Method and system for making antenna measurements in a hyperechoic chamber using scattered modulation

Abstract

Systems and methods are disclosed for measuring electric fields of an RF transmit antenna under test. The antenna under test is placed in a hyperechoic chamber, along with one or more modulating scatterers and a receive antenna. A modulator applies a modulation signal to the modulating scatterer(s) during operation of the antenna under test, and the receive antenna gathers RF energy propagated by the antenna under test and modulated RF energy scattered by the modulating scatterer(s) from an interaction with the electric field of the antenna under test. A correlation receiver discriminates the modulated RF energy portion of the received RF energy, and integrates that portion over a stirring interval during which modes present in the hyperechoic chamber are stirred.

Description

METHOD AND SYSTEM FOR MAKING ANTENNA MEASUREMENTS IN AHYPERECHOIC CHAMBER USING SCATTERED MODULATIONCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit and priority' of U.S. Prov. Pat. App. Ser. No. 63 / 430,629, titled Electric Field Mapping via Acoustically Modulated Scattering, filed December 6, 2022, and U.S. Prov. Pat. App. Ser. No. 63 / 472,514, titled Method and System for Making Antenna Measurements in a Hyperechoic Chamber Using Scattered Modulation, filed June 12, 2023, each of which is incorporated by reference in their entireties herein.BACKGROUND

[0002] Mapping arbitrary fields on free-running planar circuits is of growing importance as these circuits increasingly lack port-level access and are pushing toward mm- wave frequencies. Most reported field mapping techniques, however, whether using electrical or electro-optical instrumentation, rely on phase coherence between sample and probe, necessitating port-level access to the device under test (DUT).

[0003] Such coherent mapping of evanescent fields by direct sensing is widely used for near- field antenna pattern measurements. It has even been demonstrated at sub-mm geometries on planar circuits using localized field probes, often in combination with scanning force microscopes (SFMs) to maintain constant tip-sample distance.

[0004] Direct sensing is limited by an aperture (usually coaxial) that must be scaled down to desired field resolution, a correspondingly lossy transmission-line connection to a receiver, and — importantly and unavoidably — parasitic coupling to parts of the probe not intended to sense the fields, such as the coaxial shield.

[0005] While an alternate way to directly sense local microwave fields involves frequency mixing between the DUT fields and a signal injected onto an SFM cantilever, this is limited toconductive cantilevers whose mechanical resonance is at the difference frequency between the DUT and the signal injected onto the cantilever, thus requiring phase coherence.

[0006] Antenna characterization of small-scale antenna packages is ty pically performed in an anechoic chamber, with reflected energy' tightly controlled to avoid corrupting the line-of-sight signal between a transmitting antenna and a receiving antenna (it is possible that either is an antenna under test). For extremely small antenna packages, particularly with near-field measurements, it becomes difficult to perform tests that avoid unwanted interactions between the two antennas.

[0007] As opposed to an anechoic chamber, a hyperechoic (or reverberation) chamber has surfaces that tend to reflect, rather than absorb, radio frequency7(RF) energy in a frequency band of interest. Several researchers have proposed performing antenna field measurements in a hyperechoic chamber, using a variety' of signal processing techniques to sort the original signal propagating at a line-of-sight (LOS) angle between a transmitting antenna and a receiving antenna from the reflected signals that bounce around the chamber and arrive at the receiving antenna. For instance, in P. Besnier et al.. Radiation pattern measurements in reverberation chamber based on estimation of coherent and diffuse electromagnetic fields, 2014 IEEE Conference on Antenna Measurements and Applications, a group of researchers investigated a system similar to system 100, depicted in FIG. 1. A hyperechoic chamber 110 is fitted with vertical and horizontal mode stirrers, 112 and 114, which rotate during measurement to continually change standing wave patterns that may exist in the chamber. A device under test 120 is positioned on a rotatable stand 116. such that it may be oriented at various line-of-sight angles with respect to a receive antenna 130. Receive antenna 130 is placed close enough to device 120 that the LOS distance between the two is less than the distance from either to any wall of the chamber. Under these specific conditions, using coherent drive / receive circuitry 140 coupled to both antennas, the researchers demonstratedthat it is possible to coherently detect a LOS signal in the presence of a significant stirred, reflected receive component, given long enough integration times.SUMMARY OF ILLUSTRATIVE EMBODIMENTS

[0008] A variety of wireless systems are now being fielded where the antenna package is an array of patch antennas w ith no direct way to inject signals from a testbed into any of the antennas of the array. For instance, small wireless communication devices may operate at 10 GHz or more, based on transmit signals generated autonomously on an integrated circuit with integrated patch antennas, and antenna characterization within the configuration of the assembled product is desired. For such a system, it would be advantageous to perform near- field antenna measurements. Such is difficult in an anechoic chamber, due to the lack of ability to control the device under test (DUT) and the small size of the device, which makes it difficult to probe the device without affecting its field. Using a system such as discussed above with respect to FIG. 1 is also difficult, as without control of the DUT it may be impossible to coherently detect a LOS component of the antenna field pattern in the presence of a much larger reflected energy component.

[0009] In one aspect, the present disclosure relates to systems for electric field mapping using a mechanically modulated scattering probe. The system can comprise an electric field probe needle coupled to an acoustic modulator that vibrates the probe needle in the z-axis (i.e., normal to the surface of a planar DUT), a scanning platform that can perform relative x- and y-axis positioning translations between the electric field probe needle and a DUT, and a collection circuit that gathers and characterizes sideband energy generated by the acoustically modulated probe. It is not necessary that the system have port access or otherwise control the DUT, or be operated in frequency or phase coherence with the DUT, although in some embodiments the collection circuit can include a coherent detector that locks to a primaryfrequency emanated by the DUT (or an intermediate frequency formed by mixing the primary frequency with a slightly offset reference frequency) in order to improve detection accuracy.

[0010] In another aspect, the present disclosure relates to a method for electric field mapping using a mechanically modulated scattering probe. The tip of the scattering probe is brought within a tip-sample distance of the surface of a DUT. An acoustic modulator vibrates the scattering probe in the z-axis (i.e., normal to the surface of a planar DUT) at a given acoustic modulation frequency and amplitude. The vibration modulation creates sideband energy around the primary' frequency / frequencies of the electric field at the position of the probe tip, with the sidebands separated from the primary' frequency by the acoustic modulation frequency. At least a portion of this sideband energy is collected by the collection circuit and characterized, e.g., to determine a sideband strength. The tip of the scattering probe and / or DUT are then translated to a second measurement position and the sideband strength is once again determined. This process is repeated for a desired two-dimensional scan pattern to map field strength in a near-field region above the surface of the DUT.

[0011] In another aspect, a system and method for antenna characterization are disclosed that may be suitable for heretofore relatively intractable measurement scenarios such as the ones just described, as well as for other measurement scenarios that have other workable solutions in place. It is recognized herein that using a combination of a mode-stirred hyperechoic chamber, a modulating scatterer or scatterers under control of the system, and a receiver that can lock to a modulated component of DUT energy scattered by the modulating scatterer, it is possible to map near-field components of a free-running device under test. Each scatterer generates a response based on the DUT antenna field present at a relatively small point in space, but the hyperechoic chamber allows a significant portion of that energy to be collected by a receive antenna. Because the modulation signals are at a low frequency compared to the size of the chamber, one wavelength of phase shift in the demodulated component of thesignal may literally require thousands of reflections, and as this is unlikely, it is possible to recover phase information from the demodulated scattering signal. During a collection time, a mode stirring mechanism is operated so as to average out variability in the percentage of the modulated and scattered signal that constructively interferes and reaches the receiver. Frequency discrimination allows the modulated component of the received energy to be amplified while the unmodulated (and much larger) component of the energy' emitted by the DUT is filtered out.

[0012] In some embodiments described herein, modulated scatterers are operated concurrently at different points in space surrounding the DUT and / or different field polarities, and the received signal comprises the superposition of scattered energy received from each scatterer. Different, preferably uncorrelated modulation patterns are applied to the different scatterers, allowing correlation receivers to separate the individual received signals from each other and make concurrent estimates of the DUT field pattern at multiple spatial positions and / or polarities.

[0013] The foregoing general description of the illustrative implementations and the following detailed description thereof are merely exemplary aspects of the teachings of this disclosure and are not restrictive.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. The accompanying drawings have not necessarily been drawn to scale. Any values or dimensions illustrated in the accompanying graphs and figures are for illustration purposes only and may or may not represent actual or preferred values or dimensions. Where applicable, some or all features may not be illustrated to assist in the description of underlying features. In the drawings:

[0015] FIG. 1 illustrates a configuration of a prior art direct line-of-sight coherent detection system that operates in a hyperechoic chamber;

[0016] FIG. 2 illustrates a configuration of an embodiment of the present disclosure, deployed with a hyperechoic chamber;

[0017] FIGs. 3A and 3B illustrate a three-scatterer module according to an embodiment;

[0018] FIG. 4 contains a block diagram of a low-frequency modulator usable with the FIG. 3A module;

[0019] FIG. 5 illustrates a two-scatterer module according to an embodiment;

[0020] FIG. 6 contains a time-domain illustration of three orthogonal modulation codes usable with the modulator of FIG. 4;

[0021] FIG. 7A through FIG. 7D show frequency spectra illustrating signals present at respective points in the system;

[0022] FIG. 8 contains a block diagram for a correlation receiver usable with the FIG. 2 configuration and FIG. 4 modulator;

[0023] FIG. 9 illustrates a configuration of a spherical sampling embodiment deployed with a hyperechoic chamber;

[0024] FIG. 10 contains a block diagram for a field mapping system usable with the FIG. 9 configuration; and

[0025] FIG. 11 shows a block diagram for another correlation receiver usable with, e.g., the FIG. 2 configuration and FIG. 4 modulator.DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0026] The description set forth below in connection with the appended drawings is intended to be a description of various, illustrative embodiments of the disclosed subject matter.Specific features and functionalities are described in connection with each illustrativeembodiment; however, it will be apparent to those skilled in the art that the disclosed embodiments may be practiced without each of those specific features and functionalities.

[0027] Several embodiments were originally tested on a test bench to characterize the performance of candidate scattering elements. In one embodiment, a synthesizer, which need not be a part of the system, provides a 10.77 GHz signal to a split-ring resonator (SRR) fabricated on a microstrip circuit board (the DUT). Alternately, the DUT may create electromagnetic signals using wholly internal signaling circuitry. A function generator feeds a 600 Hz sinusoidal signal to an acoustically isolated voice coil actuator on which a steel needle is mounted. The voice coil actuator and steel needle are positioned above the SRR under test, with the needle perpendicular to the top surface of the SRR and approximately 0.25 mm separating the tip of the needle from the top surface of the SRR. The voice coil is then actuated to vibrate the needle in the vertical direction at 600 Hz with a peak-to-peak vibration amplitude of ~0.2 mm. The vibrating needle serves as a modulated scatterer, which modulates the normal component of electric field bound to the split-ring resonator. The reradiated electric field is in turn collected by a co-polarized X-band horn antenna located -90 mm from the needle. This signal from the horn antenna is fed into a spectrum analyzer (an Agilent E4448A in this example) to measure the power of one of the two 600 Hz sidebands detected. This value is proportional to the magnitude squared of the normal component of the electric field at the location of the needle probe.

[0028] In one exemplary measurement procedure, the SRR circuit is excited at 10.77 GHz and -10 dBm, and the function generator is set to 600 Hz and 6 Vrms. The needle probe is gradually lowered to within -0.2 mm of the microstrip circuit board until sidebands can be observed on the spectrum analyzer. Once sidebands are observed on a spectral display, the entire region of interest of the DUT is scanned in the x- and y-directions with a desired step size (here set to 0.635 mm but step size is adjustable), and the power of the upper sideband isrecorded at each step to create a two-dimensional mapping of sideband strength.Alternatively, the lower sideband power may be recorded, or both upper and lower sideband power readings may be summed or averaged.

[0029] A model was developed to examine the quantitative relationship between the sideband level and the scattered / re-radiated component of Ez. The same measurement procedure as described above was used to scan an open-ended 50-ohm microstrip line fabricated on Rogers RT5800LZ substrate, and the peak sideband power when the needle probe is at the center of the microstrip line at its open-circuited termination is used as a reference. The measured (double) sideband power is then related to the output from a simulation of Ezin CST Studio as detailed below.

[0030] The peak single sideband power when the needle probe is on the centerline of the termination of the open-ended microstrip line is -96 dBm. or -93 dBm for both sidebands. The simulated Ezfor the same drive conditions at the same position is - 10.9 dBV / m or 285 mV / m. If the amplitude of needle vibration is ~0.2 mm, assuming free-space impedance the power of the re-radiated signal at the tip is -4 nW, which in the model is uniformly spread over a hemisphere with tip-to-hom aperture radius of 86 mm. The hom aperture is 79x91 mm, subtending -15% of the hemisphere’s area, collecting that fraction of the power radiated from the tip, -91.8 dBm, which is close to the double-sideband measured value of power. Using such simulations as a calibration metric, the relative sideband measurements can be related to the absolute electric field strength at the point of measurement.

[0031] Using this system, an electric field map is generated by using the electric field mapping system to scan the SRR sample, which is excited at the resonant frequency of one SRR structure, (10.77 GHz), and applying the cubic interpolation function in MATLAB to smooth the contour of the results.

[0032] Although the mechanical scattering element in the preceding example was a steel, electrically conductive needle, various embodiments may substitute a material with different electrical properties. For example, a bulk semiconductor material or carbon fiber needle can possess dielectric / electrical conductivity' properties that vary' strongly with frequency, due at least in part to a characteristic dielectric relaxation time of the material.

[0033] When the mechanical scattering element is a good electrical conductor and is exposed to a polarized electric field, the field induces a current in the element, and that current regenerates a field that is transmitted in all directions (or “scattered”) in accordance with a radiation pattern of the scattering element. It is believed that inhibited movement of charge carriers in, e.g., appropriately doped bulk semiconductor materials or carbon fiber, may beneficially combine with mechanical excitation of the scattering element to couple into a resonance that tends to bunch charge carriers at one end or the other of the scattering element. This “bunching” behavior may actually create a larger scattered field than is created by a fully conductive element with a similar physical configuration. When a bulk semiconductor is used, a doping level of the semiconductor can be selected for a carrier mobility' matched to the frequency of scattering for an unbiased element, or the carrier mobility / density can be adjusted via a bias cunent.

[0034] In an illustrative embodiment, carbon fiber needles with lengths of 5, 8, and 13 mm w ere mechanically modulated with an acoustic driver at 210 Hz, and used to sample an antenna aperture at various x, y positions and tw o polarization directions. Two passes of the driver / translation mechanism were made, one without the scattering element and one with, and the two field maps as measured from the passes at a receive antenna were differenced to create a scatterer signal field map. The peak detected scattered signal for the 8 mm carbon fiber needle was roughly 47 pV, significantly higher than the peak detected scattered signalfor the 5 mm carbon fiber needle. The 13 mm carbon fiber needle performed even better, generating a peak detected scattered signal of over 300 mV.

[0035] In the test configurations above, undesired scattering behavior from the scatterer driver mechanism and translation mechanisms can create non-stationary interference signals that undesirably affect the Signal to Noise and Interference Ration (SNIR) of the detection circuit. Also affecting the SNIR, the receive antenna aperture may only gather a small fraction of the scattered energy', which must then be detected in the presence of and discriminated from the larger carrier signal. As a partial solution to these issues, several embodiments below dispose elements of the measurement system in a hyperechoic chamber.

[0036] FIG. 2 illustrates an embodiment comprising a hyperechoic chamber 210 that provides an exemplary' test environment, and a set of components that provide modulated scattering and measure its strength. The chamber itself is preferably sized for use with a range of carrier frequencies generated by the DUT, as will be understood by those skilled in the art. For instance, rectangular chambers with an internal volume in the range of 0.5 to 5 m3typically provide good reverberation performance for signals with frequencies present in 4G LTE and 5G cellular band signaling, e.g., sub-1 GHz to below 20 GHz; millimeter-wave antennas (e.g., 30-300 GHz) can also be characterized in chambers with comparable dimensions. The wavelength of such signals is small enough, compared to the distances between the opposing walls of the chamber, that hundreds to thousands of resonant modes may exist at any given point in time when a transmitter is active in the chamber, allowing statistically even power density within the chamber for stirred signals. Like a prior art chamber, a vertical mode stirrer 112 and a horizontal mode stirrer 114 are provided and are used to statistically vary the standing wave modes present in the chamber during a measurement interval. These may be stepped to a variety of deterministic positions during a measurement interval or rotated slowly and continuously, preferably at mutually non-harmonic rotation rates. It may be preferable, where multiple measurements are to be taken, to move the mixers through a same sequence of orientations during each measurement interval. Other mode-stirring mechanisms may altematively / additionally be employed, such as moving the DUT 120 and / or receive antenna 130 during signal collection, and / or changing the physical configuration of the chamber itself.

[0037] Although in preferred embodiments, the hyperechoic chamber / environment is completely enclosed and has walls that are efficient reflectors at the frequency of interest, a hyperechoic chamber as used herein may include substantial openings or absorber wall sections, for instance, while still collecting sufficient scattered energy' to be of use in an embodiment. Depending on the efficiency of the chamber, it may be desirable to deploy one or more absorbers within the chamber to adjust an extinction coefficient for radiated energy.

[0038] A rotatable stand 1 16 holds the DUT 120. The rotatable stand 116 may articulate the device in one or more articulation planes (for instance, azimuth and elevation), or may simply hold the DUT 120 at a desired position. Although the rotatable stand 1 16 may allow control signals to be fed through one or more connectors and cables (not shown) to DUT 120, this is not required in all embodiments.

[0039] A reflecting baffle 222 preferably shields a receive antenna 230 from receiving direct LOS energy from the DUT 120, and possibly also from a modulating scatterer 250.

[0040] A modulating scatterer 250 is placed in hyperechoic chamber 210 at a desired relative position from the DUT 120. The position may be in a near-field measurement region of the DUT 120, and the position may be adjustable under control of a measurement system, as will be described, but a positioning apparatus is not illustrated in FIG. 2, so as to allow a clear preliminary explanation of several basic operating principles. A single modulating scatterer may preferentially scatter RF energy of a desired polarity. As will be described below, ascatterer module may be used that allows for scattering of the antenna field at a small point in space using two, or even three, co-aligned but orthogonally polarized modulating scatterers.

[0041] Modulating scatterer 250 receives a modulation signal from a low-frequency (LF) modulator 240. Low frequency in this context is related to the size of the chamber. For instance, a chamber with an internal volume in the range of 0.5 to 5 m3is typically too small to support standing wave modes for RF energy at audio frequencies (20 Hz to 20 kHz) or ty pical ultrasonic frequencies (20 kHz to 10 MHz), and such are typically readily absorbed by the chamber walls. The modulation signal may be a pure tone, a combination of tones, or a wider spectral signal that conveys information useful in detection, as will be explained below. It should be noted that the small size of the modulating scatterer 250 typically makes the scatterer a very poor direct RF radiator within the band of the modulating signal.

[0042] It is presently believed that an appropriately configured electrically or mechanically modulated scatterer in the electric field of a radiating antenna induces an interaction with that field that re-radiates modulated RF energy. The re-radiated modulated RF energy has a strength that is related to both the strength of the electric field and the strength of the modulation signal applied to the scatterer. Spectrally, the re-radiated signal up-converts the spectrum of the modulation signal to a double-sideband signal centered on the carrier frequency of the radiating antenna (assuming the radiating antenna is transmitting a pure tone).

[0043] Receive antenna 230 gathers RF energy present in the chamber and not otherwise absorbed, and will generally be selected to efficiently gather RF energy at the operating frequency range of the DUT 120. The received energy includes RF energy propagated by the DUT 120 and modulated RF energy generated by the interaction between a portion of the RF energy propagated by the DUT 120 and the modulating scatterer 250 (i.e., the portion of that energy that sets up a field at the spatial location of the modulating scatterer). Some of thatRF energy is direct LOS energy from DUT 120, and it has not been reflected by the chamber 210 or its internal components. The remainder of the RF energy reaching the location of the modulating scatterer 250 is reflected energy present in the multitude of resonant modes present in the chamber 210 (and changing as the mode stirrers (112, 114) reposition). Over an appropriate stirring interval, the reflected energy7reaching the modulator constructively and destructively interferes, allowing such energy7to be averaged out by subsequent signal processing, and to be statistically similar at various points in the chamber. Like the chamber itself, receive antenna 230 is typically a high loss antenna at the modulating signal frequency supplied by the LF modulator, e.g., due to an impedance mismatch it readily reflects incident LF energy. Note that if a scatterer is not presently being modulated, it just becomes another reflective surface within the chamber for RF energy^ propagated by the DUT 120.

[0044] Receiver 260 operates on a signal from receive antenna 230, with knowledge of the modulation signal supplied by LF modulator 240 to the modulating scatterer 250. Receiver 260 receives a signal from receive antenna 230, the signal corresponding to both the RF energy directly propagated by the DUT 120 and the modulated RF energy produced by the interaction between modulating scatterer 250 and the field present at its location. Advantageously, although the scattered signal is small, the reverberation chamber allows a significant portion of that signal to be collected and supplied to receiver 260. Based on information related to the modulation signal (such as access to a modulation carrier and any coded information impressed on that carrier), receiver 260 discriminates the modulated portion of the signal from the receive antenna 230. Over an integration time interval, that involves repositioning the mode stirrers (112, 114) to multiple different stirring positions, receiver 260 integrates the discriminated portion of the signal. The integrated signal allows the receiver to infer a relative direct LOS field strength at the position of modulating scatterer250 due to the operation of the DUT 120.

[0045] FIGs. 3A and 3B show an embodiment of a modulating scatterer 250, in plan view and in cross-section respectively, that includes three differently polarized elongated modulation elements. Scatterer 250 is built on a circular planar dielectric substrate 310, with four plated conductors 332, 336, 342, 346 arranged symmetrically in a cross on its front side, and a series of pads (as described below) formed on its back side. At the center of the substrate, an aperture 324 allows an elongated conductive member 322 to protrude through substrate 310, normal to the plane of the substrate. Various filled vias (two of which are shown in FIG. 3B as vias 362, 366) allow electrical connectivity between the four plated conductors and other components and the backside pads. Other backside conductors or ground planes may be present in a given embodiment, as w ell as intermediate conductors or planes routed on an interior conductive layer of substrate 310.

[0046] A first modulation element is formed by the combination of conductor 332, a switching element 334, and conductor 336. A modulating signal is applied to this combination through signal connections to pads 330 and 338, which may connect, e g., to two twisted pair leads from LF modulator 240 and present within a larger shielded signal harness. When switching element 334 is an RF diode, it may be reverse biased by modulator 240 to turn “off” the first modulation element, and forward biased to allow' a modulation signal to be applied between pads 330 and 338. Switching element 334 may be located on either the front side or the back side of substrate 310, so long as it electrically connects conductors 332 and 336. Switching of modulating elements can also be effected by light (e.g., through an optical fiber) or other means.

[0047] A second modulation element is formed perpendicular to the first modulation element, by the combination of conductor 342, switching element 344. and conductor 346. A separate second modulating signal is applied to this combination through signal connections to pads340, 348. Although operated similarly to the first modulation element, the two elements are substantially responsive to orthogonal field polarizations.

[0048] Elongated conductive member 322 functions as a mechanical scatterer that, in operation, vibrates along its elongated axis responsive to a third modulation signal from LF modulator 240. In this embodiment, a piezoelectric transducer 350 (most commonly embodied as a quartz tuning fork) is supplied in a housing 320, which mounts to the underside of substrate 310. Two signal pads 326, 328 allow connection between a pair of signal wires from LF modulator 240 and piezoelectric transducer 350. Piezoelectric transducer 350, in response to the third modulation signal, displaces conductive member 322 vertically (as seen in FIG. 3B) due to a mechanical connection between the two, inducing a modulation of the field present at that position. In other embodiments, a different means (such as a voice coil) may be used to vertically displace member 322. As described above, conductive member 322 may be formed of, e g., a metallic element that is a good electrical conductor, or a material that can support a significant number of charge carriers but that has a dielectric relaxation time significantly longer than a metallic conductor, such as a carbon fiber structure or a doped bulk semiconductor. Alternately, a third electrical modulator may be substituted for the mechanical modulator of this embodiment.

[0049] In FIG. 3B, a scattering element is formed by mounting a conductive tip 322 on a piezoelectric transducer 350 such as a quartz tuning fork. In other embodiments, one or both of the tines of the tuning fork 350. which can have overlying plated conductive electrodes, are used directly as a scatterer without a separate conductive tip. In this embodiment, any shielding is removed from at least a side of the fork that faces the DUT, and potentially all shielding that could obstruct the vibrational elements of the tuning fork can be removed. The fork is then oriented such that the vibrational mode of the tines (i.e., towards and away fromeach other) is oriented in a desired direction to scatter incident RF energy of a desired polarity.

[0050] Preferably, each of the three scattering elements may be modulated concurrently, allowing contemporaneous measurement of different field polarities at a point in space occupied by modulating scatterer 250. FIG. 4 contains a block diagram for an embodiment of LF modulator 240 useful for contemporaneous measurement. LF modulator 240 comprises a code generator 402, a carrier signal generator 440, and three drivers 410, 420, and 430.

[0051] Code generator 402 generates three code sequences 412, 422, 432 to be applied respectively to drivers 410, 420, and 430. The codes may be, for example, Walsh codes, pseudorandom noise (PN) sequences, or other codes that are substantially mutually orthogonal and have a small cross-correlation coefficient.

[0052] Each driver 410, 420, 430 produces a respective modulation signal based on a combination of its respective supplied code 412, 422, 432 and a reference signal supplied by carrier signal generator 440. For instance, in one embodiment, each driver produces a binary phase-shift keyed (BPSK) version of the carrier signal based on the code supplied to it. Each driver produces a respective modulation signal to corresponding pads of scatterer 250, e.g., driver 410 supplies a signal to pads 326, 328.

[0053] In other embodiments, other modulation schemes may be substituted, including amplitude and quadrature phase modulation. The amplitude of the signal supplied by each driver may be independently variable and or independently enabled. For instance, if after an initial measurement, one or more signals are found to predominate the response, a second measurement may be taken with the larger-response modulators turned off. the drive strength of the smaller-response modulators increased, or some combination of the two. In still other embodiments, a different modulation such as an orthogonal frequency-division multipleaccess (OFDMA) solution may be included to provide each scatterer its own frequency subspectrum, which can shift during a measurement interval and be rebalanced such that some scatterers are dynamically provided more spectrum than others to increase their sensitivity. In some embodiments, multiple elements of a modulating scatterer may be operated together with a same drive signal to create a joint signal that represents the total electric field at the location of that modulating scatterer.

[0054] Other scatterer embodiments are possible. For instance, FIG. 5 shows a scattering module 500 similar to the FIG. 3 A / 3B embodiment, but consisting of only two crosspolarized scatterers, with no third scatterer.

[0055] FIG. 6 illustrates a simple embodiment for three modulating codes useful in an embodiment where the modulating scattering elements are operated concurrently. Over a code interval 620, each code varies according to a different sequence (many other more complex codes are possible in any given embodiment). At the end of the code interval 620, the codes repeat. Alternately, with a code such as a PN sequence, the code may repeat less often or not at all during a stirring interval 610. The stirring interval 610 may occupy several seconds, or at least long enough to allow statistical mode mixing of signals present in the hyperechoic chamber.

[0056] FIG. 7A through FIG. 7D present four spectral diagrams to aid in understanding operation of an embodiment. In FIG. 7A, modulation codes are applied to a carrier at a modulation frequency fm, which is typically many orders of magnitude less than a carrier frequency fcbeing produced by a DUT. For instance, the modulation frequency could be 30 kHz, and the carrier frequency could be 10 GHz. The modulation codes cause spectral spreading of a modulated signal about the modulation frequency. The frequency of the output of the code generator and the modulation frequency can be varied over a considerable range in a given embodiment to achieve a desired modulation spectrum.

[0057] FIG. 7B shows the spectrum reflected by a modulating scatterer with the inputs shown in FIG. 7A. A portion of the carrier fcis simply reflected. Additionally, the introduction of the modulating signal at the scatterer, in the presence of the carrier energy', produces two modulated sideband spectra distributed about the carrier frequency fc.

[0058] At a receiver, it is desirable to detect and discriminate the modulated sideband spectra from the carrier energy' reaching the receive antenna. In this embodiment, the carrier frequency is detected and, as shown in FIG. 7C, used to downconvert the received signal to baseband. In other embodiments, the signal may be downcon verted to an intermediate frequency and bandpass filtered to filter out one of the sidebands as well as the carrier signal.

[0059] In FIG. 7D, a lock-in amplifier uses frequency and phase information from the same reference modulation carrier fmsupplied to the LF modulator to lock onto a sideband signal, allow ing recovery of the time-vary ing amplitude and phase information produced via the modulation codes. This allows separate detection of each modulated field signal, as will be described next.

[0060] FIG. 8 contains a block diagram for an embodiment of a correlation receiver 260 useful with an embodiment, e.g., as shown in the previous figures, with three concurrent modulation signals present. A signal detected by a receive antenna 230 is supplied to a mixer 810, which mixes the supplied signal with a earner frequency estimated by a frequency source 830. The frequency source 830 may use the same signal from the receive antenna 230 to estimate the carrier frequency.

[0061] The output of mixer 810 is passed through a bandpass filter 820 and supplied to a lock-in amplifier 840. Lock-in amplifier 840 uses a signal from the same frequency generator 440 that produces a modulation carrier signal for LF modulator 240 to downconvert the modulated spectra to a baseband signal. The baseband signal is supplied to three correlators 850-1, 850-2. and 850-3, which are also supplied respectively with the three codesequences 412, 422, and 432 produced by the code generator 402 (this could be the same code generator used by modulator 240, or a replica that can reproduce the same code sequences). The correlators use the codes to separate the signals resulting from each scattering modulation, and each correlator supplies its instantaneous estimate of a respective scattered signal to a corresponding integrator 860-1, 860-2, 860-3. At the beginning of a stirring interval 610, each integrator is reset, and at the end of the stirring interval 610 each integrator produces a respective output signal SI, S2, S3 representing its estimate of a respective scattered field component.

[0062] In a given embodiment, it may be advantageous to capture field information contemporaneously from a variety of near-field positions surrounding a DUT, and / or to speed measurement over many sampling points of a larger solid angle of the antenna field pattern. FIG. 9 shows an embodiment 900 that uses a plurality’ of modulating scatterers to realize multiple simultaneous measurements and an extended field capture.

[0063] In FIG. 9, a hyperechoic chamber much like the chamber of FIG. 2 is shown. What has been added to the chamber is a rotational mechanism 910 connected to a semicircular arm 920. Arm 920 supports a large plurality of scattering modules tangentially affixed on an arc about the DUT 120. Each scattering module may be, e.g., similar to the design shown in FIG. 3A / 3B or the design shown in FIG. 5. Cabling routed through arm 920 connects each of the scattering modules to a field mapping system 1000 provided outside of the chamber through a signaling harness 930. which provides a path for the field mapping system 1000 to supply modulation signals to the plurality of scattering modules. Hamess 930 may also provide signals that allow system 1000 to control a rotational position of mechanism 910 and / or detect its current angular position.

[0064] In operation, field strength estimates may be fully gathered from one angular position of arm 920 over a stirring interval, and then the arm 920 can be stepped to another angularposition and the process repeated for another stirring interval, until a hemisphere or larger solid angle has been mapped. Alternately, partial measurements can be taken at one stirrer position for each of many angular positions of arm 920, with this process repeated for many stirrer positions in order to build up a profile for a given antenna field pattern.

[0065] FIG. 10 contains a block diagram for field mapping system 1000 according to an embodiment. Field mapping system 1000 contains many of the same components already described with respect to FIGs. 4 and 8, and which perform similar functions in FIG. 10. As one difference, however, code generator 402 now produces N mutually orthogonal codes 412- 1 to 412-N, and supplies these respectively to N drivers 410-1 to 410-N, and also to N correlators 850-1 to 850-N. Each of the drivers supplies one of N modulating signals through a pair of leads (pairs 930-1, 930-2 to 930-N, 932-N) to a corresponding scattering element on rotating arm 920. Each of the correlators is paired with a corresponding one of N integrators 860-1 to 860-2, which produce respective field estimate signals S I to SN.

[0066] A controller 1020 synchronizes operation of the system 1000. This may include supplying a positioning signal Step_0 to rotating mechanism 910, and supplying individual enable signals of an enable signal bundle EnableX to the individual drivers. As alluded to earlier, it may be desirable to selectively and adaptively enable, disable, and or adjust the relative modulation strengths of various ones of the scatterers to allow weaker regions of the antenna lobe pattern to be adequately measured. Further, scatterers near the ‘'poles” of arm 920 may not need to make as many measurements as those near the “equator” of arm 920, and thus controller may selectively enable different combinations of scatterers at each arm position.

[0067] A logger 1030, which can be located in an attached or remote computing system, is also supplied with field estimate signals SI to SN, and can perform further functions such asvisualization, normalization, or near-field to far-field estimation techniques for field estimate signals SI to SN, as are understood by those skilled in the art.

[0068] FIG. 11 contains a block diagram for an alternative embodiment of a correlation receiver 1160 useful with an embodiment, e.g., as shown in the previous figures, with three concurrent modulation signals present. A signal detected by a receive antenna 230 is supplied to a mixer 810, which mixes the supplied signal with a mixing frequency estimated by a frequency source 1130. Instead of using the carrier frequency as a mixing frequency, as in FIG. 8, the mixing frequency is equal to the carrier frequency + the modulation frequency.

[0069] The output of mixer 810 is passed through a bandpass filter 820 and supplied to a lock-in amplifier 840. In this case, however, one sideband has been shifted to DC and the other sideband has been shifted to 2iom. Lock-in amplifier 840 uses a signal from a frequency generator 1140 that produces twice the frequency of a modulation carrier signal for LF modulator 240 to downconvert the modulated spectra upper sideband to a baseband signal. The baseband signal is supplied to three correlators 850-1, 850-2, and 850-3, which are also supplied respectively with the three code sequences 412, 422, and 432 produced by the code generator 402 (this could be the same code generator used by modulator 240, or a replica that can reproduce the same code sequences). The correlators use the codes to separate the signals resulting from each scattering modulation, and each correlator supplies its instantaneous estimate of a respective scattered signal to a corresponding integrator 860-1, 860-2, 860-3. At the beginning of a stirring interval 610, each integrator is reset, and at the end of the stirring interval 160 each integrator produces a respective output signal SI, S2, S3 representing its estimate of a respective scattered field component.

[0070] Other techniques may also be used to enhance the SNIR of the received signal. For instance, if one or more modulating scatterers are located on a structure such as arm 920. mechanical modulation frequencies and modulation paterns can be selected to avoidmechanical resonance frequencies of the transport system. In some instances, a modulation frequency that produces a lower scattered signal may be preferable over a modulation frequency that produces a higher scattered signal but also excites mechanical vibration modes of the transport system.

[0071] Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. Further, it is intended that embodiments of the disclosed subject matter cover modifications and variations thereof.

[0072] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context expressly dictates otherw ise. That is, unless expressly specified otherwise, as used herein the words “a,” “an,” “the,” and the like carry the meaning of “one or more.” Additionally, it is to be understood that terms such as “left,” “right,” “top,” “bottom,” “front,” “rear,” “side,” “height,” “length,” “width,” “upper,” “lower,” “interior,” “exterior,” “inner,” “outer,” and the like that may be used herein merely describe points of reference and do not necessarily limit embodiments of the present disclosure to any particular orientation or configuration.Furthermore, terms such as “first,” “second.” “third,” etc., merely identify one of a number of portions, components, steps, operations, functions, and / or points of reference as disclosed herein, and likewise do not necessarily limit embodiments of the present disclosure to any particular configuration or orientation.

[0073] Furthermore, the terms “approximately,"’ “about,” “proximate,” “minor variation,” and similar terms generally refer to ranges that include the identified value within a margin of 20%, 10% or preferably 5% in certain embodiments, and any values therebetween.

[0074] All of the functionalities described in connection with one embodiment are intended to be applicable to the additional embodiments described below except where expressly stated or where the feature or function is incompatible with the additional embodiments. For example, where a given feature or function is expressly described in connection with one embodiment but not expressly mentioned in connection with an alternative embodiment, it should be understood that the inventors intend that that feature or function may be deployed, utilized or implemented in connection with the alternative embodiment unless the feature or function is incompatible with the alternative embodiment.

[0075] Reference has been made to illustrations representing methods and systems according to implementations of this disclosure. Aspects thereof may be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general-purpose computer, special purpose computer, or other programmable data processing apparatus and / or distributed processing systems having processing circuitry, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions / operations specified in the illustrations.

[0076] One or more processors can be utilized to implement various functions and / or algorithms described herein. Additionally, any functions and / or algorithms described herein can be performed upon one or more virtual processors. The virtual processors, for example, may be part of one or more physical computing systems such as a computer farm or a cloud drive.

[0077] Aspects of the present disclosure may be implemented by software logic, including machine readable instructions or commands for execution via processing circuitry. The software logic may also be referred to, in some examples, as machine readable code, software code, or programming instructions. The software logic, in certain embodiments, may be coded in runtime-executable commands and / or compiled as a machine-executable program or file. The software logic may be programmed in and / or compiled into a variety of coding languages or formats.

[0078] Aspects of the present disclosure may be implemented by hardware logic (where hardware logic naturally also includes any necessary signal wiring, memory elements and such), with such hardware logic able to operate without active software involvement beyond initial system configuration and any subsequent system reconfigurations (e.g., for different object schema dimensions). The hardware logic may be synthesized on a reprogrammable computing chip such as a field programmable gate array (FPGA) or other reconfigurable logic device. In addition, the hardware logic may be hard coded onto a custom microchip, such as an application-specific integrated circuit (ASIC). In other embodiments, software, stored as instructions to a non-transitory computer-readable medium such as a memory device, on-chip integrated memory unit, or other non-transitory computer-readable storage, may be used to perform at least portions of the herein described functionality.

[0079] Various aspects of the embodiments disclosed herein are performed on one or more computing devices, such as a laptop computer, spectrum analyzer, function generator, tablet computer, mobile phone or other handheld computing device, or one or more servers. Such computing devices include processing circuitry embodied in one or more processors or logic chips, such as a central processing unit (CPU), graphics processing unit (GPU), field programmable gate array (FPGA). application-specific integrated circuit (ASIC), or programmable logic device (PLD). Further, the processing circuitry may be implemented asmultiple processors cooperatively working in concert (e.g., in parallel) to perform the instructions of the inventive processes described above.

[0080] The process data and instructions used to perform various methods and algorithms derived herein may be stored in non-transitory (i.e., non-volatile) computer-readable medium or memory7. The claimed advancements are not limited by the form of the computer-readable media on which the instructions of the inventive processes are stored. For example, the instructions may be stored on CDs, DVDs, in FLASH memory7, RAM, ROM, PROM, EPROM, EEPROM, hard disk or any other information processing device with which the computing device communicates, such as a server or computer.

[0081] These computer program instructions can direct a computing device or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable medium produce an article of manufacture including instruction means which implement the function / operation specified in the illustrated process flows.

[0082] The computing device, in some embodiments, further includes a display controller for interfacing with a display, such as a built-in display or LCD monitor. A general purpose I / O interface of the computing device may interface with a keyboard, a hand-manipulated movement tracked I / O device (e.g., mouse, virtual reality glove, trackball, joystick, etc.), and / or touch screen panel or touch pad on or separate from the display.

[0083] Moreover, the present disclosure is not limited to the specific circuit elements described herein, nor is the present disclosure limited to the specific sizing and classification of these elements. For example, the skilled artisan will appreciate that the circuitry described herein may be adapted based on changes in battery sizing and chemistry or based on the requirements of the intended back-up load to be powered.

[0084] The functions and features described herein may also be executed by various distributed components of a system. For example, one or more processors may execute these system functions, where the processors are distributed across multiple components communicating in a network. The distributed components may include one or more client and server machines, which may share processing, in addition to various human interface and communication devices (e.g., display monitors, smart phones, tablets, personal digital assistants (PDAs)). The network may be a private network, such as a LAN or WAN, or may be a public network, such as the Internet. Input to the system, in some examples, may be received via direct user input and / or received remotely either in real-time or as a batch process.

[0085] Although provided for context, in other implementations, methods and logic flows described herein may be performed on modules or hardware not identical to those described. Accordingly, other implementations are within the scope that may be claimed.

[0086] While certain embodiments have been described, these embodiments have been presented by w ay of example only, and are not intended to limit the scope of the present disclosures. Indeed, the novel methods, apparatuses and systems described herein can be embodied in a variety of other forms: furthermore, various omissions, substitutions and changes in the form of the methods, apparatuses and systems described herein can be made without departing from the spirit of the present disclosures. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the present disclosures.

[0087] While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the present disclosures. Indeed, the novel methods, apparatuses and systems described herein can be embodied in a variety of other forms; furthermore, various omissions, substitutions andchanges in the form of the methods, apparatuses and systems described herein can be made without departing from the spirit of the present disclosures. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the present disclosures.

Claims

CLAIMSWhat is claimed is:

1. A system for measuring electric fields of a radio frequency (RF) antenna under test, the system comprising: a hyperechoic chamber for performing antenna field testing of a device comprising an RF antenna under test, the hyperechoic chamber comprising at least one mode stirring mechanism operable during an antenna field test; a first modulating scatterer disposed in the hyperechoic chamber; a modulator to apply a first modulation signal to the first modulating scatterer, the first modulation signal having an average frequency more than two orders of magnitude lower than a carrier frequency at which the RF antenna under test is operated during the antenna field test; a receive antenna to collect RF energy during the antenna field test, including first RF energy propagated by the RF antenna under test and modulated RF energy generated by an interaction between a portion of the first RF energy propagated by the RF antenna under test and the first modulating scatterer; and a correlation receiver configured to receive a signal from the receive antenna, the signal corresponding to the first RF energy and the modulated RF energy, based on information related to the first modulation signal, discriminate a modulated portion of the signal corresponding to the modulated RF energy, and integrate the discriminated modulated portion of the signal over an integration time interval, during which the at least one mode stirring mechanism is repositioned to a plurality’ of different stirring positions.The system of claim 1, wherein the relative position of the first modulating scatterer with respect to the RF antenna under test is substantially stationary during the integration time inter al. The system of claim 1, wherein the at least one mode stirring mechanism is a rotationally powered mode stirrer that undergoes rotational motion during the integration time interval. The system of claim 1, wherein the first modulating scatterer comprises a plurality of differently polarized modulation elements. The system of claim 4, wherein the plurality of differently polarized modulation elements comprises first and second elongated modulating elements oriented in a first plane substantially transversely to a vector from the RF antenna under test to the first modulating scatterer, the first and second elongated modulating elements arranged in a cross configuration with their respective elongated dimensions substantially 90 degrees apart in the first plane. The system of claim 5, wherein the first elongated modulating element comprises a first electric switching element responsive to the first modulation signal from the modulator, and the second elongated modulating element comprises a second electric switching element responsive to a second modulation signal from the modulator. The system of claim 5, wherein the plurality of differently polarized modulation elements further comprise a third elongated modulating element having an elongated axis oriented in substantial alignment with the vector from the RF antenna under test to the first modulating scatterer. The system of claim 7. wherein the third elongated modulating element comprises a mechanical scatterer that vibrates along its elongated axis responsive to a third modulation signal from the modulator.The system of claim 8, further comprising a piezoelectric transducer mechanically coupled to the third elongated modulating element, the piezoelectric transducer mechanically displacing the third elongated modulating element in response to the third modulation signal. The system of claim 1, further comprising a translation system to reposition the relative positions of the first modulating scatterer and the RF antenna under test to a plurality of measurement positions during the antenna field test, the correlation receiver configured to perform a separate integration for each of the plurality' of measurement positions. The system of claim 10, wherein the translation system comprises a rotating arm that repositions the first modulating scatterer via rotation of the rotating arm such that the plurality of measurement positions are arranged in an arc about the RF antenna under test. The system of claim 11, wherein the first modulating scatterer is one of a plurality of modulating scatterers arrayed on the rotating arm, each of the plurality of modulating scatterers coupled to receive a respective modulation signal from the modulator. The system of claim 12, wherein the plurality of modulating scatterers are arrayed on the rotating arm in the arc about the RF antenna under test, such that the plurality’ of measurement positions allow the plurality of modulating scatterers to sample a substantially spherical surface surrounding the RF antenna under test. The system of claim 1, wherein the average frequency of the first modulation signal is in an audio frequency range, and the carrier frequency is a microwave frequency. The system of claim 1. wherein the average frequency of the first modulation signal is below 1 MHz. The system of claim 1. wherein the carrier frequency at which the RF antenna under test is operated during the antenna field test lies in a range of 1 GHz to 20 GHz.The system of claim 1, wherein the carrier frequency at which the RF antenna under test is operated during the antenna field test lies is a millimeter-wave frequency. The system of claim 1, wherein: the first modulating scatterer is one of a plurality7of modulating scatterers arrayed within the hyperechoic chamber, each of the plurality' of modulating scatterers coupled to receive a respective modulation signal from the modulator; and the modulator comprises a code generator to generate a respective modulation code for each of the plurality7of modulating scatterers, and a corresponding driver for each of the plurality' of modulating scatterers, the corresponding driver for a given scatterer of the plurality of modulating scatterers generating the respective modulation signal for the given scatterer based on the respective modulation code for the given scatterer, wherein a plurality7of the corresponding drivers are concurrently active during the integration time interval. The system of claim 18, wherein each of the corresponding drivers use the respective modulation code for that driver to modulate a same modulation carrier reference signal. The system of claim 19, wherein the respective modulation codes are mutually orthogonal. The system of claim 19, wherein the respective modulation codes are pseudorandom noise sequences. The system of claim 18, wherein the respective modulation codes are generated according to an orthogonal frequency-division multiple access apportionment of a modulation spectrum. The system of claim 18, wherein the correlation receiver is further configured to:in a first phase of a measurement, estimate relative strengths of scattered modulation from each of the plurality of modulating scatterers; and in a second phase of a measurement, adjust the corresponding drivers such that modulating scatterers with lower relative strengths estimated in the first phase produce relatively greater modulation in the second phase, and modulating scatterers with higher relative strengths estimated in the first phase produce relatively less modulation in the second phase. The system of claim 1, wherein the first modulating scatterer comprises a modulation element that modulates a tangential electric field, at a position of the first modulating scatterer, received from the RF antenna under test. The system of claim 1, wherein the first modulating scatterer modulates a total electric field, at a position of the first modulating scatterer, received from the RF antenna under test. A method for characterizing an electric field of a radio frequency (RF) antenna under test, the method comprising: while an RF antenna under test is transmitting RF energy at a carrier frequency in a hyperechoic environment, concurrently supplying respective modulation signals to respective scatterers of a plurality of modulating scatterers, wherein the plurality of modulating scatterers are arrayed in respective near-field positions relative to the RF antenna under test; collecting modulated RF energy produced by an interaction between an electric field of the RF antenna under test and the modulating scatterers; generating a receive signal related to the collected modulated RF energy and the transmitted RF energy of the RF antenna under test;based on information related to each of the respective modulation signals. discriminating a respective portion of the receive signal related to each of the modulation signals; and separately integrating each of the discriminated respective portions of the receive signal over an integration time interval, during which a mode stirring operation is repeatedly performed in the hyperechoic environment. A system for measuring electric fields comprising: a radio frequency (RF) scattering needle having a tip; an electromechanical driver coupled to the RF scattering needle such that a drive signal applied to the electromechanical driver at a sideband frequency causes mechanical motion of the tip of the RF scattering needle at the sideband frequency; and an energy collection circuit to sense electromagnetic energy radiated from a vicinity of the tip at a scattering frequency, wherein the scattering frequency represents a modulation frequency formed by the interaction of the tip under mechanical motion with an electric field of a device under test. The system of claim 27, wherein the RF scattering needle is fabricated from an electrically conductive material. The system of claim 27, wherein the RF scattering needle is fabricated from a carbon fiber material. The system of claim 27, wherein the RF scattering needle is fabricated from a doped bulk semiconductor material.

Images