Abstract: A noise filter array includes filter elements including an LC parallel resonant circuit and an LC series resonant circuit each of which includes a coil and a capacitor provided in proximity in an array and integrally provided with one another. The LC series resonant circuits include ground capacitors having signal-side electrodes. Inductance adjustment conductors are connected to signal-side electrodes of the capacitors defining the respective filter elements, and a ground electrode of the capacitors is commonly arranged so as to oppose the signal-side electrodes.

Abstract: A tunable RF filter comprises a signal transmission path having an input and output, a plurality of resonant elements disposed along the signal transmission path between the input and output, and a set of non-resonant elements coupling the resonant elements together to form a stop band having a plurality of transmission zeroes corresponding to respective frequencies of the resonant elements, and at least one sub-band between the transmission zeroes. The set of non-resonant elements comprises a first plurality of non-resonant elements respectively coupled in parallel with the resonant elements, and a second plurality of non-resonant elements respectively coupled in series with the resonant elements. The first plurality of non-resonant elements comprises at least one variable non-resonant element for selectively introducing at least one reflection zero within the stop band to create a pass band in one of the one sub-band(s) without varying any of the second plurality of non-resonant elements.

Abstract: One embodiment of the invention relates to a dynamically adjustable differential band-pass filter. This band-pass filter includes a first leg that has an input portion and an output portion with a first inductor therebetween. It also includes a second leg in parallel with the first leg, where the second leg has an input portion and an output portion with a second inductor therebetween. The first inductor is symmetrically inter-woven with the second inductor. In some embodiments, the band pass filter is configured to compensate for losses due to the inductors. Other band-pass filters and methods are also disclosed.

Abstract: A method and apparatus to provide a bi-directional constant impedance filter. Embodiments include electric networks which give filter responses while presenting a constant impedance to a signal received at either an input or an output port for a plurality of frequencies.

Abstract: A left-handed filter of the present invention includes an interstage coupling element connected to a first capacitor and a ground, a second capacitor connected to the interstage coupling element, a third capacitor connected to the second capacitor, a first inductor connected to the connection point of a fourth capacitor and the second capacitor, and the ground, a second inductor connected to a fifth capacitor and the ground, a first input and output coupling element connected to a sixth capacitor, and a second input and output coupling element connected to the third capacitor. The first and second capacitors, the third and sixth capacitors, the first and second inductors, and third and fourth inductors are respectively arranged in symmetrical positions with respect to an interstage coupling element.

Abstract: A tunable RF filter comprises a signal transmission path having an input and output, a plurality of resonant elements disposed along the signal transmission path between the input and output, and a set of non-resonant elements coupling the resonant elements together to form a stop band having a plurality of transmission zeroes corresponding to respective frequencies of the resonant elements, and at least one sub-band between the transmission zeroes. The set of non-resonant elements comprises a first plurality of non-resonant elements respectively coupled in parallel with the resonant elements, and a second plurality of non-resonant elements respectively coupled in series with the resonant elements. The first plurality of non-resonant elements comprises at least one variable non-resonant element for selectively introducing at least one reflection zero within the stop band to create a pass band in one of the one sub-band(s) without varying any of the second plurality of non-resonant elements.

Abstract: A phase shifter circuit includes a plurality of first series circuits each comprised of a series connection of one capacitor and one resistor, a first circuit element including at least inductance connecting between a first end and a second end of a chain structure made by connecting the plurality of first series circuits in series, the first circuit element and the chain structure together constituting a first loop circuit, a plurality of second series circuits each comprised of a series connection of one capacitor and one resistor, a second circuit element including at least inductance connecting between a first end and a second end of a chain structure made by connecting the plurality of second series circuits in series, the second circuit element and the chain structure together constituting a second loop circuit, and a plurality of connection lines connecting between the first loop circuit and the second loop circuit.

Abstract: Tunable radio frequency (RF) devices, such as phase shifters and filters, are formed by depositing thin film layers on a substrate and patterning the thin film layers by various lithography techniques. A thin film metal layer is patterned to form a plurality of capacitors and inductors, leaving at least two grounding regions that lie closely adjacent the capacitors and inductors. As patterned portions of the grounding regions are electrically isolated from each other. Performance of the devices are improved by electrically bridging the differential potential grounding regions.

Abstract: A bandpass filter is described herein. The bandpass filter has two parallel signal branches, each connected, on an input side, to an input node and, on an output side, to an output node. The two signal branches form a ring resonator, having a wave mode with a complex amplitude of ?CW=|?|exp{?j?CW} propagating in a clockwise direction and a wave mode with a complex amplitude of ?CCW=|?CCW|exp{?j?CCW} propagating in a counterclockwise direction. The vector sum of a resulting wave ?out at the output node of the bandpass filter at two or more stop frequencies is: ?out=?CW+?CCW=0, and |?CW|=|UCCW| and |?CW??CCW|=180°. The stop frequencies are arranged such that a passband is formed between two stop frequencies.

Abstract: A multi-band band pass filter, including: first and second multi-order asynchronous resonators connected to each other by a coupling resonator and connected to respective first and second matching resonators, the first matching resonator connected to a signal-in terminal and second matching resonator connected to a signal-out terminal respectively; a first reference resonator connected between the signal-in terminal and a reference-in terminal and a second reference resonator connected between the signal-out terminal and a reference-out terminal, the first multi-order asynchronous resonator connected between the first matching resonator and the coupling resonator to the first reference terminal and the second multi-order asynchronous resonator connected between the second matching resonator and the coupling resonator to the reference-out terminal, the reference-in terminal connected to the reference-out terminal; and a feedback resonator connected between the signal-in terminal and the signal-out terminal.

Abstract: There is provided with a filter circuit, including: an input terminal configured to input signals; a band stop filter configured to have a center frequency of input signals from the input terminal in a stop band and configured to reflect signals in the stop band that is included in the input signals and pass signals outside the stop band; a band pass filter configured to have a pass band including the stop band, and configured to pass signals in the pass band out of the signals having passed through the band stop filter; a synthesis circuit configured to synthesize the signals reflected on the band stop filter and the signals having passed through the band pass filter to obtain synthesis signals; and an output terminal configured to output the synthesis signals.

Abstract: A multi-band band pass filter, including: first and second multi-order asynchronous resonators connected to each other by a coupling resonator and connected to respective first and second matching resonators, the first matching resonator connected to a signal-in terminal and second matching resonator connected to a signal-out terminal respectively; a first reference resonator connected between the signal-in terminal and a reference-in terminal and a second reference resonator connected between the signal-out terminal and a reference-out terminal, the first multi-order asynchronous resonator connected between the first matching resonator and the coupling resonator to the first reference terminal and the second multi-order asynchronous resonator connected between the second matching resonator and the coupling resonator to the reference-out terminal, the reference-in terminal connected to the reference-out terminal; and a feedback resonator connected between the signal-in terminal and the signal-out terminal.

Abstract: An LC filter structure and method for its fabrication, in which multiple shunt capacitors, multiple shunt inductors and multiple coupling inductors are printed on a metal layer formed on a thin dielectric substrate. The capacitors have first electrodes that are formed as spatially separated regions of the metal layer, and a common second electrode formed by a ground plane on the substrate. The shunt inductors are formed as spiral traces connected to the separated regions and to the ground plane, through conductive vias. The coupling inductors are similarly formed as spiral traces in the gaps between the separated regions, the ends of each coupling inductor being connected to respective adjacent regions of the metal layer.

Abstract: An antenna device which is connected to a radio module performing radio communications with a system using a first band and a system using a second band, has an antenna element which transmits/receives radio signals in the first and second bands. The antenna device has first and second matching circuits corresponding to the first and second bands, and also disposes a switching circuit between the first and second bands and the radio module. A first filter circuit is connected between the first matching circuit and the antenna element. The first filter circuit passes the radio signal in the first band and also attenuates the radio signal in the second band. Meanwhile, a second filter circuit is connected between the second matching circuit and the antenna element. The second filter circuit passes the radio signal in the second band and also attenuates the radio signal in the first band.

Abstract: A bandpass filter having a transmission coefficient—frequency spectrum characteristic curve with a notch on each side of the passband is disclosed.

Abstract: First, second, third, and fourth resistors are coupled to first, second, third, and fourth capacitors. A first positive in-phase terminal is coupled to the first resistor and the first capacitor. A first negative in-phase terminal is coupled to the fourth resistor and the fourth capacitor. A positive quadrature-phase terminal is coupled to the second resistor and the first capacitor. A second positive in-phase terminal is coupled to the first resistor and the second capacitor. A second negative in-phase terminal is coupled to the fourth resistor and the third capacitor. A negative quadrature-phase terminal is coupled to the third resistor and the fourth capacitor. The first and fourth resistors, the second and third resistors, the first and fourth capacitors, and the second and third capacitors are equal distances from and on a same side of respective axes, and are equal distances from and on opposite sides of a symmetry axis.

Abstract: A constant impedance filter maintains a constant input impedance for frequencies that are both inside the filter passband and outside the filter passband. The constant input impedance appears as a pure resistance. The constant impedance filter includes a plurality of filter poles that are connected in series. Each of the filter poles include an inductor, a capacitor, and a resistor. The value of the inductor, the capacitor, and the resistor are selected to provide a constant input impedance over frequency for each pole of the filter, which produces a constant input impedance for the entire filter over frequency. The constant impedance filter can be implemented as a low pass filter, a high pass filter, or a bandpass filter. Furthermore, the constant impedance filter can be implemented in a single-ended configuration or a differential configuration.

Abstract: A constant impedance filter maintains a constant input impedance for frequencies that are both inside the filter passband and outside the filter passband. The constant input impedance appears as a pure resistance. The constant impedance filter includes a plurality of filter poles that are connected in series. Each of the filter poles include an inductor, a capacitor, and a resistor. The value of the inductor, the capacitor, and the resistor are selected to provide a constant input impedance over frequency for each pole of the filter, which produces a constant input impedance for the entire filter over frequency. The constant impedance filter can be implemented as a low pass filter, a high pass filter, or a bandpass filter. Furthermore, the constant impedance filter can be implemented in a single-ended configuration or a differential configuration.

Abstract: A constant impedance filter maintains a constant input impedance for frequencies that are both inside the filter passband and outside the filter passband. The constant input impedance appears as a pure resistance. The constant impedance filter includes a plurality of filter poles that are connected in series. Each of the filter poles include an inductor, a capacitor, and a resistor. The value of the inductor, the capacitor, and the resistor are selected to provide a constant input impedance over frequency for each pole of the filter, which produces a constant input impedance for the entire filter over frequency. The constant impedance filter can be implemented as a low pass filter, a high pass filter, or a bandpass filter. Furthermore, the constant impedance filter can be implemented in a single-ended configuration or a differential configuration.

Abstract: A component that includes a first structure that is integrated with a second structure. The first structure is a lumped balanced to unbalanced circuit (balun) and the second structure is a resonator-based filter. The function of the balun is to move a signal from a differential form to a single ended form or from a single ended form to a differential form. The component has an area that is comparable to the area needed to implement the balun alone. However, the component now provides a filter function. Preferably, the substrate of the first structure also acts as a substrate for the second structure.

Abstract: A polyphase filter having metal-insulator-semiconductor (MIS) capacitors in which the whole body of the polyphase filter is fabricated as an IC. Resistors are individually combined with the capacitors to form serial connection circuits and every four signal connection circuits individually form bridge circuits that are connected in a cascade manner. When the MIS capacitors are fabricated into an IC, capacitors that are parasitic to the MIS capacitors are connected to the input sides of the resistors.

Abstract: A filter for suppressing noise surges and noise frequencies in ac power lines to an audio video load includes at least one noise suppression circuit. The noise suppression circuit in turn includes first and second inductors coupled to respective power lines in series with the load. A capacitor is coupled between the power lines at the input of the first inductor and the output of the second inductor. With this arrangement, the capacitor suppresses noise surges in the power line, while being protected by the second inductor from being damaged by the surges. Also, the filter suppresses two noise frequencies using only the single capacitor. If desired, additional noise suppression circuits can be provided, with each noise suppression circuit suppressing two noise frequencies using only a single capacitor.

Abstract: The present invention provides for a super-narrow band filter using frequency dependent L-C components. The invention utilizes a frequency dependent L-C circuit with a positive slope k for the inductor values as a function of frequency. The positive k value allows the realization of a very narrow-band filter.

Abstract: A constant impedance filter maintains a constant input impedance for frequencies that are both inside the filter passband and outside the filter passband. The constant input impedance appears as a pure resistance. The constant impedance filter includes a plurality of filter poles that are connected in series. Each of the filter poles include an inductor, a capacitor, and a resistor. The value of the inductor, the capacitor, and the resistor are selected to provide a constant input impedance over frequency for each pole of the filter, which produces a constant input impedance for the entire filter over frequency. The constant impedance filter can be implemented as a low pass filter, a high pass filter, or a bandpass filter. Furthermore, the constant impedance filter can be implemented in a single-ended configuration or a differential configuration.

Abstract: An arrangement of differential amplifying-type gyrators is used to implement a signal-filtering circuit with significantly reduced power consumption. One specific example implementation is directed to a signal-filtering circuit arrangement that uses a gyrator-type signal-filtering circuit to simulate a multiple-section LC ladder implementation. A plurality of transconductance cells are arranged to simulate the first inductance ladder section and at least one subsequent inductance ladder section. The first inductance ladder section is adapted to provide a gain of two or three. By setting the gain of the first section in this manner, the noise contribution of the subsequent sections is significantly lessened relative to the conventional implementation in which the gain of the first section is unity.

Abstract: A method of producing filters using lower unloaded Q factor components than filters with the same performance characteristics but requiring higher unloaded Q factor components is disclosed. The method includes the steps of defining a desired filter characteristic and applying an algorithm which provides a filter having infinite Q factor elements and having a theoretical characteristic corresponding to the desired characteristic transformed to a compensate for the difference between finite Q factor and infinite Q factor elements.

Abstract: A novel circuit packaging structure including a combination of printed lumped elements within a multi-layer stripline architecture is provided and methods of making the same. The lumped elements are provided by printing the individual passive elements, including interdigital capacitors and spiral inductors, onto the supporting substrate and then embedding these elements into the multi-layer stripline structure. The stripline structure consists of a signal layer sandwiched between at least two ground layers separated by two dielectric substrates of equal thickness. This ground-signal-ground approach confines the electromagnetic fields within the multi-layer structure, thereby minimizing radiation from coupling to nearby components. This approach offers a unique combination of stripline technology and printed lumped elements resulting in miniaturized radio frequency and microwave circuits at operating at frequencies well below 1 GHz, up to several GHz.

Abstract: A piezoelectric filter is one not changing in the holding position of resonator in spite of impulse, small in the number of parts, easy to assemble from one direction, small, thin, and stable in characteristic. For example, as shown in FIG. 7, on electrode patterns on a package 10 forming wiring electrode patterns 11, holding protrusions 13 for holding resonators, and partition boards 14 for positioning resonators, a rectangular plate type piezoelectric resonators 12 are adhered with conductive adhesive. The rectangular plate type piezoelectric resonators 12 and the package 10 are adhered and reinforced from above with a vibration absorbing member 15, and wiring is formed from above the resonators by a wire bonding 16, thereby wiring resonators in a ladder form. Or, as shown in FIG.

Abstract: A diplexer for use in communication of telephony and high-speed data transmission, including television, comprises two filters, each of which is constructed with plural stages in the form of a ladder network, one of the filters being a high-pass filter and the other of the filters being a low-pass filter. The filter components, inductors and capacitors, are located on one or opposed sides of a dielectric substrate. The filter components are arranged to minimize interference between signals traveling in the low-pass filter and the high-pass filter. In the case of a parallel branch of the ladder network in each of the filters, wherein the parallel branch comprises a resonant circuit of inductor and capacitor, the branches of the respective filters are located at opposite parts of the array of the filter components.

Abstract: The invention provides a tuned filter of reduced overall length in which two coils (air core inductors) are placed side by side, in mounting locations that are transverse with respect to the longitudinal direction of the filter, instead of in-line or diagonal to each other. The transverse mounting of the coils, depending on the number of poles in the filter, results in a reduction in the overall length of the filter to at least 25% to greater than 50% than can be realized in comparison to conventional in-line filter designs. In addition, the transverse mounting configuration results in an improvement in the electrical performance for both the in band and high frequency responses of the tuned notch filter.

Abstract: Lumped element electrical components, such as inductors and capacitors, are formed to include high temperature superconducting materials. In the preferred embodiment, thin film epitaxial high temperature superconductors are patterned to form capacitors and inductors on low loss substrates. Preferably, a ground plane is formed on the back side of the substrate, most preferably being formed of high temperature superconducting material, or other highly conductive materials such as gold or copper. Various advantageous structures include a planar spiral structure, a zig-zag serpentine structure, a single coil structure and a double coil structure. Single layer and multilayer structures are included. Improved narrow bandpass filters, bandreject filters and high Q resonator structures are formed. Chebyshev bandpass filters are formed from multiple series connected zig-zag filter structures.

Abstract: A small (about 8 inches long) low pass, high power (4000 watts) electric filter may include a tubular outer conductor separated from an dielectric sleeve by an outer dielectric that may be an air-filled gap. The dielectric sleeve surrounds a series of alternating capacitive elements and inductive elements. Each of the capacitive elements includes an inner conductor, such as a copper slug, that contacts the dielectric sleeve. Each of the inductive elements has a helical wire coil surrounded by a dielectric silicone compound that contacts the dielectric sleeve and the inner conductor. The silicone compound in the inductive elements facilitates heat transfer from the inductive elements and increases capacitive capacity. The air gap between the outer conductor and the dielectric sleeve adds another capacitor. The outer conductor and the dielectric sleeve may touch to improve heat transfer from the filter.

Abstract: A tunable filter (100) includes first and second resonant structures (140, 145) coupled by a capacitive coupler (120). The capacitive coupler (120) includes two tunable elements (121, 126) coupled by a capacitive circuit formed at least in part by a capacitive pi-network (130).

Abstract: A selected inductance can be provided by a three-layer structural spiral inductor having an inductor conductive part disposed between two ground electrodes within an electric insulating substrate. This is carried out by eliminating a particular portion of one of those ground electrodes formed on the outer surface of the substrate until a desired inductance is obtained, beginning at a portion of such electrode corresponding to an exposed part of a through hole joint which is electrically connected to an inner peripheral end of a spiral inductor conductive part.

Abstract: This invention provides a method and circuit for separating a signal of interest from a composite signal including a signal of interest and an angle modulated interfering signal, in which an in-phase component of a reference signal representative of a composite signal is mixed with the composite signal to produce a first compound signal, selected components from the first compound signal are removed to produce a first filtered signal, and the in-phase component of the reference signal is mixed with the first filtered signal to produce a second compound signal. Similarly, a quadrature component of the reference signal is mixed with the composite signal to produce a third compound signal, selected components from the third compound signal are removed to produce a second filtered signal, and the quadrature component of the reference signal is mixed with the second filtered signal to produce a fourth compound signal.

Abstract: A high percentage bandwith microwave filter network, according to an embodiment of the present invention, comprises a T-network of capacitors implemented with thin-film techniques by applying a groundplane to one side of an alumina substrate and a first plate to the other side. A silicon nitride dielectric layer having a high density of capacitance is deposited over the first plate, and then a second and third capacitor plate are deposited on the dielectric over the first plate.

Abstract: A saddle coil for multiple tuned response is realized by a segmented sequence of first LC elements each tuned to exhibit a low impedance at .omega..sub.1 and a second set of LC elements, each tuned to exhibit a low impedance at .omega..sub.2, at least some of said first set of LC elements having one of said second LC elements in parallel therewith.

Abstract: In a multi-stage, varactor-tuned bandpass filter utilizing electromagnetic field coupling between successive filter stages, the effective area of a coupling window provided in the RF shielding separating the filter stages is varied as a function of the varactor tune voltage to provide a flat frequency response over the filter frequency range. The bias current through a PIN diode connected across the coupling window is controlled by varactor tune voltage to effectively decrease the window area with increasing filter frequency.

Abstract: A combination of passive electrical filters and active electrical filters in a series arrangement forming active passive band pass or high pass or low pass or band reject filters. The preferred embodiment includes an amplifier stage to include high input impedance. These filter combinations are free of internal electrical noise.

Abstract: The RF data communication band pass filter is a fixed device that filters both RF multi-level data signals and binary-level data signals. The RF data communication band pass filter comprises a low pass filtering means having a corner frequency of approximately 2900 Hertz and as high pass filtering means of approximately 340 Hertz. By using precision components the overall group delay envelope is such that both multi-level RF data signals and binary-level RF data signals are adequately filtered without substantial harmonic distortion or spectral distortion. In addition, the communication band pass filter meets the strictest of international RF data communication requirements.

Abstract: A low pass filter system for filtering from an electrical circuit unwanted high frequency signals while allowing passage to a passband of wanted signals of lower frequency wherein the filter system comprises a low pass filter (1) and a group delay equalizer made up of at least two group delay means (2,3) each of which imparts delay to substantially all of the passband signals. It has been found that by sharing the group delay function between two or more group delay means enables a much higher degree of group delay equalization to be achieved with filters having a transition sharpness of below 1.17.

Abstract: A combined RFI filter and low pass filter includes two sections, both of which serve as both radio frequency interference filters and low pass filters, or dc integrators. Both sections have substantially equal time constants and employ non-polarized capacitors.

Abstract: A thin-film, lumped element filter utilizing spiral inductors and capacitive pi networks is disclosed. The filter is fabricated from a planar dielectric substrate having a ground plane on one side thereof and two thin-film metal layers and an insulation layer disposed therebetween on the opposite side of the dielectric substrate. The metal and insulation layers are configured to form one or more capacitive pi networks and spiral inductors, which are electrically interconnected to form the filter.

Abstract: The filter comprises a series branch formed by an inductance coil (L) in parallel with a resistor (R1) and a parallel branch formed by a capacitor (C) in series with a resistor (Rc). For frequencies such that Rc>>1/.omega.C and R1<<.omega.L, the ratio between the output and input powers of the filter has a predetermined maximum value strictly lower than 1 and independent of the load impedance.

Abstract: Integrated capacitors provide more than one capacitor on a single dielectric substrate. A dielectric substrate has a conductive backplane on one side and two or more conductive capacitor plates on the opposite side. The integrated capacitors may be mounted on a shield wall which has a through hole so that one capacitor is connected to the shield wall and another forms a coupling capacitor via the hole between two stages on opposite sides of the shield wall.

Abstract: A continuous time electrical filter fabricated as an integrated circuit includes capacitors (CF,CN) and resistors (R1,R2). Since capacitors and resistors are difficult to integrate with accurately defined values, a trimming circuit is provided which operates switches (S1-SN) to select appropriate ones of the capacitors (CN) to accurately define the cut-off frequency of the filter. The trimming circuit comprises a capacitor (TC2) which is charged through a resistor TR1 during a first period and which is discharged in incremental steps by capacitor (TC1). The number of incremental steps is counted by a counter (11) and transferred to a register (13). The outputs (S1-SN) of the register (13) control the switches (S1-SN). Alternatively the values of the filter resistors may be adjusted, a convenient procedure being to short out selected portions of the resistors. More than one capacitor or resistor may be adjusted using a single counter and register.

Abstract: A microwave bandpass filter for frequencies in the range of 30-3000 MHz comprises a substrate formed from dielectric ceramic sheets fused together and stacked according to a dielectric constant required of the filter capacitors, the plates of which are formed on the substrate. Discrete air core coils, to provide greater frequency selectivity in this particular frequency range, are attached into the circuit on a top surface of the substrate, and a drawn metal can covers the circuitry on the top surface and is soldered to a metallized coating extending over substantially all of the rest of the substrate except for surface mounting lead portions and associated conductive paths. Thus, EMI and RFI shielding is provided by the combination of the metal can and the ground plane of the circuit.

Abstract: An EMI filter for use in a low frequency radio receiver. The device is inserted in the signal line between the receiver and its antenna to provide both common mode and differential mode filtering of out-of-band interfering HF signals which are picked up on the antenna as well as on the signal line and attendant cable shields.

Abstract: In a line filter having a first balun, first Y capacitors, a first X capacitor, a second balun, second Y capacitors and a second X capacitor connected between the input and output terminals of the line filter in the order named, the inductance of the first balun is larger than that of the second balun. The total capacitance of the first Y capacitors is larger than that of the second Y capacitors. The line filter can be divided into a front stage containing the first balun and a rear stage containing the second balun, the two stages being coupled via a wiring. The second balun can be constituted by a plurality of baluns.

Abstract: The present invention provides a track circuit for AC electrified railways, formed by the two rails (R.sub.1,R.sub.2) of a railway section and comprising a transmitter (E) connected to one end of the track circuit for generating therein an alternating signal of given frequency and a receiver (R) connected to the other end for controlling the operation of a track relay (RV). A number of capacitive impedances (Z.sub.1,Z.sub.2, . . . Z.sub.n) with parallel inductance and capacitor (each having a middle point (3) connected to the ground) are connected between the rails (R.sub.1,R.sub.2) while being spaced apart over the whole length of the track circuit with a given pitch (P), depending on the values of the impedances, so that the transmission attenuation of the circuit at the operating frequency is always less than the transmission attenuation of a circuit having the same length formed by a rail with ground return.