DESC:FORM 2
THE PATENTS ACT, 1970
(39 OF 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(SEE SECTION 10, RULE 13)

A HIGH POWER LOW PASS FILTER FOR HARMONIC SUPPRESSION

BHARAT ELECTRONICS LIMITED
WITH ADDRESS:
OUTER RING ROAD, NAGAVARA, BANGALORE 560045, INDIA

THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

TECHNICAL FIELD
The present invention mainly relates to radio frequency electronic circuits and more particularly to radio frequency low pass filters with high power handling capacity.

BACKGROUND
A filter is used to selectively pass or attenuate a particular band of frequencies, and can be constructed of LC, RC, LCR, LR or distributed (microstrip) components and can be either active or passive in nature ( here L means inductor, C means capacitor, R means resistor). Active filters will contain some sort of amplifier combined with any of the above lumped passive components, while passive filters will simply employ lumped or distributed components.
The RF spectrum contains quite a broad range of frequencies. So that we would not interfere with, or be interfered by, other communications channels, a method had to be found that would allow us to segregate a small chunk of this wide spectrum for transmission and reception. This can be accomplished with the use of untuned and tuned filters. For an inductor of fixed value as the frequency is raised its reactance increases at the same time for a capacitor of fixed value as the frequency is increased its reactance decreases. A passive untuned LC filter circuit can easily function as a lowpass or highpass filter. This makes the untuned LC filter frequency selective. To act as a lowpass filter and attenuate higher frequencies an inductor will be arranged in series, blocking the high frequencies, while a capacitor is located in shunt, “shorting out” the higher frequencies. A highpass filter, which attenuates the lower frequencies has a capacitor that is in series, blocking the low frequencies, and a shunt inductor, shorting out the lower frequencies. These primitive filters may be cascaded to increase the sharpness of their skirts. Constant-K refers to a filter that not only rejects or passes specific frequencies, but will also match impedances between the generator and its load throughout its entire operational passband. The m-derived is another filter that displays a superior cutoff response, while maintaining a constant impedance across most of its passband.
Filters can be designed to meet a variety of requirements like in band ripple, fastest transition to the ultimate roll off and highest out of band rejection. The common types of filters are butterworth filter, bessel filter, chebyshev filter and elliptic filter. The butterworth filter provides the maximum in band flatness, although it provides a lower stop-band attenuation than a chebyshev filter. However it is also able to provide better group delay performance, and hence lower overshoot. The bessel filter provides the optimum in-band phase response and therefore also provides the best step response. It is often used where signals incorporate square waves, etc as the shape is maintained best of all. The chebyshev filter provides fast roll off after the cut off frequency is reached. However this is at the expense of in band ripple. The more in band ripple that can be tolerated, the faster the roll off. The elliptic filter also known as the cauer filter has significant levels of in band and out of band ripple, and as expected the higher the degree of ripple that can be tolerated, the steeper it reaches its ultimate roll off.
The patent application CN102820503 describes Half-lumped miniaturized microwave filter and design method thereof wherein the invention discloses a half lumped miniaturized microwave filter. According to the prototype circuit of a low-pass filter, the conventional distributed capacitors are replaced by a lumped capacitor in the middle of a micro-strip transmission line, so that the size of the microwave filter is effectively reduced, and the low pass filtering characteristic of a wide stop-band is generated at the same time. By using the half-lumped miniaturized microwave filter, the problem of the structure miniaturization of the microwave filter in a modern communication system is solved effectively; and the half-lumped miniaturized microwave filter is small in structural size, low in pass-band insertion loss, and wide in stop-band frequency range, low in cost and high in precision, and can be machined by adopting a standard printed circuit board technology conveniently.
The patent application JP2008005277 describes low pass filter wherein, it is designed to provide a compact low pass filter having low loss and steep attenuating characteristics. This low pass filter is provided with a first filter 1, a second filter 2, a third filter 3 and a fourth filter 4. The first filter 1 is configured of a two-terminal pair SAW resonator 11. The second filter 2 is configured of a one-terminal pair SAW resonator 21 and an inductor 22. The third filter 3 is configured of a capacitor 33 and a one-terminal pair SAW resonator 31 and an inductor2. The fourth filter 4 is configured of a two terminal pair SAW resonator 41.
The patent application CN101662057 describes Compact micro-band resonant unit low-pass filter with low insertion loss and wide stopband wherein, the invention relates to a compact micro-band resonant unit low-pass filter with low insertion loss and a wide stopband, which comprises a dielectric plate (1), a micro-band feed port (2), a micro-band feed line (3), low-resistance open-circuit stub lines (4), a crossed micro-band line (5) and gradually-changed trapezoidal resonant units (6), wherein the micro-band feed port (2), the micro-band feed line (3), the low-resistance open-circuit stub lines (4), the crossed micro-band line (5) and the gradually-changed trapezoidal resonant units (6) are positioned on the dielectric plate (1); the micro-band resonant units are respectively in left-and-right symmetrical structures and up-and-down symmetrical structures; the upper gradually-changed trapezoidal resonant unit (6) and the lower gradually-changed trapezoidal resonant unit (6) are connected by the crossed micro-band line (5) in the middle, wherein the left part of the crossed micro-band line (5) is connected with the micro-band feedline (3) through the low-resistance open-circuit stub lines (4), and the outer end of the micro-band feed line (3) is the micro-band feed port (2); and the characteristics of the upper gradually changed trapezoidal resonant unit (6) and the lower gradually-changed trapezoidal resonant unit (6) are tuned by the left low-resistance open-circuit stub line (4) and the right low-resistance open-circuit stub line (4), and finally, the connection with the outside is realized through the micro-band feed port (2).
The patent application CN203312429 describes Compact type wide-stop-band low pass filter wherein, the utility model discloses a compact type wide-stop-band low pass filter. By using the advantages of a ground plane defective structure and a surface intersecting branch shaped structure, the filter achieves a sharp cut-off frequency response performance and a wide stop band performance. Four U-shaped microstrip branches are attached on microstrip lines. The stop band width is increased. A combination structure of intersecting branches wrapped between two U-shaped structures has gaps reserved in the center. The structure with slow-wave effects can acquire sharp cut-off frequency responses. A cross-shaped intersecting dumbbell-like groove is arranged on a ground plane. In this way, a structure with a relatively small size and ideal insertion loss can be made. Sharper cut-off frequency responses can be acquired. The designed structure is simple to process and easy to achieve. The miniaturization of microwave circuits can be facilitated. A cost decrease can be further facilitated.
The patent application JP2018078471 describes lowpass filter wherein, In a lowpass filter constituted of multiple capacitors and multiple coils, the multiple capacitors are formed of a dielectric substrate having a first surface where multiple insular pattern electrodes are formed, and a second surface where electrodes are formed oppositely to the multiple pattern electrodes, the multiple coils are formed of a linear conductor bent in meander, the linear conductor is bent so that multiple conductor parts between opposite ends can be connected, respectively, with the multiple pattern electrodes, the conductor parts are connected, respectively, with the multiple pattern electrodes, and the opposite ends of the linear conductor are extending up to the same plane as the second surface of the dielectric substrate.
The patent application US005142252A describes audio signal transmission line with a low pass filter wherein it relates to audio signal transmission systems in general and in particular to an audio signal transmission line having a low pass filter circuit incorporated therein. This is directed to an audio signal transmission line with low pass filtering for eliminating undesired low level, low frequency oscillation in an audio signal transmission system. An audio signal transmission system comprises an audio signal generator and a load coupled to the generator by means of an audio signal transmission line. For example, the generator may comprise an acoustic transducer, such as a microphone, or an amplifier; the load may comprise an amplifier or a speaker; and the transmission line may comprise a pair of twisted or untwisted, single or multistrand wires or a coaxial cable. Even in systems comprising expensive and high quality components, it has been found that signals generated in the audio frequency range for transmission to the load can generate noise in the audio frequency spectrum on conventional transmission lines and cables which can in turn result in a significant and detectable distortion of the audio signals being transmitted thereon.
The patent application US005463346A describes fast response low pass filter wherein it relates to low-pass filters for attenuating the high-frequency components of a signal, and more particularly to a low-pass filter that is capable of responding to rapid and substantial changes in the amplitude of an input signal. The primary object of the invention is to provide a low-pass filter that is capable of adequately filtering ripple and other high-frequency signals while allowing the filter to respond to relatively rapid and substantial changes in the signal to be filtered. These and other objects of the invention are provided by a fast response low-pass filter formed by first and second low-pass filters each of which receive a common input signal and output a respective output signal. The first filter has a relatively high cutoff frequency so that it accurately follows the input signal but fails to significantly attenuate ripple. The second filter has a cutoff frequency that is changed from a relatively low frequency to a relatively high frequency in response to a control signal. When the cutoff frequency of the second filter is relatively low it effectively attenuates ripple, although it may not always respond fast enough to accurately follow the input signal. When the cutoff frequency of the second filter is relatively high it, like the first filter, fails to significantly attenuate ripple, although it accurately follows the input signal. A comparator connected to the first and second filters compares the respective values of the first and second output signals to each other and generates the control signal when either their difference or the absolute value of their difference exceeds a predetermined value. As a result, the fast response lowpass filter has a relatively low cutoff frequency to attenuate ripple until the input signal changes at a sufficient rate to cause the comparator to generate the control signal. The response time of the second filter then changes to the relatively high cutoff frequency so that it can accurately follow the input signal. The fast response low-pass filter may be implemented using either analog circuitry or digital filtering techniques.
The patent application US20120169436A1 describes microwave filter wherein it relates to that includes a transmission line having a signal input port and a signal output port, a stub connected to the transmission line between the input port and the output port, and a spurline embedded in the stub. The microwave filter is configured to substantially attenuate a frequency while substantially passing at least one predetermined odd harmonic of the frequency. In accordance with the teachings of the present application, a notch filter blocks a central notch frequency but passes at least one select odd harmonic of the central frequency without requiring the complex circuit structures of the prior art. A circuit is coupled to a transmission line and configured to appear to the transmission line as a short circuit at the central frequency and to appear as an open circuit at the at least one select odd harmonic of the central frequency. The currently preferred circuit structure is a stub line that has a spurline embedded therein so that a simple structure is still provided for the notch filter. According to one aspect of the present invention, a microwave filter comprises a transmission line comprising a signal input port and a signal output port. A stub is connected to the transmission line between the input port and the output port and a spurline is embedded in the stub. The microwave filter is configured to substantially attenuate a frequency while Substantially passing a predetermined odd harmonic of the frequency. The stub may have a first electrical length and the Spurline a second electrical length with the first and second electrical lengths being fractions of a wavelength of the frequency. The predetermined odd harmonic may comprise the third harmonic of the frequency.
The patent application US 9667063B1 describes harmonic filter for multipulse converter systems wherein an improved scheme to filter harmonics and damp resonance for multipulse converter systems using a single characteristic passive filter branch without a plurality of non characteristic harmonic filters is disclosed. There is provided a filter for a multipulse converter system that has characteristic harmonic currents and causes non-characteristic harmonic resonance, the filter comprising: a first series capacitor, a first series inductor, a frequency-dependent resistor block providing high equivalent resistance for damping non-characteristic harmonic resonance, including a second capacitor in series with a resistor and a second inductor; and the first series capacitor, first series inductor and the frequency-dependent resistor block being jointly tuned to filter the characteristic harmonic currents. In a further embodiment, there is provided a method for filtering harmonic currents generated by a multipulse converter system comprising providing a first series capacitor, a first series inductor and a frequency-dependent resistor block, the frequency-dependent resistor block including a second capacitor in series with a resistor and a second inductor; damping non characteristic harmonic resonance of the multipulse converter system by the frequency dependent resistor block providing high equivalent resistance for non-characteristic harmonic resonance; and filtering the characteristic harmonic currents by the first series capacitor, first series inductor and the frequency-dependent resistor block being jointly tuned to filter the characteristic harmonic Currents. In further embodiments, the characteristic harmonic currents comprises the 11th harmonic or higher; and the non characteristic harmonic comprises the 3rd , 5th and 7th harmonics etc.
For power amplifiers the output contains amplified version of the input signal and also the harmonics and inter modulation distortion (IMD) products of it. The first harmonic is the fundamental itself. So the second harmonic, the third harmonic, the fourth harmonic etc., are called the harmonic distortion of the power amplifier. This is the called out of band distortion. The IMD products are called the in band distortion. They both the harmonic distortion and the IMD products are caused by the inherent nonlinearity of the transistor. The harmonic distortion or the out of band distortion at the output of the power amplifiers can be filtered with the help of the filters.
Therefore there is a need for a passive, analog, lumped element, elliptical low pass filter comprising a very high transition ratio between the pass band and stop band, a very low insertion loss, high power handling capability in the pass band of interest, realized in a very small size on a microwave dielectric substrate which is easy to fabricate and assemble, also which has low manufacturing cost, whose main application is to suppress the harmonics at the output of power amplifiers and also high harmonic suppression achieved across very wideband.

SUMMARY
An aspect of the present invention is to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. The assembly disclosed herein addresses the above stated need of a passive, analog, lumped element, elliptical low pass filter coupled to output of a power amplifier for harmonic suppression. The filter assembly comprises a microwave dielectric substrate coupled to a power amplifier, the microwave dielectric substrate comprising a top surface and a bottom surface opposing each other. The microwave dielectric substrate comprises an input port and an output port on the top side of the microwave dielectric substrate. A first transmission line corresponding to the input port and a second transmission line corresponding to the output port realized on the top side of said microwave dielectric substrate. One or more metal pads are realized between the first transmission line and said second transmission line on the top side of the microwave dielectric substrate for electrical contact between the first transmission line and the second transmission line. One or more lumped elements are soldered on the top side of the microwave dielectric substrate for realizing a low pass filter of maximum capacitor topology. One or more capacitors of the low pass filter optimized for achieving low insertion loss. A copper cladding portion on the bottom side of the microwave dielectric substrate is configured as a ground. The microwave dielectric substrate mounted on an extension of heat sink of the power amplifier.
Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS
The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which.
FIG. 1 illustrates a two element low pass filter.
FIG. 2 illustrates an elliptic analog low pass filter.
FIG. 3 illustrates an embodiment of the elliptic analog low pass filter with a different topology.
FIG. 4 illustrates an nth order elliptic analog low pass filter with one type of topology.
FIG. 5 illustrates an mth order elliptic analog low pass filter with a different type of topology.
FIG. 6 illustrates a higher order analog elliptic low pass filter.
FIG. 7A illustrates a parallel LC section as a tank circuit in the higher order analog elliptic low pass filter.
FIG. 7B illustrates a microwave dielectric substrate for realizing the higher order analog elliptic low pass filter.
FIG. 8 is a graph illustrating measured insertion loss (dB), Input return loss (dB), and Output return loss (dB) of the higher order analog elliptic low pass filter.
FIG. 9 is a graph illustrating measured results of second and third harmonics of the higher order analog elliptic low pass filter.
FIG. 10 is a graph illustrating measured results of second, third, fourth, and fifth harmonics of the higher order analog elliptic low pass filter.
FIG. 11 is a graph illustrating measured results of input return loss of of the higher order analog elliptic low pass filter.
FIG. 12 is a graph illustrating measured results of output return loss of the higher order analog elliptic low pass filter.
FIG. 13 is a graph illustrating pass band cut-off frequency of the higher order analog elliptic low pass filter.
FIG. 14 is a graph illustrating magnitude and angle of S21 of the higher order analog elliptic low pass filter.
It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative methods embodying the principles of the present disclosure. Similarly, it will be appreciated that any flow charts, flow diagrams, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.
Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure. Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.

DETAILED DESCRIPTION
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.
The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces.
By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic is intended to provide.
The present disclosure describes about passive, analog, lumped element, elliptical low pass filter.
In the following description, for purpose of explanation, specific details are set forth in order to provide an understanding of the present disclosure. It will be apparent, however, to one skilled in the art that the present disclosure may be practiced without these details. One skilled in the art will recognize that embodiments of the present disclosure, some of which are described below, may be incorporated into a number of systems.
However, the systems and methods are not limited to the specific embodiments described herein. Further, structures and devices shown in the figures are illustrative of exemplary embodiments of the presently disclosure and are meant to avoid obscuring of the presently disclosure.
It should be noted that the description merely illustrates the principles of the present invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described herein, embody the principles of the present invention. Furthermore, all examples recited herein are principally intended expressly to be only for explanatory purposes to help the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.
FIGS. 1 through 14, discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way that would limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged communications system. The terms used to describe various embodiments are exemplary. It should be understood that these are provided to merely aid the understanding of the description, and that their use and definitions, in no way limit the scope of the invention. Terms first, second, and the like are used to differentiate between objects having the same terminology and are in no way intended to represent a chronological order, unless where explicitly stated otherwise. A set is defined as a non-empty set including at least one element.
FIGS. 1-14 are merely representational and are not drawn to scale. Certain portions thereof may be exaggerated, while others may be minimized. FIGS. 1-14 illustrate various embodiments of the invention that can be understood and appropriately carried out by those of ordinary skill in the art.
In the foregoing detailed description of embodiments of the invention, various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments of the invention require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the detailed description of embodiments of the invention, with each claim standing on its own as a separate embodiment.
FIG. 1 discloses an L-configuration of an analog low pass filter 103 that comprises reactive elements. The L-configuration of the passive analog low pass filter 103 comprises a single series inductor La 101 and a single shunt capacitor Ca 102. The elements 101 and 102 are lumped elements and are connected between impedances Z1 and Z2 to form the analog low pass filter 103.
FIG. 2 discloses a p-configuration of a passive third order elliptic analog low pass filter 204. The passive third order elliptic low pass filter 204 has two shunt arms 202 and 203, where the shunt arms 202 and 203 comprise capacitors C1 and C2 respectively. The passive third order elliptic low pass filter 204 comprises a series arm 201 that encompasses a tank circuit. The series arm 201 comprises an inductor Lt and a capacitor Ct parallel to each other. The elements used in the passive third order elliptic low pass filter 204 are lumped elements. 201, 202, and 203 are connected between impedances Z1 and Z2 to form a filter.
The elliptic or elliptical filter is also known as a Cauer filter or a Zolotarev filter. The filter is used in radio frequency (RF) applications where a transition between the passband and stopband frequencies is required. The elliptic filter produces the faster transition between the passband and stopband frequencies compared to other types of filters, for example, Butterworth, Chebyshev, Bessel, etc. The elliptical filter exhibits gain ripple in both passband and stopband. One or more spurious signals may be present just outside the required bandwidth and the spurious signals have to be removed. The levels of ripple in the passband and stopband are independently adjustable during the design. As the ripple in the stopband approaches zero, the response of the elliptic filter becomes similar to response of a Chebyshev type I filter, and as the ripple in the passband approaches zero, the response of the elliptic filter becomes similar to response of a Chebyshev type II filter. If the ripple in both stopband and passband become zero, then the response of the elliptic filter becomes similar to response of a into a Butterworth filter. The embodiments of the elliptic filter can be realized using two circuit configurations used for the analog low pass filter versions of the Cauer elliptic filter. In an embodiment, one configuration has a capacitor and inductor in parallel across each other (also called tank circuit) in series path of the signal line. This embodiment is called as maximum capacitor topology elliptic filter. In another embodiment, a series inductor and capacitor branch connected in shunt between signal line and ground reference as shown in the figure FIG. 3. This embodiment is called as maximum inductor topology elliptic filter.
FIG. 3 discloses a T-configuration of a passive third order elliptic analog low pass filter 304. The T-configuration of a passive third order elliptic analog low pass filter 304 has two series arms 301 and 302 and the series arms 301 and 302 comprise inductors L1 and L2 respectively. The filter 304 comprises one shunt arm 303 that encompasses a series inductor L3 and a series capacitor C3. The elements in the passive third order elliptic analog low pass filter 304 are lumped elements. 301, 302, and 303 are connected as shown in the figure between impedances Z1 and Z2 to form a filter.
FIG. 4 discloses an nth order elliptic analog low pass filter 408. The nth order elliptic analog low pass filter 408 comprises shunt arms 406, 407,- - - - , 107, 108 that comprise capacitors C4, C5, - - - - , C(n-2) , C(n-1) respectively. The series arms of the filter are 405, - - - -, 106 encompasses tank circuits. The nth order elliptic analog low pass filter 408 comprises elements that are lumped elements. FIG. 2 is an embodiment of FIG. 4. 405, 406, 407, - - - - , 106, 107, 108 are connected between impedances Z1 and Z2 to form the nth order elliptic analog low pass filter 408.
FIG. 5 discloses an mth order elliptic analog low pass filter 504. The mth order elliptic analog low pass filter 504 comprises the series arms are 502, 503, - - - - , 207, 208 that comprise inductors L5, L6, - - - - , L(m-2), L(m-1) respectively. The shunt arms of the mth order elliptic analog low pass filter 504 are 501, - - - -, 206 that encompass series inductor and capacitor. The elements used in the mth order elliptic analog low pass filter 504 are lumped elements. FIG. 3 is an embodiment of FIG. 5. 501, 502, 503, - - - - , 206, 207, 208 are connected as shown in the figure between impedances Z1 and Z2 to form the mth order elliptic analog low pass filter 504.
FIG. 6 discloses a higher order analog elliptic low pass filter 612 comprising elements that are lumped elements. FIG. 2 is an embodiment of FIG. 6 in the present invention where in plurality of the topology in FIG. 2 are cascaded to get the desired filter as shown in FIG. 6. 601, 603, 605, 607, and 609 are shunt arms of the higher order analog elliptic low pass filter 612 that encompass capacitors C8, C9, C10, C11, and C12 respectively. 602, 604, 606, 608, 611 are the series arms that encompass tank circuits T1, T2, T3, T4, T5 respectively. 601, 602, 603, 604, 605, 606, 607, 608, 609, 611 are connected between input and output to form a eleventh order analog elliptic low pass filter. The filter input and output impedances both are 50 ohms.
FIG. 7A discloses a parallel LC section, comprising an inductor 701 and a capacitor 702. The LC section connected between an input port and an output ports is a tank circuit. The tank circuits T1, T2, T3, T4, T5, as disclosed in FIG. 6 comprise a tank circuit as shown in FIG. 7A. The inductor 701 and the capacitor 702 are connected in parallel to perform the desired function of creating a transmission zero. The tank circuit comprising the inductor 701 and the capacitor 702 creates a transmission zero in the low pass filter. The transmission zero of the filter is described as a frequency at which the transfer function of a filter has zero transmission. The transmission zeroes at infinite frequency may be found in low-pass filters.
For a low-pass filter, the zeros are located at infinite frequency. To create an elliptical filter finite frequency transfer function zeros are added to poles. The elliptical filter has a shorter transition region than the Chebyshev filter as the elliptical filter allows ripples in both the stop band and the pass band. The addition of zeros in the stop band causes ripple in the stop band and provides a higher rate of attenuation corresponding to the number of poles. An inductor and a capacitor are connected in parallel to form a tank circuit. The tank circuit performs the desired function of creating a transmission zero in the low pass filter. In another embodiment, inclusion of a shunt branch comprising an inductor and capacitor in series creates zero in the low pass filter transfer function. The standard elliptical filter parameters are the filter order, the passband ripple, the stopband ripple, the pass band cutoff frequency (?p), the stop band cutoff frequency (?s), the transition ratio (?p/?s), and the stop band attenuation.
The low pass filter with stop-band rejection of -60dB is cascaded with power amplifier to achieve desired harmonic suppression levels. Introduction of multiple transmission zeros in the filter aids in achieving harmonic rejection. Low passband insertion loss is achieved by using high Q components like air core inductors. The air core inductors with thicker copper wire have higher current carrying capacity. The choice of the capacitors is based on maximum working voltage and current carrying capacity. Circuit analysis with multi-tone RF source assists in finding currents across each component in the filter. The circuit analysis is inevitable for designing filters rated for power as high as 150W.
For reducing the complexity of design, all the inductors in the higher order analog elliptic low pass filter have the same value while capacitors are optimized for low insertion loss and high rejection. In an embodiment, the value of the inductors is about 6.6 nano henries. The microwave dielectric substrate is thermally conductive and is used for realizing the higher order analog elliptic low pass filter. The higher order analog elliptic low pass filter is an elliptical low pass filter with five sections. The higher order analog elliptic low pass filter is compact and has an insertion loss of about -0.4dB. However, the high stop-band rejection of -60dB was achieved till the 3rd harmonic frequency due to series resonant frequency limitation of lumped components.
However, if the inductors are of different values and not maintained at a same value, while optimizing the said filter lumped components for the desired results, the sizes of all the inductors shall not be the same. A few of the inductors may be larger than the few others. To solder these inductors on to the PCB, the microstrip copper pads cannot be realized equidistant from each other thereby increasing the size of the higher order analog elliptic low pass filter. Further, all inductor values with desired current handling capability may not be available with the inductor component vendor. However based on the standard values available, the inductor of 6.6 nano henries is considered from such values available with him. Furthermore the capacitors chosen are of 100B size and 800B size and there are five tank circuits in the higher order analog elliptic low pass filter. The tank circuit comprises components such as one inductor and one capacitor parallel to each other. All the tank circuit capacitors are of 100B size. The inductors are maintained at same value in the filter and the capacitors are varied with several combinations of their values (optimized) to obtain low insertion loss and high rejection. The insertion loss is about -0.183 dB minimum to -0.384 dB maximum across the desired passband of interest which is 960-1240 MHz. Therefore the RF (radio frequency) power dissipated in the filter is low and therefore can be used for high power applications. The percentage bandwidth achieved is 25.54%.
The filter is capable of handling high radio frequency (RF) power of about 150W continuous wave in the band of interest that is 960 MHz – 1240 MHz. And the power dissipated in the higher order analog elliptic low pass filter is 12.73W. The higher order analog elliptic low pass filter comprises five aircore inductors and ten ceramic capacitors that are soldered on the microwave dielectric substrate. The five aircore inductors and the ten ceramic capacitors along with the microwave dielectric substrate dissipate 12.73W. The higher order analog elliptic low pass filter has high current carrying capacity and has high working voltage. The microwave dielectric substrate is placed on the heat sink for conducting away heat.
The elliptic low pass filter in the invention, wherein the magnitude of the achieved input reflection coefficient or the achieved input voltage standing wave ratio in the passband of interest of 960 - 1240MHz is such that it allows more than 95% of the incident signal power at its input to flow into it. The so allowed signal is processed inside the filter and is allowed to flow out. Furthermore the magnitude of the achieved output reflection coefficient or the achieved output voltage standing wave ratio of this filter in the said invention is such that it allows more than 95.5% of the processed signal inside it to flow out.
FIG. 7B discloses a microwave dielectric substrate for realizing the higher order analog elliptic low pass filter 612 as disclosed in the FIG. 6. The microwave dielectric substrate along with the higher order analog elliptic low pass filter 612 is the filter assembly that is coupled to the power amplifier for achieving harmonic suppression. The microwave dielectric substrate comprises an input port a01 and an output port a02. The microwave dielectric substrate comprises a plurality of metal pads P1, P2, P3, P4, P5, and P6 wherein the metal pads are copper pads. The metal pads are of equal size and are equidistant from each other. The microwave dielectric substrate further comprises a first transmission line b01 corresponding to the input port a01 and a second transmission line b02 corresponding to the output port a02. The characteristic impedance of the microstrip transmission line b01 and the microstrip transmission line b02 is 50 ohm. The transmission lines b01 and b02 are microstrip transmission lines realized to facilitate soldering of the SubMiniature version A connectors at both ends. The metal pad P1 is connected to the transmission line b01 and the metal pad P6 is connected to the transmission line b02 and the metal. In an embodiment, a common ground plane GP is formed wherein one or more ground vias are realized.
The higher order analog elliptic low pass filter has a higher transition ratio between the pass band and stop band, a low insertion loss, higher power handling capability in the pass band of interest in comparison with earlier mentioned low pass filters. The higher order analog elliptic low pass filter realized in a small size microwave dielectric substrate is fabricated and assembled and has low manufacturing cost. The goal of the higher order analog elliptic low pass filter is to suppress harmonics at the output of power amplifiers. A high harmonic suppression is achieved across a wideband. The size of the 6.6 nano henries inductor is comparable to the size of the 100B size capacitor that enables maintenance of equal distances between pads P1, P2, P3, P4, P5 and P6, wherein in an embodiment, the distance between the pads is 1.9 mm. In an embodiment, the pads P1, P2, P3, P4, P5 and P6 are of the size 3mm x 8.8 mm each. The parallel inductor-capacitor pairs of the tank circuit are soldered between the pads P1, P2, P3, P4, P5 and P6.
The higher order analog elliptic low pass filter is realized in a compact form-factor of size 37.5mm × 20mm on the microwave dielectric substrate with lumped elements. All the components, such as the lumped elements, are multiturn aircore inductors and ceramic capacitors that are soldered on the top side of the microwave dielectric circuit, that is, printed circuit board (PCB). The complete bottom side copper cladding of the microwave dielectric substrate is a complete ground for enabling the microwave dielectric substrate to be put over a metallic heat sink directly for rapidly conducting away the heat, thereby enhancing continuous wave (CW) power handling capacity of filter. The insertion loss of the filter is low and the corresponding RF power dissipated in it is low, therefore the higher order analog elliptic low pass filter can be used for high power applications. The top side ground portion and the bottom side ground portion are merged with multiple ground vias. The microwave dielectric substrate has a relative dielectric constant of 3.48, the height of the dielectric material is 0.762mm, the copper cladding thickness of the substrate is 0.035mm on both the sides and the loss tangent of the substrate is 0.0031.
In another aspect the higher order analog elliptic low pass filter is put at the output of the power amplifier for harmonic suppression which is the main application. In an embodiment, the heat sink is a small extension of the power amplifier heat sink for mounting the microwave dielectric substrate of the higher order analog elliptic low pass filter as the size of the microwave dielectric substrate is small. The heat sink is used for conducting away the heat under radio frequency (RF), and continuous wave (CW) conditions. The topology employed is the maximum capacitor topology that aids in reducing the size of the higher order analog elliptic low pass filter. The transmission lines are converted into metal pads or landing pads for the components such as the lumped elements. The microstrip lines have been extended at ends to couple RF signal. The metal pads or landing pads are similar and equidistant and the inductors used in all the sections of the higher order analog elliptic low pass filter are of the same size. The higher order analog elliptic low pass filter provides a fractional bandwidth of 25% due to sharp transmission zero created by the tank circuit. Sharpness of cut-off is due to five series tank circuits in the higher order analog elliptic low pass filter.
The maximum capacitor topology aids in reducing the size of the filter as the high current aircore inductors are bulky when compared to the capacitors and there would not be a compromise on the performance when compared to maximum inductor topology.
FIG. 8 is a graph illustrating measured insertion loss (dB), Input return loss (dB), and Output return loss (dB) of the higher order analog elliptic low pass filter disclosed in the FIG. 6. The Y-axis is in dB(decibel) scale and the X-axis is in megahertz frequency. The graph illustrates insertion loss (dB), Input return loss (dB) and Output return loss (dB) of the higher order analog elliptic low pass filter. The insertion loss is the trace dB(S21) plotted. The band of interest is 960MHz to 1240MHz. The insertion loss at 960MHz is -0.1827dB also the insertion loss at 1090MHz is -0.3844dB and further the insertion loss at 1240MHz is -0.3086dB as marked and shown. Since the insertion loss is low in the desired band, the power dissipated in the filter is also low and so this filter can be used for high power applications.
FIG. 9 is a graph illustrating measured results of second and third harmonics of the higher order analog elliptic low pass filter. The graph has dB(S21) of the filter plotted on it against frequency from 100MHz to 5500MHz. The harmonic rejection or the harmonic suppression is shown in the trace dB (S21) plotted. The Y-axis is in dB scale and the X-axis is in megahertz frequency. The band of interest is 960MHz to 1240MHz. The elliptical low pass filter designed will be put at the output of the power amplifier. The first harmonic of the input signal is the fundamental itself. The second and third harmonics generated at the output of the power amplifier are suppressed by the higher order analog elliptic low pass filter. As the desired passband edges are 960MHz and 1240MHz, their second harmonics fall at frequencies of 1920MHz and 2480MHz, they are suppressed by -85.13dB and -66.22dB respectively. Furthermore their third harmonics fall at frequencies of 2880MHz and 3720MHz, they are suppressed by -63.41dB and -58.97dB respectively.
FIG. 10 is a graph illustrating measured results of second, third, fourth, and fifth harmonics of the higher order analog elliptic low pass filter. The complete response to the input signal of the analog elliptical low pass filter is shown from 100MHz to 7000MHz. The graph has a dB(S21) of the filter plotted on it against frequency showing the harmonic rejection or harmonic suppression achieved. The elliptical low pass filter designed is put at the output of the power amplifier. As the desired passband is 960 - 1240MHz, their second harmonics fall in the band 1920MHz to 2480MHz, they are suppressed by better than -60dB. Also the third harmonics that fall in the band 2880MHz to 3720MHz, are suppressed by around -60dB. In another aspect the fourth and fifth harmonics generated at the output of the power amplifier are also called as harmonic distortion and are suppressed by the the higher order analog elliptic low pass filter. The fourth harmonics fall in the band 3840MHz to 4960MHz, they are suppressed by around -18dB minimum. Also the fifth harmonics fall in the band 4800MHz to 6200MHz, they are suppressed by better than -33dB.
FIG. 11 is a graph illustrating measured results of input return loss of the higher order analog elliptic low pass filter. The Y-axis is in dB scale and the X-axis is in megahertz frequency. In this graph, the input return loss of the higher order analog elliptic low pass filter is highlighted to show the value at different frequency points. The input return loss is the trace dB (S11) plotted. The band of interest is 960MHz to 1240MHz. The input return loss at 960MHz is -18.93dB also the input return loss at 1081MHz is -13.07dB and further the input return loss at 1226MHz is -22dB and also the input return loss at 1240MHz is -21.09dB as marked.
FIG. 12 is a graph illustrating measured results of output return loss of the higher order analog elliptic low pass filter. In this plot the output return loss of the filter is highlighted showing its value at different frequency points The Y-axis is in dB scale and the X-axis is in megahertz frequency. The output return loss is the trace dB (S22) plotted. The band of interest in the said invention is 960MHz to 1240MHz. The output return loss at 960MHz is -19.56dB also the output return loss at 1085MHz is -13.45dB and further the output return loss at 1228MHz is -27.85dB and also the output return loss at 1240MHz is -24.73dB as marked.
FIG. 13 is a graph illustrating pass band cut-off frequency of the higher order analog elliptic low pass filter. The graph has a dB(S21) of the higher order analog elliptic low pass filter plotted on it against frequency from 100MHz to 1600MHz It is also called the 3dB cutoff frequency of the filter. It is also called the half power point of the filter. This pass band cutoff frequency of the filter in the said invention is at 1499MHz as shown in the present figure. At this point the insertion loss of the filter is -3dB. FIG. 14 is a graph illustrating magnitude and angle of S21 of the higher order analog elliptic low pass filter plotted on it against frequency from 100MHz to 1600MHz wherein 1499MHz is the passband cut off frequency. The graph displays a phase response of the passive, elliptical, eleventh order, low pass filter.
The higher order analog elliptic low pass filter with transition ratio of 0.78 and stop-band attenuation of around 60dB, which is achieved till third harmonic frequency, has been designed in L band frequency. The higher order analog elliptic low pass filter is cascaded at the output of the power amplifier. A high current rated air core coil is used for inductors. The air core inductors with thicker copper wire have higher current carrying capacity. The choice of capacitors is based on the maximum working voltage and current carrying capacity. Performing circuit analysis with multi-tone RF power source ensures that the current across components is within an accepted limit. Since the high current aircore inductors are bulky when compared to capacitors, the maximum capacitor topology of elliptic filter has been chosen for harmonic suppression. The inductors are of equal value and capacitors are optimized for obtaining low insertion loss and high rejection. The higher order analog elliptic low pass filter is realized with 5 sections on the microwave dielectric substrate with high thermal conductivity in a compact form-factor of 37.5mm × 20mm. The higher order analog elliptic low pass filter has an insertion loss of around -0.4dB and high stop-band rejection of around -60dB achievable till 3rd harmonic frequency due to series resonant frequency limitation of lumped components.
The arms 601, 602, 603, 604, 605, 606, 607, 608, 609, 611, disclosed in the detailed description of FIG. 6, between the input and the output to form a eleventh order analog elliptic low pass filter. The higher order analog elliptic low pass filter input impedance is 50 ohms and output impedance is 50 ohms.
The passband of interest in the higher order analog elliptic low pass filter is 960MHz to 1240MHz. The insertion loss at 960MHz is -0.1827dB, also the insertion loss at 1090MHz is -0.3844dB, and further the insertion loss at 1240MHz is -0.3086dB as marked and shown in the FIG. 8. The insertion loss is quite low in the desired band therefore the power dissipated in the filter is low. This allows the higher order analog elliptic low pass filter to be used for high power applications. The elliptical low pass filter designed is cascaded at the output of the power amplifier. The first harmonic of the input signal is the fundamental itself. The second and third harmonics generated at the output of the power amplifier are called harmonic distortion and that are to be suppressed. This higher order analog elliptic low pass filter suppresses the harmonics generated by the power amplifier. As the desired passband is 960 - 1240MHz, the second harmonics fall in the band 1920MHz to 2480MHz that are suppressed by around -60dB as shown in the FIG. 9. The third harmonics fall in the band 2880MHz to 3720MHz that are suppressed by around -60dB as shown in the FIG. 9. In another aspect the fourth and fifth harmonics generated at the output of the power amplifier are also called harmonic distortion that are to be suppressed by the higher order analog elliptic low pass filter. The fourth harmonics fall in the band 3840MHz to 4960MHz, they are suppressed by around -18dB minimum. Also the fifth harmonics fall in the band 4800MHz to 6200MHz, they are suppressed by around -33dB as shown in the FIG. 10.
The pass band cutoff frequency of the higher order analog elliptic low pass filter is at 1499MHz as shown in the figure FIG. 13 and the insertion loss of the filter is -3dB. In FIG. 14 a pass band phase response of the filter is plotted.
In the foregoing detailed description of embodiments of the invention, various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments of the invention require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the detailed description of embodiments of the invention, with each claim standing on its own as a separate embodiment.
It is understood that the above description is intended to be illustrative, and not restrictive. It is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein,” respectively.

,CLAIMS:We Claim:
1. A high power low pass filter assembly coupled to a power amplifier for harmonic suppression, said filter assembly comprises:
a thermally conductive microwave dielectric substrate coupled to a power amplifier, said microwave dielectric substrate comprising a top surface and a bottom surface;
said microwave dielectric substrate comprising a ground plane, an input port and an output port on said top side of said microwave dielectric substrate;
a first transmission line corresponding to said input port and a second transmission line corresponding to said output port realized on said top side of said microwave dielectric substrate;
one or more metal pads realized between said first transmission line and said second transmission line on said top side of said microwave dielectric substrate for electrical contact between said first transmission line and said second transmission line;
with one or more tank circuits and also comprising a plurality of lumped elements soldered on said top side of said microwave dielectric substrate for realizing a low pass filter of maximum capacitor topology, wherein an input end of said tank circuit comprises atleast one capacitor shunted to ground;
one or more capacitors of said low pass filter optimized for achieving low insertion loss;
a copper cladding portion on said bottom side of said microwave dielectric substrate configured as a ground; and
said microwave dielectric substrate mounted on an extension of heat sink of said power amplifier.
2. The filter assembly of claim 1, wherein said low pass filter is of eleventh order.
3. The filter assembly of claim 1, wherein said low pass filter is analog and passive.
4. The filter assembly of claim 1, wherein said low pass filter is an elliptical filter.
5. The filter assembly of claim 1, wherein characteristic impedance of said first transmission line and said second transmission line is 50 ohm.
6. The filter assembly of claim 1, wherein said filter is realized on said microwave dielectric substrate of form-factor of about 37.5 mm length and 20 mm width and height of said microwave dielectric substrate is 0.762 mm.
7. The filter assembly of claim 1, wherein dielectric constant of said microwave dielectric substrate is 3.48.
8. The filter assembly of claim 1, wherein the sizes of the said consecutive metal pads are equal and the distances between the said consecutive metal pads are equal.
9. The filter assembly of claim 1, wherein insertion loss is from about -0.183 dB to about -0.384dB.
10. The filter assembly of claim 1, wherein passband of interest is from about 960 MHz to about 1240 MHz.
11. The filter assembly of claim 1, wherein power handling capability is about 150W.
12. The filter assembly of claim 1, wherein said low pass filter is realized with upto five sections on said microwave dielectric substrate.
13. The filter assembly of claim 1, wherein said lumped elements are aircore inductors and ceramic capacitors.
14. The filter assembly of claim 13, wherein said values of said inductors are equal and values of said capacitors are different.
15. The filter assembly of claim 13, wherein said lumped elements are upto five aircore inductors and upto ten ceramic capacitors.
16. The filter assembly of claim 1, wherein said transition ratio is about 0.78.
17. The filter assembly of claim 1, wherein stop band attenuation of around -60dB is achieved till third harmonic frequency.
18. The filter assembly of claim 1, wherein said ground plane on said top surface and said ground on said bottom surface are connected by one or more vias.
Dated this 30th day of March, 2019

For BHARAT ELECTRONICS LIMITED,
By their Agent,

(D. Manoj Kumar) (IN/PA – 2110)
KRISHNA & SAURASTRI ASSOCIATES LLP