Abstract: A scheme for counteracting subsynchronous resonance in an AC power system that comprises a passive device in combination with a line inductive shunt reactor (L10). The device is connected from the neutral end (N) of the reactor (L10) to ground and comprises a plurality of circuit branches: one of those branches comprises a condenser (C10), another one includes an inductor (L12) in series with at least one tank circuit (C14, L14). The combination of these elements results in circuit paths from the AC line to ground of selected admittance values for at least one selected frequency as well as in a solid grounding of the shunt reactor neutral at synchronous frequency.

Abstract: A filter network adapted to be placed in parallel with a power supply which provides a high impedance to the 60 Hz utility frequency but acts like a short circuit to all higher frequencies. The network comprises a plurality of parallel circuits, each of which comprise a capacitor, an inductor and a resistor in series with a second resistor coupled in parallel with the inductor. Signal levels above 60 Hz in a prototype circuit using seven such parallel circuits with a parallel capacitor coupled thereto shows that above 60 Hz the signal from the power supply is attenuated by 25-30 dB at 1 KHz and that the impedance is substantially resistive beyond 1 KHz.

Abstract: In this digital signal measurement apparatus, an approach is employed to deliver a measurement signal from a measurement signal generator to a measured circuit to transform the signal on the time base through the measured circuit to a signal on the frequency base by a frequency base transform circuit and to further obtain a signal on the time base by a time base transform circuit. Thus, a difference between the signal on the time base through the measured circuit and the signal on the time base from the time base transform circuit is employed. Thus, for example, even if the measured circuit is a linear system, a measured result in the digital region and a result of the analog measurement can be in correspondence with each other. Accordingly, gain correction of a measurement signal is unnecessary, thus making it possible to prevent an increase in an error of a measured result of S/N.

Abstract: A line filter assembly adapted to be inserted in an electrical power supply circuit of various electronic appliances for avoiding an invasion and leakage of noise include first and second core members each having a pair of opposite arms connected together through a common bridge. Coils are formed on the arms of each of the core members in such a manner that directions of magnetic fluxes produced in the closed magnetic circuit as a result of flow of line currents counteract while the same coils are so arranged that the magnetic fluxes produced in the closed magnetic circuit as a result of the flow of the line currents are oriented in the same direction.

Abstract: A filter network adapted to be placed in parallel with a power supply which provides a high impedance to the 60 Hz utility frequency but acts like a short circuit to all higher frequencies. The network comprises a plurality of parallel circuits, each of which comprise a capacitor, an inductor and a resistor in series with a second resistor coupled in parallel with the inductor. Signal levels above 60 Hz in a prototype circuit using seven such parallel circuits with a parallel capacitor coupled thereto shows that above 60 Hz the signal from the power supply is attenuated by 25-30 dB at 1 KHz and that the impedance is substantially resistive beyond 1 KHz. To maximize the power factor of the circuit comprising the network and consequently the dynamic range of audio equipment coupled thereto, the magnitude of the total capacitance of the network is chosen to be as close as possible to the magnitude of the input inductance of the audio equipment.

Abstract: An image band-stop filter circuit includes a transmission line formed on a substrate of GaAs and a T circuit connected in parallel to the transmission line and having a high pass filter characteristic. The T circuit includes series-connected first and second capacitances connected in parallel to the transmission line and a stub having one end connected to a node between the first and second capacitances and the other end short-circuited. The stub has a line length which becomes inductive in an image signal band to be removed. As a result, the lengths of the transmission line and the stub can be reduced to enable the band-stop filter circuit to be made into a MMIC with ease while achieving an excellent signal transmission characteristic.

Abstract: A technique is disclosed to independently tune different sections of a filter by momentarily frequency shifting a first tuned section, preferably by shorting certain nodes in the first tuned section, such that response of a second section can be determined and independently tuned. The first section is returned to normal operation after the second section is tuned.

Abstract: A filter system which may be configured either as a stopband (notch) or passband filter having controlled gain and low ripple throughout its stopband or passband. The system includes a filter network which is operative for splitting an input signal spectrum into a pair of complimentary spectrum components and a coupler for proportionately combining these spectrum components in accordance with a prescribed gain factor. The filter network may either comprise a pair of complimentary band reject and band pass filters connected to a common input terminal or a diplexer including identical filters coupled between a splitter and a combiner.

Abstract: A TVRO receiver for receiving frequency-modulated video signals comprises a tuner including a super-heterodyne circuit having a voltage-controlled oscillator (VCO), means for supplying a cotnrolling input voltage to the VCO, and a mixer for combining incoming 1st intermediate frequency (IF) sigals within a 1st predetermined IF frequency range with the output of the VCO to reduce the frequency of the 1st IF signals to generate 2nd IF signals within a 2nd predetermined IF frequency range, and a filter for selectively rejecting undersired interference signals. The interference filter comprises separate trap means for respectively blocking out interfering signals having selected bottom and top trap frequencies, tuning means for adjusting the bottom and top trap frequencies, means for increasing the quality factor of the trap means, and means for selectively switching the trap means into and out of the receiver based on the presence or absence of signals which interfere with the received signals.

Abstract: A notch filter having increased pass band characteristics is provided by insertion of an inductor between a normally-grounded capacitor and ground, the capacitor being connected to an inductive-capacitive filter network. By this construction, the equivalent capacitor of the high-frequency equivalent circuit is not connected directly to ground, but rather has the inductor interposed between it and ground. This significantly increases the pass band of the circuit.

Abstract: A line filter assembly adapted to be inserted in an electrical power supply circuit of various electronic appliances for avoiding an invasion and leakage of noise includes first and second core members each having a pair of opposite arms connected together through a common bridge. Coils are formed on the arms of each of the core members in such a manner that directions of magnetic fluxes produced in the closed magnetic circuit as a result of flow of line currents counteract while the same coils are so arranged that the magnetic fluxes produced in the closed magnetic circuit as a result of the flow of the line currents are oriented in the same direction.

Abstract: A filtering cell including a resonant circuit having an inductor and capacitor connected in parallel, the inductor being separated into two identical inductors which are connected in series with a midpoint therebetween. A second capacitor is connected at the midpoint and has a quality factor substantially equal to a quality factor of the first inductor, resulting in attenuation at the resonant frequency. Each of the inductors and capacitors are formed by micro-ribbon technology.

Abstract: A monolithically realizable radio frequency (RF) bias choke implemented as a parallel inductor/capacitor arrangement connected between a DC supply node and an RF circuit bias point.

Abstract: A monolithically realizable harmonic trapping circuit that is a shunt connected series-resonant inductor-capacitor combination which has a resonant frequency designed to coincide with an undesired harmonic frequency of a desired fundamental frequency signal.

Abstract: Networks which remove the jamming signal from the television signal supplied to subscribers for a premium T.V. channel in a cable (CATV) or other subscription television system utilizes surface accoustic wave (SAW) devices arranged to provide a broadband all pass response across the entire band covered by all of the transmitted television channels except for a notch at the frequency of the jamming signal. The narrow band response of the filter in the premium channel is such that the notch presents insertion loss at the jamming signal frequency and over a frequency range to accommodate shifts in the frequency response of the SAW devices with respect to the frequency of jamming signal, which insertion loss is about 100 times the insertion loss at the picture carrier frequency of the television signal, even though the picture carrier and the jamming signal frequency may be within about 200 KHz apart from each other.

Abstract: A bandpass filter is disclosed which may be used in a superheterodyne receiver in front of the first mixer. The bandpass filter provides a narrow front end filter which is tuned automatically as the local oscillator frequency is changed. In the present exemplary embodiment, a series tuned reflective notch filter is coupled to a node of a balanced resistive bridge in such a manner that the tuned circuit presents a short circuit to the node at the filter's resonant frequency to thereby assure a maximum transfer of signal from the input to the output of the circuit at a desired frequency. At all other frequencies, the series tuned reflective notch filter will present essentially an open circuit across the node of the resistive bridge so that the node is balanced and no transmission occurs. The inclusion of the series reflective notch filter thereby produces a bandpass characteristic from the circuit input to output.

Abstract: A band pass filter for radio frequencies uses helical resonators arranged to have a transmission notch on the high frequency side of the pass band. A plurality of resonators housed in a shield are formed by quarter-wavelength transmission lines wound in a helix, with adjacent resonators being wound in opposite directions and grounded at opposite ends to provide a transmission notch at the high frequency side of the passband. One or more grounded shielding pins are arranged between adjacent resonators to adjust the frequency of the notch to be at or near an image frequency. The grounded end of each of the resonators may be coupled to a varicap diode network for electronically tuning the filter.

Abstract: A notch filter comprises a helical transmission line enclosed in a conductive activity, and a high Q capacitor coupled to form a series-resonant circuit. The capacitor is fabricated from a portion of coaxial cable having an outer conductor forming a first plate of the capacitor and an inner conductor forming the second plate of the capacitor. The notch filter is particularly suited for rejecting terrestrial interference in a C-band tuner at an intermediate frequency of about 400 MHz.

Abstract: A notch filter for reducing clock signal feedthrough effects in an acoustic charge transport (ACT) device. This notch filter is included on the same substrate as an ACT device. This notch filter may be employed to remove clock signals which have greater voltage than the data signals in an ACT device. This notch filter may be employed either at the input or the output of the ACT device. The notch filter includes a bridge connection of surface acoustic wave transducers and RC networks on opposite sides of the bridge configuration. Each side of the bridge is balanced so that at the notch filter frequency, the voltage output by each side of the bridge is one-half of the input voltage and in phase. These voltages produced by each half of the bridge are differenced and a zero output voltage is produced at approximately a notch or center frequency.

Abstract: Disclosed is a microwave notch filter which includes a plurality of serially connected YIG tuned resonators, each YIG resonator including a coupling wire and a PIN diode connected in parallel with the coupling wire. A first interconnect interconnects one end of the plurality of serially connected YIG resonators to an RF signal source, and a second inter-connect interconnects another end of the plurality of serially connected YIG band reject filters to a load. An inductor connects one of the interconnects to a circuit ground potential, and an inductor means connects the other interconnect to a DC voltage source, whereby the PIN diodes in the YIG resonators can be forward-biased or reverse-biased by an applied DC voltage. The DC voltage source selectively provides a positive DC voltage and a negative DC voltage which effectively controls the functioning of said YIG band reject filters.

Abstract: A noise suppression inductor including a bobbin having a spool divided into a plurality of spool portions, a core mounted on the bobbin, at least one coil wound around the spool portions into a plurality of coil portions and a plurality of terminal pins connected to opposite ends of the coil, in which the coil portions have different winding widths such that stray capacities produced at the coil portions are made different from each other, whereby the noise suppression inductor has a plurality of self-resonance frequencies.

Abstract: A circuit compensating for the attenuation caused on a line by an LS series type notch filter at frequencies less than the filter cut-off frequency, with such filter connected across said line and a reference voltage, includes a first resistor (R) in series on the line, a series circuit comprising a capacitor (C1) and a second resistor (R1) connected across the line and reference voltage, the capacitor being connected on the side of a first terminal of the first resistor, and an amplifier, the input of which is connected to the connection point of a capacitor and second resistor and the output of which is connected to the second terminal of the first resistor, such amplifier having a transconductance at said lower frequencies capable of ensuring the injection of a current equal to the sum of currents deviated by the series circuit and the filter.

Abstract: A filter comprises components arranged in a plurality of compartments which are magnetically shielded from each other. Each compartment includes components for filtering two frequencies wherein these frequencies are separated by an amount such that inductive coupling is substantially prevented. The compartments are combined to increase the attenuation, and any number of compartments may be utilized to filter the desired number of frequencies.

Abstract: In a bridge T type trap circuit having CR delta connection circuit and a variable LC tank circuit, the CR delta connection circuit is connected to the LC tank circuit via a variable inductor (19; 31, 33; 40). In one embodiment, a capacitor (18) is coupled with the variable inductor (19; 31, 33) to form another LC tank circuit. In another embodiment, a variable inductor (39) used in the first-mentioned LC tank circuit has a tap which is grounded. Impedance values of the first-mentioned LC tank circuit and the variable inductor (40) are adjusted so that absolute values of real and imaginary parts thereof have a given relationship. A resistor (22) of the CR delta connection circuit is used in common in a double-trap structure having two CR delta connection circuits and two series circuits each having two LC tank circuits.

Abstract: A noise suppression inductor including a bobbin having a spool divided into a plurality of spool portions, a core mounted on the bobbin, at least one coil wound around the spool portions into a plurality of coil portions and a plurality of terminal pins connected to opposite ends of the coil, in which at least one of the coil portions is composed of a first part having one or two winding layers and a second part having one winding layer and at least another one of the coil portions has not less than two winding layers.

Abstract: A power filter resonant frequency modulation network for coupling between a load and a variable voltage direct current power source which is subjected to superposition of alternating current thereon at various frequencies and at various voltage levels, the filter including first and second series inductor arrangements in the positive and negative buses between the power supply and the load, with capacitive elements in parallel between the two buses. Each series inductor arrangement includes a first inductor in series with a frequency and voltage responsive inductive load device, which is a second inductor having a tertiary winding loaded with a coil wound about the core of a saturable reactor. The firing angles of the reactors are directly proportional to voltage and inversely proportional to voltage to provide a phase staggered change of inductance in the network.

Abstract: Scrambling and descrambling systems utilizing jamming signals provide improved pictures with minimal distortion due to ghosts and artifacts. The television signal is combined with a jamming signal or signals located preferably in the vestigial sideband (VSB) of the modulated picture carrier and at about a null in the spectrum of the modulation of the picture carrier by the horizontal sync. The jamming signal or signals also preferably are placed between the harmonic components of the horizontal sync which define the horizontal sync spectrum and at frequencies which comply with governmental regulations respecting frequency locations of strong signals of CATV purposes. The jamming signals are generated by phase locking to the horizontal sync signals of the television signals or to a submultiple of the horizontal sync frequency.

Abstract: For reducing the amount of circuitry and losses in double highpasses for filter arrangements in networks with a high voltage level, the two resonance frequencies and Q-factors of the double highpass are tuned so that in a critical harmonic range, at most a harmless slight resonance peak can occur, for which purpose a series capacitor together with a series choke are connected in series with a shunt circuit grounded, in the case of high voltage and ungrounded in the case of medium voltage, of ohmic, inductive and capacitive circuit elements connected to the network.

Abstract: An enhanced quadrature notch filter which is capable of rejecting an undesired signal component of an input signal, generally comprising a quadrature phase splitter for producing first and second reference signals which have substantially the same frequency as the undesired signal component but which are 90.degree. out of phase with each other, a pair of amplitude control loops for adding the first and second reference signals to the input signal in amplitudes which will cancel the undesired signal component, and a phase locked loop circuit for maintaining a predetermined phase relationship between the undesired signal component and one of the first and second reference signals.

Abstract: Absorption circuits (C.sub.f,L.sub.F) are used for filtering out unwanted harmonic currents in electric lines (R, S, T), particularly for rated power within the range of 1 MVar-1 GVar, for example in the case of high-voltage direct-current transmission systems. So that the absorption circuit holds the virtual capacitance of the filter capacitor circuit (C.sub.f) at its rated value at a tuned frequency even when one or more capacitors (C11, C21, C12, C22) fail, a compensating choke (L) is provided in the cross branch of an "H" circuit of the capacitors. The capacitance of each capacitor (C11 . . . C22) is equal to the total capacitance C of the filter capacitor circuit (C.sub.f). The inductance L of the compensating choke (L) is equal to 1/.omega..sup.2 C, where .omega.=angular frequency of the harmonic. The compensating choke (L) can be designed for 5%-10% of the power of the filter choke (L.sub.F).

Abstract: A radio frequency signal coupler includes first and second conductors formed on an insulating body for coupling signals between respective input and output couplings. A series resonant circuit is formed on the insulating body between the first and second conductors for shunt attenuating interfering signals. The series resonant circuit includes a capacitance part that also functions as a protective spark gap.

Abstract: A notch filter which utilizes a quadrature coupler with the input signal on one of its four terminals, a first tuned surface acoustic wave device coupled to a second on of the terminals for receiving the input signal at 0.degree. phase shift, a second tuned surface acoustic wave device coupled to a third one of the terminals for receiving the input signal with a 90.degree. phase shift and the fourth terminal being utilized as the output signal terminal.

Abstract: Undesirable RF coupling via the outside of an outer coaxial cable conductor to/from RF coils in a magnetic resonance imaging apparatus is minimized by employing a parallel resonance tuned RF choke in the circuit. The choke is realized by forming a short coiled section of the coaxial cable with a lumped fixed capacitance connected in parallel thereacross and a conductive tuning rod positioned within the center of the coiled section so as to trim the parallel resonant frequency to the desired value.

Abstract: A tunable notch filter for the front end of an RF receiver includes magnetically coupled input and output tunable stages each responsive to a tuning voltage provided thereto. Each of the input and output stages includes a respective series and parallel resonant circuit wherein the parallel resonant circuits are tuned to a selected RF frequency and the series resonant circuits are tuned to a frequency approximately equal to that of a local oscillator. By using the series resonant circuits to create a notch at a frequency approximating that of the local oscillator, conducted local oscillator leakage from the receiver's antenna is substantially reduced and local oscillator image frequency rejection is substantially improved.

Abstract: An input filter for coupling television signals from a signal source such as an antenna to an RF signal processing stage includes series resonant input and output sections shunting the signal path. The input section forms a trap at the video IF frequency. The output section resonates at a frequency other than the sound IF frequency, below the band of television signal frequencies to be processed by the RF signal processing stage. The filter serves to attenuate unwanted stray signals such as video and sound IF components otherwise capable of interfering with and distorting the input television signals.

Abstract: A device generates a signal corresponding to a variable magnitude associated with the reactive power of an arc furnace, in order to control a reactive power compensator. It comprises a reactive power or voltage demodulator and a set of filters connected to the output of the demodulator. The set of filters comprises active bandstop filters having a bandwidth of .+-.F relative to frequencies of 2f.sub.O, 4f.sub.O and 6f.sub.O and narrowband active bandstop filters having a bandwidth of substantially .+-.2 Hz relative to frequencies of f.sub.O and 3f.sub.O. f.sub.O is the supply frequency and F is approximately f.sub.O /2. The device further comprises a phase advance corrector circuit operative in the frequency band O-F connected to the output of the set of filters.

Abstract: Selected interference frequency signals are inserted into the video or RF signal at the transmitter for scrambling purposes, either singly or in pairs. At the receiving end the signals are decoded by use of filters matched to the transmitted signal. The system allows different scrambling for each pay program thereby increasing the security relative to the prior art single frequency system.

Abstract: In a superheterodyne receiver, such as a typical television receiver, a radio frequency coupling arrangement includes a first resonant circuit coupled to receive the input signal and further coupled to a second resonant circuit by a first coupling. Both resonant circuits are tuned to the desired signal frequency. The capacitive branch of the first resonant circuit includes a circuit impedance for providing a neutralizing voltage, at least a portion of which is coupled by a second coupling to the inductive branch of the second resonant circuit. The circuit impedance is selected to cause cancellation in the inductive branch between image frequency components coupled by the first coupling and image frequency components coupled by the second coupling, without substantially affecting the desired signals.

Abstract: An impedance element formed from a single-port, surface acoustic wave transucer which generates a conductance versus frequency response characteristic having first and second peaks separated by a valley at a predetermined frequency and which generates a susceptance versus frequency response characteristic having a substantially constant portion over a finite bandwidth on both sides of said predetermined frequency.

Abstract: A two-cavity notch filter including an input terminal inductively coupled to an output terminal having a low impedance connection therebetween, a first relatively high impedance resonant tank circuit including the series combination of a circuit grounded capacitor and circuit grounded inductor having a tap point thereon, a capacitor and optionally in series therewith an inductor connected between the input terminal and tap point. The notch filter also includes a second relatively high impedance resonant tank circuit including the series connection of a circuit grounded capacitor and circuit grounded inductor having a tap thereon, and a capacitor and, optionally, in series therewith an inductor between the output terminal and tap point of the second tank circuit inductor. There is capacitive coupling between the capacitors forming the first and second tank circuits.

Abstract: An improved notch filter having a single port, surface acoustic wave transducer in parallel with an inductor and forming a first circuit which primarily blocks the signal frequency at all frequencies except the notch frequency at which point the first circuit becomes essentially resistive and at that point is cancelled by a signal from a second circuit in parallel with said first circuit and which is 180.degree. out-of-phase thereby providing a notch filter.

Abstract: A radio frequency switching circuit device wherein an LC circuit is inserted between two terminals, while a diode is shunted, and wherein the diode is controlled into its nonconductive state and its conductive state by a D.C. voltage, whereby the path between the two terminals is switched into a conductive state and a nonconductive state with respect to a radio frequency signal; characterized in that a transistor which turns "on" upon the application of the D.C. voltage is inserted in series with a D.C. voltage terminal for applying the D.C. voltage to the diode.

Abstract: A switched bandpass filter includes a plurality of triple tuned bandpass filter sections coupled in parallel between an input and an output terminal and complemented by input and output parallel resonant circuits. Each bandpass filter section includes a switch such as a switching diode by means of which the filter section is either rendered active for passing or inactive for suppressing the frequency band to which it is tuned. Portions of the inactive bandpass filter sections contribute to the resulting frequency response with notches at frequencies outside of the passband of the active filter section. Thus, some components of the inactive bandpass filter sections are reused with the active filter section to gain steep out-of-band rejection slopes. In one embodiment, the switched bandpass filter employs only one switching diode per frequency band and, by reusing two-thirds of the filter components, provides a highly efficient switched bandpass filter arrangement.

Abstract: An electronically tunable solid-state microwave frequency source comprises a transmission-absorption filter incorporated within a magnetic structure. The transmission-absorption filter is employed with a tunable solid-state oscillator and tunable solid-state multiplier to provide a continuously tunable microwave signal source with enhanced spurious signal attenuation over a multiple-octave tuning range. The filter structure comprises a sphere of monocrystalline garnet such as yttrium iron garnet (YIG) and two coupling loops disposed in the field region of an adjustable field DC magnet. The coupling loops are disposed orthogonal to the magnetic field and to each other. In a specific embodiment the first coupling loop is operative to receive at its input the fundamental frequency signal and at its output is grounded, and the second coupling loop is operative to receive a harmonic input signal at its input and to convey a desired output signal at its output.

Abstract: A protective circuit for reducing subsynchronous natural resonant frequency signals in a multiphase power supply system includes a damping resistor connected in series with each phase winding of the system. A resonant circuit tuned to the rated line frequency of the system is connected in parallel with each resistor so that signals having frequencies other than the rated line frequency will be damped by the resistors. An inductor can also be placed in parallel with each resistor to maximize the damping effect of each resistor at predetermined subsynchronous frequencies.

Abstract: An impedance bond for use in an electrified railway to provide a path for traction current includes a housing and a core positioned within the housing. The core is formed of a band of silicon steel wound into a closed loop construction which is then severed to form a cap and a U-shaped body. A pair of coils are positioned each upon a leg of the U-shaped core prior to repositioning the cap upon the core. Each of the coils is substantially identical in electrical resistance and each is formed of a flat copper sheet, identical in length, and wound into coils identical in configuration. There are terminal straps attached to the coil which extend outwardly from the housing. The terminal straps are preferably cast copper.

Abstract: An intermediate frequency band-pass filter for providing a pass band, comprising a frame forming a hollow surrounded region, a shield plate mounted in the frame and dividing the hollow surrounded region into two spaces, the shield plate having a coupling window, and two resonators disposed respectively in the spaces and coupled through the coupling window, the shield plate having in an edge thereof a step serving as a trap which provides a frequency suppression capability in the vicinity of the pass band of the filter.

Abstract: Spaced conductive strips are formed on a substrate. A first transmission line transformer is connected between the input and one end of the first strip. A second transmission line transformer is connected between the other end of the first strip and the output. The second strip is connected to ground. A shunt tuning circuit is connected between the strips. The circuit includes first and second MOS capacitors mounted on and connected to the grounded strip. An RF choke interconnects the other side of each capacitor. A varactor diode is mounted on and connected to the first capacitor. An etched spiral inductor connects the varactor and the first strip. A bias signal is applied to the junction between the choke and the second capacitor in order to tune the filter.

Abstract: An input signal is supplied to a first port of a non-reversible circuit and those frequency components of the input signal applied to a second port of the non-reversible circuit which differ from the resonance frequency of a resonance element connected to the second port are absorbed by a resistive terminating element connected to the second port. On the other hand, a frequency component of the input signal which coincides with the resonance frequency is derived at the third port of the non-reversible circuit. By changing the resonance frequency of the resonance element, the frequency component of the input signal which are provided to the third port can be altered.

Abstract: A band-pass filter circuit has series elements, comprising a parallel resonant circuit and a first inductance, and shunt elements comprising a series resonant circuit and a second inductance, the resonant circuits comprising variable capacitances to permit variation of the frequency over a frequency band. The second inductance may be a transformer. The variable capacitances may be constituted by a plurality of switchable component capacitances, having switches connected between the capacitances and ground.