A new narrowband electrically tunable filter

Abstract

The present invention discloses a novel narrowband electrically tunable filter designed using a resonator-coupled filter method. Unlike traditional filters, which are difficult to implement, this novel narrowband electrically tunable filter is composed of N resonators, N-1 tuning voltages, and N-1 coupling elements K. The novel narrowband electrically tunable filter comprises: a first resonator, a second resonator, a coupling element, a third resonator, and a fourth resonator, which are connected in sequence. The first, second, third, and fourth resonators are all connected to a single-chip microcomputer, and the capacitance of the varactor diodes therein is tuned using the same tuning voltage. The present invention can improve the selectivity of a radio frequency receiving circuit, eliminating more strong interference from being out of band.

Description

Technical Field

[0001] The present invention relates to the field of filtering, and in particular to a novel narrowband electrically tunable filter. Background Art

[0002] Ultra-short wave communication is the main means of ground-to-air line-of-sight communication. With the increase in the types and number of services, the number of ultra-short wave communication equipment has also gradually increased. Various interferences during use, especially co-site interference, have become increasingly prominent and have become an important factor restricting the effectiveness of radio stations.

[0003] Co-site interference impacts ultra-shortwave communication equipment in two primary ways: first, it disrupts the receiver's ability to correctly demodulate the target signal, manifesting as excessive noise during voice transmission and high bit error rates during data transmission. Second, it shields weaker target signals, shortening communication range and preventing distant signals from being received. The receiver is often unaware of this situation. This is due to the poor performance of the radio receiver's front-end RF preselector, known as the electronically tunable filter circuit. This circuit has an excessively wide bandwidth and insufficient suppression of strong interfering signals.

[0004] Therefore, in order to meet the requirements of co-location and co-operation of ultra-short wave radio stations, the inventors of the present invention have designed a new narrowband electrically tunable filter to solve the above problems. Summary of the Invention

[0005] In order to solve the above problems, the purpose of the present invention is to provide a novel narrowband electrically tunable filter, which can improve the selectivity of the radio frequency receiving circuit and exclude more strong interference from the band of the radio frequency receiving circuit.

[0006] Based on this, the present invention provides a novel narrowband electrically tunable filter, the filter comprising:

[0007] a first resonator, a second resonator, a coupling element, a third resonator, and a fourth resonator connected in sequence;

[0008] The first resonator, the second resonator, the third resonator and the fourth resonator are all connected to the single chip microcomputer.

[0009] The first resonator includes: a first inductor and a first capacitor. The first inductor and the first capacitor form a series resonant circuit. The first capacitor is composed of two variable capacitance diodes connected in parallel. The first inductor is composed of an inductor with a first inductance value and an inductor with a second inductance value connected in series.

[0010] The model of the varactor diode is BB439.

[0011] The first resonator includes: a first inductor and a first capacitor. The first inductor and the first capacitor form a series resonant circuit. The first capacitor is composed of a single varactor diode. The first inductor is composed of a first inductance value inductor and a second inductance value inductor connected in series.

[0012] The model of the varactor diode is BB535.

[0013] The second resonator includes: a second inductor and a second capacitor. The second inductor and the second capacitor form a series resonant circuit. The second capacitor is composed of two variable capacitance diodes connected in parallel or a single variable capacitance diode.

[0014] The third resonator includes: a third inductor and a third capacitor. The third inductor and the third capacitor form a series resonant circuit. The third capacitor is composed of two varactor diodes connected in parallel or a single varactor diode.

[0015] Among them, the fourth resonator includes: a fourth inductor and a fourth capacitor, the fourth inductor and the fourth capacitor form a series resonant circuit, the fourth capacitor is composed of two variable capacitance diodes in parallel or a single variable capacitance diode, and the fourth inductor is composed of a third inductance inductor and a fourth inductance inductor connected in series.

[0016] The output end of the DC voltage of the digital-to-analog converter DAC inside the single chip is connected to the negative input end of the varactor diode in the first resonator, the second resonator, the third resonator and the fourth resonator.

[0017] The coupling element is composed of a coupling inductor, an input end of the coupling inductor is connected to an output end of the second resonator, and an output end of the coupling inductor is connected to an input end of the third resonator.

[0018] The present invention is a unique and easy-to-implement filter consisting of four resonators and a coupling element. All four resonators use a single tuning voltage to tune the capacitance of their varactor diodes. This differs from conventional filters, which are difficult to implement and consist of N resonators, N-1 tuning voltages, and N-1 coupling elements. In terms of suppression, the new narrowband electrically tunable filter achieves the same performance as conventional filters of its kind. The tuning voltage applied to the varactor diodes in this new filter is achieved via the DC output of the DAC (digital-to-analog converter) within the microcontroller circuit.

[0019] The present invention achieves the design of a novel narrowband electronically tunable filter consisting of four resonators tuned by the same tuning voltage and a coupling element, a feat unachievable with conventional filters. In the 108MHz to 174MHz frequency band, this filter achieves an insertion loss A0 of less than 4dB, a 3dB bandwidth BW3 of less than 6MHz, and a suppression ratio greater than 15dBc at the frequency point f0±5%f0. In the 225MHz to 400MHz frequency band, the filter achieves an insertion loss A0 of less than 5dB, a 3dB bandwidth BW3 of less than 15MHz, and a suppression ratio greater than 18dBc at the frequency point f0±5%f0. This solves the problems of conventional electronically tunable filters with poor circuit performance, excessive bandwidth, and insufficient suppression of strong interfering signals. The novel narrowband electronically tunable filter improves the selectivity of radio frequency receiving circuits, eliminating more strong interference out of band and resolving the mutual interference problem when multiple radio stations operate at the same location. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0021] Figure 1 Schematic diagram of a novel narrowband electrically tunable filter provided by an embodiment of the present invention;

[0022] Figure 2 Schematic diagram of an original filter with a center frequency of 140 MHz, composed of two resonators and a coupling element, provided in an embodiment of the present invention;

[0023] Figure 3 The embodiment of the present invention provides Figure 2 The simulation results of the original filter are shown in Figure 2.

[0024] Figure 4 The embodiment of the present invention provides Figure 2 A new narrowband filter with a center frequency of 140MHz is derived from the original filter, which consists of four resonators and a coupling element;

[0025] Figure 5 This is a diagram showing the simulation results of a novel narrowband filter with a center frequency of 140 MHz provided by an embodiment of the present invention using ADS software;

[0026] Figure 6 This is a circuit diagram of a novel narrowband filter with a center frequency of 108 MHz provided by an embodiment of the present invention;

[0027] Figure 7 The embodiment of the present invention provides Figure 6 ADS software simulation results of a new narrowband filter with a center frequency of 108MHz;

[0028] Figure 8 This is a circuit diagram of a novel narrowband filter with a center frequency of 174 MHz provided by an embodiment of the present invention;

[0029] Figure 9 The embodiment of the present invention provides Figure 8 ADS software simulation results of a new narrowband filter with a center frequency of 174MHz;

[0030] Figure 10 This is a circuit diagram of a novel narrowband electronically tunable filter with a frequency range of 108 MHz to 174 MHz provided by an embodiment of the present invention;

[0031] Figure 11 This is a block diagram of the principle of inductor π→T conversion provided by an embodiment of the present invention.

[0032] Figure 12 This is a circuit diagram of a novel narrowband electrically tunable filter with a frequency range of 108 MHz to 174 MHz provided by an embodiment of the present invention;

[0033] Figure 13 This is a circuit diagram of a novel narrowband filter with a center frequency of 225 MHz provided by an embodiment of the present invention;

[0034] Figure 14 The embodiment of the present invention provides Figure 13 ADS software simulation results of a new narrowband filter with a center frequency of 225MHz;

[0035] Figure 15 This is a circuit diagram of a novel narrowband filter with a center frequency of 400 MHz provided by an embodiment of the present invention;

[0036] Figure 16 The embodiment of the present invention provides Figure 15 ADS software simulation results of a new narrowband filter with a center frequency of 400MHz;

[0037] Figure 17 This is a circuit diagram of a novel narrowband electrically tunable filter with a frequency range of 225 MHz to 400 MHz provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0038] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0039] The electronically tunable filter changes the network frequency response by changing the variable capacitance of the varactor diode in the resonator, and uses voltage to change the capacity of the variable capacitance of the varactor diode to achieve the required frequency response.

[0040] In the full frequency band, the frequency conversion ratio K1 = 400MHz / 108MHz≈3.7, so the frequency conversion ratio of the corresponding varactor diode needs to be K2 = K1 2 =3.7 2 = 13.69. Due to limitations in semiconductor materials, hyperabrupt junction diodes are still unavailable. Therefore, a single electrically tunable filter is required for both the 108MHz-174MHz and 225MHz-400MHz bands to meet these requirements. This paper only details the design of a novel narrowband electrically tunable filter for the 108MHz-174MHz band. The design principles and steps for the novel narrowband electrically tunable filter for the 225MHz-400MHz band are similar, so only the results are presented here without further elaboration.

[0041] Figure 1 : is a schematic diagram of a novel narrowband electrically tunable filter provided by an embodiment of the present invention, the filter comprising:

[0042] A first resonator 101, a second resonator 102, a coupling element 103, a third resonator 104, and a fourth resonator 105 connected in sequence;

[0043] The first resonator 101 , the second resonator 102 , the third resonator 104 , and the fourth resonator 105 are all connected to a single chip microcomputer 106 .

[0044] The novel narrowband electrically tunable filter is a two-port reciprocal network with interchangeable inputs and outputs. Here, it is described from the perspective of the input: the output end of the first resonator 101 is connected to the input end of the second resonator 102, the output end of the second resonator 102 is connected to the input end of the coupling element 103, the output end of the coupling element 103 is connected to the input end of the third resonator 104, the output end of the third resonator 104 is connected to the input end of the fourth resonator 105, and the output end of the single-chip microcomputer 106 is connected to the negative input end of the varactor diodes of the first resonator 101, the second resonator 102, the third resonator 104, and the fourth resonator 105.

[0045] The design process of the present invention is as follows:

[0046] The present invention utilizes the coupled design method within the predistortion design approach, specifically the predistortion k and q parameter tables. To accurately achieve various responses even with lossy components, the component losses must be factored in beforehand, and then the design parameters for these losses are synthesized to derive. In this case, the response curve remains the same as for the lossless case, except for a fixed loss added within the passband. This is the predistortion design approach. In this embodiment, only one coupling element is used. Initially, a normalized k and q parameter table is consulted based on the number of resonator elements, n = 2, to design a basic electrically tunable filter consisting of two resonators and a coupling element. This is then transformed into a filter network with n = 4 resonators.

[0047] In the new narrowband electronically tunable filter in the 108MHz to 174MHz frequency band, the intermediate frequency 140MHz is selected as the starting point for design. Assume the input conditions are:

[0048] 1. Flattest Butterworth bandpass filter;

[0049] 2. Center frequency: f0 = 140 MHz;

[0050] 3. BW3≤8MHz;

[0051] 4. Insertion loss A0≤3dB;

[0052] 5. Input and output impedance R S =R L =50 euros.

[0053] When A0≤3dB, n=2, we can find from the table that q0=14.142, A0=0.915, q1=1.4142; q2=1.4142; k 12 =0.7071.

[0054] Therefore, the minimum unloaded Q0 of the resonator is:

[0055]

[0056] Generally speaking, a coil with large inductance must also have large stray capacitance, so its own resonant frequency is also very low. On the other hand, due to the large inductance, the number of turns of the coil must be large, and its equivalent series resistance must be large, which leads to the coil's Q value being low. In order to make Q0 as large as possible and minimize the influence of distributed parameters, a compromise is made here, and L is selected. p =60nH.

[0057] Loaded quality factor:

[0058]

[0059]

[0060] Coupling coefficient

[0061]

[0062] Assuming the inductance in each resonator is Lp, the capacitance in the resonator is:

[0063]

[0064] Input tap ratio

[0065] Because q1=q2, and R S =R L , so the output tap ratio p L The tap ratio p of the input s same.

[0066] Calculating tapped inductance L s2 =L p -L s1 =48.8nH; similarly L L1 =11.2nH, L L2 =48.8nH.

[0067] The designed circuit is Figure 2 As shown in the figure, it is the original filter. Since the present invention is to design an electrically tunable filter, the capacitance of the varactor diode used in the resonator changes with the applied tuning voltage. Therefore, the slight change of the resonator resonant frequency caused by the coupling element is not considered here, and the center frequency of the filter is allowed to vary slightly. Figure 3 This is the ADS simulation result for the original filter, using an ideal lumped parameter model for component selection. As shown in the figure, the center frequency f0 is 143.1MHz, the insertion loss A0 is -0.04dB, and the 3dB bandwidth BW3 is 7.9MHz.

[0068] Next, in order to maintain the coupling element L 12 unchanged, only one coupling element is used; the tap ratio of input and output remains unchanged, that is, L s1 , L s2 , L L1 , L L2 The value of remains unchanged, and each resonator is changed from a parallel resonant circuit to two series resonant circuits in cascade. That is, the filter is derived into a new narrowband filter consisting of four resonators and a coupling element. The final circuit diagram is as follows Figure 4 As shown, Figure 5 for Figure 4 The circuit diagram of the new narrowband filter is simulated by ADS software. Figure 5 As shown in the figure, the new narrowband filter has a center frequency f0 of 141.6MHz, an insertion loss A0 of -0.008dB, and a 3dB bandwidth BW3 of 3.9MHz. Compared to the original filter, the new narrowband filter has almost no change in insertion loss, but a narrower 3dB bandwidth. The simulation graph also shows improved out-of-band rejection.

[0069] The design of a new narrowband electronically tunable filter in the 108MHz to 174MHz frequency band is to Figure 4 C in p1 , C p2 , C p3 , C p4 The center frequency of the filter will change with the change of the capacitance of the varactor diode. However, at the operating frequency of 108MHz, due to the inductance of the coupling element L 12 The value remains unchanged, the coupling becomes stronger, and it is in an over-coupling state. The filtering characteristic curve will change from a single peak to a double peak, with a concave point in the middle and the bandwidth becomes wider. Therefore, the coupling strength should be reduced and the component inductance L should be increased. 12 The inductance value can be set as L 12 =1800nH. According to the formula

[0070] It is found that (L = 60nH), when the operating frequency is 108MHz, the capacitance C≈36pF; when the operating frequency is 174MHz, the capacitance C≈13.94pF; the specific capacitance value can be corrected through simulation, and the correction and simulation results are as follows:

[0071] f0=108MHz:capacitor C P =36.8pF; Insertion loss A0 = -0.058dB; 3dB bandwidth BW3 = 2.4MHz; @f0±5% suppression: >15dBc. Circuit diagram as shown Figure 6 The simulation results are shown in Figure 7 shown.

[0072] f0=174MHz:capacitor C P =14.24pF; Insertion loss A0 = -0.443dB; 3dB bandwidth BW3 = 4.1MHz; @f0±5% suppression: >15dBc. Circuit diagram as shown Figure 8 The simulation results are shown in Figure 9 shown.

[0073] Varactor diode BB439, when the tuning voltage V R =3~25(V), capacitor C P=26~4.3(pF), which cannot cover the capacitance value C used for f0=108MHz~174MHz P Therefore, two BB439 varactor tubes need to be connected in parallel. The corresponding circuit diagram of the new narrowband electronically tunable filter of 108MHz to 174MHz is as follows: Figure 10 shown.

[0074] The first resonator 101 includes: a first inductor L S , the first capacitor C P1 , the first inductor L S With the first capacitor C P1 Forming a series resonant circuit, the first capacitor C P1 It is composed of two varactor diodes connected in parallel. The first inductor L S By the first inductance value L s1 , the second inductance value L s2 The first inductance value L s1 , the second inductance value L s2 The inductance ratio is determined by the input tap ratio, which acts as an impedance converter. The input of the first resonator 101 is connected to a peripheral device with a 50-ohm output impedance; the output of the first resonator 101 is connected to the input of the second resonator 102. The varactor diode is BB439.

[0075] The second resonator 102 includes: a second inductor L P3 , the second capacitor C P3 , the second inductor L P3 With the second capacitor C P3 Forming a series resonant circuit, the second capacitor C P3 The input end of the second resonator 102 is connected to the output end of the first resonator 101 , and the output end of the second resonator 102 is connected to the input end of the coupling element 103 .

[0076] The third resonator 104 includes: a third inductor L P4 , the third capacitor C P4 , the third inductor L P4 With the third capacitor C P4 Forming a series resonant circuit, the third capacitor C P4 The input end of the third resonator 104 is connected to the output end of the coupling element 103 , and the output end of the third resonator 104 is connected to the input end of the fourth resonator 105 .

[0077] The fourth resonator 105 includes: a fourth inductor L L, the fourth capacitor C P2 , the fourth inductor L L With the fourth capacitor C P2 Forming a series resonant circuit, the fourth capacitor C P2 The fourth inductor L is composed of two varactor diodes connected in parallel. L By the third inductance L L1 , the fourth inductance value L L2 At the same time, the third inductance value L L1 , the fourth inductance value L L2 The inductance ratio is determined by the output tap ratio, which plays an impedance conversion role. The input end of the fourth resonator 105 is connected to the output end of the third resonator 104, and the output end of the fourth resonator 105 is connected to a peripheral device with an input impedance of 50 ohms.

[0078] The DC voltage output of the DAC within the microcontroller 106 is connected to the negative input terminals of the varactor diodes BB439 in the first resonator 101, the second resonator 102, the third resonator 104, and the fourth resonator 105. In other words, the resonant center frequencies of the four resonators are tuned using the same tuning voltage, and the required values ​​of the DAC registers within the microcontroller are determined by the operating frequency.

[0079] The coupling element 103 is composed of an inductor L 12 Composition, L 12 =1800nH. Its input is connected to the output of the second resonator 102, and its output is connected to the input of the third resonator 104. Since there is capacitance between each turn of the coil and the coil wire has distributed resistance, the actual inductor coil will show different equivalent circuits at low and high frequencies. At low frequencies, these distributed parameters are very small and the coil can be equivalent to an ideal inductor. However, at high frequencies, these distributed parameters cannot be ignored. The capacitance between turns, the skin effect that increases the wire resistance and causes the Q value to decrease, etc. must all be taken into account. And L 12 , the inductance is 1800nH, and there must be many turns. In order to reduce the influence of these distributed parameters at high frequencies, the inductance must be reduced and the inductance value can be reduced. The inductance L in the second resonator can be used to reduce the inductance. P3 and the inductor L in the third resonator P4 This is accomplished through π→T transformation, as shown in the schematic diagram. Figure 11 shown.

[0080]

[0081]

[0082]

[0083] The final new narrowband electronically tunable filter circuit diagram from 108MHz to 174MHz is shown in the figure below: Figure 12 shown.

[0084] The design principles and steps for the new narrowband electronically tunable filter in the 225MHz to 400MHz frequency band are the same as those for the 108MHz to 174MHz band, so they will not be described in detail here. The results are as follows:

[0085] f0=225MHz:capacitor C P =10.2pF; Insertion loss A0 = -0.058dB; 3dB bandwidth BW3 = 4.9MHz; @f0±5% suppression: >18dBc. Circuit diagram as shown Figure 13 The simulation results are shown in Figure 14 shown.

[0086] f0=400MHz:capacitor C P =3.24pF; Insertion loss A0 = -0.61dB; 3dB bandwidth BW3 = 9.3MHz; @f0±5% suppression: >18dBc. Circuit diagram as shown Figure 15 The simulation results are shown in Figure 16 shown.

[0087] Varactor diode BB535, when the tuning voltage V R =2~25(V), capacitor C P =15~2.24(pF), which can cover the capacitance value C used for f0=225MHz~400MHz P Therefore, only one varactor tube BB535 is needed to meet the design requirements of the new narrowband electronically tunable filter of 225MHz~400MHz. The corresponding final circuit diagram is as follows Figure 17 shown.

[0088] Through the above technical solution, two new narrowband electronically tunable filters have been designed for the 108MHz-174MHz and 225MHz-400MHz ultrashortwave frequency bands. In the 108MHz-174MHz band, insertion loss A0 is less than 4dB, 3dB bandwidth BW3 is less than 6MHz, and suppression is greater than 15dBc at f0±5%f0. In the 225MHz-400MHz band, insertion loss A0 is less than 5dB, 3dB bandwidth BW3 is less than 15MHz, and suppression is greater than 18dBc at f0±5%f0. This solves the problems of poor performance, excessive bandwidth, and insufficient suppression of strong interfering signals in traditional electronically tunable filters. The new narrowband electronically tunable filters improve the selectivity of RF receiver circuits, eliminating more strong interference out of band and resolving the mutual interference problem when multiple radio stations are co-located.

[0089] 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 substitutions can be made without departing from the technical principles of the present invention. These improvements and substitutions should also be regarded as the scope of protection of the present invention.

Claims

1. A novel narrowband electrically tunable filter, characterized in that: include: a first resonator, a second resonator, a coupling element, a third resonator, and a fourth resonator connected in sequence; The first resonator, the second resonator, the third resonator, and the fourth resonator are all connected to a single-chip microcomputer, and the output end of the DC voltage of the digital-to-analog converter DAC inside the single-chip microcomputer is connected to the negative input end of the varactor diode in the first resonator, the second resonator, the third resonator, and the fourth resonator; The first resonator includes: a first inductor and a first capacitor, wherein the first inductor and the first capacitor form a series resonant circuit, the first capacitor is composed of two varactor diodes connected in parallel, and the first inductor is composed of an inductor of a first inductance value and an inductor of a second inductance value connected in series; The second resonator includes: a second inductor and a second capacitor, the second inductor and the second capacitor form a series resonant circuit, and the second capacitor is composed of two variable capacitance diodes connected in parallel or a single variable capacitance diode; The third resonator includes: a third inductor and a third capacitor, wherein the third inductor and the third capacitor form a series resonant circuit, and the third capacitor is composed of two varactor diodes connected in parallel or a single varactor diode; The fourth resonator includes: a fourth inductor and a fourth capacitor, wherein the fourth inductor and the fourth capacitor form a series resonant circuit, the fourth capacitor is composed of two varactor diodes connected in parallel or a single varactor diode, and the fourth inductor is composed of an inductor of a third inductance value and an inductor of a fourth inductance value connected in series; The coupling element is composed of a coupling inductor, an input end of the coupling inductor is connected to the output end of the second resonator, and an output end of the coupling inductor is connected to the input end of the third resonator.

2. The novel narrowband electrically tunable filter according to claim 1, characterized in that: The model of the varactor diode is BB439.

3. The novel narrowband electrically tunable filter according to claim 1, characterized in that: The first resonator includes: a first inductor and a first capacitor. The first inductor and the first capacitor form a series resonant circuit. The first capacitor is composed of a single varactor diode. The first inductor is composed of an inductor with a first inductance value and an inductor with a second inductance value connected in series.

4. The novel narrowband electrically tunable filter according to claim 1, characterized in that: The model of the varactor diode is BB535.

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