EP2207237A1 - Tiefpassfilter - Google Patents

Tiefpassfilter Download PDF

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Publication number
EP2207237A1
EP2207237A1 EP09290011A EP09290011A EP2207237A1 EP 2207237 A1 EP2207237 A1 EP 2207237A1 EP 09290011 A EP09290011 A EP 09290011A EP 09290011 A EP09290011 A EP 09290011A EP 2207237 A1 EP2207237 A1 EP 2207237A1
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EP
European Patent Office
Prior art keywords
section
frequency
filter
lowpass filter
capacitive
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Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
EP09290011A
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English (en)
French (fr)
Inventor
Dieter Pelz
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Alcatel Lucent SAS
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Alcatel Lucent SAS
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Publication date
Application filed by Alcatel Lucent SAS filed Critical Alcatel Lucent SAS
Priority to EP09290011A priority Critical patent/EP2207237A1/de
Publication of EP2207237A1 publication Critical patent/EP2207237A1/de
Ceased legal-status Critical Current

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01P—WAVEGUIDES; RESONATORS, LINES, OR OTHER DEVICES OF THE WAVEGUIDE TYPE
    • H01P1/00—Auxiliary devices
    • H01P1/20—Frequency-selective devices, e.g. filters
    • H01P1/201—Filters for transverse electromagnetic waves
    • H01P1/203—Strip line filters
    • H01P1/2039—Galvanic coupling between Input/Output

Definitions

  • the present invention relates to a filter for filtering radio frequency waves, in particular high frequency (HF) or ultrahigh frequency (UHF) waves.
  • the filter comprises a transmission line structure in the form of a thin conducting stripline structure centered between two conducting planes.
  • the stripline comprises a first lowpass filter section having a first lowpass filter element.
  • radio frequency is a frequency or rate of oscillation within the range of about 3 kHz to 300 GHz. This range corresponds to frequency of alternating current electrical signals used to produce and detect radio waves.
  • High frequency (HF) radio frequencies are between 3 and 30 MHz.
  • Very high frequency (VHF) is the radio frequency range from 30 MHz to 300 MHz.
  • Ultra high frequency (UHF) designates a range (band) of electromagnetic waves with frequencies between 300 MHz and 3 GHz (3,000 MHz).
  • Filters for radio frequency waves are traditionally used in radio frequency transmitters such as high power UHF TV broadcast transmitters.
  • the signals generated by the transmitter may comprise frequency components, which fall into the frequency band of a neighboring (co-located) communication network.
  • lowpass filters are used in order to suppress frequency components, which may cause disturbance in neighboring communication networks.
  • a lowpass filter is a filter that passes low-frequency signals but attenuates (reduces the amplitude of) signals with frequencies higher than the cutoff frequency.
  • the cutoff frequency represents a boundary in the lowpass filters' frequency response at which energy entering the system begins to be attenuated by reflection instead of being transmitted.
  • the 3-dB cutoff frequency is the frequency above which the output power of the electronic filter is one half of the power of the passband signals. Because power is proportional to the square of voltage, the voltage signal is 70.7% of the passband voltage at the cutoff frequency. Hence, the corner frequency is also known as the -3 dB point because 70.7% is close to -3 decibels.
  • a stripline is a TEM (transverse electromagnetic) transmission line media.
  • a stripline circuit uses a flat strip of metal which is sandwiched between two parallel ground planes. The insulating material of the substrate forms a dielectric. The width of the strip, the thickness of the substrate and the relative permittivity of the substrate determine the characteristic impedance of the stripline. The central conductor need not be equally spaced between the ground planes.
  • a stripline transmission line network represents a complex electrical filter, which may be described using a distributed element model.
  • the distributed element model or transmission line model of electrical circuits assumes that the attributes of the circuit (resistance, capacitance, inductance) are distributed across the whole geometry according to the theory of transmission lines.
  • the overall stripline may be divided into sections, which roughly correspond to a capacitance or inductance for frequencies within the passband. Therefore, standard lumped element models may be used in the process of designing a transmission line lowpass filter.
  • the filter elements are identified with shunt capacitances or series inductances together producing the overall properties of the filter.
  • a filter element providing a positive series reactance in the desired passband frequency range is identified with a series inductance; a filter element having a negative shunt reactance is identified with a shunt capacitance in the lumped element model of the stripline structure.
  • the lowpass filter designed according to the lumped element network model does not represent an ideal lowpass filter.
  • the main reason for this is that the transfer function of the stripline modeled according to the distributed element network model is a periodic function. This means that the passband section of the lowpass filter re-occurs at a higher frequency. The lowpass frequency section of the transfer function re-occurs periodically. The period of re-occurrence is determined by the lengths of the transmission line sections. Long transmission line elements result in a smaller period of re-occurrence. Therefore, the attenuation of high frequency signals is limited by the inherent periodicity of the transmission line elements used in the lowpass filter.
  • the lowpass filter's pass band usually extends from DC to a cutoff frequency fc.
  • the resulting lumped element network has relatively large series inductances, which in turn leads to long transmission line series elements in the distributed element filter network.
  • the pass band characteristic re-occurs after a relatively short frequency period, because the period of re-occurrence is smaller for long transmission line elements.
  • the filter for filtering radio frequency waves, in particular HF or UHF waves, as defined in appended claim 1.
  • the Filter comprises a transmission line structure having a thin conducting stripline structure arranged between two conducting planes.
  • the stripline structure comprises a first lowpass filter section.
  • the first lowpass filter section has a transfer function H(f) that resembles a lowpass filter characteristic having a cutoff frequency fc within a predetermined frequency band.
  • the pass band of the low pass filter is limited to a frequency band ranging from the cutoff frequency fc to a lower frequency fl greater than zero.
  • the passband of the lowpass filter is constricted to the relevant frequency range.
  • Frequencies lower than the lower cutoff frequency fl do not have to be attenuated, since the transmitter is not likely to generate frequency components below the nominal transmitter frequency.
  • the constricted passband enables using smaller series inductances in a lowpass filter prototype - resulting in shorter high impedance sections in the distributed element realization, which in turn reduces the transmission periodicity, i.e. the frequency period within which the transfer function H(f) is smaller is extended.
  • the short high impedance sections each have a length that is shorter than the quarter-wavelength at the cutoff frequency fc.
  • the high-Z sections have a characteristic impedance of about 100 Ohm.
  • the low-Z sections are concerned Z ⁇ 50 Ohms applies such that the characteristic impedance is 10 Ohm or below.
  • the first lowpass filter section is adapted to generate transmission zeros at predetermined stop band frequencies (fn). Transmission zeros are critical frequencies where signal transmission between input and output is stopped (zero).
  • Finite-frequency transmission zeros may be realized in lumped element form either as parallel resonators in series or series resonators to ground. In both cases - since resonance is at a frequency above the lowpass filter passband - the resonators still have exactly the required series-inductive or shunt-capacitive character for passband frequencies.
  • the parallel resonator in series stops signal flow by being an open circuit at the resonant frequency while the series resonator to ground becomes a short circuit at the resonant frequency.
  • shunt-capacitive open ended stubs act like series resonators to ground at a frequency where they become exactly a quarter of a wavelength long. Thereby, the passbands occurring at higher frequencies due to the periodicity introduced by the lowpass filter's inductive series elements may be suppressed. The transmission zero virtually stops any signal transmission at the critical stop band frequency.
  • the first filter section of the present invention may represent a shunt capacitance, which is realized in the distributed element lowpass filter as double open-end stub.
  • the length of the stub is preferably chosen to produce a transmission zero at the desired stopband frequency. Thereby, signal transmission at high frequencies due to the periodicity of the transfer function of the inductive lowpass filter elements is suppressed.
  • the first filter section comprises a capacitive section of negative reactance followed by a series inductive section of positive reactance.
  • the capacitive sections are adapted to generate a transmission zeros at predetermined stop band frequencies. This layout is preferable because it allows determining the desired transmission zeros simply by choosing appropriate lengths of capacitive sections. At the same time the required capacitance of the shunt capacitive sections may be adjusted by the width of the open ended stripline stubs.
  • the inductive section has a length I1 in the direction of wave propagation, which is smaller than a quarter of the wavelength corresponding to the cut off frequency fc of the low pass filter.
  • the inductive section acts as a series inductance.
  • the inductive section changes its reactance from a positive reactance to a zero reactance at a wavelength of twice the length of the section.
  • the capacitive section of the filter has a capacitive length I2 perpendicular to the direction of wave propagation.
  • the capacitive length I2 is equal to a quarter of the wavelength corresponding to the frequency (fn) of the transmission zero.
  • the capacitive section also creates a notch at a desired stopband frequency, thereby suppressing the aforementioned transmission periodicity.
  • the first lowpass filter section of the present invention may comprise a plurality of lowpass filter sections with positive and negative reactance connected alternately in line in order to generate plurality transmission zeros in the predetermined stop band.
  • additional transmission zeros may be arranged in the transfer function. These transmission zeros may correspond to the known carrier frequencies of other communication networks.
  • a second lowpass filter section having a higher cutoff frequency than the first low pass filter section may be added to the filter according to the present invention.
  • the second low pass filter section is connected in line with the first low pass filter section.
  • the second lowpass filter section may provide for an extended stopband rejection in a higher frequency range while not contributing to the stopband rejection at stopband frequencies near the cutoff frequency of the first lowpass filter section.
  • the short input and output lines of the lowpass filter have a characteristic impedance of approximately 50 Ohm and their purpose is to generate a transition from the coaxial plug interfaces with a radial E-field to a stripline having a parallel E-field.
  • the whole low pass filter is made of several elements that have a periodicity of the reactance due to their transmission line character. Because they have different lengths, a combination of their periodic character appears and therefore the period is not constant.
  • a periodicity of a transmission may very well have a changing period.
  • the periodicity of a single element however has a constant period.
  • the recurring part is the transmission at approx. 0 dB insertion loss.
  • a concentrated series inductive element continuously approaches an infinite attenuation.
  • the quasi-equivalent transmission line element is periodic (above a certain frequency, which is conventionally chosen in such a way that it corresponds to the cutoff frequency of the filter, in which this element is used).
  • the constricted passband relates to the passband, which is characterized by the smallest attenuation and very low reflection. In some filters of this kind, this is not readily recognizable in the transmission, however in the reflection the constricted passband is apparent.
  • FIG. 1 shows the design of a stripline filter for filtering radio frequency waves.
  • the stripline structure comprises a conducting stripline 30 centered between a first and second conductive plane 30 and 40.
  • This stripline represents a waveguide for a transverse electro-magnetic wave.
  • the conductive planes and the size and dimension of the stripline pose boundary conditions, which define the transverse modes for wave propagation.
  • a transverse mode of a wave of electromagnetic radiation is a particular electromagnetic field pattern of radiation measured in a plane perpendicular (i.e. transverse) to the propagation direction of the wave. Transverse modes occur in radio waves and microwaves confined to a waveguide.
  • the transmission line comprises a first and a second lowpass filter section 10 and 20.
  • Both the first and second lowpass filter sections 10 and 20 represent individual lowpass filters, which are connected in series to each other.
  • the first lowpass filter element 10 comprises an input element 60.
  • the input element 60 acts essentially as a short transmission line establishing the transition from the coaxial input socket of the first lowpass filter 10.
  • This is followed by a first lowpass filter element 70, which represents a shunt-capacitive element in the form of a double end-open stub.
  • Figure 1 further shows that the first lowpass filter section 10 also comprises an inductive element 80 and a second capacitive element 90.
  • the inductive elements have a characteristic impedance of approximately 100 Ohm.
  • the capacitive elements 90 have a characteristic impedance of approximately 10 Ohm.
  • the capacitive elements 70 and 90 are designed to generate transmission zeros at predetermined stopband frequencies fn.
  • a length I1 of the inductive element 80 is depicted in Figure 1 . This length I1 is parallel to the direction of propagation of a transverse electromagnetic wave travelling along the stripline. Preferably, the length I1 is designed to be smaller than ⁇ fc /4. For f ⁇ fc the inductive element 80 acts like a regular series inductance connected to the output capacitance 60a.
  • the inductive element 80 looses its inductive character. This leads to a reduced signal rejection from the series inductive elements and therefore the shunt capacitive sections 70, 90 and so on are designed to increase signal rejection by producing multiple transmission zeros.
  • the first lowpass filter section 10 comprises additional inductive element 80 and capacitive elements 90 similar to the above mentioned capacitive and inductive elements 80 and 90.
  • the additional capacitive elements 90 generate additional transmission zeros in the desired stopband.
  • the second lowpass filter section 20 is cascaded to the transmission line filter according to the preferred embodiment. This second lowpass filter section 20 constitutes a lowpass filter having a higher cutoff frequency than the first lowpass filter section 10. Thereby, the transmission of signals is rejected at the end of the first filter section's stopband and beyond.
  • the design of the lowpass filter according to the preferred embodiment has several advantages.
  • the first lowpass filter section 10 comprises relatively short inductive elements 80 for reducing the transmission periodicity and capacitive elements 90. These capacitive elements 90 provide at the same time transmission zeros at the desired notch frequencies fn.
  • FIG. 2 shows a plan view of the stripline filter of Figure 1 next to a transmission line equivalent circuit.
  • Each element of the stripline corresponds to one element of the equivalent circuit, which consists of high-impedance series lines and low-impedance open-end double-stubs.
  • the lowpass filter comprises two filter sections, a first filter section 10 and a second lowpass filter section 20.
  • the first and second filter sections 10 and 20 both constitute lowpass filters in and of themselves.
  • the first lowpass filter section 10 comprises an input element 60 and a first lowpass filter element 70.
  • the stripline lowpass filter according to the preferred embodiment is designed for UHF frequency band of 470 to 860 MHz. A stopband extending from 1.2 GHz to 3 GHz is demanded.
  • An open-ended and electrically short piece of transmission line has the electrical properties of a shunt capacitance. If such a piece of transmission line is utilized over a relatively wide frequency range it may be designed in such a way that, while acting as a simple shunt capacitance at relatively low frequencies, it becomes a transmission zero producing short circuit at a high frequency. These properties can be exploited in stripline lowpass filters.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Control Of Motors That Do Not Use Commutators (AREA)
EP09290011A 2009-01-07 2009-01-07 Tiefpassfilter Ceased EP2207237A1 (de)

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EP09290011A EP2207237A1 (de) 2009-01-07 2009-01-07 Tiefpassfilter

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EP09290011A EP2207237A1 (de) 2009-01-07 2009-01-07 Tiefpassfilter

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9369107B2 (en) 2012-06-18 2016-06-14 Thomson Licensing Apparatus and method for filtering singals in a receiver
RU2708342C1 (ru) * 2019-04-05 2019-12-05 Федеральное государственное автономное образовательное учреждение высшего образования "Сибирский федеральный университет" Высокоселективный полосковый фильтр нижних частот
CN112864555A (zh) * 2021-01-25 2021-05-28 南通大学 均匀阻抗枝节加载的悬置带线腔体合路器

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0373452A2 (de) * 1988-12-02 1990-06-20 Fujitsu Limited Filter zur Unterdrückung der zweiten Harmonischen
EP1058336A1 (de) 1998-11-12 2000-12-06 Mitsubishi Denki Kabushiki Kaisha Tiefpassfilter
US20030001697A1 (en) 2001-06-20 2003-01-02 The Boeing Company Resonance suppressed stepped-impedance low pass filter and associated method of fabrication
US20050077984A1 (en) 2003-10-08 2005-04-14 Lee Byoung Hwa Laminated low pass filter

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0373452A2 (de) * 1988-12-02 1990-06-20 Fujitsu Limited Filter zur Unterdrückung der zweiten Harmonischen
EP1058336A1 (de) 1998-11-12 2000-12-06 Mitsubishi Denki Kabushiki Kaisha Tiefpassfilter
US20030001697A1 (en) 2001-06-20 2003-01-02 The Boeing Company Resonance suppressed stepped-impedance low pass filter and associated method of fabrication
US20050077984A1 (en) 2003-10-08 2005-04-14 Lee Byoung Hwa Laminated low pass filter

Non-Patent Citations (3)

* Cited by examiner, † Cited by third party
Title
G. L. MATTHAEI, L. YOUNG, E. M. T. JONES: "Microwave Filters, Impedance matching networks, and coupling structures", 1 November 1985, ARTECH HOUSE BOOKS, North Bergen, NJ, USA, ISBN: 0-89006-099-1, pages: 365 - 372 *
QUENDO C ET AL: "OPTIMAL DESIGN OF LOW-PASS FILTERS USING OPEN STUBS TO CONTROL THE OUT-OF-BAND", 30TH EUROPEAN MICROWAVE CONFERENCE PROCEEDINGS. PARIS, OCT. 3 - 5, 2000; [PROCEEDINGS OF THE EUROPEAN MICROWAVE CONFERENCE], LONDON : CMP, GB, vol. CONF. 30, 5 October 2000 (2000-10-05), pages 336 - 339, XP001061045, ISBN: 978-0-86213-212-5 *
QUENDO ET AL.: "Optimal design of low pass filters using open stubs to control the out-of-band", 30TH EUROPEAN MICROWAVE CONFERENCE PROCEEDINGS, PARIS, vol. 30, 3 October 2000 (2000-10-03), pages 336 - 339, XP001061045

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9369107B2 (en) 2012-06-18 2016-06-14 Thomson Licensing Apparatus and method for filtering singals in a receiver
RU2708342C1 (ru) * 2019-04-05 2019-12-05 Федеральное государственное автономное образовательное учреждение высшего образования "Сибирский федеральный университет" Высокоселективный полосковый фильтр нижних частот
CN112864555A (zh) * 2021-01-25 2021-05-28 南通大学 均匀阻抗枝节加载的悬置带线腔体合路器

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