Patent Publication Number: US-2023144500-A1

Title: High frequency heterodyne mixer

Description:
TECHNICAL FIELD 
     This disclosure relates to high frequency electronics and detection systems, and in particular, heterodyne mixers and methods of fabricating the same. 
     BACKGROUND 
     A mixer is an electronic device that can be used to generate output signals having frequencies different than the frequencies of received signals. For example, where two input signals are applied to a mixer, the mixer may generate output signals having frequencies at the sum and difference of the input signals, or harmonics thereof. In this way, a mixer can be used to down-convert a detected signal to a lower frequency, for instance, to simplify subsequent processing. 
     A mixer is shown in WO 2007/125326 that receives an incoming radio frequency (RF) signal, receives a local oscillator (LO) signal, and produces an intermediate frequency (IF) signal from a diode chip. This design uses separate RF, LO and IF circuit pathways, as well as wire-based stub tuners to optimize conversion efficiency and for power matching. 
     However, there remains a need for a mixer design that is effective across a wide range of frequencies (e.g., at very high frequencies) and that is compatible with the fabrication and assembly techniques used for small components, such as the small components needed when circuit elements scale with the reduction in signal wavelength for a given system. 
     SUMMARY 
     According to embodiments, a mixer apparatus, such as a heterodyne mixer, is disclosed. The mixer may comprise a first input, such as a local oscillator (LO) input; a second input, such as a radio frequency (RF) input; an output, such as an intermediate frequency (IF) signal output; a suspended signal transmission element comprising a plurality of suppression slots and filter elements; and a diode circuit mounted on the suspended signal transmission element. The suspended signal transmission element may be, for instance, waveguide. In certain aspects, the suspended signal transmission element is a waveguide comprising a substrate having a first and second surface and a transmission line on the first surface, where the substrate is a quartz substrate and the transmission line is a patterned metal microstrip (also referred to as stripline) transmission line. Further, the first input (e.g., LO) can be located between the second input (RF) and the output (e.g., IF), and the transmission element can be configured to transmit an IF signal generated in the diode circuit along a longitudinal axis of the transmission element from the diode circuit to the output. 
     According to embodiments, a heterodyne mixer apparatus has a waveguide component, where the waveguide component comprises filtering means, transmission means, and suppression means. One or more semiconductor devices can be mounted on the surface of the waveguide component. 
     According to embodiments, a mixer with one or more slotted filter elements is provided. In certain aspects, slots in the filter are relatively small compared to the free-space wavelength. For example, the slots may be less than λ/10. 
     According to embodiments, a mixer is provided having an RF signal input at a first end, an IF signal output at a second end, and an LO signal input between the two ends (e.g., in the middle). In certain aspects, a waveguide transmission element runs from the first end to the second end. The transmission element may be suspended, and may also have a plurality of suppression slots and filter elements. Further, a diode circuit may be mounted at the first end of the mixer and proximate the RF signal input. In some embodiments, IF signals generated in the diode circuit propagate along the transmission element to the IF signal output at the other end of the mixer. The mixer may be arranged in a detector block with RF and LO input channels along a first axis, and IF outputs along a second axis that is perpendicular to the first. 
     According to some embodiments, a detector, such as a terahertz camera, is provided. The detector may include, for example, one or more of the mixers described herein; one or more feedhorns coupled to the input (e.g., providing RF signal) of at least one of the mixers; a local oscillator coupled to another input of at least one of the mixers; a coaxial line (or other transmission element) coupled to the output (e.g., IF) of at least one of the mixers; and one or more of a low noise amplifier (LNA), power detector, and analog-to-digital converter (ADC) circuit elements connected to the coaxial line and configured to process the output IF signal. The detector may further include additional filtering and image processing, for instance, to generate an image of an object or scene from which the input radiation was received. In certain aspects, the one or more mixers comprise a plurality of mixers optimized at 250 GHz and a plurality of mixers optimized at 375 GHz. The set of mixers can further comprise a plurality of mixers optimized at 125 GHz. 
     According to some embodiments, a method of operating a mixer is provided. The method can include, for example, the steps of receiving an RF signal at a first input of a mixer apparatus having a suspended transmission line element, a plurality of suppression elements, and a diode circuit; providing a local oscillator signal to the mixer; and outputting an IF signal generated by the diode circuit, wherein outputting the IF signal comprises propagating the signal along a longitudinal direction of the mixer through a plurality of filters. 
     According to some embodiments, a method of fabricating a mixer apparatus, such as one or more of the mixers described herein, is provided. The method may begin with machining steps, including: (1) machining a first housing component comprising a first radio frequency channel portion, a first local oscillator portion, a first intermediate frequency channel portion, a first cavity portion, a first mounting shelf portion, and a second mounting shelf portion; and (2) machining a second housing component comprising a second radio frequency channel portion, a second local oscillator portion, a second intermediate frequency channel portion, a second cavity portion, a third mounting shelf portion, and a fourth mounting shelf portion. A signal transmission element (e.g., waveguide) can be suspended in the first and/or second cavity portion by mounting the waveguide element on the first and second mounting shelf portions and/or the third and fourth mounting shelf portions. Additionally, the first housing component can be attached to the second housing component to form a block. The block may be, for example, a receive array block of a detector, such as a terahertz camera. 
     According to embodiments, a detector block is provided that comprises a housing and one or more mixers mounted in the housing. The housing may comprise one or more feedhorns coupled to an RF input of at least one of the mixers, one or more local oscillator channels coupled to an LO input of at least one of the mixers, and one or more output channels coupled to an IF output of at least one of the mixers, One or more of the mixers may comprise a suspended signal transmission element mounted on first and second mounting structures of the housing. In some embodiments, the one or more feedhorns and the one or more local oscillators channels extend along a first axis of the housing, the one or more output channels extend along a second axis of the housing, and the first axis and second axis are perpendicular. Additionally, the suspended signal transmission element may comprise: a substrate having a first and second surface; and a transmission line on the first surface, wherein the transmission line is a patterned metal microstrip transmission line comprising one or more suppression slots and the filter elements. 
     According to embodiments, a terahertz camera is provided that comprises: one or more mixers comprising an IF output, an LO input, and an RF input; one or more feedhorns coupled to an RF input of at least one of the mixers; a local oscillator coupled to an LO input of at least one of the mixers; and one or more of a low noise amplifier, power detector, and analog-to-digital converter connected to an IF output and configured to process an output IF signal from at least one of the mixers. Additionally, at least one of the mixers may further comprise: a suspended signal transmission element comprising a plurality of suppression slots and filter elements; and a diode circuit mounted on the suspended signal transmission element and configured to generate the IF output signal based on radiation received on one or more of the feedhorns and a power signal from the local oscillator. In some embodiments, the received radiation is in a band centered around 125 GHz, 250 GHz or 375 GHz. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and form part of the specification, illustrate various embodiments. 
         FIG.  1    illustrates a mixer apparatus according to some embodiments. 
         FIGS.  2 A and  2 B  illustrate a mixer apparatus according to some embodiments. 
         FIGS.  3 A and  3 B  illustrate details of a mixer apparatus according to some embodiments. 
         FIG.  4    is a flow chart illustrating a process according to some embodiments. 
         FIG.  5    is a schematic of a detection system according to some embodiments. 
         FIGS.  6 A- 6 C  illustrate detector blocks according to embodiments. 
         FIG.  7    is a flow chart illustrating a process according to some embodiments. 
         FIGS.  8 A and  8 B  illustrate microstrip fields and currents according to some embodiments. 
         FIG.  9    illustrates a detection system, such as a terahertz camera, according to embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The sensing of microwave, millimeter, and/or submillimeter signals from an object or scene can require extremely sensitive detectors. For instance, the power levels of received signals can be in the femtowatt range. 
     In some instance, for example at certain microwave frequencies, a low noise amplifier can be used to increase the power level of the signal by orders of magnitude (e.g., 100-1000 fold) such that it can then be processed to produce a DC voltage. This DC voltage can then be further amplified until it is sufficiently strong that it can be converted to a digital value that can then be used to create image data. Other detection schemes can be used such as bolometers, which are effectively highly sensitive thermometers that produce a voltage directly proportional to a scene&#39;s power level. Where power levels are quite low, bolometers may require cooling to very low temperatures, typically less than 20 degrees Kelvin, so that the weak received signal is enhanced against the intrinsic noise of the bolometer itself (caused by the random movement of the electrons caused by thermal vibration of the atoms/molecules in the material used for the bolometer). This cooling requirement can require complex cryogenic technology, which not only adds significant cost, volume, and mass but may also require significant power levels. Thus, the applications for bolometer-based imaging may be limited. For example, many security and material inspection applications require a high degree of portability from the system which in turn requires low mass, volume, and power. 
     At higher frequencies, such as millimeter and submillimeter wavelengths, high gain, low noise amplifiers can be difficult to operate at room temperature with sufficiently high performance to achieve passive imaging in real time. Additionally, as the frequency increases, so too does the path length of the amplifier gate, requiring smaller and smaller devices. This in turn leads to high resistance and increased signal loss and noise, with a reduction in gain. If the operational frequency is increased by a factor of two, the area of a circuit is typically reduced by a factor of four, and its volume by a factor of eight. Thus, even modest increases in frequency can introduce fabrication and assembly challenges, which in turn degrade reproducibility and yield thereby increasing cost. Such frequency increases can also reduce the extent to which conventional assembly tools are practical for machining of high frequency components. 
     By way of example, a quartz filter used to separate the RF, LO and IF signals can become extremely small and difficult to handle at frequencies in excess of 250 GHz. Also, if bond wires are needed to “tune” the three signals of a heterodyne mixer, the wires and connections become difficult to realize in a routine manner, which is compounded by the need for bond wires to be correctly dimensioned in order for all three signals to be optimized. Additionally, if a filter, such as an IF filter, is formed by high and low quarter wave impedance sections realized on an IF output pin, it can complicate the assembly, increasing costs and assembly time. 
     According to embodiments, certain problems with existing systems are solved through the use of a mixer to down-convert a detected signal to a lower (e.g., lower microwave) frequency such that it can then be amplified and processed. According to embodiments, a mixer can achieve high down-conversion efficiency from the RF signal to the IF signal, while using a minimum amount of LO signal power. In certain aspects, the disclosed designs can minimize the amount of signal reflected at the IF output (i.e., there is good IF match). In certain aspects, a good IF match may mean that a mixer circuit under LO pumped conditions (i.e., during operation) is at approximately 50 ohms. In some instance, this may be the same impedance as one or more IF amplifiers. 
     According to embodiments, designs at higher frequencies can be routinely manufactured and assembled, with a circuit that simultaneously provides improved performance by isolating the effect of circuit elements on each other while providing increased scope for circuit optimization. By increasing the size available on the filter substrate, there can be reductions in the ohmic resistance of the filter metallization. This can in turn reduce the amount of signal that would otherwise be lost in a smaller circuit. 
     In some embodiments, wideband transverse waveguide is used at both LO and RF microstrip junctions to provide a simplified circuit layout and topology. Odd-mode and transverse-mode suppression slots can be used in conjunction with an air suspended microstrip filter topology having low impedance sections of one or more filter circuits. In certain aspects, this allows wide filter channels to be used, easing machining tolerances, improving fabrication yields, and reducing assembly complexity. 
     A quartz filter circuit can be achieved using the microstrip circuit configuration. According to embodiments, the signals propagate in the longitudinal direction in the fundamental order Transverse Electromagnetic Mode (TEM-00), in which there are no components of the electromagnetic field existing in the longitudinal direction even though all transfer of power is in that direction. In order to propagate a TEM-00 mode, and in some embodiments, the microstrip configuration consists of two conductors that are sufficiently small as to prevent the propagation of higher order modes. These conductors can comprise the outer wall of the filter channel and the inner filter metallization. In this geometry, the fields can be calculated using finite element analysis for each cross section and their combination. As the size of the outer conductor increases with respect to the signal wavelength, which may be desirable for the reasons highlighted herein, higher order TEM-nn modes such as the odd modes (e.g., TEM-01 and TEM-10) can be generated. As the signals propagate down the filter channel, such higher order odd modes can create blocking resonances between filter cross sections that degrade the broadband performance. Wave propagation in the transverse direction can also result in a loss of power transfer between the mixer circuit elements. Moreover, use of the widest possible cross section can increase the possibility of odd-order modes and transverse wave propagation. However, for odd-order TEM modes and transverse wave propagation to occur, there is a need for lateral surface currents across the geometry (e.g., the filter conductor geometry). Thus, by incorporating longitudinal slots, for example in a first filter element, unwanted modes and propagation can be suppressed. 
     According to some embodiments, the use of a wide longitudinal filter common center line feed can minimize ohmic losses in all three signals (LO, RF, IF). Additionally, the use of a capacitive, inductively-tuned short circuit pad used in conjunction with a physical DC ground can lock all three signal paths to ground in the shortest length, thereby increasing the intrinsic bandwidth of the entire circuit. This can negate, for instance, the need for individual tuning bonds/wires for each frequency, further minimizing ohmic losses. 
     Referring now to  FIG.  1   , a cross-sectional illustration of a mixer  100  according to some embodiments is provided. The mixer  100  may be, for example, a heterodyne mixer, including a superheterodyne, harmonic, and/or subharmonic mixer. In some embodiments, a balanced mixer design may be used. 
     The mixer  100  may include a first input  102 , such as an LO input, a second input  104 , such as an RF input, and a signal output  106 , such as an output for an IF signal. According to embodiments, the first input  102  may be the portion of mixer  100  configured to couple a signal from a local oscillator via an LO waveguide  114 , which delivers the LO signal to the mixer  100 . In some embodiments, waveguide height reduction and matching elements can be used to impedance match the LO signal to the mixer. Additionally, the second input  104  may be the portion of the mixer  100  configured to couple a signal from a radio frequency source via an RF waveguide  116 . In certain aspects, the RF waveguide  116  delivers RF signal as part of a detection system, for instance, via one or more feedhorns of a high frequency camera. In certain aspects, the inputs  102 ,  104  may be antennas. An example of machined signal paths coupled to the mixer inputs and outputs is provided in  FIG.  6 A . Although illustrated with waveguide inputs and antennas, one or more of the LO and RF signals may be delivered to the mixer  100  using other transmission elements, such as co-axial line or co-planar line. As shown in the example of  FIG.  1   , a mixer is provided with an RF input at one end, an IF output at an opposite end, and an LO input in between (e.g., in a central portion of the mixer). 
     The signal output  106  provides the output signal from the mixer circuitry, such as diode circuit  110 . In some embodiments, the diode circuit  110  comprises two Schottky diodes, in an anti-parallel configuration. However, other circuits may be used. In some embodiments, an IF signal from the diode circuit  110  is output from the mixer  100  via a coaxial connection, for instance, connection element  118  in an output channel. Alternatively, the IF signal could be output using one or more of waveguide or other wired connections depending on frequency. The IF signals can be transmitted from the diode circuit  110  to the output  106  along the length of the suspended transmission line element  108 , as illustrated by direction arrow  134 . In this arrangement, the LO input  102  is interposed between the RF input  104  and the IF signal output  106 . According to embodiments, the RF input  104  is situated at a first distal end of the mixer  100  and waveguide element  108 , the IF output  106  is situated at a second distal end of the mixer  100  and waveguide element  108 , and the LO input  102  is situated in a center region of the mixer  100  and waveguide element  102 . 
     According to embodiments, mixer  100  comprises a suspended element  108  that is waveguide, and which is mounted via substrate mounts  120   a  and  120   b  in a cavity of the mixer housing  132 . In this way, an airgap  130  is provided on at least two sides of the suspended waveguide element  108  (e.g., above and below). As another example, and as illustrated in  FIG.  2 A , the suspended waveguide element  108  may have an airgap  130  on all four sides. One or more of the substrate mounts may be flat, such as mount  120   a,  or stepped, such as mount  120   b . The waveguide element  108  may comprise a substrate portion and a patterned metal surface portion. For instance, the substrate may be quartz (or another suitable dielectric, such as aluminum nitride) with a patterned gold layer forming a microstrip transmission line. Other suitable transmission line materials could include other metallic or otherwise conductive materials. The diode circuit  110  may be mounted on the patterned metal surface of the waveguide element  108 , for instance, at least partially within RF input  104 . In some embodiments, the suspended waveguide element  108  includes suppression slots, which are configured to suppress unwanted transmission and resonance modes. In some embodiments, the housing  132  may comprise a block or portion of a block, such as a detection array block. For instance, blocks  514 ,  600 ,  620 , and  650  may comprise the housing illustrated in  FIGS.  1 - 3   . 
     According to embodiments, the signals may propagate on the surface of the suspended waveguide element  108  along the transmission line through one or more filters. For instance, the suspended waveguide element  108  may include an LO filtering/suppression portion  124 , an RF filtering/suppression portion  126 , and an RF reception element (e.g., antenna)  128 , which may be part of RF input  104 . The LO filtering/suppression portion  124  may filter LO signals from reaching the IF output  106 . This can enable, in some embodiments, the removal of the need for a separately machined LO and/or IF coaxial filter pin. The RF filtering/suppression portion  126  may filter RF signals from reaching the IF output  106  and/or filter spurious signals from reaching the diode circuit  110 . Additionally, the RF reception element  128  may be configured with a plurality of slots (e.g., suppression slots). In some embodiments, the suppression slots used along the surface of the suspended waveguide element  108  extend in a longitudinal direction and suppress odd mode propagation. For example, slotted features within a stripline can inhibit electric current from flowing across the width of the suspended waveguide element  108 , thereby preventing un-wanted odd-modes from propagating along the stripline towards IF output  106 . According to embodiments, a desired even-mode propagation of the IF signal is unaffected by the suppressions slots. 
     In some embodiments, the slots have a width of less than λ/10, and a length of at least ¼λ. 
     According to embodiments, slotted features within the microstrip circuit inhibit electric currents from flowing across the width of the microstrip metal and prevent unwanted odd-modes from propagating along the microstrip circuit. However, the desired even-mode propagation can be left unaffected by the slots. 
     As illustrated in  FIG.  1   , the mixer  100  comprise one or more mounting structures  120   a,    120   b  for the suspended waveguide element  108 . Thus, and according to embodiments, the filters of the mixer are suspended at the ends of the stripline rather than on the sides. Thus, there can be less dielectric substrate within the cavity so that the higher modes are moved above the operating frequency of the mixer. Also, and in some embodiments, the sides of the channel cavity are flat and less likely to perturb the filter characteristics. 
     According to embodiments, one or more of the filters of mixer  100  couple to the top and/or bottom of the housing cavity in which the mixer is mounted. For instance, the suspended-stripline filters can be designed such that the filter sections couple primarily to the top or bottom of the cavity rather than the sides of the cavity. This improves the manufacturability of the mixer because the filter characteristics are more dependent upon the size of the filter features than the size of the machined channels (or assembled position within the channel). The filter features can be easier to control accurately because they are defined by photolithography, for instance. In some embodiments, the suspended waveguide&#39;s width comprises 90% or more of the housing  132  cavity&#39;s width. In certain aspects, the presence of the dielectric under the edges of the metallization can cause the filter sections to couple primarily to the sides of the cavity. According to embodiments, the cavity walls are moved further away from the filtering elements by suspending the substrate. However, the dielectric sucks the field lines to the sidewall at the edges of the metallization. The associated current density is therefore higher at the edges where there are no slots, minimizing the ohmic loss they would otherwise cause. For example, a filter section  124  has a wide center strip  204  with slots that are spaced apart from the edges of the suspended transmission line  108 , as illustrated in the example of  FIG.  2 A . 
     According to embodiments, through the use of one or more slotted features near the transition between the microstrip quartz circuit and the waveguide (e.g., where the diode sits across a gap), suppression of higher-order transverse electromagnetic (TEM) modes that could propagate longitudinally down the microstrip circuit is provided. According to embodiments, this is achieved by breaking the transverse surface current path across the centre conductor by introducing gaps. However, the longitudinal propagation of the desired fundamental mode is unaffected because there is no transverse current for the fundamental mode. Aspects of this feature are illustrated in  FIGS.  8 A and  8 B . In some implementations, without these features, the higher order modes would introduce resonances into the low impedance section of the filter&#39;s passband, which can restrict the bandwidth limiting the filters operational bandwidth and thereby the bandwidth of the overall mixer. 
     In some embodiments, the mixer  100  further includes ground connection  112   a  and  112   b.  This may be, for instance, via a wired connection. The mixer  100  may further include a tuned short-circuit pad  122 . In some embodiments, a ground connection  112   b  is made from the pad  122  to housing  132 . Additionally, one side of diode circuit  110  may be mounted to the pad  122 . 
     With respect to tuned short-circuit pad  122 , and according to some embodiments, one end of the stripline circuit is capacitively coupled to ground via a rectangular pad of metal, through the stripline substrate, to the metal shelf it is resting on. This connects the circuit to ground as an effective short circuit at the detected signal (RF) and LO millimeter-wave frequencies through capacitive coupling. This capacitive coupling can reduce the effect of bondwire assembly variations on the ground point for the LO and RF signals. The bond wire from this pad to the metal step connects the circuit to ground at lower frequencies (low microwave) and DC. This allows a much shorter length of line, thus providing broader bandwidth that can be achieved with minimum bond lengths and number. 
     According to embodiments, a separately machined LO and/or IF coaxial filter pin is not required. 
     In certain aspects, in order to achieve the widest bandwidth possible, a ground connection is presented to all signals with a minimum length close to the ground-side of the diode circuit  110 . Such a connection was traditionally achieved using a “virtual” reactive comprising tuning stubs using bond wires that are multiples of ¼ th  λ, at the LO and signal frequencies. However, and according to embodiments, the IF signal also has a short ground length in order to achieve a broadband match, which may be incompatible with traditional techniques. To achieve the ideal ground at all frequencies, including at the LO and signal frequencies which are sufficiently high, a virtual broadband short is achieved in some embodiments at both frequencies using a combination of the capacitance of the bond wire pad and the inductance of the short bond wire. In this embodiment, the IF signal is too low in frequency to be affected by the capacitance of the bond pad or the inductance of the bond wire and as the physical length is very short compared to the IF wavelength (e.g., less than λ/10) and a broadband physical short is achieved. 
       FIGS.  2 A and  2 B  show the upper surface of suspended waveguide  108  within the housing  132  of mixer  100 . For instance, the figures illustrate a patterned metal layer on a substrate to form a waveguide path for signals within the mixer. In  FIG.  2 B , the diode circuit  110  is omitted to illustrate the pad structure. 
     As shown in  FIG.  2 A  the upper surface of the suspended waveguide  108  can include a microstrip transmission line  202 , which includes a wide center strip  204 . In this example, the airgap  130  in the housing  132  is provided on four sides around the suspended waveguide  108  to form a cavity. The sides of the cavity are shown, for instance, as cavity sidewalls  206   a - d.  In the top view image of mixer  100  shown in  FIG.  2 B , the coupling of waveguides  114  and  116 , and coaxial connection  118 , are further shown. While a coaxial connection is shown as an example, other output channels may be used. According to embodiments, the input channels can extend along a first direction, while the outputs extend along a perpendicular direction, as shown for instance in  FIG.  6 A . The waveguides  114  and  116  may be in a first plane, while the output channel (e.g., with coaxial connection  118 ) is in a second, orthogonal plane. In this example, the LO input waveguide  114  bisects the mixer. The slotted features illustrated in  FIGS.  2 A and  2 B  can correspond, in some embodiments, to an LO filtering/suppression portion  124 , an RF filtering/suppression portion  126 , and an RF reception element (e.g., antenna)  128 . 
       FIGS.  3 A and  3 B  illustrate further details of the surface of the waveguide element  108  according to embodiments. 
       FIG.  3 A  illustrates an example of LO filtering portion  124  and RF filtering portion  126  on either side of the LO waveguide input  114 , according to some embodiments. As shown in the example of  FIG.  3 A , the patterned transmission line on the surface of suspended waveguide  108  comprises a plurality of slots, which can provide one or more of unwanted mode suppression, filtering, and matching. For instance, a suppression slot can simultaneously provide suppression and filtering. In some embodiments, hammerhead filters  302  can be used and extended in a longitudinal direction to filter LO signals from reaching the IF output  106 . Similarly, the half-hammerhead structures  304  provide filtering of spurious RF. In certain aspects, hammerhead (or half-hammerhead) structures can be inductive capacitance combinations that act as effective virtual short circuits spaced at  212 , which block the signal from passing, thereby providing filtering. 
     According to embodiments, the inclusion of slotted suppression features enables a wide center path through the transmission line, which in turn, reduces ohmic losses along the length of the suspended waveguide element as IF signal is passed from the diode circuit  110  to the IF signal output  106 . According to embodiments, the thickness of the metallization layers are at least  3  times the skin depth. 
       FIG.  3 B  illustrates an enlarged view of the RF reception element (e.g., antenna)  128 , which may be part of RF input  104 , according to some embodiments. In this example, the reception element  128  comprises a first set of slots and a second set of slots, wherein the first set  306  are provided for filtering of spurious RF and the second set  308  help to suppress unwanted transmission modes by providing a current break for higher order signals. According to embodiments, the first slots  306  are wider than the second slots  308 . Further, and in some embodiments, both the first and second sets  306 ,  308  are sufficiently narrow that the reception element  128  can provide wideband coupling to the RF input waveguide  116 . In the view of FIG.  3 B, the diode circuit  110  is not shown, which makes the various metallization layers (e.g., pads) visible, such as pad  122 . 
     According to embodiments, the mixer  100  can be optimized for a particular frequency range. For example, the width of the patterned metal features (e.g., filter widths) on the surface of the waveguide element  108  may be set to approximately 0.3-0.4λ, where λ, is the free-space wavelength. For instance, for operation at 375 GHz, the filter channel width may be approximately 0.3 mm. In some embodiments, the mixer is optimized at one of 125 GHz, 250 GHz, and 375 GHz. While specific frequencies/wavelengths are provided in this example, the design and manufacture of the disclosed mixers may be optimized across a wide band of frequencies/wavelengths. For example, mixer  100  may be applicable as a mixer in the microwave, millimeter, and sub-millimeter ranges. 
     According to embodiments, the center part of the LO and RF filter arrangement is designed to be as wide as possible in order to minimize ohmic losses. In some embodiments, the width of the center line is 30%-50% of the width of the cavity in which the mixer is mounted. In some embodiments, the width of the center line is 50% or more of the width of the waveguide substrate. In some embodiments, the width of the line is between 0.1-0.2λ, at the operating frequency. For example, for optimization at 375 GHz, the width of the center line running from the signal end to the IF output end may be approximately 100 microns wide. 
     In some embodiments, the patterned metal microstrip transmission line comprises a center line that extends from the diode circuit to the output and through one or more filter elements, the width of the center line is equal to or greater than 0.1λ, and less than or equal to 0.2λ, the width of the suspended waveguide element is approximately 0.2λ, and the thickness of the substrate is 0.04λ. The housing cavity may have a width (e.g., in the transverse direction) of approximately 0.35λ, (e.g., between 0.3-0.4λ), and the patterned microstrip transmission line is arranged in the cavity to couple an even transmission mode to a sidewall of the housing  132  cavity. 
     According to embodiments, the mixer  100  can be implemented without separate tuning stubs (e.g., wires) or a machined LO filter pin. This can be contrasted, for instance, with the design of WO 2007/125326, which requires one or more stub tuners in the form of extra wires as well as an orthogonal coaxial filter at the IF output to handle LO signal filtering. 
     According to embodiments, and in order to achieve the highest bandwidth in the RF and LO input circuit, an open circuit waveguide probe is used. This may provide an improvement, as short circuited waveguide are inherently narrow band, often resulting in poor power coupling across the desired band. 
     Referring now to  FIG.  4   , a method  400  of operating a mixer according to some embodiments is provided. The mixer may be, for instance, mixer  100  or any other mixer have features shown with respect to  FIGS.  1 ,  2 A,  2 B,  3 A,  3 B, and  6 A . 
     The method  400  may begin with step s 410 , which comprises receiving an RF signal at a first input of a mixer apparatus having a suspended waveguide element, a plurality of suppression elements, and a diode circuit. In step s 420 , a local oscillator signal is provided to the mixer. In step s 430 , an intermediate frequency (IF) signal generated by the diode circuit is outputted. According to embodiments, outputting the IF signal comprises propagating the signal along a longitudinal direction of the mixer (e.g., mixer  100 ) through a plurality of filters. The filter may be, for example, a part of the plurality of suppression elements. According to some embodiments, the RF signal and LO signal are received and provided along a first axis with respect to the mixer, while the IF is output along a different, perpendicular axis. 
     Referring now to  FIG.  5   , a detection system  500  according to some embodiments is provided. In this example, an array of feedhorns  502  provide RF signal to one or more mixers  504 , such as mixer(s)  100  described above. The RF signal can be mixed with a LO signal  512  from source  508 , such as one or more Gunn oscillator sources. The resultant IF can then be fed to processing circuitry  506  for subsequent processing, including one or more of amplification, filtering, analog-to-digital conversion, and image generation. In some embodiments, the output IF is provided via a coaxial line from the mixers to one or more of the processing circuitry  506 . In some embodiments, the feedhorns  502 , mixers  504 , and signal paths (e.g.,  512 ) can be housed in a block  514 . A block for use with one more mixers described herein is illustrated, for instance, with respect to  FIGS.  6 A- 6 C . 
     According to embodiments, the set of mixers  504  includes at least three types of mixers, where a first mixer is optimized a first frequency, a second mixer is optimized at a second frequency, and a third mixer is optimized at a third frequency. In this example, all types of mixers follow the design and configuration of mixer  100  as set forth in  FIGS.  1 - 3   . In some embodiments, the frequencies are 125 GHz, 250 GHz, and 375 GHz. In some embodiments, a fourth type of mixer is included using the design and configuration of mixer  100 , which is optimized at 500 GHz. 
     Referring now to  FIG.  6 A , two machined halves of a detector block  600  are illustrated according to some embodiments. In this example, the first machined portion  602  can be combined with the second machined portion  604  to form a detector block. The block may comprise, for instance, the housing  132  of mixer  100  and block  514 . As shown in  FIG.  6 A , each of the halves can include one or more feedhorn portions  606 , which can serve as channels to deliver RF signals to a mixer  100 . Each of the halves may also include input signal (e.g., LO) delivery portions  608 , as well mixer cavities, shown in the detail  610 . Additionally, and in some embodiments, the mixer can couple its output into a signal output  612  (e.g., containing a coaxial or other transmission line), which can be formed in the respective block halves such that a coaxial cable can run through the mixer housing  132 . The output  612  may, for instance, carry IF signals generated by the mixer. According to embodiments, the feedhorns  606  and LO input  608  can be machined along the same axis (e.g., the x-axis) or in the same plane (e.g., the plane formed by the x- and z-axes). Additionally, in some embodiments, the mixer output  612  can be machined in a perpendicular/orthogonal fashion (e.g., along the y-axis). In this respect, the output signals from the mixer can be delivered in a different direction than the direction in which the input signals are received. For instance, signals may be output in a direction perpendicular to the plane in which signals are received. 
     Referring now to  FIGS.  6 B and  6 C , one or more detector array blocks are illustrated according to some embodiments. These may correspond, for instance, to arrays  502  and blocks  514 . A detector element, such as block  514  with an array of feedhorns  502 , can be formed as a single block (e.g., as shown in  FIGS.  6 A and  6 B ). Also it can be formed from multiple blocks (e.g., with two sub-blocks as shown in  FIG.  6 C ) to form a combined detection block with multiple rows and columns of feedhorns. Additionally, and according to embodiments, each sub-block can be formed from two parts as illustrated with respect to  FIG.  6 A , which shows the attachment of a half-block portion  602  with another half-block portion  604  to form block  600 . 
     Referring now to  FIG.  6 B , a block  620  according to some embodiments is provided. In some instance, one or more of detector arrays  502  and block  514  may comprise block  620 . Block  620  includes one or more signal inputs  622 , one or more secondary inputs  624 , and one or more outputs  626 . For instance, signal inputs  622  may correspond to a plurality of detector feedhorns as illustrated in  FIGS.  2 ,  5 ,  6 A, and  9   . Similarly, the secondary input(s)  624  may be an LO signal input, such as described with respect to  FIGS.  2 ,  5 ,  6 A, and  9   . The output(s)  626  may be detected signal outputs, such as one or more IF signals from a mixer  100 , as described in connection with  FIG.  9   , for instance. According to embodiments, the inputs are provided in the same plane while the outputs are provided in a transverse plane. In the example of  FIG.  6 B , the inputs  622 ,  624  are provided along the x-axis while the outputs  626  are provided along the y-axis. According to embodiments, processing circuitry may be connected to the outputs  626 . For example, a plurality of processing circuitry units may be stacked in the z-axis direction to process the signals output from block  620 . Likewise, additional blocks may be stacked in the z-axis direction to expand the detector. A similar arrangement is shown in  FIG.  6 C  with elements  660   a - n.  Such circuitry could include, for instance, one or more LNAs, wide-band power detectors, and/or ADCs as illustrated with respect to  FIG.  9   . In some embodiments, the power detectors are used to convert the broadband microwave power into base-band signals by thermal means (e.g. a bolometer) or by using an amplitude modulation (AM) detector. Additionally, the processing circuitry can include one or more filters, for instance, on either side of the LNA or ADC. According to embodiments, the power detector, such as  921   a,    921   b,    921   c,  detects power across the entire band, and outputs a voltage related to the power. 
     Referring now to  FIG.  6 C , and according to embodiments, a first block (e.g., block  620 ) can be combined with a second block (e.g.,  650 ) to form a single detection array block, such as block  514  with array  502 . In this example, the inputs  652  and  654  of block  650  are provided in a single plane along the x-axis and the outputs  656  are provided in a perpendicular direction extending along the y-axis. Given the arrangement of the inputs and outputs, the processing circuitry  660   a - n  can be stacked at an outer surface of the blocks  620 , 650  in the z-axis direction without interfering with signal capture from a source by inputs  652 , or blocking delivery of the LO to input  654 . Although the circuitry  660   a - n  is shown on the block  620 -side of the array, the stack may also be provided on the other side of the array (e.g., the block  650 -side). In this respect, a first stack  660   a - n  of processing circuitry may be provided on one side of the array, a second stack  660   a - n  of processing circuitry may be provide on a second side of the array, an input signal (e.g., RF input) may be provided on a third side of the array, and in some embodiments, another input signal (e.g., LO) can be provided on a fourth side of the array. The circuitry  660   a - n  may comprise, for instance, one or more LNAs, AM (envelope) power detectors, and/or ADCs, and in some cases, filters. 
     Although the array of  FIG.  6 C  is depicted with two, connected blocks, embodiments are not so limited. For instance, according to embodiments, detector arrays can be formed of blocks that are not directly connected, and with more than two blocks. For example, processing circuitry  660   a - n  could be interposed between two sub-blocks of an array (e.g., between blocks  620  and  650 ). In some instances, three sub-blocks could be used with circuitry interposed between the first and second block, and between the second and third block. In such an arrangement the circuitry between the first and second block could process signals from one or both of the first or second blocks, while circuitry between the second and third block could process signals from one or both of the second and third blocks. Additionally, another circuitry stack could be provided on one or more of the outer sides of the array, for instance, to process signals from the first or third block. While described using three blocks, according to embodiments, this arrangement could be repeated to expand the width of the array beyond three blocks. Detection arrays can also be expanded in the z-direction, for instance, by stacking additional blocks  620 ,  650 . According to embodiments, there are no inputs or outputs on the upper and lower surfaces of an array block  620 , 650 , and thus, it can be stacked with another block without interference. Some embodiments include a block  620 , 650  with six primary sides, with inputs and/or outputs exposed on outer sides (e.g., inputs and/or outputs in the x- and y-directions through the block), but not on the top or bottom (e.g., in the z-direction). 
     According to embodiments, a detection system may be optimized for up to four detection frequency bands. According to embodiments, the width of the band may be set by a combination of the mixer, IF amplifier, and/or an envelope detector. In some embodiments, the band is +/−20% of the center frequency for the waveguide. 
     Referring now to  FIG.  7   , a method  700  of fabricating a mixer apparatus is provided according to some embodiments. This method may be used, for instance, to fabricate a mixer  100  as illustrated with respects to  FIGS.  1 - 3   , as well as to form the machined portions  602 ,  604  shown in  FIG.  6 A . 
     The method  700  may begin with step s 710 , which comprises machining a first housing component (e.g.,  602 ) comprising a first radio frequency channel portion, a first local oscillator portion, a first intermediate frequency channel portion, a first cavity portion, a first mounting shelf portion, and a second mounting shelf portion. In step s 720 , a second housing component (e.g.,  604 ) is machined, which comprises a second radio frequency channel portion, a second local oscillator portion, a second intermediate frequency channel portion, a second cavity portion, a third mounting shelf portion, and a fourth mounting shelf portion. The method may further comprise suspending (s 730 ) a waveguide element in the first and/or second cavity portion by mounting the waveguide element on the first and second mounting shelf portions and/or the third and fourth mounting shelf portions. This could be, for example, waveguide element  108 . The method  700  may further comprise attaching (s 740 ) the first housing component to the second housing component to form a block. For instance, this method may be used to form a block (or blocks) as illustrated in  FIGS.  6 B and  6 C . 
     According to embodiments, two halves of a complete feedhorn are machined in each half of a block using a bullnose cutter and a five axis machine. This allows for flexibility in horn geometries to be used, such as an oval section horn. This can allow the feedhorn beam pattern to be adjusted to achieve optimal optics coupling. For instance, a circular horn provides an oval beam, which must be corrected in the optics otherwise the image can have improper stigmatism. An oval feedhorn, however, can achieve a circular beam which does not need correcting. Thus, and according to some embodiments, a feedhorn (e.g., to deliver received RF to the mixer  100 ) may have an oval shape. 
     Referring now to  FIGS.  8 A and  8 B , electric and magnetic fields are illustrated. For instance,  FIG.  8 A  illustrates transverse electromagnetic modes  802 . Where the microstrip outer channel is less than one-half of the guide wavelength, typically, only the fundamental Transverse EletroMagnetic mode (TEM-00) mode can exist. In this instance the electric field lines and magnetic field lines are transverse and propagate from the inner conductor to the outside of the metal channel in the same direction or loop around the filter metallization in the same direction respectively as shown by  802  (conductor in air, in this example). Operation in the fundamental TEM-00 mode can result in a very smooth impedance versus frequency response, for instance, as shown at  804 , which depicts the transmission passband. This arrangement is further illustrated by  808 , with a circuit line (e.g., gold)  810  on a quartz substrate  812 . 
     The effective microstrip filter metallization width and outer channel width dimensions scale inversely as a function of frequency and so at higher frequencies the width of the filter metallization pattern and the channel it sits in gets very small, and both become challenging to manufacture, thereby increasing cost if the dimensional condition that the outer width of the channel stays below one half guide wavelength. If this condition cannot be met (e.g., either is more than one half guide wavelength), higher order transverse and longitudinal modes can be excited (e.g., by discontinuities in the channel or filter) and propagate. This situation is illustrated by  814 . The electric and magnetic field lines are then no longer uniform and resonant conditions can be created, resulting in non-uniform behavior as a function of frequency and resulting in reflected power as shown by  806 , where resonance appears in the passband. These resonances can reduce the smooth, broadband operation of the mixer. 
     Often a microstrip filter will cascade quarter-wavelength low and high impedance sections. The finesse of the filter is best when the ratio of the high to low impedance sections is greatest, which is achieved using the narrowest and widest lines. This is illustrated by  822  and  824 , respectively, in  FIG.  8 B . The performance of such filters is therefore fundamentally limited when the generation of higher order modes is considered as there is a finite width before the higher order modes are excited and resonant reflections are introduced into the filters frequency response. Aspects of the disclosure can provide the benefits of wider lines (e.g., lower impedance and improved filtering) without the negative effects of higher order mode propagation. This can be achieved, for instance, through the inclusion of current breaks or slots along the length of filter sections as shown in  830  of  FIG.  8 B . This is contrasted with  826 , where the current flows un-interrupted and generates non-ideal passband performance as illustrated in  828 . When higher order modes are created the current flow no longer occur just along the longitudinal direction as in the case of the fundamental mode, but also run across the lateral direction of the filter metallization. With the introduction of current breaks, these transverse currents are interrupted and the excitation of a higher order mode suppressed, even though the width of both the filter section and channel would otherwise allow it. A much lower impedance filter section is achieved along with a smooth filter frequency behavior as shown in  832 . 
     Referring now to  FIG.  9   , a schematic depiction of a detection system  900  according to some embodiments is provided. In some embodiments, system  500  can be implemented using the arrangement of  FIG.  9   . 
     Radiation can be received at inputs  902 ,  904 , and  906 , which may be feedhorns of block  908 ,  910 , and  912 , respectively. In some embodiments, blocks  908 ,  910 , and  912  may each form a part of detector array  502 ,  514 ,  600 ,  620 , and/or  650 . The radiation signals received at inputs  902 ,  904 ,  906  are each passed to a mixer element, depicted as elements  914   a,    914   b,  and  914   c.  This could be, for instance, a mixer  100 . In some embodiments, one or more of the received signals may be rotated before processing by a mixer, for instance, by optional polarization rotation element  916 . The processed signals from the mixer, such as one or more intermediate frequency (IF) signals, are passed to the output ( 918   a,    918   b,    918   c ) of each block for subsequent processing by a low noise amplifier (LNA) ( 920   a,    920   b,    920   c ), power detector ( 921   a ,  921   b,    921   c ), and analog to digital converter (ADC) ( 922   a,    922   b,    922   c ). Additional processing may include filtering. The digital signals may then be passed to image generator  924  to form an image based on the detected radiation, such a composite image of an object or scene using the radiation received at  902 ,  904 , and  906 . The image processing  924  may be coupled to, or part of, a viewing system  936 , such as a computer or monitor. In some embodiments, the IF signals are outputted using coaxial cable within each block. 
     Although this system is depicted with three mixer elements or inputs, it could be implemented with more or less. For instance, the system  900  could be extended to include a fourth set of input feedhorns, each with a corresponding set of mixers and processing circuitry. Additionally, each of blocks  908 ,  910 , and  912  may comprise an array of inputs, and multiple mixers  100 . Information may be processed by a stack of circuitry as illustrated in  FIG.  6 C . 
     In some embodiments, the local oscillator (LO) signal is provided to the mixers of system  900  using fewer LO sources than mixers. For instance, in the example illustrated in FIG.  9 , a single LO source  926  provides an LO signal to each of mixers  914   a - c.  According to embodiments, this is accomplished using one or more power dividers  928   a,    928   b,  and in some instances, one or more frequency multipliers  990   a - c,  such as doublers or triplers. For example, an original LO signal from source  926  could be provided to divider  928   a,  which in turn provides signal to divider  928   b  and an optional multiplier  990   a.  The multiplier  930  may be used to provide mixer  914   a  with an LO signal at a higher frequency than is provided by source  926 . Similarly, divider  928   b  may provide signal to optional multipliers  930   b  and  930   c  for mixers  914   b  and  914   c.  In this respect, each of blocks  908 ,  910 , and  912  can be configured for operation at different frequencies/wavelengths. Such configuration could include, for instance, the mixer design as well as the shape and dimensions of the input feedhorns. Accordingly, different wavelength radiation signals may be detected, processed, and used by the processing circuitry  924  to form a composite image using data at multiple wavelengths. One or more amplifiers, such as amplifier  992  may be used to boost a signal between a source, divider, multiplier, and/or mixer. Other amplifiers are not illustrated in the example of  FIG.  9   . 
     The feedhorn size should be selected to provide the most optimum coupling to the chosen optics, and the spacing should be selected to provide the optimum coverage depending on the application. 
     In certain aspects, components are optimized to minimize the amount of LO power reflected from the RF coupling circuit over as wide range of circuit operating conditions as possible. This limits reflected LO signal propagating back into one or more LO power splitters, thus limiting interference between channels. To further reduce reflection issues, a 90 degree 3 dB hybrid power splitter such as a Magic-T or a 3 dB hybrid branch guide coupler could be implemented. While illustrated with a single LO  926 , multiple LOs may be used according to embodiments. 
     According to some embodiments, one or more of dividers  928   a  and  928   b  is a non-equal divider. According to some embodiments, multipliers  930   a  and  930   b  are doublers while multiplier  930   c  is a tripler. In some embodiments one or more of the multipliers is not needed. For example, multiplier  930   a  may be omitted in some embodiments. 
     According to some embodiments, a common local oscillator source  926  is provided, multiplied in frequency using different order frequency multipliers (e.g., doublers and triplers), and subharmonic mixers are implemented. For example, mixers  914   a - c  may be operating in bands centered on the frequencies 125 GHz, 250 GHz and 375 GHz, respectively, which can use local oscillator frequencies of 62.5 GHz, 125 GHz and 187.5 GHz. In this example, the base local oscillator source  926  may be a Gunn oscillator providing approximately 100 mW operating at 62.5 GHz. This is split into two equal signals using a splitter, such as divider  928   a.  This may be an equal power splitter, such as a standard magic-tee, or a non-equal splitter. One half of the splitter output pumps the 125 GHz array (block  908  in this example), and the second half is input into a power amplifier (e.g., amplifier  992  in this example) to give a signal of approximately 400 mW that is then used to pump frequency multipliers  930   b  and  930   c.  Given the use of a subharmonic mixer at the example frequencies, the multiplier  930   a  is not required in this instance. In some embodiments, the system is optimized by using a configurable, non-equal power divider to provide the optimum power to the 125 GHz array and power amplifier. Each frequency mixer array requires similar local oscillator power, 30-40 mW in this example. However the typical efficiency of a doubler is 40% whereas a tripler is 15%. This means that use of an equal power splitter such as a magic-tee can result in too much power (80 mW) being generated for the 250 GHz array (e.g., block 910 in this example) and marginal power for the 375 GHz array (e.g., block  912  in this example). Traditional power splitters such as magic-tees rely on the input power being split equally in half, however, if such an approach is used for this embodiment, it may result in too much power being supplied to the 125 GHz doubler (e.g., multiplier  930   b ) and not enough power being supplied to the 187.5 GHz tripler (e.g., multiplier  930   c ). Therefore in order to supply sufficient power to the tripler, which is typically less efficient than a doubler, the power amplifier would need to be over specified and the power to the doubler would be unnecessarily attenuated (otherwise it can be damaged). Thus, and according to embodiments, a configurable power divider is used that can split the local oscillator pump power arbitrarily into two or more pathways, thereby dividing the power for each local oscillator arm&#39;s multipliers according to their respective power requirements. According to embodiments, dividers  928   a  and/or  928   b  are configurable. Thus, a system can be provided in which the overall system LO power requirement (and thereby cost) is minimized by matching the power split to each frequency arm so that the power delivered matches the peak efficiency input power requirement for each respective multiplier. 
     According to some embodiments, one or more blocks that is configured to receive and process radiation at a fourth frequency may be provided. For instance, one or more blocks may be configured at 500 GHz. The mixer for such a block could similarly operate using the same LO source, for instance, with an additional divider and or doubler in one or more of the LO paths shown in  FIG.  9   . In some embodiments, the block may be a sub-block or array  514 ,  620 , and/or  650 . 
     According to some embodiments, the LO source  926  may have multiple outputs at one or more frequencies, and initial divider  928   a  and/or multiplier  930   a  may not be required. In some embodiments, one or more of divider  928   a  and multiplier  930   a  are integrated into the local oscillator source  926 , such that source  926  provides multiple frequency signals at configurable powers. Although two outputs are used by way of example, this implementation could be extended through the use of additional non-equal power splitters and/or multipliers. 
     Earth&#39;s atmosphere provides unique illumination characteristics according to the observed frequency transmission through the atmosphere, which in turn is determined by the amount of water vapor present above the scene and the wavelength being used for detection. Two phenomena dominate this effect. Firstly, a water molecule has rotation resonances in the millimeter to terahertz domain so that at particular frequencies it absorbs photons. At frequencies away from these lines it passes photons, which may be understood as the water window. In addition, as the wavelength shortens the effective path length through the atmosphere becomes longer increasing the attenuation. At frequencies where the transmission is highest, Earth&#39;s atmosphere is transparent and so the cold background of space provides very high contrast during millimeter-wave detection. Thus, an outdoor scene under observation is dominated by the “cold” sky illumination. At frequencies where the transmission through Earth&#39;s atmosphere is low, the scene is dominated by the “warm” illumination from the column of air above the scene. According to embodiments, and in order to provide a color scene where different contrast types are achieved, a mixture of high transmission moderate transmission and low transmission wavelengths can be used. Through experimental research, these have been determined to be ˜35 GHz wide bands centered around 125 GHz, 250 GHz and 375 GHz. 
     Although embodiments use 125 GHz, 250 GHz, 375 GHz, and 500 GHz as examples, other sets of frequencies may be used. For instance, 60, 120, 240, 360 and 480 GHz may be used. This set of frequencies relates to the absorption properties of oxygen in the atmosphere. 
     While various embodiments of the present disclosure are described herein, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments. Generally, all terms used herein are to be interpreted according to their ordinary meaning in the relevant technical field, unless a different meaning is clearly given and/or is implied from the context in which it is used. All references to a/an/the element, apparatus, component, means, step, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. Any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context. 
     Additionally, while the processes described above and illustrated in the drawings are shown as a sequence of steps, this was done solely for the sake of illustration. Accordingly, it is contemplated that some steps may be added, some steps may be omitted, the order of the steps may be re-arranged, and some steps may be performed in parallel. That is, the steps of any methods disclosed herein do not have to be performed in the exact order disclosed, unless a step is explicitly described as following or preceding another step and/or where it is implicit that a step must follow or precede another step.