Three-order tuning low-pass filter

Abstract

The invention relates to the technical field of microwave radio frequency, in particular to a third-order tuning low-pass filter. Comprising an input coupling inductor L1, a first interstage coupling inductor L2, a second interstage coupling inductor L3, an output coupling inductor L4, a first tuning capacitor C1, a second tuning capacitor C2 and a third tuning capacitor C3, the first tuning capacitor C1, the second tuning capacitor C2 and the third tuning capacitor C3 are all adjustable capacitor arrays, and each tunable capacitor is composed of a plurality of capacitors and PIN diodes. By changing the control signal into a high level or a low level, the conduction or cut-off state of the PIN diode is changed, and then the equivalent capacitance value of the whole tuning capacitor is changed, so that the frequency of the filter is correspondingly changed, and different working frequency bands are obtained. The three-order tuning low-pass filter can effectively restrain third and more harmonic waves at the rear ends of the frequency hopping filter and the power amplifier, and the cut-off frequency of the three-order tuning low-pass filter can change along with control signals.

Description

Technical Field

[0001] The present invention relates to the field of microwave radio frequency technology, and particularly to a third-order tuned low-pass filter. Background Art

[0002] In the field of electronic communication, as a key component for signal processing, the basic function of a low-pass filter is to allow low-frequency signals to pass through unobstructed while effectively blocking high-frequency signals. In recent years, with the wide application of high-power devices such as high-power transmitters and 5G base stations, the communication system has put forward higher-level requirements for the performance of low-pass filters.

[0003] The output power of high-power transmitters continues to increase, and the signal output power of some power amplifiers has reached hundreds of watts or even thousands of watts. In this case, the low-pass filter not only needs to maintain stable operation under high-power conditions to ensure signal quality, but also needs to effectively cope with various challenges brought about by increased power. For high-power communication facilities such as 5G base stations, the high-power signals transmitted often contain complex and diverse interference signals, which requires the low-pass filter to have strong interference filtering capabilities to ensure the high-quality transmission of high-power signals and maintain the stable operation of the communication network.

[0004] In modern communication and electronic countermeasure systems, the harmonic output generated by high-power radio frequency transmitters will cause serious interference to surrounding electronic devices, greatly affecting the electromagnetic compatibility between subsystems and the overall performance of the system. To solve this problem, filters are introduced in microwave circuit design to enable the transmitter signal to pass through smoothly while effectively suppressing its multiple harmonic signals. At present, LC filters have been widely used in low-frequency high-power transmitters due to their advantages of simple design, easy implementation, and relatively far parasitic passbands. However, with the continuous development of technology, existing filters gradually expose some deficiencies when facing scenarios such as high power, complex interference, and high-frequency applications. For example, in a high-power environment, signal stability is difficult to guarantee, and performance attenuation is likely to occur due to power overload; the ability to suppress harmonics is limited, and it is difficult to meet the suppression requirements of multiple orders and wide frequency bands; when applied in the high-frequency band, the parasitic effects and passband characteristics of traditional filters limit the system spectrum utilization rate; in a complex electromagnetic environment, the anti-interference ability needs to be further improved. Therefore, it is necessary to improve the performance of existing low-pass filters. Summary of the Invention

[0005] The purpose of the present invention is to provide a third-order tuned low-pass filter. Based on a Chebyshev function low-pass filter and an adjustable capacitor array, this filter can effectively suppress the third and higher harmonics at the back end of the frequency hopping filter and the power amplifier, and further realize a third-order tuned low-pass filter whose cut-off frequency can vary with the control signal.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A third-order tuned low-pass filter includes an input coupling inductor L1, a first inter-stage coupling inductor L2, a second inter-stage coupling inductor L3, an output coupling inductor L4, a first tuning capacitor C1, a second tuning capacitor C2, and a third tuning capacitor C3;

[0008] The input coupling inductor L1 serves as the input end of the entire third-order tuned low-pass filter and is connected to the input-end radio frequency signal RF in , and the output end is sequentially connected to the first inter-stage coupling inductor L2, the second inter-stage coupling inductor L3, and the output coupling inductor L4. The output coupling inductor L4 serves as the output end of the entire third-order tuned low-pass filter and is used to output the RF out signal;

[0009] One end of the first tuning capacitor C1 is connected to the common connection point of the input coupling inductor L1 and the first inter-stage coupling inductor L2, and the other end is grounded; one end of the second tuning capacitor C2 is connected to the common connection point of the first inter-stage coupling inductor L2 and the second inter-stage coupling inductor L3, and the other end is grounded;

[0010] One end of the third tuning capacitor C3 is connected to the common connection point of the second inter-stage coupling inductor L3 and the output coupling inductor L4, and the other end is grounded.

[0011] Furthermore, the first tuning capacitor C1, the second tuning capacitor C2, and the third tuning capacitor C3 are all adjustable capacitor arrays, and the three have the same structure.

[0012] Furthermore, the first tuning capacitor C1 includes: a first capacitor, a second capacitor, a third capacitor, a fourth capacitor, a first PIN diode, a second PIN diode, a first resistor, and a second resistor;

[0013] One end of the first capacitor and one end of the second capacitor are connected together and then electrically connected to the common connection point of the input coupling inductor L1 and the first inter-stage coupling inductor L2;

[0014] The other end of the first capacitor is electrically connected to one end of the first resistor, one end of the second resistor, the negative electrode of the first PIN diode, and the positive electrode of the second PIN diode respectively;

[0015] The other end of the second capacitor is grounded;

[0016] The other end of the first resistor and the positive electrode of the first PIN diode are connected together and then electrically connected to one end of the third capacitor;

[0017] The other end of the second resistor and the negative electrode of the second PIN diode are connected together and then electrically connected to one end of the fourth capacitor;

[0018] The other ends of the third capacitor and the fourth capacitor are both grounded;

[0019] The power supply VCC is connected to the other end of the first resistor, the positive electrode of the first PIN diode, and one end of the third capacitor;

[0020] The external device is connected to the other end of the second resistor, the negative electrode of the second PIN diode, and one end of the fourth capacitor.

[0021] Furthermore, the capacitance values of the first tuning capacitor C1 and the third tuning capacitor C3 are the same.

[0022] The technical effects achieved by the present invention: It is used at the back end of a 108 - 174 MHz, 225 - 400 MHz frequency hopping filter and a power amplifier. The operating frequency band changes with the control signal to 108 - 174 MHz or 225 - 400 MHz. Since the first tuning capacitor C1, the second tuning capacitor C2, and the third tuning capacitor C3 are all adjustable capacitor arrays, and each adjustable tuning capacitor is composed of multiple capacitors and PIN diodes, by changing the control signal to a high level or a low level, the on - off state of the PIN diode is changed, and then the equivalent capacitance value of the entire tuning capacitor is changed, so that the frequency of the filter changes accordingly, obtaining different operating frequency bands. At the same time, in terms of the circuit structure, a Chebyshev - type low - pass filter design is adopted. Through the optimization of the pass - band ripple approaching the ideal low - pass characteristic, the signal energy loss in the pass - band is reduced to achieve low insertion loss; its impedance matching design and flat frequency response reduce signal reflection to ensure the standing - wave performance; the steep cut - off characteristic and the pole distribution characteristic enable it to rapidly attenuate in the out - of - band region, thereby obtaining high suppression ability. The insertion loss ≤ 0.8 dB, the standing - wave ≤ 1.6, and the third - harmonic out - of - band suppression 3f0 ≥ 40 dBc, where f0 is the center frequency of the operating frequency band of the 108 - 174 MHz, 225 - 400 MHz frequency hopping filter. Brief Description of the Drawings

[0023] Figure 1 It is the schematic diagram of the third - order tuning low - pass filter of the present invention.

[0024] Figure 2 It is the circuit diagram of the third - order tuning low - pass filter of the embodiment. Detailed Embodiments

[0027] The technical solution of the present invention will be described in detail below in conjunction with the drawings and embodiments. According to the following description, the advantages and features of the present invention will be clearer. It should be noted that the drawings are all in a very simplified form and use non - precise scales, only for conveniently and clearly assisting in explaining the purpose of the embodiments of the present invention.

[0028] Figure 1This is the schematic diagram of the third-order tuned low-pass filter of the present invention. As Figure 1 shown, a third-order tuned low-pass filter provided in this embodiment includes an input coupling inductor L1, a first inter-stage coupling inductor L2, a second inter-stage coupling inductor L3, an output coupling inductor L4, a first tuning capacitor C1, a second tuning capacitor C2, and a third tuning capacitor C3;

[0029] The input coupling inductor L1 serves as the input end of the entire third-order tuned low-pass filter and is connected to the input end radio frequency signal RF in , and the output end is sequentially connected to the first inter-stage coupling inductor L2, the second inter-stage coupling inductor L3, and the output coupling inductor L4. The output coupling inductor L4 serves as the output end of the entire third-order tuned low-pass filter and is used to output the RF out signal;

[0030] One end of the first tuning capacitor C1 is connected to the common connection point of the input coupling inductor L1 and the first inter-stage coupling inductor L2, and the other end is grounded; one end of the second tuning capacitor C2 is connected to the common connection point of the first inter-stage coupling inductor L2 and the second inter-stage coupling inductor L3, and the other end is grounded;

[0031] One end of the third tuning capacitor C3 is connected to the common connection point of the second inter-stage coupling inductor L3 and the output coupling inductor L4, and the other end is grounded.

[0032] In this embodiment, the input coupling inductor L1 and the output coupling inductor L4 have the same inductance value of 27 nH, and the first inter-stage coupling inductor L2 and the second inter-stage coupling inductor L3 have the same inductance value of 54.7 nH.

[0033] The first tuning capacitor C1, the second tuning capacitor C2, and the third tuning capacitor C3 are all adjustable capacitor arrays, and the three have the same structure. As Figure 2 shown, the first tuning capacitor C1 includes: capacitors C4 to C7, PIN diodes D1 to D2, and resistors R1 to R2; one ends of capacitors C4 and C5 are electrically connected to the connection point between the input coupling inductor L1 and the first inter-stage coupling inductor L2. The other end of capacitor C4 is electrically connected to one end of resistor R1, one end of resistor R2, the negative electrode of PIN diode D1, and the positive electrode of PIN diode D2. The other end of capacitor C5 is grounded. The other end of resistor R1 and the positive electrode of PIN diode D1 are respectively electrically connected to one end of capacitor C6. The other end of resistor R2 and the negative electrode of PIN diode D2 are respectively electrically connected to one end of capacitor C7. The other ends of capacitor C6 and capacitor C7 are both grounded; the first power supply VCC is connected to the other end of resistor R1, the positive electrode of PIN diode D1, and one end of capacitor C6; the external device is connected to the other end of resistor R2, the negative electrode of PIN diode D2, and one end of capacitor C7.

[0034] The second tuning capacitor C2 includes: capacitors C8 to C11, PIN diodes D3 to D4, and resistors R3 to R4; one ends of the capacitors C8 and C9 are electrically connected to the connection point between the first inter-stage coupling inductor L2 and the second inter-stage coupling inductor L3, the other end of the capacitor C8 is respectively electrically connected to one end of the resistor R3, one end of the resistor R4, the negative electrode of the PIN diode D3, and the positive electrode of the PIN diode D4, the other end of the capacitor C9 is grounded, the other end of the resistor R3 and the positive electrode of the PIN diode D3 are respectively electrically connected to one end of the capacitor C10, the other end of the resistor R4 and the negative electrode of the PIN diode D4 are respectively electrically connected to one end of the capacitor C11, and the other ends of the capacitor C10 and the capacitor C11 are both grounded; the second power supply VCC is connected to the other end of the resistor R3, the positive electrode of the PIN diode D3, and one end of the capacitor C10; the external device is connected to the other end of the resistor R4, the negative electrode of the PIN diode D4, and one end of the capacitor C11.

[0035] The second tuning capacitor C3 includes: C12 to C15, PIN diodes D5 to D6, and resistors R5 to R6; one ends of the capacitors C12 and C13 are electrically connected to the connection point between the second inter-stage coupling inductor L3 and the output coupling inductor L4, the other end of the capacitor C12 is respectively electrically connected to one end of the resistor R5, one end of the resistor R6, the negative electrode of the PIN diode D5, and the positive electrode of the PIN diode D6, the other end of the capacitor C13 is grounded, the other end of the resistor R5 and the positive electrode of the PIN diode D5 are respectively electrically connected to one end of the capacitor C14, the other end of the resistor R6 and the negative electrode of the PIN diode D6 are respectively electrically connected to one end of the capacitor C15, and the other ends of the capacitor C14 and the capacitor C15 are both grounded; the third power supply VCC is connected to the other end of the resistor R5, the positive electrode of the PIN diode D5, and one end of the capacitor C14; the external device is connected to the other end of the resistor R6, the negative electrode of the PIN diode D6, and one end of the capacitor C15.

[0036] The capacitance values of the first tuning capacitor C1 and the third tuning capacitor C3 are the same, ranging from 8 - 34 pF, which are different from only the capacitor C8 (37 pF) in the second tuning capacitor C2 and the capacitor C4 (26.5 pF) in the first tuning capacitor C1; C9 (7 pF) is different from C5 (6.8 pF) in the first tuning capacitor C1.

[0037] In the embodiment, if the equivalent grounded capacitance value when the three adjustable capacitor arrays are cutoff is too small, less than 2 pF, the equivalent grounded capacitance value can be increased by adding the capacitor C5 to the first capacitor C1, adding the capacitor C9 to the second capacitor C2, and adding the capacitor C13 to the second capacitor C3, and a low-pass filter with a cutoff frequency of 400 MHz can be obtained.

[0038] During encapsulation, all capacitors in the third-order tuned low-pass filter of this embodiment adopt high-Q capacitors, that is, capacitors with a Q value greater than 1500 and 0805 package. All inductors are wound with a polytetrafluoroethylene skeleton and 0.8 mm wide enameled wire, and PIN diodes that can withstand a maximum signal of 50 dBm are used, with a safety margin designed.

[0039] When the control signal is 0, the OUT outputs a low level, and the PIN diodes D1~D6 are turned on. Capacitors C6 and C7 are connected in parallel and then in series with C4, and then in parallel with C5 to obtain the first tuned capacitor C1; capacitors C10 and C11 are connected in parallel and then in series with C8, and then in parallel with C9 to obtain the second tuned capacitor C2; capacitors C14 and C15 are connected in parallel and then in series with C12, and then in parallel with C13 to obtain the third tuned capacitor C3. At this time, a third-order tuned low-pass filter proposed by the present invention is in the operating frequency band of 108 - 174 MHz. In the frequency band of 108 - 174 MHz, its insertion loss ≤ 0.8 dB, voltage standing wave ratio ≤ 1.6, and third harmonic out-of-band rejection 3f0 ≥ 40 dBc, where f0 is the center frequency of the operating frequency band of the 108 - 174 MHz frequency hopping filter.

[0040] When the control signal is 1, the OUT outputs a high level, and the PIN diodes D1~D6 are turned off. Capacitor C6 is connected in series with the junction capacitance of PIN diode D1, and at the same time capacitor C7 is connected in series with the junction capacitance of PIN diode D2. Then the two capacitors are connected in parallel, then in series with C4, and finally in parallel with C5 to obtain the first tuned capacitor C1; capacitor C10 is connected in series with the junction capacitance of PIN diode D3, and at the same time capacitor C11 is connected in series with the junction capacitance of PIN diode D4. Then the two capacitors are connected in parallel, then in series with C8, and finally in parallel with C9 to obtain the second tuned capacitor C2; capacitor C14 is connected in series with the junction capacitance of PIN diode D5, and at the same time capacitor C15 is connected in series with the junction capacitance of PIN diode D6. Then the two capacitors are connected in parallel, then in series with C12, and finally in parallel with C13 to obtain the third tuned capacitor C3. At this time, the third-order tuned low-pass filter of this embodiment is in the operating frequency band of 225 - 400 MHz. In the frequency band of 225 - 400 MHz, the insertion loss ≤ 0.8 dB, voltage standing wave ratio ≤ 1.6, and third harmonic out-of-band rejection 3f0 ≥ 40 dBc, where f0 is the center frequency of the operating frequency band of the 225 - 400 MHz frequency hopping filter.

[0041] In summary, the third-order tuned low-pass filter of this embodiment can effectively suppress the third and higher harmonics at the back end of the frequency hopping filter and the power amplifier, and thus realize a third-order tuned low-pass filter whose cut-off frequency can change with the control signal.

[0042] The present invention is not limited to the foregoing specific embodiments. For those of ordinary skill in the technical field to which this application belongs, without departing from the inventive concept of this application, several simple deductions or substitutions can also be made.

Claims

1. A third-order tuned low-pass filter, characterized in that, This filter adopts a Chebyshev low-pass filter structure, including an input coupling inductor L1, a first inter-stage coupling inductor L2, a second inter-stage coupling inductor L3, an output coupling inductor L4, a first tuning capacitor C1, a second tuning capacitor C2, and a third tuning capacitor C3; The input coupling inductor L1 serves as the input terminal of the entire third-order tuned low-pass filter and is connected to the input RF signal RF. in The output terminal is sequentially connected to the first inter-stage coupling inductor L2, the second inter-stage coupling inductor L3, and the output coupling inductor L4. The output coupling inductor L4 serves as the output terminal of the entire third-order tuned low-pass filter and is used to output the RF out signal. One end of the first tuning capacitor C1 is connected to the common point of the input coupling inductor L1 and the first inter-stage coupling inductor L2, and the other end is grounded; One end of the second tuning capacitor C2 is connected to the common point of the first inter-stage coupling inductor L2 and the second inter-stage coupling inductor L3, and the other end is grounded; One end of the third tuning capacitor C3 is connected to the common point of the second inter-stage coupling inductor L3 and the output coupling inductor L4, and the other end is grounded.

2. The third-order tuned low-pass filter according to claim 1, characterized in that The first tuning capacitor C1, the second tuning capacitor C2, and the third tuning capacitor C3 are all adjustable capacitor arrays, and the three have the same structure.

3. A third-order tuned low-pass filter according to claim 2, characterized in that, The first tuning capacitor C1 includes: a first capacitor, a second capacitor, a third capacitor, a fourth capacitor, a first PIN diode, a second PIN diode, a first resistor, and a second resistor; One end of the first capacitor and one end of the second capacitor are connected and then electrically connected to the common point of the input coupling inductor L1 and the first inter-stage coupling inductor L2; The other end of the first capacitor is electrically connected to one end of the first resistor, one end of the second resistor, the negative electrode of the first PIN diode, and the positive electrode of the second PIN diode respectively; The other end of the second capacitor is grounded; The other end of the first resistor and the positive electrode of the first PIN diode are connected and then electrically connected to one end of the third capacitor; The other end of the second resistor and the negative electrode of the second PIN diode are connected and then electrically connected to one end of the fourth capacitor; The other ends of the third capacitor and the fourth capacitor are both grounded; The power supply VCC is connected to the other end of the first resistor, the positive electrode of the first PIN diode, and one end of the third capacitor; The external device is connected to the other end of the second resistor, the negative electrode of the second PIN diode, and one end of the fourth capacitor.

4. A third-order tuned low-pass filter according to claim 2, characterized in that, The capacitance values of the first tuning capacitor C1 and the third tuning capacitor C3 are the same.

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