Abstract:
A RF pick-up probe, RF choke, and DC output line that simultaneously receives RF radiation from a waveguide and provides a detected DC voltage provided by a diode RF detector disposed in said waveguide to one or more output video lines. The RF pick-up probe, RF choke, and DC output line are preferably disposed with an antenna transition element for coupling a horn antenna to a matched diode detector which provides the aforementioned DC voltage. The transition preferably includes a ridged waveguide operatively coupled to the horn antenna; a substrate for supporting a diode chip, carrying said matched diode detector, adjacent the waveguide, the substrate also supporting a pair of RF pick-up probes, each RF probe having a portion which is coupled with the diode chip, the substrate also supporting conductors coupled to the diode chip and to the pair of RF pick-up probes; and a waveguide short circuit at least partially enclosing the diode chip and disposed adjacent the substrate.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The US Government has a paid up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of W911QX-04-C-0127 awarded by DARPA. 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to the disclosure of U.S. patent application Ser. No. 12/172,481 filed 14 Jul. 2008, the disclosure of which is hereby incorporated herein by this reference. 
     TECHNICAL FIELD 
     This invention relates to passive imaging technologies where detectors rely on ambient millimeter wave radiation naturally radiated by an object to detect its presence. The present invention may be used to couple an antenna, such as a horn antenna, directly to a detector diode without the need for intermediate pre-amplification. The detectors may be arranged in a two dimensional array. 
     BACKGROUND 
     Millimeter wave imaging technology, particularly at frequencies from about 70-150 GHz, is actively being pursued for concealed weapons detection, all-weather landing aids, and imaging of building interiors. Passive imaging, where no active source is used (such as compared to radar technologies), has the advantage of not requiring a transmitter thus reducing the cost of the system. It relies on detection of the various levels of millimeter wave radiation naturally radiated by an object (that is its&#39; emissivity) to differentiate between the object and its&#39; background. Detection can be direct to a DC voltage which is proportional to the received integrated noise power, or else the received noise can be mixed down to a lower frequency and then detected. Direct detection has the advantage that it requires fewer parts, but the very small millimeter wave noise levels before detection generally require amplification (see L. Yujiri, “Passive Millimeter Wave Imaging,” IEEE MTT-S International Microwave Symposium Digest, 2006, pp. 98-101, June 2006). HRL Laboratories of Malibu, Calif. has developed a Sb-heterostructure diode that has been optimized to operate as a direct detector without bias voltage (see H. P. Moyer, R. L. Bowen, J. N. Schulman, D. H. Chow, S. Thomas, J. J. Lynch, and K. S. Holabird, “Sb-Heterstructure Low Noise W-Band Detector Diode Sensitivity Measurements,” IEEE MTT-S international Microwave Symposium Digest 2006, pp, 826-829, June 2006). Thus, direct detection without pre-amplification is possible (see J. Lynch, H. Moyer, J. Schulman, P. Lawyer, R. Bowen, J. Schaffner, D. Choudhury, J. Foschaar, and D. Chow, “Unamplified Direct Detection Sensor for Passive Millimeter Wave Imaging,” Proc. Of SPIE on Passive Millimeter-Wave Imaging Technology, eds. R. Appleby and D. Wilkner, Vol. 6211, 2006), which could enable a low-cost millimeter wave focal plane array if a suitable means for coupling an arrayable antenna to an array of the aforementioned Sb-heterostructure diodes could be devised. The present disclosure is directed to techniques for coupling an antenna, such as a horn antenna, to a diode without the need for intermediate pre-amplification. 
       FIGS. 1A-1C  shows an initial effort at a solution to this problem.  FIG. 1A  shows a top the basic concept of a low-cost millimeter wave passive imaging array. Only two antennas are shown in this view for ease of illustration, but the array, which you typically be a two dimensional array, can be of any size desired.  FIG. 1B  is a side sectional view, the section being taken along line  1 B- 1 B shown in  FIG. 1A . In order to make the device shown in  FIGS. 1A and 1B , diode chips  1  are mounted onto a printed circuit board  2  preferably using a flip-chip attachment process. The printed circuit board  2  has a conductive bottom surface  4   a  typically formed of a metal such as copper. The top surface of the printed circuit board  2  is patterned so that wiring  4   c  is formed by pattering the typical metallic surface of the printed circuit board  2 . The wiring  4   c  on the top surface can be seen in  FIG. 2A . Vias  4   b  conduct RF energy from the diode chip  1  and through the printed circuit board  2  to the bottom side thereof. A molded metal horn array  3  is soldered onto the topside wiring  4   c  on circuit board  2  preferably for efficient W-band image noise collection.  FIG. 1C  is a close up view of a diode chip  1 , which has a pair of diodes  5   a ,  5   b . The conductors  4   d  coupled respectively to diodes  5   a  and  5   b  pass each other without making electrical contact with each other in region  7  so as to make contact with the connectors  8  shown on opposite edges of chip  1 . A thin layer of an insulator  6  allows the wiring from the diodes  5   a ,  5   b  to pass other each other with making connection. The connectors  8  can be bonded to the wiring  4   c  on the circuit board  2  using flip-chip bonding techniques known in the art. 
     While there are some common features between these initial efforts and the technology described subsequently herein, the present disclosure addresses some shortcomings of the this initial effort. In particular, the original diode chip  1  had RF pick-up antennas on the diode chip  1 . It was subsequently discovered through electromagnetic simulation that the RF pick-up antennas needed to be on a printed circuit board substrate for wide band operation. Also, a back-short tuning cavity was fabricated using the printed circuit board itself, whereas in the present disclosure, an air-filled back-short cavity is explicitly made and used for increased operational bandwidth. The other major difference in these initial efforts is that the video output for a particular input polarization is single-ended, whereas in the present disclosure a differential output is described that can reduce interference on the DC lines, although for single linearly polarized field. 
       FIGS. 2A-2D  shows a prior art (see J. Lynch, et. al., “Unamplified Direct Detection Sensor for Passive Millimeter Wave Imaging.”  Proc. of SPIE on Passive Millimeter - Wave Imaging Technology , eds. R. Appleby and D. Wilkner, Vol. 6211, 2006) passive millimeter wave imaging transition that this invention improves upon. It can be seen in the plan view of  FIG. 2A  and the perspective view of  FIG. 2B  that the diode chip  1  is flip-chip mounted onto a fused silica substrate  2  that forms part of the back-short cavity. The video output  4  is taken off of the chip with a coplanar strips (ground-signal-ground) transmission line that is orthogonal to the RF pick-up antennas. The DC signal line is then bonded to a coaxial centerline pin/conductor.  FIG. 2C  shows a close view of the chip  1 , while  FIG. 2D  is a bottom view of the chip  1  showing connections to conductors disposed on the substrate  2 . 
     The new technology described in this disclosure integrates an RF choke into the RF pick-up probes (antennas) so that the DC lines can come directly off of the probe. This eliminates a lot of excess metal within the transition that causes parasitic reactance and DC/RF isolation in the DC lines. Also, the use of an air-filled back-short cavity of this disclosure rather than a fused silica filled cavity enables broader bandwidths to be achieved. 
     BRIEF DESCRIPTION OF THE DISCLOSURE 
     This disclosure teaches how to make a very wide-band millimeter wave transition from a ridged waveguide input to a millimeter wave imaging diode detector. This transition is designed for operation from 70 GHz to greater than 140 GHz. Novel features of this disclosure are believed to include:
         A wide-band transition that takes an input millimeter wave signal from ridged waveguide to a millimeter wave impedance matched diode detector chip.   An integrated RF pick-up probe, RF choke, and DC output line that simultaneously receives millimeter wave radiation from a waveguide and provides the detected DC voltage the millimeter wave diode detector to an output video line.   A differential DC output with high RF isolation.   A substrate with the integrated transition contained within unit cell of a passive millimeter wave detector array that enables the array to be scalable to any size.   A method of using a fused silica substrate and standard thin film processing techniques to create the transition.   A method of using an alumina substrate and standard thin film processing techniques to create the transition.   By carefully integrating the antenna transition elements and the detector, the conventional requirement for a Low Noise Amplifier (LNA) is eliminated.       

     The transition is designed to couple to a ridged waveguide which is know in the art to have a wider bandwidth than a standard rectangular waveguide (for coupling to the pick-up horn antenna). 
     The DC output lines come directly off of the RF pick-up probes, thus minimizing parasitic RF pick-up by the DC line and facilitating a differential DC output. 
     This invention has improved RF isolation from the DC line due to the RF choke and a cut-off DC output waveguide channel. 
     Two embodiments are described below, one for fused silica and one for alumina. Alumina substrates are not typically used at frequencies 70 GHz+. 
     In one aspect the present invention provides a transition for coupling a horn antenna to a matched diode detector. The transition preferably comprises a ridged waveguide operatively coupled to the horn antenna; a substrate for supporting a diode chip (carrying the matched diode detector) adjacent the waveguide, the substrate also supporting a pair of RF pick-up probes, each RF probe having a portion which is coupled with the diode chip; and a waveguide short circuit at least partially enclosing the diode chip and disposed adjacent said substrate. 
     In another aspect the present invention provides a combination of a RF pick-up probe, RF choke, and DC output line that simultaneously receives RF radiation from a waveguide and provides a detected DC voltage provided by a diode RF detector disposed in said waveguide to one or more output video lines via the RF choke. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1C  shows some initial efforts at a solution to the problem of coupling an antenna, such as a horn antenna, to a diode without the need for intermediate pre-amplification; 
         FIGS. 2A-2D  show a prior art passive millimeter wave imaging transition (see J. Lynch, et. al., “Unamplified Direct Detection Sensor for Passive Millimeter Wave Imaging.”  Proc. of SPIE on Passive Millimeter - Wave Imaging Technology , eds. R. Appleby and D. Wilkner, Vol. 6211, 2006); 
         FIG. 3A  depicts a focal plane array of individual pixels elements of a type shown in  FIG. 3B , for example; 
         FIG. 3B  is an exploded view of an individual passive millimeter wave imaging pixel that uses the wideband transition disclosed herein and which may be grouped in an array as depicted by  FIG. 3A ; 
         FIG. 4A  depicts the preferred internal arrangement, in a perspective view, of the cavity and differential video signal output coaxial lines arranged in the block  20  shown in  FIG. 3B ; 
         FIG. 4B  depicts one embodiment of the substrate  16  also shown in  FIG. 3B  and also in a perspective view—this figure depicts the elements disposed on and in it in greater detail than done in  FIG. 3B ; 
         FIG. 4C  a top view of block  18  also shown in  FIG. 3B ; 
         FIG. 4D  is a side elevational view taken along section line  4 D- 4 D of  FIG. 4C  showing the internal arrangement of the waveguide and the horn antenna; 
         FIG. 5A  is a perspective view and  FIG. 5B  is top view of one embodiment of the transition of the present disclosure; 
         FIG. 6  is a perspective close-up view of the detector chip of  FIGS. 5A and 5B  mounted on a first embodiment of the substrate; 
         FIG. 7   a  is a graph of reflection from the transition looking in from the waveguide according to a simulation of the first embodiment; 
         FIG. 7   b  is a graph of the RF isolation from the video output line according to a simulation of the first embodiment; 
         FIG. 8  is a perspective view of the substrate (and elements formed or mounted thereon) according to a second embodiment thereof; 
         FIG. 9   a  is a graph of reflection from the transition looking in from the waveguide according to a simulation of the second embodiment; and 
         FIG. 9   b  is a graph of the RF isolation from the video output line according to a simulation of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An exploded system level view of a passive millimeter wave imaging pixel  10  that utilizes the transition disclosed herein is shown in  FIG. 3B . This pixel can be replicated in a periodic fashion in two directions to create a two dimensional array of pixels  10  as shown by  FIG. 3A . The array of pixels  10  may be a planar array as shown in  FIG. 3B  or it may conform to a non-planar shape and hence a surface defined by the leading surfaces of the pixels  10  may assume a three dimensional shape. 
     As can be seen in  FIG. 3B , each pixel  10  preferably comprises three basic parts: 
     (1) a horn antenna  12  that collects incoming millimeter wave energy and transitions the incoming electromagnetic fields from free-space to a ridged waveguide  14 . The horn antenna  12  is depicted in an exploded perspective in the upper portion  FIG. 3B  and greater detail in  FIGS. 4C  and  4 D which present a top down view and a cross sectional view, respectively, of the horn antenna  12 . The sectional view of  FIG. 4D  is taken along lines  4 D- 4 D shown in  FIG. 4C . The horn antenna  12  is preferably formed from a block  18  of electrically conductive material such as a metal. Lines  15  represent small indentations which may (or may not) occur on the interior walls of the horn antenna  12  as a byproduct of machining the horn antenna from a block of metallic material. These indentations  15  do not seem to affect the RF performance of the antenna  12  in any appreciable way and therefor the indentations may be omitted. The block  18  has the horn antenna  12  formed at one end thereof and a ridged waveguide  14  coupled to a distal end or throat of the horn antenna  12 . 
     (2) a transition substrate  16  preferably contains a detector diode chip  17 , RF pick-up probes  26  which receive millimeter wave energy from the ridged waveguide  14  and brings it to the detector chip  17  via conductors  26   c , and differential DC video lines  22  for carrying a rectified millimeter wave signal to pads  32 ′ which are coupled to the center conductor of coaxial lines  32  depicted in  FIG. 4A . The detector diode chip  17  is preferably flip-chip bonded to conductors on the transition substrate  16 . The transition substrate  16  is depicted in perspective in the middle portion of  FIG. 3B  and greater detail in a perspective view of  FIG. 4B . It ends up being sandwiched between block  18  and a block  20 . The transition substrate  16  is preferably formed of a dielectric material, such as fused silica (quartz), alumina, liquid crystal polymer or any other suitably rigid material preferably having a millimeter wave loss tangent less than 0.01. 
     (3) a pixel back structure formed by a electrically conductive block  20  with an cavity  24  therein that forms a waveguide tuning short circuit. The block  20  also has differential video signal output coaxial lines  32  for connection to post processing electronics (not shown). The pixel back structure  20  may be formed of a metal and is depicted in perspective view in the lower portion of  FIG. 3B  and its preferred internal arrangement is depicted in greater detail in a perspective view of  FIG. 4A . In order to simply  FIG. 4A , this figure shows the internal structure of the block  20  without showing its external shape or configuration, as its external shape or configuration is of less importance than its internal shape or configuration. This figure also shows that the detector chip  17  is received in cavity  24  when the transition substrate  16  is positioned adjacent block  20 .  FIG. 4A  also shows the two co-axial transmission lines  32  whose center conductors are each coupled to an associated pad  32 ′ shown in  FIG. 4B . Pads  32 ′ mate with conductors  22  as shown. The exterior shields of the two co-axial transmission lines  32  are preferably formed by the body of block  20 , which is preferably metallic. The two co-axial transmission lines  32  completely penetrate block  20  for connection to the aforementioned post processing electronics, while the cavity  24  does not penetrate the depth of block  20  and thus is open on one side of block  20  for receiving the detector chip  17  as already mentioned. The space between the coaxial transmission lines  32  to the block  20  may be filled with a dielectric material as is common with coaxial cables. 
     The size of the cavity  24  may be bigger than needed to just accommodate the detector chip  17 . The cavity  24  preferably acts as a short circuit at the frequencies of interest to the antenna. It can be best sized using software such as Ansoft HFSS® to simulate the transition  10 . 
     This pixel  10  can be part of a larger array, such as that depicted by  FIG. 3A , which would typically be located at the focus of an optical lens or reflector system as part of a millimeter wave imaging camera, for example. An exemplary the use of the millimeter wave pixels  10  described herein is in a two dimensional focal plane array  40  of pixels  10  as shown in  FIG. 3A . The blocks  18  forming the horn antenna  12  and ridged waveguide  14  may be formed as one larger integral block when formed in an array such as that described with reference to  FIG. 3A . Similarly, blocks  20  and insulating substrates  16  make likewise be formed as larger electrically conductive block and a larger insulating sheet when disposing a plurality of pixels in an array. 
       FIG. 5A  is a perspective view and  FIG. 5B  is top down view of one embodiment of the transition of the present disclosure. The horn portion  18 B of block  18  (see  FIG. 4D ) is not shown in these figures for ease of illustration. These figures in combination with  FIGS. 4C and 4D  provide a close-up view of the region of the pixel  10  that includes the ridged waveguide  14  that connects to the horn antenna  12  in the horn portion  18 B of block  18 . Also not shown in these figures, for ease of illustration, are the video output connection pads  32 ′ and the video output coaxial lines  32  shown in  FIGS. 4A and 4B , for example. In this embodiment substrate  16  is preferably formed of 0.125 mm thick fused silica disposed between two electrically conductive plates or blocks  18  and  20  which may be made of a metal such as aluminum, copper or brass. The diode detector chip  17  is preferably flip-chip bonded onto the fused silica substrate  16  preferably using known techniques such as those disclosed by Virk, R. S.; Maas, S. A.; Case, M. G.; Matloubian, M.; Lawyer, P.; Sun, H. C.; Ngo, C.; Rensch, D. B. in “A lowcost W-band MIC mixer using flip-chip technology”,  IEEE Microwave and Guided Wave Letters , Vol. 7, No. 9, September 1997, pp. 294-496 or by H. Kusamitsu, Y. Morishita, K. Maruhashi, M. Ito, and K. Ohata in “The Flip-Chip Bump Interconnection for Millimeter-Wave GaAs MMIC”,  IEEE Trans. On Electronics Packaging Manufacturing , vol. 22, No. 1, January 1999, pp. 23-28. The disclosures of these documents is hereby incorporated herein by reference. 
     Other dielectric materials than fused silica may be used for the substrate  16  which supports detector chip and its associated conductors  22  and RF probes  26 . As will be seen, openings may be placed in substrate  16  in order to accommodate different dielectric constants of the substrate  16  when different insulating materials are used. 
     The horn antenna  12  is preferably formed in electrically conductive plate or block  18  as shown in  FIGS. 4C and 4D , but  FIG. 5A  only shows the ridged waveguide portion  18 A of block  18  and not the horn antenna portion  18 B of block  18  for ease of illustration.  FIGS. 5A and 5B  are also drawn as if the depicted structure were transparent for ease of illustration and understanding the internal structures of and adjacent waveguide  14 . A ridge  14   r  preferably occurs in the waveguide  14  on either side thereof projecting in an inwardly direction as can perhaps be best seen in  FIG. 4C  so that the throat of the waveguide  14  preferably assumes what might be called a figure eight configuration. The ridge  14   r  may extend all the way up the horn antenna with a more or less constant width as shown in  FIG. 4C  as opposed to decreasing to a knife edge as shown in  FIG. 4D . Likewise, the edge  21  of the horn antenna  12  may decrease to a knife edge as also shown in  FIG. 4D  or it may have a flatten surface as shown in  FIG. 4C . 
     A perspective close-up view of the detector chip  17  mounted on the substrate  16  is shown in  FIG. 6 . In this figure, the detector chip  17  and those portions of the elements on and in layer  16  are drawn in solid lines, while elements in or on the underside of chip  17  or hidden by layer  16  are shown in dotted or dashed lines. For the most part, block  20  is omitted, but openings  20   c  forming channels in block  20  are shown, and in dashed lines, to show their arrangement relative to conductors  22  defined on substrate  16 . 
     The detector chip  17  may have monolithic delay line inductors and silicon nitride capacitors (shown in dashed lines on  FIG. 6 ) for impedance matching of the detector diode in chip  17  to the transmission line  32  and the aforementioned post processing electronics. The monolithic matching circuit for the diode in chip  17  is preferably of the type disclosed in related U.S. patent application Ser. No. 12/172,481. For a particular diode chip  17 , the dimensions of the transition are preferably determined simultaneously with the dimensions of the MMIC tuning elements on the chip  17  in order to create an impedance match from the horn antenna input to the diode in the detector chip  17 . 
     The transition shown in  FIGS. 5A and 5B  preferably uses the following distributed tuning features to achieve a wideband impedance matched transformation from the horn antenna  12  to the diode in chip  17 . First, the ridged waveguide  14  is formed in metal plate or block  18 . The ridged waveguide  14  is used to expand the bandwidth over what is available for a rectangular waveguide by decreasing the cut-off frequency of the waveguide (see S. Ramo, J. R. Whinnery, and T. Van Duzer, “Field and Waves in Communications Electronics,” 1st edition, John Wiley and Sons, 1965, pp. 465-467). In  FIGS. 5A and 5B , the ridged waveguide  14  is defined by two intersecting cylinders  14   c  (although other geometric shapes could be used) formed in a metal plate or block  18 , the cylinders  14   c  intersecting each other to help to define ridges  14   r . The ridges  14   r  are elongated flat surfaces formed between the two cylinders (or other geometric shapes) near where they intersect. The two elongated flat surfaces  14   r  in the throat of the waveguide  14  are each disposed parallel to, but spaced from, a plane intersecting the centers of the two cylinders  14   c . The two intersecting cylinders  14   c  form a “figure eight” configuration in with waveguide  14 . 
     The operational frequency of the input signal to a pixel  10  and the bandwidth of the input signal to a pixel  10  as well as its impedance match to the RF pick-up probes  26  on the fused silica substrate  16  are controlled by the dimensions of the ridged waveguide  14 . The maximum bandwidth of the input signal to pixel  10  is constrained on the lower frequency end by the cutoff frequency of the ridged waveguide  12   r  and on the higher frequency end by the cutoff frequency of the next order mode (which is typically the second order mode). The reason for limiting the higher frequency end of the bandwidth is that otherwise going into the next (typically second) order mode would allow energy from a direction away from the imaged target to enter the pixel  10 . 
     For the particular embodiment shown in  FIGS. 5A and 5B , the cylinder  14   c  radii are preferably 0.5 mm and their centerline-to-centerline distance is preferably maintained at 1.0 mm. A preferably 0.4 mm gap forms the ridges  14   r  which are located in-between the cylinders  14   c  on opposing sides of the waveguide  14  facing one another. This approach to creating ridged waveguide  14  in electrically conductive plate or block  18  should facilitate machining of the ridged waveguide  14  with standard metal working tools when block or plate  18  is made of a metal. Alternatively, block  18  and  20  could be injection molded out of a plastic material and then metal coated—see the related copending U.S. Patent application referred to above. Second, a waveguide cavity  24  is also used to help tune the transition to the detector diode chip  17 . Waveguide cavity  24  is formed in electrically conductive plate or block  20 . For the particular embodiment of  FIGS. 5A and 5B , the waveguide cavity  24  dimensions are preferably 1.85 mm×1.0 mm×0.7 mm. Finally, the fused silica substrate  16  is disposed between the electrically conductive plate or block  18  containing the ridged waveguide  14  and the electrically conductive plate or block  20  containing the back-short waveguide cavity  24 . In order to prevent RF losses by parallel plate electromagnetic modes within the substrate  16 , conductive via posts  19  are preferably located around the cavity/ridged waveguide. These conductive via posts  19  may be fabricated using known thin film processing techniques (see, for example American Technical Ceramics “Thin Film Products Guideline,” at www.atceramics.com/products/thinfilm.asp). 
     The arrangement and design of the detector diode chip  17  is depicted and described in greater detail in the above-mentioned U.S. patent application Ser. No. 12/172,481 filed 14 Jul. 2008. The diode attachment and RF probe metallization of the detector diode chip  17  is disposed on the side of the substrate  16  facing the back-short cavity  25 . No metal RF signal connection is needed from the side of the substrate  16  attached to the ridged waveguide  14  to the side of the substrate  16  attached to the back-metal cavity  24 . Posts  19  tie the ground planes on both sides of the substrate and prevent spurious substrate modes. The details of the RF probes  26  of this embodiment is best shown in  FIGS. 5B and 6 . The two RF probes  26  are disposed symmetrically on the substrate  16  and receive the millimeter wave signal from the ridged waveguide  14  and also serve as differential terminals for the video output lines  22 . In the related U.S. Patent Application referred to above, the video lines come off a capacitor on the chip  17 . The technique shown here improves the bandwidth of the antenna. The dimensions of each RF probe  26  are adjusted for the optimum impedance match looking in from the ridged waveguide  16 —this adjustment can be made using software such as Ansoft HFSS® to simulate the transition  10 . Each probe  26  in this embodiment is preferably 0.265 mm long, 0.25 mm wide near the chip  17 , and 0.3 mm wide near the edge of cavity  24 . Slots  26   s  cut into each probe  26  form a short slotted transmission line that serves as an RF choke. The length of the slotted line is optimized for maximum isolation between the RF and video signals along the video output lines  22  preferably by using the simulation software noted above. The video output signal supplied to lines  22  originates on the diode chip  17  (see  FIG. 6 ) and is transmitted on conductors  26   c  and via the probes  26  and thence to the video output connection pads  32 ′ via conductors  22 , as shown in  FIG. 4B . Conductors  22  are preferably arranged to pass through the back-side block  20  using channels  20   c  (see  FIGS. 4A and 6 ) cut or otherwise formed in the back-side block  20 . The video lines  32  are preferably 0.025 mm wide and the channel  20   c  dimensions in the back metal block  20  are preferably 0.08 mm×0.1 mm. This channel  20   c  is preferably sized to be too small for the millimeter wave signal of interest to propagate therethrough and therefore contributes to the RF isolation of the video output lines  22 , yet each channel  20   c  is big enough to accommodate one of the conductors  22 . 
     This structure was simulated using Ansoft HFSS® for a Sb-heterostructure diode in chip  17  that was 0.8 μm×0.8 μm in diameter (see the above-mentioned U.S. patent application Ser. No. 12/172,481 filed 14 Jul. 2008). The reflection from the transition  10  looking in from the waveguide is shown in the graph of  FIG. 7   a , and the RF isolation from the video output line is shown in the graph of  FIG. 7   b . The −5 dB bandwidth is 45 GHz, and an effective bandwidth (see J. Lynch, H. Moyer, J. Schulman, P. Lawyer, R. Bowen, J. Schaffner, D. Choudhury, J. Foschaar, and D. Chow, “Unamplified Direct Detection Sensor for Passive Millimeter Wave Imaging,” Proc. Of SPIE on Passive Millimeter-Wave Imaging Technology, eds. R. Appleby and D. Wilkner, Vol. 6211, 2006) of 68.4 GHz for an average detector sensitivity of about 7000 V/W over the frequency band from 70 GHz to 140 GHz. The diode was modeled as a parallel combination of junction resistance of 1300Ω and junction capacitance of 8 fF. A parasitic series resistance was also included in the diode model of 25Ω. Monolithic delay line inductors and capacitors were used to match the diode chip to the transition as taught by related U.S. patent application Ser. No. 12/172,481; for this particular design the series delay line were 0.06 mm long, the parallel serpentine line had 0.02 mm ripples, and the parallel capacitor area was 0.055 mm×0.055 mm with a 1900 Å layer of SiN. 
     Another embodiment of the wideband transition is fabricated with an alumina substrate  16  is shown in  FIG. 8 . In this embodiment, the transition uses an alumina substrate  16 , preferably 0.1 mm thick, which substrate  16  has two openings  16   o  therein to account for the higher dielectric constant of alumina as compared to the fused silica shown in the embodiment of  FIG. 4B . Electromagnetic simulation of the transition on a solid slab of alumina revealed spurious in-band resonances in the frequency dependent reflection coefficient. This was caused by the high dielectric constant of alumina, which is 9.8, that makes the substrate  16  appear electrically larger than that of the fused silica substrate  16  (which has a dielectric constant of 3.8) of the first described embodiment. This problem is solved in this latter embodiment by removing much of the alumina substrate  16  in way of the waveguide  14  of transition  10  using standard commercial processes, such as laser drilling (see, for example American Technical Ceramics “Thin Film Products Guideline,” at www.atceramics.com/products/thinfilm.asp). Compare  FIG. 8  with  FIG. 4B  noting the openings  16   o  which occur in the alumina embodiment of  FIG. 8  and not the fused silica embodiment of  FIG. 4B . 
     In  FIG. 8 , the detector diode chip  17  and transition probes  26  are located on a bridge  16   b  of alumina, as shown in  FIG. 8 , which occurs between openings  16   o . In this figure, the width of the bridge  16   b  is preferably 0.4 mm. The RF probe  26  dimensions are the same as for earlier fused silica embodiment, however, the slots  26   s  for the RF choke need not be as long because of the higher dielectric constant of alumina compared to fused silica. All other waveguide and cavity dimensions are preferably the same as in the earlier fused silica embodiment. The simulated reflection coefficient as seen from the ridged waveguide and video line RF isolation is shown in  FIG. 9   a . For this case the detector model was again the 0.8 μm×0.8 μm diode, with monolithic diode matching elements of 0.08 mm for the series delay line inductors, 0.02 mm for the parallel delay line inductors, and a capacitor size of 0.05 mm×0.05 mm. The −5 dB bandwidth is 45 GHz and the effective bandwidth is 62.4 GHz.  FIG. 9   b  is a graph of the RF isolation from the video output line according to a computer simulation of this second embodiment. 
     The openings  16   o  in the embodiment of  FIG. 8  are depicted as being rectangular, but any convenient and preferably geometric shape will likely serve the intended purpose of reducing the bulk dielectric constant of substrate  16 . 
     It should be understood that the above-described embodiments are merely some possible examples of implementations of the presently disclosed technology, set forth for a clearer understanding of the principles of this disclosure. Many variations and modifications may be made to the above-described embodiments of the invention without departing substantially from the principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.