Jonathan J. Lynch

Passive Millimeter-Wave Imaging Module With Preamplified Zero-Bias Detection

An analytical model and supporting measured data are presented for a preamplified W-band radiometer with a zero-bias detector appropriate for commercial millimeter-wave imaging cameras. Basic radiometer parameters, including RF bandwidth, are computed directly from simple low-frequency measurements and compare well with those obtained from RF measurements. A detailed analytical model shows how radiometer performance depends on internal component parameters, such as low-noise amplifier gain, noise factor, reflection coefficient, detector responsivity, etc. The measurements suggest that performance is sufficient for operation without a Dicke switch or mechanical chopping. A measured noise equivalent temperature difference of 0.45 K was obtained, assuming a single sensor is scanned across a focal plane, forming 32 pixels with 3.125-ms integration time per pixel. This sensitivity is considered sufficient by commercial manufacturers to obtain quality images in low-contrast (e.g., indoor) environments.

A Low Noise Chipset for Passive Millimeter Wave Imaging

Technology for passive millimeter wave imaging has been maturing over the last 4-5 years. One key piece of technology that will allow for large scale production is a low cost front-end receiver. We have developed a two chip solution that addresses this need at W-band. A four stage InP LNA with a pre-matched Sb-heterostructure diode provides a low noise equivalent temperature difference (NETD). Our fabricated chipset provides sensitivities of 10,000 mV/muW over a 22 GHz noise equivalent bandwidth and an NETD of 0.8 K. To our knowledge, this is the best performance to date of a two chip solution for a passive millimeter wave radiometer.

W-Band Sb-Diode Detector MMICs for Passive Millimeter Wave Imaging

A W-band monolithic microwave integrated circuit (MMIC), including an Sb-heterostructure diode on a GaAs substrate, has been demonstrated. The MMIC also includes the RF choke and output shorting capacitor essential to detector circuits. Additional input matching has yielded peak sensitivities on the order of 10 000 V/W and equivalent bandwidths of 40 GHz. Using these circuits in conjunction with current W-band low-noise amplifier technology can achieve the sub-1degK noise equivalent temperature difference necessary for producing discernible images with W-band passive imaging cameras.

Sb-Heterostructure Low Noise W-Band Detector Diode Sensitivity Measurements

Sb-heterostructure diodes have become the detector of choice for W-band millimeter wave imaging cameras. Here we demonstrate lower impedance versions that optimize noise equivalent power (NEP). The goal is to decrease the gain required of the RF pre-amplifier, ideally to zero. Measured W-band sensitivities for three diodes are 3500, 5500, and 6000 V/W. Their zero bias differential resistance values imply Johnson noise limited NEPs of 0.98, 0.83, and 0.79 pW/Hzfrac12, respectively, much less than obtained from conventional Schottky diodes. A wideband transition from a horn antenna to the 6000 V/W detector has produced an integrated bandwidth of 30 GHz with implied temperature sensitivity (NEDeltaT) close to 10degK

Low Noise Direct Detection Sensors for Millimeter Wave Imaging

As millimeter wave electronic component technologies have matured, passive millimeter wave (mmW) imaging using direct detection is emerging as a cost effective approach to imaging through obscuring materials such as clothing for concealed weapons detection or sand for rotorcraft landing. This paper reviews the basic operation of W band direct detection receivers, and describes progress made at HRL in the development of mmW front end IC components. InP HEMT LNAs provide high gain and low noise, appropriate for high sensitivity front ends that require relatively short integration times. Sb-based MMIC detectors provide high sensitivity and low noise, relaxing the gain requirements of the LNA. Finally, promising results of the ongoing DARPA MIATA program demonstrate reasonable sensitivity of a purely passive imaging module, the first step toward imaging without the need for LNAs

Modeling polarization mode coupling in frequency-selective surfaces

An analysis of scattering from frequency-selective surfaces (FSS) is developed that correctly accounts for coupling between scattered polarization modes. This is useful for the design of FSS structures with obliquely incident radiation, such as polarization converters (polarizers) for direct broadcast satellite reception by linearly polarized ground terminal antennas. In the past, the neglect of polarization mode coupling limited the performance range of various FSS structures, such as meander-line polarizers, to near normal incidence where mode coupling is weak. The following analysis shows how a suitable pair of polarization modes can be selected to maintain modal independence. The result is a pair of independent equivalent circuits representing scattering of the two modes, together with mode transformation networks. To illustrate the theory, a four-layer meander-line polarizer is designed for an incident angle of 45/spl deg/ off normal. The resulting axial ratio is measured to be 0.3 dB at 12.45 GHz.

A wideband radiometer module for an unamplified direct detection scalable W-band imaging array

A W-band unamplified direct detection radiometer module is described that provides a wideband response and is scalable to large arrays. The radiometer design is intended to provide sufficient sensitivity for millimeter wave imaging applications with a goal of 2K noise equivalent temperature difference (NETD) at a 30 Hz frame rate. This effort leverages previously reported device scaling to increase sensitivity. We present a radiometer module designed for 60 GHz RF bandwidth that utilizes HRLs antimonide-based backward tunnel diode. An impedance matching circuit with on- and off-chip elements, as well as ridged waveguide, provides a wideband match to the detectors. Modules were designed with two different microwave substrates: 125 micron thick quartz and 100 micron thick alumina. flip-chip bonding of the detectors is amenable to automated pick-and-place for high volume manufacturing. The modular nature of the array approach allows large arrays to be manufactured in a straightforward manner. We present the design approach along with both electromagnetic simulations and measured performance of the modules. This work was supported by phase II of DARPAs MIATA program.

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The 1/f noise of a series of Sb-heterostructure diodes with varying area has been measured. Standard power law formulas for the frequency and voltage dependence were found adequate to summarize the data. An inverse dependence of voltage noise spectral density on the area was determined, consistent with the simple resistor model. Simulations using the noise formula predict that pre-amplification gain in the 30 to 35 db range can produce a sub-1degK noise equivalent temperature difference in realistic imaging cameras

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We have demonstrated a high efficiency package for zero bias Sb-based backward tunnel diodes developed for passive millimeter wave imaging. Flip-chip mounting of detector MMICs onto quartz substrates permit placement of the detector directly within the WR-10 waveguide feeds for diagonal horn antennas. This arrangement minimizes the losses between the detectors and antennas while providing an impedance match over a majority of W band. A 2x2 array of radiometers was fabricated, assembled, and measured using coherent measurement techniques. The resulting noise equivalent temperature difference, calculated assuming a 30 Hz frame rate is 10 degrees K.

Noise of Sb-Heterostructure Diodes for Pre-Amplified Detection

Sb-heterostructure diodes have become the detector of choice for W-band millimeter wave imaging cameras now being commercialized or in prototype development. Here we optimize the diode impedance to yield a minimum noise equivalent power (NEP). The goal is to decrease the gain required of the front-end LNA. Measured W-band sensitivities for two diodes are 3500 and 5500V/W. Their zero bias differential resistance values imply Johnson noise limited NEPs of 0.98 and 0.83pW/Hz1/2, respectively, much less than obtained from conventional biased Schottky diodes. A MMIC version of the diode detector has been simulated with an integrated bandwidth of ~ 30 GHz at W-band. The simulated temperature sensitivity (NEΔT) with an HRL W-band LNA on the front end is <1°K.