033] Embodiment 1: As Figures 6 - 9 shown, a circuit structure of a low insertion loss electrically tunable filter includes a first path filter circuit, a second path filter circuit, a third path filter circuit, a radio frequency switch U1, and a radio frequency switch U2. The two ends of the first path filter circuit, the second path filter circuit, and the third path filter circuit are respectively connected to the radio frequency switch U1 and the radio frequency switch U2. The first path filter circuit is a circuit with models of 30 - 90 MHz and 90 - 225 MHz, the second path filter circuit is a circuit with models of 225 - 650 MHz, 650 - 1200 MHz, 1200 - 1650 MHz, and 1650 - 2100 MHz, and the third path filter circuit is a circuit with a model of 2100 - 2500 MHz.

[0034] Further, as Figure 7As shown in the figure, the above-mentioned first filter circuit includes inductors LIN, L19, L18, L17, L16, L15, L10, L11, LIN1, capacitors C5 and C3. Inductors LIN, L18, L17, L15, L16, L10 and LIN1 are connected in series in sequence. One end between the series connection of inductors LIN and L18 is connected to one end of inductor L19. One end between the series connection of inductors L17 and L15 is connected to one end of inductor L11. One end between the series connection of inductors L18 and L17 is connected to one end of capacitor C5. One end between the series connection of inductors L16 and L10 is connected to one end of capacitor C3. The other ends of inductors L19, L11, capacitor C5 and capacitor C3 are grounded. The other ends of inductors LIN and LIN1 are respectively connected to radio frequency switches U1 and U2.

[0035] Further, as Figure 8 shown in the figure, the above-mentioned second filter circuit includes inductors L5, L3, L1, L2, L4, L6, capacitors C1 and C2. Inductors L5, L1, L2 and L6 are connected in series in sequence. The other ends of inductors L5 and L6 are respectively connected to radio frequency switches U1 and U2. One end between the series connection of inductors L5 and 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 between the series connection of inductors L2 and 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 capacitors C1 and C2 are grounded.

[0036] Further, as Figure 9 shown in the figure, the above-mentioned third filter circuit includes inductors L5’, L3’, L1’, L2’, L4’, L6’, L7, L8, capacitors C1’ and C2’. Inductors L5’, L1’, L2’ and L6’ are connected in series in sequence. The other ends of inductors L5’ and L6’ are respectively connected to radio frequency switches U1 and U2. One end between the series connection of inductors L5’ and 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 between the series connection of inductors L2’ and 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 capacitors C1’ and C2’ are grounded. One end of inductor L7 is connected to the connection end between inductor L3’ and capacitor C1’. The other end of inductor L7 is connected to one end of inductor L8 and grounded. The other end of inductor L8 is connected to the connection end between inductor L4’ and capacitor C2’.

[0037] Embodiment 2: As Figures 1 - 13As shown in the figure, a low insertion loss electrically tunable filter includes a PCB circuit board 1 and a metal housing 2. The PCB circuit board 1 arranges the electrical components of the electrically tunable filter. The PCB circuit board has six layers. Each layer of the PCB circuit board is grounded through a large area. In the space outside the component pads and traces of the PCB circuit board, a large ground is laid. The traces on the first, third, and fifth layers of the PCB circuit board, and the second, fourth, and bottom layers are all complete grounds. The RF pin leads of the RF switches U1 and U2 arranged on the PCB circuit board are wired with a 50-ohm impedance on the first and third layers; the filters arranged on the PCB circuit board adopt a left-right symmetric layout.

[0038] Further, as Figure 6 shown in the figure, the circuit of the above-mentioned low insertion loss electrically tunable filter includes a first filter circuit, a second filter circuit, a third filter circuit, an RF switch U1, and an RF switch U2. The two ends of the first filter circuit, the second filter circuit, and the third filter circuit are respectively connected to the RF switch U1 and the RF switch U2. The first filter circuit is a circuit with models of 30 - 90 MHz and 90 - 225 MHz. The second filter circuit is a circuit with models of 225 - 650 MHz, 650 - 1200 MHz, 1200 - 1650 MHz, and 1650 - 2100 MHz. The third filter circuit is a circuit with a model of 2100 - 2500 MHz.

[0039] Further, as Figure 7 shown in the figure, the above-mentioned first filter circuit includes inductors LIN, L19, L18, L17, L16, L15, L10, L11, LIN1, capacitors C5, and C3. The inductors LIN, L18, L17, L15, L16, L10, and LIN1 are connected in series in sequence. One end between the series connection of LIN and L18 is connected to one end of L19. One end between the series connection of L17 and L15 is connected to one end of L11. One end between the series connection of L18 and L17 is connected to one end of C5. One end between the series connection of L16 and L10 is connected to one end of C3. The other ends of L19, L11, C5, and C3 are grounded. The other ends of LIN and LIN1 are respectively connected to the RF switch U1 and the RF switch U2.

[0040] Further, as Figure 8As shown in the figure, the second filter circuit described above includes inductors L5, L3, L1, L2, L4, L6, capacitors C1 and C2. Inductors L5, L1, L2, and L6 are connected in series in sequence. The other ends of inductors L5 and L6 are respectively connected to radio frequency switches U1 and U2. One end between the series connection of 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 between the series connection of 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 capacitors C1 and C2 are grounded.

[0041] Further, as Figure 9 shown in the figure, the third filter circuit described above includes inductors L5’, L3’, L1’, L2’, L4’, L6’, inductor L7, inductor L8, capacitors C1’ and C2’. Inductors L5’, L1’, L2’, and L6’ are connected in series in sequence. The other ends of inductors L5’ and L6’ are respectively connected to radio frequency switches U1 and U2. One end between the series connection of 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 between the series connection of 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 capacitors C1’ and C2’ are grounded. One end of inductor L7 is connected to the connection end between inductor L3’ and capacitor C1’. The other end of inductor L7 is connected to one end of inductor L8 and grounded. The other end of inductor L8 is connected to the connection end between inductor L4’ and capacitor C2’.

[0042] The electrically tunable filter has the following advantages:

[0043] The PCB circuit board uses a large number of ground vias to enhance the grounding effect. Moreover, the filters arranged on the PCB circuit board adopt a left-right symmetrical layout, making the parasitic capacitance and inductance generated by the left and right pads equal, which is convenient for the later production of the product. The remaining specific effects are as follows:

[0044] 1. The 30 - 2500 MHz frequency band is divided into 7 filters for implementation (30 - 90 MHz, 90 - 225 MHz, 225 - 650 MHz, 650 - 1200 MHz, 1200 - 1650 MHz, 1650 - 2100 MHz, 2100 - 2500 MHz);

[0045] 2. For the high-frequency bands of 225 - 650 MHz, 650 - 1200 MHz, 1200 - 1650 MHz, 1650 - 2100 MHz, and 2100 - 2500 MHz, air-core coils are used to make inductors to improve the Q value of the product. The larger the Q value, the better the various indicators of the product; at the same time, size and cost are saved.

[0046] 3. For the frequency bands of 30 - 90 MHz and 90 - 225 MHz, a larger inductance value is required, and a magnetic core is needed to achieve it.

[0047] 4. The product has a small size and is a 6-layer board. The 50-ohm microstrip line is routed on the first layer, and the 50-ohm strip line is routed on the third layer, reducing the connection loss between the RF ports of the RF switch and the input / output of the filter. Especially for the high-frequency part (1000 - 2500 MHz), a 50-ohm RF line must be used for connection, otherwise it will have a great impact on the various indicators of the product and may cause the indicators not to be achieved.

[0048] 5. For the filters in the frequency bands of 1650 - 2100 MHz and 2100 - 2500 MHz, air-core coil inductors are connected in parallel to reduce the inductance value, so that the filter frequency can reach 2100 MHz and 2500 MHz.

[0049] 6. The PCB base is used to dock with the metal shell to achieve the purpose of shielding external interference and reduce the size of the product at the same time.

[0050] Example 3: As Figures 1 - 13 shown, a manufacturing method of a 30 MHz - 2500 MHz low insertion loss electrically tunable filter, the method includes the following steps:

[0051] Step 1. PCB circuit board layout: The PCB circuit board has six layers. Each layer of the PCB circuit board is grounded through a large area (such as Figure 1 , Figure 2 , Figure 3 , Figure 4 , Figure 5 ). In the space outside the component pads and traces, a large ground is laid. The first, third, and fifth layers of the PCB circuit board are routed, and the second, fourth, and bottom layers are all complete grounds. The PCB circuit board uses a large number of grounding vias to increase the grounding effect. The grounding requirement for 2500 MHz high frequency is very strict. And the lead-out wires of the RF pins of the RF switches U1 and U2 arranged on the PCB circuit board are routed with a 50-ohm impedance on the first and third layers; the filters arranged on the PCB circuit board are arranged symmetrically left and right, so that the parasitic capacitance and inductance generated by the left and right pads are equal, facilitating the manufacture of the product in the later stage.

[0052] Step 2. The manufacturing method includes the following steps:

[0053] 2.1) After preparing the chip inductors and capacitors, solder them according to the circuit diagram of the electrically tunable filter. Use leaded solder paste Sn40Pb60 for SMT machine soldering and reflow soldering to obtain the semi-finished chip product;

[0054] 2.2) Prepare the corresponding inductors according to the parameter table in Table 1;

[0055]

[0056] For the high-frequency bands of 225 - 650 MHz, 650 - 1200 MHz, 1200 - 1650 MHz, 1650 - 2100 MHz, and 2100 - 2500 MHz, use air-core coils to make inductors;

[0057] 2.3) Use leaded solder wire Sn40 / Pb60, 0.5 mm to solder the inductors prepared in step 2.2) to the corresponding positions on the PCB circuit board according to the pad diagram;

[0058] 2.4) Assemble the semi-finished product onto the test fixture to test the insertion loss, 3 dB relative bandwidth, out-of-band rejection, and voltage standing wave ratio. Debug the filter inductors (by adjusting the size of the inductor, the position of the magnetic ring, and forming an LC match to optimize the filter performance) until the set conditions for optimal performance are met (insertion loss ≤ 4.5 dB, 3 dB relative bandwidth ≥ 5%, out-of-band rejection: center frequency ± 10% ≥ 8 dBc, voltage standing wave ratio ≤ 1.8), and then use Nanda 704 silicone for fixation;

[0059] 2.5) Seal the qualified semi-finished product with a metal shell. When sealing, use leaded solder wire Sn40 / Pb60, 0.5 mm for sealing;

[0060] 2.6) After sealing, conduct tests and electrical performance index tests to form a data report, and then remove the product from the test fixture, clean it, conduct external inspection, and package it to form a finished product.

[0061] Furthermore, as Figure 6 shown, the circuit of the 30 MHz - 2500 MHz low insertion loss electrically tunable filter prepared above includes a first path filter circuit, a second path filter circuit, a third path filter circuit, RF switch U1, and RF switch U2. The two ends of the first path filter circuit, the second path filter circuit, and the third path filter circuit are respectively connected to RF switch U1 and RF switch U2. The first path filter circuit is a circuit with models of 30 - 90 MHz and 90 - 225 MHz. The second path filter circuit is a circuit with models of 225 - 650 MHz, 650 - 1200 MHz, 1200 - 1650 MHz, and 1650 - 2100 MHz. The third path filter circuit is a circuit with a model of 2100 - 2500 MHz.

[0062] Further, as Figure 7 shown, the first filter circuit described above includes an inductor LIN, an inductor L19, an inductor L18, an inductor L17, an inductor L16, an inductor L15, an inductor L10, an inductor L11, an inductor LIN1, a capacitor C5, and a capacitor C3. The inductor LIN, the inductor L18, the inductor L17, the inductor L15, the inductor L16, the inductor L10, and the inductor LIN1 are connected in series in sequence. One end between the series connection of the inductor LIN and the inductor L18 is connected to one end of the inductor L19. One end between the series connection of the inductor L17 and the inductor L15 is connected to one end of the inductor L11. One end between the series connection of the inductor L18 and the inductor L17 is connected to one end of the capacitor C5. One end between the series connection of the inductor L16 and the inductor L10 is connected to one end of the capacitor C3. The other ends of the inductor L19, the inductor L11, the capacitor C5, and the capacitor C3 are grounded. The other ends of the inductor LIN and the inductor LIN1 are respectively connected to the RF switch U1 and the RF switch U2.

[0063] Further, as Figure 8 shown, the second filter circuit described above includes an inductor L5, an inductor L3, an inductor L1, an inductor L2, an inductor L4, an inductor L6, a capacitor C1, and a capacitor C2. The inductor L5, the inductor L1, the inductor L2, and the inductor L6 are connected in series in sequence. The other ends of the inductor L5 and the inductor L6 are respectively connected to the RF switch U1 and the RF switch U2. One end between the series connection of the inductor L5 and the inductor L1 is connected to one end of the inductor L3. The other end of the inductor L3 is connected to one end of the capacitor C1. One end between the series connection of the inductor L2 and the inductor L6 is connected to one end of the inductor L4. The other end of the inductor L4 is connected to one end of the capacitor C2. The other ends of the capacitor C1 and the capacitor C2 are grounded.

[0064] Further, as Figure 9 shown, the third filter circuit described above includes an inductor L5’, an inductor L3’, an inductor L1’, an inductor L2’, an inductor L4’, an inductor L6’, an inductor L7, an inductor L8, a capacitor C1’, and a capacitor C2’. The inductor L5’, the inductor L1’, the inductor L2’, and the inductor L6’ are connected in series in sequence. The other ends of the inductor L5’ and the inductor L6’ are respectively connected to the RF switch U1 and the RF switch U2. One end between the series connection of the inductor L5’ and the inductor L1’ is connected to one end of the inductor L3’. The other end of the inductor L3’ is connected to one end of the capacitor C1’. One end between the series connection of the inductor L2’ and the inductor L6’ is connected to one end of the inductor L4’. The other end of the inductor L4’ is connected to one end of the capacitor C2’. The other ends of the capacitor C1’ and the capacitor C2’ are grounded. One end of the inductor L7 is connected to one end of the connection between the inductor L3’ and the capacitor C1’. The other end of the inductor L7 is connected to one end of the inductor L8 and grounded. The other end of the inductor L8 is connected to one end of the connection between the inductor L4’ and the capacitor C2’.

[0065] The circuit performance parameters are as shown in Table 2 below:

[0066] Table 2 Circuit Performance Parameters

[0067]

[0068] The existing circuit of the 30 MHz - 2500 MHz low insertion loss electrically tunable filter is as Figure 7 shown. For the conventional 30 MHz - 2500 MHz electrically tunable filter, the insertion loss cannot be made less than or equal to 5 dB as a whole at 1 GHz - 2.5 GHz, greatly reducing the sensitivity of the filter.

[0069] The circuit connects two inductors in parallel (as Figure 9 ), while ensuring the filter performance, extends the frequency conversion frequency of the filter to 2.5 GHz, and maintains a low insertion loss throughout the 30 MHz - 2500 MHz band.

[0070] The prepared electrically tunable filter has the following effects:

[0071] The PCB circuit board uses a large number of ground vias to increase the grounding effect, and the filters arranged on the PCB circuit board adopt a left - right symmetric layout, making the parasitic capacitance and inductance generated by the left and right pads equal, facilitating the later production of the product; the remaining specific effects are as follows:

[0072] 1. The 30 - 2500 MHz frequency band is divided into 7 filters for implementation (30 - 90 MHz, 90 - 225 MHz, 225 - 650 MHz, 650 - 1200 MHz, 1200 - 1650 MHz, 1650 - 2100 MHz, 2100 - 2500 MHz);

[0073] 2. For the high - frequency bands of 225 - 650 MHz, 650 - 1200 MHz, 1200 - 1650 MHz, 1650 - 2100 MHz, and 2100 - 2500 MHz, air - core coils are used to make inductors to improve the Q - value of the product. The larger the Q - value, the better the product indicators; at the same time, it saves size and cost;

[0074] 3. For 30 - 90 MHz and 90 - 225 MHz, a larger inductance needs to be removed, and a magnetic core is required to achieve it;

[0075] 4. The product has a small size and is a 6 - layer board. The 50 - ohm microstrip line is routed on the first layer and the 50 - ohm strip line is routed on the third layer, reducing the connection loss between the RF ports of the RF switch and the input / output of the filter. Especially for the high - frequency part (1000 - 2500 MHz), a 50 - ohm RF line must be used for connection, otherwise it will have a great impact on the product indicators and may cause the indicators not to be achieved;

[0076] 5. Filters in the 1650 - 2100 MHz and 2100 - 2500 MHz frequency bands use parallel - connected air - core coil inductors to reduce the inductance value, so that the filter frequencies can reach 2100 MHz and 2500 MHz.

[0077] 6. High - Q varactor diodes (Q value 4500) are used in the 1650 - 2100 MHz and 2100 - 2500 MHz frequency bands.

[0078] 7. Low - loss RF switches are used, with an average loss of 0.4 dB, an average isolation of 31 dB, and an average VSWR of 1.1. This index has little impact on the filter and can improve the overall performance.

[0079] 8. A PCB base is used to dock with the metal shell, achieving the purpose of shielding external interference and reducing the size of the product at the same time.

[0080] As Figure 10 shown, the test fixture welds the electrically - tuned filter on the pads, enabling reliable and stable testing of the newly - customized electrically - tuned filter. The data testing is accurate, the welding of the fixed pins is tested, and after the test, the welding points are loosened by a soldering gun or other means without damaging the pins. The test fixture directly welds the grounding pins to the pads without using the bottom of the product, thus avoiding affecting the product's usage function due to the welding of the grounding pins at the bottom during the grounding - pin test; effectively solving the following problems existing in the prior art when using connecting cables to connect to the power - supply end for power supply and the RF end for testing:

[0081] 1) Due to too many connecting wires, it is easy to make mistakes and the test accuracy is poor.

[0082] 2) There is a risk of pulling off the product pins.

[0083] 3) The RF cables connecting RFin and RFout need to be grounded. Without the test fixture, the bottom of the product will be used for grounding, which will affect the product appearance and leave most of the solder, affecting the customer's usage function.

[0084] In order to quickly locate the electrically - tuned filter, a limit - indication frame is drawn in the middle of the PCB board. The limit - indication frame is larger than the electrically - tuned filter. A plurality of welding pads corresponding to the pins of the electrically - tuned filter are arranged on the left and right sides of the limit - indication frame (11 on each side, a total of 22). Through the limit - indication frame, the electrically - tuned filter can be quickly positioned, and then the welding terminals can be quickly carried out, improving the welding efficiency and the test efficiency.

[0085] To avoid the shaking of the electronically tunable filter during the soldering process, two rows of limiting rolling pins are symmetrically arranged on the front and rear sides of the upper surface of the above-mentioned PCB board (the limiting indication frame is tangent to the limiting rolling pins on the outside). There are a total of four limiting rolling pins in two rows, which are symmetric in the front, rear, left, and right. The two rows of limiting rolling pins can limit the front and rear ends of the electronically tunable filter. During use, quickly place the electronically tunable filter into the two rows of limiting rolling pins, move it left and right to the corresponding soldering pads of the terminals. Before soldering, it can quickly position. During the soldering process, it can avoid shaking and make the soldering more stable and reliable.

[0086] Principle of using the test fixture:

[0087] 1. Weld and fix two RF cables respectively at the Figure 9 "RFin" and "RFout" pins on the PCB of

[0088] 2. Correspond the pins 1 - 22 of the electronically tunable filter to be tested with the soldering pads on the test fixture, and then fix the pins 1 - 22 with leaded solder wire Sn40 / Pb60, 0.5mm.

[0089] 3. Connect the "RFin" RF cable to the output port of the network analyzer, and connect the "RFout" RF cable to the input port of the network analyzer.

[0090] 4. Figure 13 The 3.3V power supply terminal plug - in row connected to the "+3.3V" pin in

[0091] (1) When the "A0" pin is connected to a high level (1.3 - 2.7V), the "A1" pin is connected to a low level (0 - 0.45V), and the "A2" pin is connected to a low level (0 - 0.45V), the electronically tunable filter to be tested switches to the 30 - 90MHz frequency band, and then perform a voltage tuning of 1 - 23V on the 23V power supply terminal plug - in row 3 connected to the "+23V" pin in Figure 9 to reach the frequency to be measured.

[0092] (2) When the "A0" pin is connected to a low level (0 - 0.45V), the "A1" pin is connected to a high level (1.3 - 2.7V), and the "A2" pin is connected to a low level (0 - 0.45V), the electronically tunable filter to be tested switches to the 90 - 225MHz frequency band, and then perform a voltage tuning of 1 - 23V on the 23V power supply terminal plug - in row 3 connected to the "+23V" pin in Figure 9 to reach the frequency to be measured.

[0093] (3) When the "A0" pin is connected to a high level (1.3 - 2.7V), the "A1" pin is connected to a high level (1.3 - 2.7V), and the "A2" pin is connected to a low level (0 - 0.45V), the electronically tunable filter to be tested switches to the 225 - 650MHz frequency band, and then perform a voltage tuning of 1 - 23V on the 23V power supply terminal plug - in row 3 connected to the "+23V" pin in Figure 9The 23V power supply terminal plug-in row 3 connected to the "+23V" pin in [device name] is tuned for voltage from 1V to 23V to achieve the frequency to be measured;

[0094] (4) Connect the "A0" pin to a low level (0 - 0.45V), the "A1" pin to a low level (0 - 0.45V), and when the "A2" pin is connected to a high level (1.3 - 2.7V), the measured electronically tunable filter switches to the 2100 - 2500 MHz frequency band, and then Figure 9 The 23V power supply terminal plug-in row 3 connected to the "+23V" pin in [device name] is tuned for voltage from 1V to 23V to achieve the frequency to be measured;

[0095] (5) Connect the "A0" pin to a high level (1.3 - 2.7V), the "A1" pin to a low level (0 - 0.45V), and when the "A2" pin is connected to a high level (1.3 - 2.7V), the measured electronically tunable filter switches to the 1650 - 2100 MHz frequency band, and then Figure 9 The 23V power supply terminal plug-in row 3 connected to the "+23V" pin in [device name] is tuned for voltage from 1V to 23V to achieve the frequency to be measured;

[0096] (6) Connect the "A0" pin to a low level (0 - 0.45V), the "A1" pin to a high level (1.3 - 2.7V), and when the "A2" pin is connected to a high level (1.3 - 2.7V), the measured electronically tunable filter switches to the 650 - 1200 MHz frequency band, and then Figure 9 The 23V power supply terminal plug-in row 3 connected to the "+23V" pin in [device name] is tuned for voltage from 1V to 23V to achieve the frequency to be measured;

[0097] (7) Connect the "A0" pin to a high level (1.3 - 2.7V), the "A1" pin to a high level (1.3 - 2.7V), and when the "A2" pin is connected to a high level (1.3 - 2.7V), the measured electronically tunable filter switches to the 1200 - 1650 MHz frequency band, and then Figure 9 The 23V power supply terminal plug-in row 3 connected to the "+23V" pin in [device name] is tuned for voltage from 1V to 23V to achieve the frequency to be measured;

[0098] (8) Connect the "A0" pin to a low level (0 - 0.45V), the "A1" pin to a low level (0 - 0.45V), and when the "A2" pin is connected to a low level (0 - 0.45V), the measured electronically tunable filter switches to through - connection.

[0099] The above is only the specific implementation manner of the present utility model, but the protection scope of the present utility model is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present utility model can easily think of changes or substitutions, which should all be covered within the protection scope of the present utility model. Therefore, the protection scope of the present utility model should be subject to the protection scope of the claimed rights.

Claims

1. Circuit structure of a low insertion loss electrically tunable filter, characterized in that: It includes a first filter circuit, a second filter circuit, a third filter circuit, RF switch U1 and RF switch U2. Both ends of the first filter circuit, the second filter circuit and the third filter circuit are respectively connected to RF switch U1 and RF switch U2. The first filter circuit is a circuit with models of 30~90 MHz and 90~225 MHz. The second filter circuit is a circuit with models of 225~650 MHz, 650~1200 MHz, 1200~1650 MHz and 1650~2100 MHz. The third filter circuit is a circuit with a model of 2100~2500 MHz. For the high frequency bands of 225~650 MHz, 650~1200 MHz, 1200~1650 MHz, 1650~2100 MHz and 2100~2500 MHz, air-core coils are used to make inductors; Among them, the first filter circuit includes inductor LIN, inductor L19, inductor L18, inductor L17, inductor L16, inductor L15, inductor L10, inductor L11, inductor LIN1, capacitor C5 and capacitor C3. Inductor LIN, inductor L18, inductor L17, inductor L15, inductor L16, inductor L10 and inductor LIN1 are connected in series in turn. One end between the series connection of inductor LIN and inductor L18 is connected to one end of inductor L19. One end between the series connection of inductor L17 and inductor L15 is connected to one end of inductor L11. One end between the series connection of inductor L18 and inductor L17 is connected to one end of capacitor C5. One end between the series connection of inductor L16 and inductor L10 is connected to one end of capacitor C3. The other ends of inductor L19, inductor L11, capacitor C5 and capacitor C3 are grounded. The other ends of inductor LIN and inductor LIN1 are respectively connected to RF switch U1 and RF switch U2; The second filter circuit includes inductor L5, inductor L3, inductor L1, inductor L2, inductor L4, inductor L6, capacitor C1 and capacitor C2. Inductor L5, inductor L1, inductor L2 and inductor L6 are connected in series in turn. The other ends of inductor L5 and inductor L6 are respectively connected to RF switch U1 and RF switch U2. One end between the series connection of 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 between the series connection of 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 and capacitor C2 are grounded; The third filter circuit includes inductors L5’, L3’, L1’, L2’, L4’, L6’, L7, L8, capacitors C1’ and C2’. Inductors L5’, L1’, L2’ and L6’ are connected in series in sequence. The other ends of inductors L5’ and L6’ are respectively connected to RF switches U1 and U2. One end between the series connection of inductors L5’ and 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 between the series connection of inductors L2’ and 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 capacitors C1’ and C2’ are grounded. One end of inductor L7 is connected to the connection end between inductor L3’ and capacitor C1’. The other end of inductor L7 is connected to one end of inductor L8 and grounded. The other end of inductor L8 is connected to the connection end between inductor L4’ and capacitor C2’.

2. The circuit structure of a low insertion loss electrically tunable filter according to claim 1, wherein: The RF pin leads of RF switches U1 and U2 arranged on the PCB board are wired with a 50-ohm impedance on the first layer and the third layer.

3. The circuit structure of a low insertion loss electrically tunable filter according to claim 2, characterized in that: The filters arranged on the PCB board adopt a left-right symmetric layout.

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