Harmonic trap filter using coupled resonators

A harmonic trap filter suppresses at least one harmonic signal produced by an amplifier and includes an input terminal and a ground terminal. The harmonic trap filter further includes a plurality of resonators electrically coupled one to another between the input terminal and the ground terminal in a spatial order defined by relative phase shift of alternating voltage bias signals respectively applied thereto. The resonators are tuned to resonate at a frequency at which a phase delay is imparted to the at least one harmonic signal by the resonators to effect cancelation of the at least one harmonic signal at the input terminal.

BACKGROUND

High power amplifiers often generate unwanted harmonic content due to nonlinearities inherent in the circuit components from which the amplifiers are built. In radar, for example, amplifier harmonics can cause elevated sidelobes as well as a mischaracterization of the actual range ambiguity function. Obviously, to avoid such system anomalies, the harmonic content must be filtered out. Such additional filtering is usually realized in series with the high power amplifier output, thus lowering the desired output level by filter insertion loss. This additional filtering is thus costly in terms of power loss and dollars.

SUMMARY

A harmonic trap filter suppresses a harmonic signal produced by an amplifier and includes an input terminal and a ground terminal. The filter further includes a plurality of resonators electrically coupled one to another between the input terminal and the ground terminal in a spatial order defined by relative phase shift of alternating voltage bias signals respectively applied thereto. The resonators are tuned to resonate at a frequency at which a phase delay is imparted to the harmonic signal by the resonators to effect cancelation of the harmonic signal at the input terminal.

A radio-frequency (RF) transmitter comprises an amplifier and a harmonic trap filter to suppress at least one harmonic signal produced by the amplifier. The harmonic trap filter includes an input terminal, a ground terminal and a plurality of resonators electrically coupled one to another between the input terminal and the ground terminal in a spatial order defined by relative phase shift of alternating voltage bias signals respectively applied thereto. The resonators are tuned to resonate at at least one frequency at which a phase delay is imparted to the at least one harmonic signal by the resonators to effect cancelation of the at least one harmonic signal at the input terminal.

A shunt harmonic trap filter comprises an input terminal, a first magnet-free isolator circuit and a second magnet-free isolator circuit electrically coupled in series with the first magnet-free isolator circuit to define a non-reciprocal circuit path that begins and ends at the input terminal.

DETAILED DESCRIPTION

The present concept is best described through certain embodiments thereof, which are described in detail herein with reference to the accompanying drawings, wherein like reference numerals refer to like features throughout. It is to be understood that the term invention, when used herein, is intended to connote the concept underlying the embodiments described below and not merely the embodiments themselves. It is to be understood further that the general concept is not limited to the illustrative embodiments described below and the following descriptions should be read in such light.

Additionally, the word exemplary is used herein to mean, “serving as an example, instance or illustration.” Any embodiment of construction, process, design, technique, etc., designated herein as exemplary is not necessarily to be construed as preferred or advantageous over other such embodiments. Particular quality or fitness of the examples indicated herein as exemplary is neither intended nor should be inferred.

FIG. 1is a schematic block diagram of a transmitter100by which the principles of the present disclosure may be embodied. Such a transmitter100may be found in numerous applications including radar and telecommunications. Transmitter100may include transmitter circuitry110by which a waveform112is generated. Waveform112is provided to amplifier150, which generates transmitter waveforms152, Transmitter waveforms152can include not only the desired amplified signal, but also undesired artifacts, e.g., harmonics of waveform112. Harmonic trap filter130may be constructed or otherwise configured to remove harmonics at its input node154by way of the principles described herein. The filtered waveform, which can be an amplified version of waveform112, may be provided to an antenna140or other mechanism for conveying the signal over a medium.

Transmitter100may include control circuitry120by which operations of transmitter100are coordinated. As illustrated in theFIG. 1, control circuitry120may provide signals to harmonic trap filter130, such as AC bias signals122and DC bias signals124. Each of AC bias signals122and DC bias signals124may be dynamically altered for filtering signals that have non-stationary frequency characteristics, e.g., signals undergoing linear frequency modulation (LFM) also referred to as chirp modulation.

FIG. 2is an electrical schematic diagram of an example harmonic trap filter130by which the principles described herein can be embodied. It is to be understood that the circuit illustrated inFIG. 2is but one possible topology by which the harmonic trap filter functionality described herein can be realized. The harmonic trap filter function may be achieved using non-reciprocal features, where, as used herein, non-reciprocity refers to the case where the response of a system is different when the source and receiver are interchanged. Coupled resonator circuits may be used in shunt with the high power amplifier output and may be tuned so that the phase delay at any harmonic frequency that needs to be suppressed uses the harmonic signal to create cancelation. Multiple coupled resonators may be used for shorting out additional harmonic frequencies. The coupled resonator circuits may also be actively tuned during a LFM waveform so the harmonic traps can move dynamically with the fundamental and lower the unwanted harmonic content.

As illustrated inFIG. 2, harmonic trap filter130may comprise an input terminal202and a ground terminal204electrically coupled to a plurality of resonators210a-210f, representatively referred to herein as resonator(s)210. Each resonator210may comprise an inductor L1-L6, respectively, and a variable capacitance C1-C6, respectively.

Turning momentarily toFIG. 3, them is illustrated an example variable capacitance circuit300that may be used to embody the principles of this disclosure. Variable capacitance circuit300may be realized at C1-C6illustrated inFIG. 2. Variable capacitance may be achieved by applying a variable DC voltage to varactor VR310by a voltage source V(A), where A is an amplitude of the DC capacitance control signal. In certain embodiments, V(A) can be varied with sufficient rapidity to dynamically modify the capacitance, and hence the resonant frequency of the corresponding resonator210, during non-stationary frequency waveforms, e.g., LFM waveforms.

Each variable capacitance circuit300may further be coupled to an AC voltage source V(θ), where θ is a phase angle relative to that of other resonators210. The purpose of this phase angle is described in more detail below. In certain embodiments, the amplitude and frequency of V(θ) is static across all resonators210, with the frequency being much lower than that of harmonic frequencies being filtered. For example, for a 1 GHz resonator, the modulation frequency may be 70-210 MHz. In other embodiments, the amplitude of AC voltage source V(A) may be varied to alter the capacitance of varactor VR310, in which case DC voltage source V(A) may be held constant.

It is to be understood that variable capacitance circuit300may be implemented in ways other than that illustrated inFIG. 3. Those having skill in microwave circuits will recognize numerous variable capacitance techniques that can be used without departing from the spirit of the principles described herein. The filtering and choke elements, C302, L304, L306, L308, C312, L314and L316may be chosen according the operating frequencies of the application being implemented. Moreover, it is to be understood that techniques other than variable capacitance can be used to realize the principles described herein, which will be apparent to skilled artisans upon review of this disclosure.

Returning toFIG. 2, it is to be observed that resonators210are coupled one to another in an order defined by the phase shift θ of the AC voltage applied thereto. For example, resonator210amay have an AC voltage V1(0) applied thereto, resonator210bmay have an AC voltage V2(120) applied thereto and resonator210cmay have an AC voltage V3(240) applied thereto. Similarly, resonator210dmay have an AC voltage V4(120) applied thereto, resonator210emay have an AC voltage V5(240) applied thereto and resonator210fmay have an AC voltage V6(0) applied thereto. The phase angle differences are applied in, for example, 120° increments around sets of resonators210to define spatiotemporally modulated loops of resonators210. It is to be understood, however, that other phase increments, e.g., 90°, may be used to embody the principles described herein. Additionally, it is to be understood that while resonators210are illustrated as being wye-connected, other topologies, such as delta-connected resonators may also implement the principles of this disclosure. These spatiotemporally modulated loops of resonators210realize non-reciprocity with respect to ports P1-P3and P4-P6. When so embodied, resonators210implement a pair of coupled circulators220aand220b, representatively referred to herein as circulator(s)220. Bandpass filters F1-F6contain the bias voltages V1-V12 within circulators220.

FIG. 4is an electrical schematic diagram of harmonic trap filter130at a higher level of abstraction than that illustrated inFIG. 2. It is to be understood that circulators220aand220bcan be implemented by spatiotemporal modulation and not by magnetic bias by permanent magnets. As used herein, such circulators are referred to herein as “magnet-free” circulators. By way of the principles described herein, resonators of circulators220aand220bcan be tuned to impart a 90° phase delay from port to port. That is, at resonance (where the losses from P1to P2in circulator220aand from P4to P5in circulator220are minimum and the isolation between P1to P3in circulator220aand between P4to P6in circulator220bare maximum) there may be a 90° phase delay from port P1to port P2of circulator220aand an additional 90° phase delay from port P4to port P5of circulator220bfor a total of 180° phase delay through both circulators220aand220b. Thus, at the chosen harmonic frequency for which harmonic trap filter130is tuned, there is a cancelation of the harmonic signal at the input node202.

As illustrated inFIG. 4, port P3of circulator220aand port P6of circulator220bare terminated in respective resistive loads, RX and RY (also shown inFIG. 2). As such, circulators220aand220bform respective isolators that define a non-reciprocal electrical path that begins and ends at input terminal202, i.e., from input terminal202, through ports P1and P2of circulator220a, through ports P4and P5of circulator220band terminating at input terminal202. As stated above, a harmonic signal component that traverses this circuit path can undergo a 180° phase delay to effect cancelation of the harmonic signal component at input terminal202. Resistors RS and RL may be used for source and load impedance matching, respectively.

FIG. 5is a graph of simulated transmission coefficient S(2,1) of the embodiment illustrated inFIG. 4. At the operating frequency of 1.0 GHz, them is little to no insertion loss, due to the shunt configuration of the harmonic trap filter130. At the second harmonic of 2.0 GHz, however, there is a reduction of almost 10 dB through cancelation.

FIG. 6is an electrical schematic diagram of another harmonic trap filter630by which the principles described herein may be embodied. Harmonic trap filter630may be constructed or otherwise configured to remove multiple harmonics from a signal applied to its input node602. To that end, harmonic trap filter630may include a first isolator pair610acomprising isolators620aand620b, and a second isolator pair610bcomprising isolators620cand620d. Isolator pairs610aand610bare representatively referred to herein as isolator pair(s)610and isolators620a-620dare representatively referred to herein as isolator(s)620(or isolator circuit(s)620). Isolators620may be constructed from circulators, each terminated at respective third ports by an appropriate load. As illustrated inFIG. 6, isolator620ais constructed from a circulator terminated in RX at its port P3, isolator620bis constructed from a circulator terminated in RY at its port P6, isolator620cis constructed from a circulator terminated in RV at its port P9and isolator620dis constructed from a circulator terminated in RW at its port P12. Resistors RS and RL serve as example source and load impedance matching circuits, respectively.

Each isolator pair610may be constructed and may operate in the manner illustrated inFIGS. 2 and 3, although, as mentioned above, other circuit topologies may be employed to realize the spatiotemporal modulated loops of resonators. Isolator pairs610define respective non-reciprocal electrical paths that begin and end at input terminal602. That is, a first electrical path proceeds from input terminal602, through ports P1and P2of isolator620a, through ports P4and P5of isolator620band terminates at input terminal602. A second electrical path proceeds from input terminal602, through ports P7and P8of isolator620c, through ports P10and P11of isolator620dand terminates at input terminal602. Each isolator pair610may be tuned to different harmonic signal components of the signal applied to input terminal602such that, at each harmonic frequency, the corresponding harmonic signal component undergoes a 180° phase delay or shift when the corresponding electrical path is traversed thereby. The 180° phase delay applied to each harmonic effects cancelation of that harmonic at input node602.

FIG. 7is a graph of simulated transmission coefficient S(2,1) of the embodiment illustrated inFIG. 6. At the operating frequency of 1.0 GHz, there is little to no insertion loss, due to the shunt configuration of the harmonic trap filter. At the second harmonic of 2.0 GHz and at the third harmonic of 3.0 GHz, however, there is a reduction of almost 10 dB through cancelation. Referring back toFIG. 6, isolator pair610ais tuned to cancel the second harmonic of 2.0 GHz and isolator pair610bis tuned to cancel the third harmonic of 3.0 GHz.

It is to be understood that the concept of canceling more than one harmonic signal component described with reference toFIG. 6may be extended to any number of harmonics by incorporating additional isolator pairs as needed.

The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description within this disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the forms disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the principles described herein.

The descriptions above are intended to illustrate possible implementations of the present concept and are not restrictive. Many variations, modifications and alternatives will become apparent to the skilled artisan upon review of this disclosure. For example, components equivalent to those shown and described may be substituted therefore, elements and methods individually described may be combined, and elements described as discrete may be distributed across many components. The scope of the principles should therefore be determined not with reference to the description above, but with reference to the appended claims, along with their full range of equivalents.