A wide-band tunable bandpass filter based on LC elements

Abstract

The present invention designs a wide-band tunable bandpass filter based on LC elements by utilizing a combination of LC elements and varactor diodes. Within the tuning range, the insertion loss and return loss of the filter do not change significantly. By utilizing mutual inductance and parallel capacitance to enhance coupling, a transmission zero point is generated on the right side of the passband while increasing the tuning range, thereby improving stopband suppression and frequency selectivity.

Description

Technical Field

[0001] The present invention relates to a radio frequency communication electronic device, in particular to a bandpass filter. Background Art

[0002] In modern wireless communication systems, spectrum resource constraints, increasing system complexity, and the rapid development toward higher levels of integration are placing higher demands on RF circuits and microwave filters. Traditional filters typically operate within a fixed frequency band and cannot meet the dynamic frequency flexibility and bandwidth requirements of multi-standard, multi-function communication systems. In contrast, tunable filters, with their flexible ability to adjust frequency and bandwidth, are becoming an essential component of communication equipment. These filters are widely used in mobile communications, satellite communications, radar communications, and other fields, providing the required frequency selectivity and interference immunity in dense electromagnetic environments.

[0003] Tunable filters typically feature tunable center frequency, bandwidth, phase response, and filter switching. Their core tuning mechanism relies on the combined use of varactor diodes and PIN diodes, dynamically controlling the spectrum through changes in parameters such as capacitance and inductance. However, current tunable filters often experience significant degradation in insertion loss and return loss as the frequency tuning range expands, resulting in a decrease in overall performance and a limited tuning range. Therefore, developing a filter that can achieve stable center frequency tunability across a wide frequency band is of great significance.

[0004] Lumped LC element filters are widely used in modern RF communication equipment due to their simple structure, flexible design, low cost, and ease of integration. Advances in lumped element manufacturing processes have made it possible to create compact, high-performance tunable filters using LC components. These filters not only improve spectrum efficiency but also enable compatibility with multiple communication standards over a wider range. However, existing LC element tunable filters still face challenges in balancing tuning range and filter performance, particularly maintaining low insertion loss and high return loss within the required wide tuning range. Summary of the Invention

[0005] Purpose of the invention: In response to the above-mentioned existing technologies, a wide-band tunable bandpass filter based on LC elements is proposed to solve the problems of existing tunable filters such as limited tuning range within a wide frequency band, difficulty in maintaining stable insertion loss and return loss, and poor spectral selectivity.

[0006] Technical solution: A wide-band tunable bandpass filter based on LC elements. The bandpass filter structure is bilaterally symmetrical. From the left input end to the right, it includes inductor 1, inductor 2, inductor 3, inductor 4, and inductor 6 connected in series, with the other end of inductor 6 being grounded. A grounded capacitor 1 is connected in parallel between inductor 1 and inductor 2, a grounded capacitor 2 is connected in parallel between inductor 2 and inductor 3, and a varactor 1 is connected in parallel between inductor 3 and inductor 4, with the other end of varactor 1 being grounded.

[0007] Symmetrically, from the right output end to the left, the circuit includes inductor 10, inductor 9, inductor 8, inductor 5, and inductor 7 connected in series, with the other end of inductor 7 being grounded; grounded capacitor 5 is connected in parallel between inductor 10 and inductor 9, grounded capacitor 4 is connected in parallel between inductor 9 and inductor 8, and varactor 2 is connected in parallel between inductor 8 and inductor 5, with the other end of varactor 2 being grounded;

[0008] It also includes capacitor three, one end of which is connected in parallel between inductor four and inductor six, and the other end is connected in parallel between inductor five and inductor seven; there is mutual inductance M1 between inductor four and inductor five, and there is mutual inductance M2 between inductor six and inductor seven.

[0009] Furthermore, the center resonant frequency of the bandpass filter is tuned by tuning the capacitance values ​​of the first varactor and the second varactor.

[0010] Furthermore, by configuring the parameters of capacitor M3 and mutual inductances M1 and M2, the bandpass filter forms a transmission zero on the right side of the passband.

[0011] Furthermore, the inductance values ​​of inductor 1, inductor 3, inductor 10, and inductor 8 are the same, all L1; the inductance values ​​of inductor 2 and inductor 9 are the same, all L2; the inductance values ​​of inductor 4, inductor 5, inductor 6, and inductor 7 are the same, all L3; and the capacitance values ​​of capacitor 1, capacitor 2, capacitor 4, and capacitor 5 are the same, all C1.

[0012] Furthermore, the center frequency of the harmonic bandpass filter is 2 GHz, L1=0.4 nH, L2=8.3 nH, L3=0.6 nH, C1=1 pF, the capacitance value of capacitor three is C2=0.45 pF, M1=0.06 nH, M2=0.06 nH, and the capacitance tuning range of varactor one and varactor two is 1.2 pF-38.5 pF.

[0013] Beneficial effects: Existing tunable filters usually have problems of increased insertion loss and deteriorated return loss when the tuning range is expanded, which limits the stability of their performance. In addition, the complex tuning structure increases the difficulty and cost of filter design, and the spectrum selectivity and miniaturization are limited, making it difficult to meet the requirements of modern communication systems for flexible adjustment of high-frequency bandwidth, low loss and compact size. These technical defects urgently need to be overcome in order to achieve a tunable filter that is suitable for a wide frequency band, high efficiency and high integration. The present invention realizes a wide center frequency tunable bandpass filter based on LC elements. Within the tuning range, the insertion loss and return loss of the filter do not change significantly; the mutual inductance and parallel capacitance are used to enhance coupling, which increases the tuning range while also generating a transmission zero on the right side of the passband, thereby improving the stopband suppression and frequency selectivity.

[0014] Specifically, five series-connected inductors 101, 102, 103, and 104, a grounded inductor 106, and parallel-connected capacitors 201, 202, and a tunable capacitor 301 form a bandpass filter response, providing preliminary attenuation and selective filtering of high-frequency components in the input signal. This circuit structure is simple and compact, and is conducive to miniaturization and integration, with a relatively small number of circuit components.

[0015] Adding mutual inductances M1 and M2 between the inductor 104 and the inductor 105 , the ground inductor 106 , and the ground inductor 107 network increases coupling between signal paths, causing phase cancellation of signals at specific frequencies, thereby forming one or more transmission zeros out of band.

[0016] The capacitor 203 connected in parallel between the left and right circuits can further enhance the coupling and form a transmission zero together with the two pairs of mutual inductors. The depth of the transmission zero deepens as the center frequency shifts to the left, which can dynamically improve the stopband suppression and the frequency selectivity of the filter.

[0017] Loading the variable capacitor 301 between the inductor 103 and the inductor 104 realizes a wide-band center frequency tunable response, and within the tuning range, the insertion loss and the return loss do not decrease significantly, and the operating bandwidth does not change significantly. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The figure is a schematic diagram of the structure of a wide-band tunable bandpass filter based on LC elements;

[0019] Figure 2 A parameter diagram of a wide-band tunable bandpass filter based on LC elements;

[0020] Figure 3 is the variable container C in the embodiment t1 Varying S parameters, where (a) corresponds to the parameter |S 11 |, (b) corresponding parameters | S 21 |;

[0021] Figure 4 The out-of-band suppression effect of the variable capacitor C in the embodiment t1 's change curve. DETAILED DESCRIPTION

[0022] The present invention will be further explained below with reference to the accompanying drawings.

[0023] like Figure 1 As shown, a wide-band tunable bandpass filter based on LC elements has a bilaterally symmetrical structure. Looking from the left input end to the right, inductor 101, inductor 102 and inductor 103 are connected in series. A grounded capacitor is connected in parallel between adjacent inductors. Specifically, a grounded capacitor 201 is connected between inductor 101 and inductor 102, and a grounded capacitor 202 is connected in parallel between inductor 102 and inductor 103. Inductor 103, inductor 104 and inductor 106 are connected in series in sequence, and the other end of inductor 106 is grounded. A varactor 301 is connected in parallel between inductor 103 and inductor 104, and the other end of varactor 301 is grounded. Symmetrically, looking from the right output end to the left, inductor 110, inductor 109 and inductor 108 are connected in series in sequence, and grounded capacitors 205 and 204 are respectively connected between adjacent inductors. Inductor 108, inductor 105, and grounded inductor 107 are connected in series, with grounded varactor 302 incorporated between inductor 108 and inductor 105. Furthermore, capacitor 203 has one end connected in parallel between inductor 104 and inductor 106, and the other end connected in parallel between inductor 105 and inductor 107. Mutual inductance M1 exists between inductor 104 and inductor 105, and M2 exists between inductor 106 and inductor 107.

[0024] The signal enters the input port and passes through five series-connected inductors 101, 102, 103, and 104, as well as a grounded inductor 106, and parallel capacitors 201, 202, and tunable capacitor 301 to form a basic bandpass filter response, which performs preliminary attenuation and selective filtering on the high-frequency components in the input signal. The signal passes through the mutual inductance effect between the inductor 104 and the inductor 105, the grounded inductor 106, and the grounded inductor 107 network, which increases the coupling between the signal paths, causing the signal to cancel out of phase at a specific frequency, thereby forming one or more transmission zeros outside the band. The formation of transmission zeros increases the attenuation of out-of-band signals and enhances the filter's stopband suppression capability. Capacitor 203 is connected in parallel between the left and right circuits. Under appropriate capacitor 203 and mutual inductance M1 and M2 parameter configurations, the circuit forms a transmission zero on the right side of the passband, further suppressing out-of-band signals and enhancing frequency selectivity. Finally, the signal is output through the output circuit port. Due to the circuit's complete left-right symmetry, phase consistency and impedance matching are ensured throughout the signal transmission path, maintaining the filter's low insertion loss and stable frequency response. The center resonant frequency can be tuned by adjusting the capacitance of variable capacitors 301 and 302.

[0025] In this embodiment, an ultra-wide center frequency tunable bandpass filter based on LC elements with a center frequency of 2 GHz is designed and simulated and verified. Figure 2 This is a parameter diagram of a bandpass filter operating at a center frequency of 2 GHz in this embodiment, wherein the inductance values ​​of inductor 1 101, inductor 3 103, inductor 10 110, and inductor 8 108 are consistent, all L1; the inductance values ​​of inductor 2 102 and inductor 9 109 are consistent, all L2; the inductance values ​​of inductor 4 104, inductor 5 105, inductor 6 106, and inductor 7 107 are consistent, all L3; the capacitance values ​​of capacitor 1 201, capacitor 2 202, capacitor 4 204, and capacitor 5 205 are consistent, all C1. Specific parameters are: L1 = 0.4 nH, L2 = 8.3 nH, L3 = 0.6 nH, C1 = 1 pF, the capacitance value of capacitor 3 203 is C2 = 0.45 pF, M1 = 0.06 nH, M2 = 0.06 nH, and the variable capacitor C t1 The tuning range is 1.2pF-38.5pF.

[0026] Figure 3 The center frequency variable capacitor C obtained by simulation in this embodiment t1Frequency response curve. The varactor's capacitance ranges from 1.2pF to 38.5pF, and the center frequency tuning range is from 0.75 to 3.2GHz, corresponding to a relative center frequency tuning range of 124%. As the capacitance increases, the filter's center frequency shifts leftward, while insertion loss remains consistently below 0.1dB and in-band return loss exceeds 16dB. Both insertion loss and return loss remain at good levels across the ultra-wide center frequency tuning range.

[0027] Figure 4 Given the |S in the range of 0-10GHz 21 |As the varactor changes, it can be seen that there is a transmission zero at a fixed position at 4GHz. t1 The center frequency is shifted to the left, the zero point is deepened, and the suppression of the high-stop band and the frequency selectivity are enhanced. In the 4-8GHz range, the out-of-band suppression is greater than 58dB, which has a very good out-of-band suppression effect.

[0028] This embodiment designs a wide-band tunable bandpass filter based on LC elements by utilizing a combination of LC elements and varactor diodes. This filter can maintain low insertion loss and stable return loss (greater than 16 dB) over a wide frequency range, achieve a tuning range of 124% relative to the center frequency, and simultaneously achieve high-frequency band suppression and good spectral selectivity. Furthermore, the filter has a simple and compact structure, meeting the requirements of modern communication equipment for miniaturization, integration, and high performance.

[0029] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A wide-band tunable bandpass filter based on LC elements, characterized in that: The bandpass filter structure is symmetrical on the left and right. From the input end on the left to the right, it includes inductor 1 (101), inductor 2 (102), inductor 3 (103), inductor 4 (104) and inductor 6 (106) connected in series in sequence, and the other end of inductor 6 (106) is grounded; grounded capacitor 1 (201) is connected in parallel between inductor 1 (101) and inductor 2 (102), grounded capacitor 2 (202) is connected in parallel between inductor 2 (102) and inductor 3 (103), and varactor 1 (301) is connected in parallel between inductor 3 (103) and inductor 4 (104), and the other end of varactor 1 (301) is grounded; Symmetrically, from the right output end to the left, it includes inductor ten (110), inductor nine (109), inductor eight (108), inductor five (105) and inductor seven (107) connected in series in sequence, and the other end of inductor seven (107) is grounded; grounded capacitor five (205) is connected in parallel between inductor ten (110) and inductor nine (109), grounded capacitor four (204) is connected in parallel between inductor nine (109) and inductor eight (108), and variable capacitor two (302) is connected in parallel between inductor eight (108) and inductor five (105), and the other end of variable capacitor two (302) is grounded; It also includes capacitor three (203), one end of which is connected in parallel between inductor four (104) and inductor six (106), and the other end of which is connected in parallel between inductor five (105) and inductor seven (107); there is a mutual inductance M1 between inductor four (104) and inductor five (105), and there is a mutual inductance M2 between inductor six (106) and inductor seven (107).

2. The wide-band tunable bandpass filter based on LC elements according to claim 1, characterized in that: The central resonant frequency of the bandpass filter is tuned by tuning the capacitance values ​​of the first varactor (301) and the second varactor (302).

3. The wide-band tunable bandpass filter based on LC elements according to claim 1 or 2, characterized in that: By configuring the parameters of capacitor three (203) and mutual inductances M1 and M2, the bandpass filter forms a transmission zero point on the right side of the passband.

4. The wide-band tunable bandpass filter based on LC elements according to claim 1 or 2, characterized in that: The inductance values ​​of inductor 1 (101), inductor 3 (103), inductor 10 (110), and inductor 8 (108) are the same, all L1; the inductance values ​​of inductor 2 (102) and inductor 9 (109) are the same, all L2; the inductance values ​​of inductor 4 (104), inductor 5 (105), inductor 6 (106), and inductor 7 (107) are the same, all L3; the capacitance values ​​of capacitor 1 (201), capacitor 2 (202), capacitor 4 (204), and capacitor 5 (205) are the same, all C1.

5. The wide-band tunable bandpass filter based on LC elements according to claim 4, characterized in that: The center frequency of the harmonic bandpass filter is 2 GHz, L1=0.4 nH, L2=8.3 nH, L3=0.6 nH, C1=1 pF, the capacitance value of capacitor three (203) is C2=0.45 pF, M1=0.06 nH, M2=0.06 nH, and the capacitance tuning range of varactor one (301) and varactor two (302) is 1.2 pF-38.5 pF.

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