Summary of the invention
(1) technical problem that will solve
For solving the above problems, the invention provides the adjustable quasi-ellipse function filters of a kind of varactor electricity, utilize four joint half-wave resonator Unit Designs to have the quasi-ellipse function filters of four transmission zeros, reach good suppress sideband effect; Make this quasi-ellipse function filters realize electricity adjustable simultaneously, be applied to the bias voltage on varactor by adjusting, make the centre frequency of this varactor electrically tunable filter along with applied voltage changes, to meet the needs of frequency agility.
(2) technical scheme
The invention provides a kind of electric tunable filter.This filter comprises: input microstrip line, the first resonator, the second resonator, the 3rd resonator, the 4th resonator and output microstrip line.Wherein, the first resonator and the 4th resonator outward opening, mirror image setting side by side, both form magnetic coupling in position adjacent to one another; The second resonator and the 3rd resonator opening are inside, mirror image is arranged at the top of the first resonator and the 4th resonator side by side, both form electric coupling in position adjacent to one another, input microstrip line and output microstrip line lay respectively at the below of the first micro-strip resonantor and the 4th micro-strip resonantor, mirror image setting side by side, both ends are respectively along spending away from the direction deviation 90 of the first micro-strip resonantor and the 4th micro-strip resonantor, and two deviation parts are parallel to each other, and form electric coupling.
(3) beneficial effect
Can find out from technique scheme, varactor electricity tunable filter of the present invention has following beneficial effect:
(1) owing to having adopted cross-couplings resonator structure, make filter there is class elliptic function characteristic, sideband has transmission zero, thereby makes filter have good squareness factor, has improved filtering performance;
(2) owing to having adopted input, output coupling circuit, utilize this input, output to be coupled as a cross-couplings into filter, cross-couplings between resonator is as another cross-couplings of filter, make filter of the present invention on identical resonator number, construct a cross-couplings than traditional quasi-ellipse function filters more, thereby increase pair of transmission zeros, four joint resonator structures have two pairs of transmission zeros, thereby make filter have better squareness factor, filtering performance is better;
(3) owing to having adopted varactor electricity to adjust structure, change the centre frequency of filtering, and keep the shape invariance of filter curve by the tuning voltage changing on varactor, the centre frequency of filter has very fast tuned speed, meets the demand of frequency agility.
For making the object, technical solutions and advantages of the present invention clearer, below in conjunction with specific embodiment, and with reference to accompanying drawing, the present invention is described in more detail.Although the demonstration of the parameter that comprises particular value can be provided herein, should be appreciated that, parameter is without definitely equaling corresponding value, but can in acceptable error margin or design constraint, be similar to corresponding value.
In Theory of Electromagnetic Field, electromagnetic wave with sinusoidal wave form transmission, can present and occur voltage wave peak dot and nodal point at ad-hoc location on transmission line, and at voltage wave peak dot, voltage reaches maximum, and now electric current is 0, shows as the form of electric field; At voltage wave node, voltage reaches minimum value, and electric current reaches maximum, shows as the form in magnetic field.The coupling that relies on electric field to carry out electromagnetic energy is called electric coupling, and the coupling that relies on magnetic field to carry out is called magnetic coupling, is called hybrid coupled in the time being coupled by Electric and magnetic fields acting in conjunction.
For any one transmission line structure, because its open end electric current cannot pass through, electric current is 0, and electromagnetic wave always presents voltage wave peak dot, shows as electric field form, the electromagnetic energy carrying out herein be coupled as electric coupling.At the earth point of transmission line, voltage reaches minimum value, now between itself and ground, there is no voltage difference, and now electromagnetic electric current reaches maximum, shows as the form in magnetic field, and that carries out herein is coupled as magnetic coupling.To a half-wave open-loop resonator, its two ends open circuit, electromagnetic wave reaches voltage max at resonator two ends, shows as electric field, can only carry out electric coupling; In the centre of open-loop resonator, due to electromagnetic periodic law, can only be voltage wave node, show as magnetic field, can only carry out magnetic coupling herein; It is hybrid coupled in other position.
Fig. 1 is that varactor electricity of the present invention is adjusted microstrip filter structure chart.For clarity sake, in Fig. 1, do not provide the bias circuit of varactor.As shown in Figure 1, this filter comprises: input microstrip line, the first resonator, the second resonator, the 3rd resonator, the 4th resonator and output microstrip line.The first resonator, the second resonator, the 3rd resonator and the 4th resonator are the resonator of identical resonance microstrip line length.Wherein, in the first resonator, micro-band resonance line is the shape of left half " work " font, the first resonator and the 4th resonator outward opening, and mirror image setting side by side, both form magnetic coupling in position adjacent to one another; In the second resonator, micro-band resonance line is the shape of the lower corner open rectangle in inner side, the second resonator and the 3rd resonator opening are inside, mirror image setting side by side, lay respectively at the top of the first resonator and the 4th resonator, both form electric coupling in position adjacent to one another, input microstrip line and output microstrip line lay respectively at the below of the first micro-strip resonantor and the 4th micro-strip resonantor, mirror image setting side by side, both ends are all along spending away from the direction deviation 90 of the first micro-strip resonantor and the 4th micro-strip resonantor, two deviation parts are parallel to each other, and form electric coupling.
In the present embodiment, input microstrip line parallel coupling, to the first micro-strip resonantor, is hybrid coupled pattern between the two, i.e. electric coupling and magnetic coupling acting in conjunction; Between the first resonator and the second resonator, it is hybrid coupled; It between the second resonator and the 3rd resonator, is electric coupling; Between the 3rd resonator and the 4th resonator, it is hybrid coupled; Output microstrip line parallel coupling to the four micro-strip resonantors are hybrid coupled pattern between the two; Between input microstrip line and output microstrip line, it is electric coupling.
This filter has two cross coupling structures: form the first cross coupling structure by the first resonator, the second resonator, the 3rd resonator and the 4th resonator, coupling between the first resonator and the 4th resonator forms cross-couplings, i.e. coupling between the transmission unit of non-adjacent sequence number; Form the second cross coupling structure by input microstrip line, the first resonator, the 4th resonator and output microstrip line, the coupling between input microstrip line and output microstrip line forms cross-couplings.Each cross coupling structure can form two transmission zeros, thereby this filter has four transmission zeros, has improved the signal suppressing ability to neighboring trace.
Below respectively each part of varactor electricity tunable filter is elaborated.
1, resonator.Resonator is the elementary cell of filter, is made up of open loop micro-band resonance line and varactor.Resonator of the present invention is: the half-wave resonator of the single-ended loading of varactor.The positive pole of varactor is received one end of open loop micro-strip resonantor, minus earth, and tuning voltage Vt is added to varactor negative pole by bias resistance R1, forms the bias voltage path of varactor, as shown in Figure 2.
A, micro-band resonance line.
Micro-band resonance line adopts 50 ohm characteristic impedance transmission lines, calculates the length d of micro-band resonance line in the single-ended loading half-wave resonator of varactor according to formula 1:
0for the centre frequency of filter, Z
0for line characteristic impedance, C is the capacitance value that loads varactor, the propagation constant that β is guided wave.
B, varactor.For reducing insertion loss, select high Q varactor as far as possible, capacitance size is got little in the situation that meeting frequency transfer scope as far as possible, because Q value and the capacitance of varactor are inversely proportional to.The in the situation that of given micro-band resonance line length, suppose that the centre frequency transfer area requirement of filter is f
2, the capacitance variation scope that requires varactor is C
2.The electric capacity of varactor is transferred scope demand and is calculated by 2 formulas:
2, input microstrip line and output microstrip line.
Input, output microstrip line adopt 50 ohm characteristic impedance microstrip transmission lines, and the electric coupling of their open circuit end structure pair of parallel, with thinking that filter provides pair of cross coupling, forms pair of transmission zeros, as shown in Figure 1.
Between input, output line, the large I of coupling coefficient is calculated according to formula 3.Its coupling size has determined the distance of the positional distance passband of transmission zero.If this coupling is excessive, transmission zero is very near apart from passband, and Out-of-band rejection performance will decline to some extent; If this coupling is too small, transmission zero is excessively far away apart from getting around regulations band, and the squareness factor of filtering is by variation.
14for the size of the coupling amount between resonator 1,4, Q
efor outside loaded quality factor, it is for representing the annexation between input/output line resonator, f
0centered by frequency, f
2the frequency that is respectively two transmission zeros that wish establishes, k is coupling factor, comprehensively going out k value is 1.597, changes position that k value will make transmission zero and produces and be offset.
3, cross-couplings resonator.
In Fig. 1, the first resonator, the second resonator, the 3rd resonator and the 4th resonator have formed a complete cross-couplings resonator element, carry out cross-couplings by the first resonator and the 4th resonator, the coupling coefficient of this cross-couplings resonator can be calculated by (4) formula.
01represent the natural frequency of the first resonator, f
04represent the natural frequency of the 4th resonator; f
p1represent this to coupled resonators the oscillation point at frequency low side, f
p2represent this to coupled resonators at the high-end oscillation point of frequency.Undertaken can emulation drawing two crest frequency points after the weak loading in outside by coupled resonators.(4) in formula, negative sign is got in electric coupling, and magnetic coupling is got positive sign, and because the cross-couplings in example of the present invention is magnetic magnetic coupling, symbol just should got.
The design process of the embodiment of the present invention is roughly as follows:
One, design parameter: centre frequency transfer scope is 1.4GHz～1.8GHz, relative bandwidth FBW is 0.04.The relevant parameter that covers the micro-band plate of copper adopting is: dielectric constant: 3.2, and dielectric thickness: 1mm;
Two, by above-mentioned design parameter and peripheral parameter, in the time that frequency is 1.8GHz, if the length d that the loading capacitance of resonator is 1.1pF obtains open loop micro-band resonance line is 48mm, the capacitance variations scope that is calculated varactor by formula 2 is at least 1.1pF to 1.4pF, according to the result of above-mentioned calculating, can select the varactor of model SMV1233, its corresponding capacitance is 1.1pF@6V, 3pF@1V.
Three, the parameter such as bending length of adjusting spacing between each resonator, input microstrip line and output microstrip line obtains best filter curve, then this filter of adjusting is extracted to its coupling coefficient and external sort factor, the filter of later same type can normalize to the centre frequency needing according to this coupling coefficient of having optimized and external sort factor and design.The coupling coefficient of this filter and external sort factor are respectively:
Be 3.3 at the dielectric constant of medium, in the situation of dielectric thickness 1mm, the concrete structural parameters of this filter are: the coupling length of I/O microstrip line is 3mm, and spacing is 1.1mm; Coupling length between the first resonator and the second resonator is 13mm, spacing 2.1mm; Coupling length between the second resonator and the 3rd resonator is 11mm, spacing 0.9mm; Coupling length between the 3rd resonator and the 4th resonator is 13mm, spacing 2.1mm; Coupling length between the 1st resonator and the 4th resonator is 13mm, spacing 3.5mm; I/O microstrip line is 23mm with the coupling length of the first resonator and the 4th resonator respectively, spacing 0.5mm, and in resonator, the total length of micro-band section is 48mm.
Fig. 3 is S parameter S 21 response curves of this filter, and wherein solid line is actual test curve, and dotted line is simulation curve, and simulation curve and actual test curve coincide good.In the time that the applied voltage of varactor is regulated by 2V～6V, the centre frequency of filter moves on to 1.8GHZ by 1.4GHz, and S21 has remained four transmission zeros simultaneously, and Out-of-band rejection is functional.
In the present embodiment, the length of resonator micro-band resonance line and the varactor of use can be changed accordingly according to formula, as long as can meet the frequency transfer scope of filter.Particularly adopt after the varactor of higher Q value, as MA4ST250, its insertion loss can obtain improvement to a certain extent.
Above-described specific embodiment; object of the present invention, technical scheme and beneficial effect are further described; institute is understood that; the foregoing is only specific embodiments of the invention; be not limited to the present invention; within the spirit and principles in the present invention all, any amendment of making, be equal to replacement, improvement etc., within all should being included in protection scope of the present invention.