EP2207237A1

EP2207237A1 - Lowpass filter

Info

Publication number: EP2207237A1
Application number: EP09290011A
Authority: EP
Inventors: Dieter Pelz
Original assignee: Alcatel Lucent SAS
Current assignee: Alcatel Lucent SAS
Priority date: 2009-01-07
Filing date: 2009-01-07
Publication date: 2010-07-14

Abstract

The present invention relates to a RF filter for filtering RF radio frequency waves, in particular UHF waves. The RF filter comprises a transmission line structure having a thin conducting stripline structure centered between two conducting planes. The stripline structure comprises a first lowpass filter element. The first lowpass filter element has a shunt capacitive character and is adapted to generate a transmission zero at a predetermined stop band frequency. The RF Filter comprises a second lowpass filter element connected to the first lowpass filter element. The second lowpass filter element has series inductive character and its length is short compared the length of a quarter of a wavelength at the cutoff frequency. The lowpass filter consists of relatively short high-Z inductive sections for reduced transmission periodicity and low-Z capacitive sections which at the same time produce transmission zeros in the stopband.

Description

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.
As understood herein, radio frequency (RF) 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. However, the signals generated by the transmitter may comprise frequency components, which fall into the frequency band of a neighboring (co-located) communication network. Hence, 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.
Conventionally, stripline technology is used in components for ultra high frequency broadcasting. 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. However, 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.
However, in general 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. Henceforth, 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. These periodically re-occurring passband sections of the filter do not allow to sufficiently attenuate the unwanted signals. Therefore, communication networks operating in these frequency bands may be disturbed.
Therefore, it is object of the present invention, to provide a stripline filter for filtering radio frequency waves, which reduces the detrimental effects of the periodic transfer function.
The object is solved by 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.
In accordance with an embodiment of the invention, 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. As far as the low-Z sections are concerned Z<<50 Ohms applies such that the characteristic impedance is 10 Ohm or below.
Advantageously, 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.
In distributed element form, 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.
Preferably, 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.
According to a preferred embodiment of the present invention, 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. For f lower than the cutoff frequency, 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.
According to a preferred embodiment of the present invention, 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.
In particular, 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. Thereby, additional transmission zeros may be arranged in the transfer function. These transmission zeros may correspond to the known carrier frequencies of other communication networks.
Additionally, 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. In this embodiment, 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.
Preferred embodiments of the present invention are described with reference to the appended drawings by way of example only:

Figure 1 shows the representation of a stripline filter according to an embodiment of the present invention.
Figure 2 shows a plan view of the stripline filter next to a transmission line equivalent circuit.
Figure 3 shows a plan view of the stripline filter next to an equivalent circuit showing the resonant nature of the shunt elements
Figure 4 shows a plan view of the stripline filter next to an equivalent circuit for passband frequencies.

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.
Figure 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 λ_fc/4. For f<fc the inductive element 80 acts like a regular series inductance connected to the output capacitance 60a. However, if the signal's wavelength λ is equal to 2I=λ, 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.
In Figure 1, 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. Finally, 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 passband of the first lowpass filter section 10 is restricted, i.e. the passband does not extend all the way down to f=0 Hz. However, this does not constitute a disadvantage, since signal transmission at these low frequencies is of no concern. Instead, the transmission periodicity may be reduced, i.e. the lowpass frequency range may be increased and the stopband rejection within the frequency range may be improved. Furthermore, 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.
Figure 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.
Again, 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 following description exemplifies how a capacitive filter element may be laid out for given notch and cutoff frequencies. 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.

f_c :: lowpass filter cutoff frequency
C_fc :: required lowpass shunt capacitance
Z₀ :: characteristic impedance of stripline
/: electrical length of stripline

The input reactance of an end open transmission line is given by $X_{in} = Z_{0} * \cot (2 * π * \frac{l}{λ_{f_{c}}})$
The reactance of a shunt capacitance is defined by: $X_{C} = \frac{1}{2 * π * f_{c} * C_{f_{c}}}$
The input reactance of a short piece of end open transmission line is capacitive and therefore it can be written: $X_{in} = X_{C}$
Therefore, the stub reactance is equal to: $Z_{0} * \cot (2 * π * \frac{l}{λ_{f_{c}}}) = \frac{1}{2 * π * f_{c} * C_{f_{c}}}$
If we demand that the end-open stripline is quarter wave long at a prescribed notch frequency f_notch (f_notch >> f_c), then we get: $Z_{0} * \cot (2 * π * \frac{\frac{λ_{notch}}{4}}{λ_{f_{c}}}) = \frac{1}{2 * π * f_{c} * C_{f_{c}}}$
The necessary characteristic impedance of the stripline is found to be: $Z_{0} = \frac{1}{2 * π * f_{c} * C_{f_{c}} * \cot (2 * π * \frac{λ_{notch}}{4 * λ_{f_{c}}})}$

or simpler $Z_{0} = \frac{1}{2 * π * f_{c} * C_{f_{c}} * \cot (2 * π * \frac{f_{c}}{f_{notch} * 4})}$

List of reference numerals

10: first lowpass filter section
20: second lowpass filter section
30: conducting stripline
40: first conducting plane
50: second conducting plane
60: input element
70: first lowpass filter element
80: inductive element
90: capacitive element
100: impedance element
110: capacitive element
120: inductive element
Z1: input transition element
Z2: middle transition element
Z3: output transition element
C1: first capacitor
C2: second capacitor
C3: third capacitor
L1: first inductor
L2: second inductor
L3: Third inductor

Claims

Filter for filtering radio frequency waves, in particular HF or UHF waves, comprising:
a transmission line filter having a thin conducting stripline structure (30) arranged between two conducting planes (40, 50), wherein said stripline structure (30) comprises a first lowpass filter section (10), said first lowpass filter section (10) having a transfer function providing a lowpass filter characteristic and having a cutoff frequency within a predetermined frequency band, characterized in that the pass band of the first low pass filter section is limited to a frequency band ranging from the cutoff frequency to a lower frequency greater than zero, and

the first lowpass filter section is adapted to generate transmission zeros at predetermined stop band frequencies,
in that the lowpass filter consists of relatively short high-Z inductive sections for reduced transmission periodicity and low-Z capacitive sections which at the same time produce transmission zeros in the stopband.
Filter according to claim 1, characterized in that
first lowpass filter section (10) comprises a capacitive section (70) of negative reactance followed by an inductive section (80), said capacitive section (70) being adapted to generate a transmission zero at the predetermined stop band frequency (f_n).
Filter according to claim 2, wherein the inductive section (80) has an inductive length (I1) in the direction of wave propagation, said inductive length (I1) being smaller than a quarter of the wavelength corresponding to the cut off frequency (fc) of the first low pass filter section (10).
Filter according to claim 3, wherein said inductive section (80) changes its reactance from a positive reactance to a negative reactance at a wavelength of one half the wavelength corresponding to the electrical length I1 of the inductive section (I1 = λ/2).
Filter according to claims 1 or 2, wherein the capacitive section (90) has a capacitive length (I2) perpendicular to the direction of wave propagation, said capacitive length (I2) being equal to a quarter of the wavelength corresponding to the frequency (f_n) of the transmission zero.
Filter according to one of the previous claims, wherein the first lowpass filter section (10) comprises a plurality of lowpass filter elements with positive and negative reactance connected alternately in line, said capacitive elements generate a plurality of transmission zeros in the predetermined stop band.
Filter according to one of the previous claims, further comprising a second lowpass filter section (20) having a higher cut off frequency than the first low pass filter section (10), said second low pass filter section being connected in line to the first low pass filter section (10).
Filter according to one of the previous claims, wherein the relatively short high-Z inductive sections each have a length which is smaller than the quarter-wavelength at the cutoff frequency.
Filter according to claim 8, wherein the length is smaller than half the quarter-wavelength at the cutoff frequency, preferably smaller than 10% of the quarter-wavelength at the cutoff frequency.
Filter according to one of the previous claims, wherein the low-Z capacitive sections have a characteristic impedance of about 10 Ohm or below 10 Ohm.