A low-band filter with high rectangle degree and small flatness

Abstract

This utility model relates to the field of communications, specifically to a low-frequency band filter with high rectangularity and low flatness. It includes a bandpass filter and an amplitude equalizer connected to the bandpass filter. The bandpass filter includes an input inductor, a multi-order quasi-elliptic circuit, and an output inductor connected in sequence. The multi-order quasi-elliptic circuit includes multiple sets of parallel-grounded resonant units connected in sequence, each set being an inductor and capacitor connected in parallel to ground; parallel resonant units connected between every two sets of parallel-grounded resonant units, each parallel resonant unit being an inductor and capacitor connected in parallel; and inter-stage grounding capacitors connected between the parallel resonant units for adjusting in-band flatness. The amplitude equalizer includes two sets of high-band equalizers and one set of low-band equalizers connected in sequence. This utility model solves the problem that existing elliptic function structure filters cannot simultaneously satisfy high rectangularity and low in-band flatness.

Description

Technical Field

[0001] This utility model relates to the field of communications, specifically to a low-frequency band filter with high rectangularity and low flatness. Background Technology

[0002] In modern communication systems, the performance requirements for filters outputting intermediate frequency (IF) signals are becoming increasingly stringent. On the one hand, filters are required to have high sensitivity within a certain bandwidth to meet the needs of signal transmission and reception; on the other hand, higher requirements are also placed on the rectangularity and in-band flatness of the filters. Filters with high rectangularity can achieve a fast transition between the passband and stopband, effectively suppressing out-of-band interference, while filters with good in-band flatness can ensure stable signal transmission within the passband and reduce signal distortion.

[0003] However, existing elliptic function structure filters often require higher orders to achieve high rectangularity. While increasing the order improves rectangularity, it also leads to increased in-band insertion loss and excessive insertion loss in the filter's sidebands, resulting in excessive in-band flatness and making it difficult to simultaneously meet the requirements of high rectangularity and low flatness. For example, the 5G bandpass filter mentioned in reference document CN110649904A, although it achieves miniaturization and integration to some extent, does not provide an effective solution for the dual requirements of high rectangularity and low in-band flatness in communication systems.

[0004] Furthermore, many existing filter designs focus on optimizing specific performance characteristics, such as insertion loss, bandwidth, frequency stability, and integration, while neglecting the issue of reducing in-band flatness while maintaining high rectangularity. This makes it difficult for existing filters to achieve ideal performance in practical applications, especially in scenarios with extremely high requirements for signal quality and transmission stability.

[0005] To overcome these shortcomings in the prior art, this application proposes a filter with high rectangularity and low flatness in the low-frequency band. Utility Model Content

[0006] The purpose of this invention is to provide a filter with high rectangularity and low flatness in the low-frequency band, so as to solve the problem that filters in the prior art cannot simultaneously meet the requirements of high rectangularity and low in-band flatness.

[0007] To achieve the above objectives, the following technical solution is adopted.

[0008] A filter with high rectangularity and low flatness in the low-frequency band, characterized in that it comprises a bandpass filter and an amplitude equalizer connected to the bandpass filter.

[0009] Optionally, the bandpass filter includes an input inductor, a multi-order quasi-elliptic circuit, and an output inductor connected in sequence.

[0010] Optionally, the multi-order quasi-elliptic circuit includes,

[0011] Multiple sets of parallel grounded resonant units connected in sequence, each set consisting of an inductor and a capacitor connected to ground in parallel.

[0012] A parallel resonant unit is connected between every two sets of the parallel grounded resonant units, and the parallel resonant unit is an inductor and capacitor in parallel circuit;

[0013] Interstage grounding capacitors connected between the parallel resonant units are used to adjust in-band flatness.

[0014] Optionally, a grounding capacitor is provided between the input inductor and the signal input terminal, and a grounding capacitor is provided between the output inductor and the signal output terminal.

[0015] Optionally, the amplitude equalizer includes two sets of high-band equalizers and one set of low-band equalizers connected in sequence.

[0016] Optionally, both the high-end equalizer and the low-end equalizer are three-branch parallel structures, wherein the first branch is a resistor, the second branch is an inductor and a capacitor connected in series, and the third branch is two resistors connected in series, with one end of a parallel resonant circuit of an inductor and a capacitor connected between the two series resistors, and the other end of the parallel resonant circuit is grounded through a resistor.

[0017] Optionally, two sets of parallel resonant units are connected in series between every two sets of parallel grounding resonant units.

[0018] Optionally, the interstage capacitor is located between every two sets of parallel resonant units connected in series.

[0019] Optionally, the multi-order quasi-elliptic circuit is of order 13, that is, there are a total of 13 groups of parallel ground resonant units and parallel resonant units.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] This novel filter design combines a bandpass filter with an amplitude equalizer. The bandpass filter, with its unique circuit structure, enhances rectangularity, while the amplitude equalizer precisely controls the in-band amplitude characteristics. This allows the filter to maintain high rectangularity while reducing in-band flatness by more than half compared to traditional designs, and less than one-third that of conventional filters. This solves a key contradiction in existing technologies and provides a superior solution for filter selection in communication systems. It not only improves the filter's performance indicators but also significantly enhances its application effectiveness in practical communication scenarios, effectively improving signal transmission quality, reducing signal distortion, and meeting the high-performance requirements of modern communication systems.

[0022] By employing an inductor-capacitor connection architecture in the bandpass filter, its near-end suppression is significantly improved, laying a solid foundation for enhanced rectangularity. Precisely defining the configuration of parameters such as parallel ground resonant units, parallel resonant units, and interstage grounding capacitors optimizes the signal processing capabilities of the bandpass filter layer by layer. For example, the combined design of parallel ground resonant units and parallel resonant units effectively adjusts the filter's frequency response characteristics, giving it better selectivity and suppression within the passband. The introduction of interstage grounding capacitors further improves in-band flatness and reduces amplitude fluctuations during signal transmission. The multi-order quasi-elliptic circuit is 13th order, solidifying the advantageous parameter combination and ensuring stable filter output performance. This filter performs excellently in the 1MHz–150MHz frequency range, meeting the performance requirements of various communication systems and possessing broad application prospects and market competitiveness. Attached Figure Description

[0023] Figure 1 This is a circuit topology diagram of an embodiment of a low-frequency filter with high rectangularity and low flatness according to the present invention.

[0024] Figure 2 This is a topology diagram of a bandpass filter circuit according to an embodiment of a low-frequency filter with high rectangularity and low flatness.

[0025] Figure 3 This is a circuit topology diagram of an amplitude equalizer for a low-frequency filter embodiment with high rectangularity and low flatness according to the present invention.

[0026] Figure 4 The graph shows the prior art fluctuation frequency, bandwidth, rectangularity, and delay curves of a low-frequency filter embodiment with high rectangularity and low flatness according to this utility model.

[0027] Figure 5 The graph shows the fluctuation frequency, bandwidth, rectangularity, and delay of a low-frequency filter embodiment with high rectangularity and low flatness according to this utility model. Detailed Implementation

[0028] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0029] The following detailed description is exemplary and intended to provide further detailed explanation of the present invention. Unless otherwise specified, all technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. The terminology used in this invention is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to the present invention.

[0030] like Figure 1 As shown, this utility model relates to a filter with high rectangularity and low flatness in the low-frequency band, mainly used in the frequency range of 1MHz to 150MHz, aiming to solve the problem that existing elliptic function structure filters cannot simultaneously satisfy high rectangularity and low in-band flatness. The specific implementation of this filter will be described in detail below.

[0031] The filter mainly consists of a bandpass filter and an amplitude equalizer. The bandpass filter is responsible for selectively passing signals, ensuring that signals within the target frequency band can pass smoothly while suppressing interference signals from other frequency bands; the amplitude equalizer is used to adjust the amplitude response characteristics of the filter to achieve the dual goals of high rectangularity and low in-band flatness.

[0032] The bandpass filter employs a 13th-order quasi-elliptic circuit design. Its core structure comprises an input inductor, a multi-order quasi-elliptic circuit, and an output inductor connected sequentially. Specifically, the multi-order quasi-elliptic circuit consists of multiple sets of parallel-to-ground resonant units and parallel resonant units connected alternately. Each set of parallel-to-ground resonant units is an inductor and capacitor connected in parallel to ground, while the parallel resonant units are inductors and capacitors connected in parallel. Furthermore, stages of inter-stage grounding capacitors are connected between the parallel resonant units for fine-tuning in-band flatness. This design allows the filter to effectively reduce in-band ripple and improve signal transmission quality while maintaining high rectangularity.

[0033] An amplitude equalizer is connected to the bandpass filter. The amplitude equalizer consists of two sets of high-band equalizers and one set of low-band equalizers connected sequentially. Each set of high-band and low-band equalizers employs a three-branch parallel structure. The first branch is a resistor, used to provide signal attenuation; the second branch is an inductor and capacitor in series, used to generate impedance changes at specific frequency bands, thereby adjusting the signal amplitude; the third branch is two resistors in series, with an inductor and capacitor connected in parallel resonant circuit between them. One end of this parallel resonant circuit is connected between the two resistors, and the other end is grounded through a resistor. This three-branch parallel structure allows for precise control of the filter's amplitude response at different frequency bands, effectively improving in-band flatness.

[0034] To further optimize filter performance, specific brand components were used. Resistors were selected from the SAMSUNG RC1608 series, known for their excellent stability and accuracy. Capacitors were chosen from the Temex SHS251 series, a high-Q capacitor that effectively reduces energy loss and improves filter performance. Inductors were selected from the Micrometals T-series, featuring high inductance and low loss, which helps achieve high rectangularity and low in-band flatness in the filter.

[0035] In practical applications, a bandpass filter was designed and fabricated using a center frequency of 21.4 MHz as an example. This filter has a 0.5 dB bandwidth ≥ 6 MHz, a rectangularity coefficient (K = BW - 50 dB / BW - 0.5 dB) ≤ 1.5, and an in-band flatness (within 21.4 ± 3 MHz) ≤ 0.5 dB. In contrast, a conventionally designed 13th-order quasi-elliptic function bandpass filter, without an equalizer, has an in-band flatness (within 21.4 ± 3 MHz) of approximately 1.8 dB. Therefore, the filter proposed in this invention maintains high rectangularity while reducing in-band flatness to less than one-third of that of conventional filters, significantly improving product performance.

[0036] To illustrate the specific circuit connections for the filter, a detailed circuit description is provided below. First, the signal input section connects to an input inductor, which in turn connects to the starting point of a multi-stage quasi-elliptic circuit. This multi-stage quasi-elliptic circuit consists of multiple parallel-grounded resonant units and parallel resonant units connected sequentially. Each group of parallel-grounded resonant units comprises an inductor and a capacitor connected in parallel, for example, L1 and C1 connected in parallel, and then connected to the next unit. Parallel resonant units are composed of inductors and capacitors connected in parallel, such as the parallel circuit of L2 and C2, with one end connected to the previous stage and the other end connected to the next stage. Inter-stage grounding capacitors (such as C14) are connected between these parallel resonant units to adjust in-band flatness. After signal processing by the multi-stage quasi-elliptic circuit, the output inductor leads the signal to the amplitude equalizer.

[0037] like Figure 1 and Figure 2 As shown, firstly, the signal input terminal has its first input path connected to ground via C19. The second input path is connected to L14. The first path of L14 is connected to a parallel ground circuit of L1 and C1. The second path of L14 is connected to a parallel circuit of L2 and C2. The first output of this parallel circuit of L2 and C2 is connected to ground via C14. The second path of this parallel circuit of L2 and C2 is then connected to a parallel circuit of L3 and C3. The first output of this parallel circuit of L3 and C3 is connected to a parallel ground circuit of L4 and C4. The second output of this parallel circuit of L3 and C3 is connected to a parallel circuit of L5 and C5. The first output of this parallel circuit of L5 and C5 is connected to ground via C15. The second output of the parallel circuit of L5 and C5 is connected to a parallel circuit of L6 and C6. The first output of the parallel circuit of L6 and C6 is connected to a parallel circuit of C7 grounded and L7 grounded. The second output of the parallel circuit of L6 and C6 is connected to a parallel circuit of C8 and L8. The first output of the parallel circuit of C8 and L8 is connected to a C16 grounded. The second output of the parallel circuit of C8 and L8 is connected to a parallel circuit of C9 and L9. The first output of the parallel circuit of C9 and L9 is connected to a parallel circuit of C10 and L10 grounded. The second output of the parallel circuit of C9 and L9 is connected to a parallel circuit of C11 and L11. C11 and L1... The first output of the parallel circuit of 1 is connected to C17 and grounded. The second output of the parallel circuit of 1 is connected to a parallel circuit of C12 and L12. The first output of the parallel circuit of C12 and L12 is connected to a parallel circuit of C13 and L13 and grounded. The second output of the parallel circuit of C12 and L12 is connected to L15. The first output of L15 is connected to C18 and grounded. The second output of L15 is connected to the output terminal.

[0038] The amplitude equalizer is connected as follows: the input is connected to a high-end equalizer in one frequency band, then another high-end equalizer in series, and finally a low-end equalizer in series. The output is connected to subsequent circuitry or a load. The specific structure of each equalizer is shown in the three-branch parallel structure described above. For example, in the high-end equalizer, the resistor in the first branch provides basic attenuation, the inductor and capacitor in series in the second branch generate impedance changes at specific high frequencies, and the two resistors in series in the third branch, along with the parallel resonant circuit in the middle, are used to finely adjust the signal amplitude, ensuring that the signal amplitude in the high-end frequency band meets design requirements. The low-end equalizer mainly performs similar amplitude adjustments for the low-end frequency band to achieve flatness optimization across the entire passband.

[0039] like Figure 1 and Figure 3 As shown, the circuit of the amplitude equalizer is as follows: the input is connected to a high-end equalizer of a frequency band, then connected in series with another high-end equalizer of a frequency band, then connected in series with a low-end equalizer of a frequency band, and then output. The circuit diagrams of these three equalizers are all: a parallel circuit consisting of three branches. The first branch is a resistor, the second branch is an inductor connected in series with a capacitor, and the third branch is two resistors connected in series, with an inductor and a capacitor connected in parallel between the two resistors. Then the parallel circuit is connected to a resistor grounded.

[0040] For the circuit board design, RF-4 board material from Dongguan Shengyi brand was selected as the base material. This board material has good electromagnetic and processing properties, and can meet the application requirements of the filter in the frequency range of 1MHz to 150MHz. The external dimensions of the circuit board were designed according to the actual layout and wiring requirements to ensure reasonable component placement, short signal transmission paths, and reduced loss and interference.

[0041] The simulation results of this application are as follows: Figure 5 As shown, from Figure 5 It can be seen that compared to before Figure 4 The flatness of the filter curve without an equalizer is greatly improved, and it can simultaneously meet the requirements of high rectangularity and in-band flatness.

[0042] Based on the proposed solution, a bandpass filter with a center frequency of 21.4MHz, a bandwidth of ≥6MHz with a 0.5dB bandwidth, a rectangularity coefficient (K=BW-50dB / BW-0.5dB) ≤1.5, and an in-band flatness (@21.4±3MHz) ≤0.5dB was designed and manufactured. In contrast, existing conventionally designed bandpass filters without an equalizer have an in-band flatness (@21.4±3MHz) of approximately 1.8dB. This invention simultaneously guarantees rectangularity and ultra-low in-band flatness within a certain signal bandwidth, and the flatness of the filter is less than 1 / 3 of that of conventional filters, greatly improving product performance.

[0043] As is known from common technical knowledge, this utility model can be implemented through other embodiments that do not depart from its spirit or essential characteristics. Therefore, the disclosed embodiments described above are merely illustrative in all respects and are not the only ones. All modifications within the scope of this utility model or its equivalents are included in this utility model.

Claims

1. A filter with high rectangularity and low flatness in the low-frequency band, characterized in that, It includes a bandpass filter and an amplitude equalizer connected to the bandpass filter; the bandpass filter includes an input inductor, a multi-order quasi-elliptic circuit and an output inductor connected in sequence.

2. The filter with high rectangularity and low flatness in the low-frequency band according to claim 1, characterized in that, The multi-order quasi-elliptic circuit includes, Multiple sets of parallel grounding resonant units are connected in sequence, and each set of parallel grounding resonant units is an inductor and capacitor connected to ground in parallel; A parallel resonant unit is connected between every two sets of the parallel grounded resonant units, and the parallel resonant unit is an inductor and capacitor in parallel circuit; Interstage grounding capacitors connected between the parallel resonant units are used to adjust in-band flatness.

3. A filter with high rectangularity and low flatness in the low-frequency band according to claim 1, characterized in that, A grounding capacitor is also provided between the input inductor and the signal input terminal, and a grounding capacitor is also provided between the output inductor and the signal output terminal.

4. A filter with high rectangularity and low flatness in the low-frequency band according to claim 1, characterized in that, The amplitude equalizer consists of two sets of high-band equalizers and one set of low-band equalizers connected in sequence.

5. A filter with high rectangularity and low flatness in the low-frequency band according to claim 4, characterized in that, Both the high-end equalizer and the low-end equalizer of the frequency band have a three-branch parallel structure. The first branch is a resistor, the second branch is an inductor and a capacitor connected in series, and the third branch is two resistors connected in series. One end of the parallel resonant circuit of the inductor and capacitor is connected between the two series resistors, and the other end of the parallel resonant circuit is grounded through a resistor.

6. A filter with high rectangularity and low flatness in the low-frequency band according to claim 2, characterized in that, Two sets of parallel resonant units are connected in series between each pair of parallel grounding resonant units.

7. A filter with high rectangularity and low flatness in the low-frequency band according to claim 6, characterized in that, The indirect capacitors are located between every two sets of parallel resonant units connected in series.

8. A filter with high rectangularity and low flatness in the low-frequency band according to claim 2, characterized in that, The multi-order quasi-elliptic circuit is of order 13, meaning that there are a total of 13 sets of parallel grounding resonant units and parallel resonant units.

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