INFRARED SCENE PROJECTOR WITH CURRENT-MIRROR CONTROL ELECTRONICS Technical Field The present invention concerns image or scene projection arrays, particularly infrared scene projectors and related control electronics.
Background of the Invention Analogous to digital slide projectors, scene projectors are devices that generate images based on input electrical signals. Infrared scene projectors, a particular class of scene projectors, generate images in the infrared portion of the electromagnetic energy spectrum. These type scene projectors are useful in testing infrared surveillance and guidance equipment which identify and track objects, such as enemy missiles, based on their infrared emissions or heat-generation characteristics.
Infrared scene projectors typically include a thermal pixel array which converts electrical input signals into a desired thermal or infrared image. The thermal pixel array is typically a rectangular array of hundreds of interconnected infrared pixels, with each pixel having a unique row and column position in the array. Each pixel, which includes a pixel control transistor and a resistor coupled between positive and ground voltage terminals, converts an electrical input voltage into a corresponding amount of heat, or thermal energy, based on the magnitude of the input voltage. Thus, by precisely controlling the input voltage, one controls the infrared emission of each pixel and thus defines an infrared image with desired characteristics.
One problem with conventional infrared scene projectors concerns how precisely one can specify the infrared emission at each pixel in typical thermal pixel arrays. In particular, thermal pixels arrays operate with large electric currents. As these large currents flow through the ground terminal (ground bus) which is distributed throughout the array, large voltages (based on the electrical resistance of the ground terminal and the current magnitude) develop at various points along the ground terminal, meaning that the ideally constant ground voltage actually varies from point to point on the ground terminal and therefore from pixel to pixel. Thus, each pixel has a unique ground voltage based on its point of connection to the ground terminal. This means, for example, that applying the same pixel control voltage to two pixels in the array would not produce two identical infrared emissions as desired, but would instead produce two significantly different infrared emissions based on the unique ground voltages of the pixels.
Conventionally, there have been two approaches for addressing this problem.
One approach entails estimating the ground voltage at each pixel and adjusting the pixel control voltage for each pixel to account for its specific ground voltage. The other approach entails biasing each pixel control transistor to operate in its saturation region and minimizing resistance of the ground bus, so that the pixels are less sensitive to ground voltage. However, neither approach has been completely successful in overcoming the ground-variance problem, since with identical pixel control voltages, pixel brightness stills varies significantly across infrared pixel arrays.
Accordingly, there remains a need for better solutions to the ground-variance problem in infrared pixel arrays.
Summary of Invention To address this and other problems,. the present inventors devised not only innovative infrared pixel arrays and related control methods, but also infrared scene projector incorporating them. One exemplary infrared pixel array includes two or more pixels, with each pixel including a resistor and its own current-mirror circuit.
A related control method entails receiving a pixel-control voltage, converting the pixel-control voltage to a current, and then coupling the current to the current-mirror circuit in one of the pixels. The current mirror applies an appropriate control current to the resistor and thus enables operation of the pixel independent of ground-voltage variations.
Brief Description of the Drawings Figure 1 is a partial schematic diagram of an exemplary thermal pixel array 100 incorporating teachings of the present invention ; and Figure 2 is a block diagram of an exemplary infrared scene projector 200 incorporating teachings of the present invention.
Description of the Preferred Embodiments The following detailed description, which references and incorporates Figures 1 and 2, describes and illustrates one or more specific embodiments of the invention. These embodiments, offered not to limit but only to exemplify and teach, are shown and described in sufficient detail to enable those skilled in the art to implement or practice the invention. Thus, where appropriate to avoid obscuring the invention, the description may omit certain information known to those of skill in the art.
Figure 1 shows a representative portion of an exemplary thermal pixel array 100 incorporating teachings of the present invention. (For clarity, the figure omits conventional features, such as row-select logic, column-select logic, timing-and- control circuitry, and analog-to-digital converters.) In addition to respective supply nodes 102 and 104 and a pixel-control-voltage input, a row-select input, and a pair of complementary column-select inputs, array 100 includes at least one input voltage-to-current converter 110, at least one current router 120, at least one pixel circuit 130.
In general operation, voltage-to-current converter 110 receives a pixel- control voltage at the pixel-control-voltage input and outputs a pixel-control current Icontrol, based on the pixel-control voltage to current router 120. Current router 120, which is located outside of pixel circuit 130 in the exemplary embodiment, routes current Icontrol, or more precisely a control current Icontrol'derived from current Icontrol, to a particular pixel circuit, such as pixel circuit 130. Pixel circuit 130 receives the current Icontrol'and produces a corresponding thermal emission, which is independent of the ground voltage at the pixel. (In some embodiments, two or more pixel circuits share the same address and are operated currently based on the same control voltage and control current.) More particularly, voltage-to-current converter 110 includes two n-channel mosfets (metal-oxide-semiconductor field-effect transistors) 112 and 114, each having three nodes or regions: a control region, called a gate, and two controlled regions, one called a source and the other called a drain. (This description refers to specific nodes or regions of transistor using the name of the node followed, by the reference numeral of the transistor itself. For example, mosfet 112 includes a gate 112, a source 112, and a drain 112. For clarity, the drawings omits these additional reference numbers.) Mosfets 112 and 114, both of which have channels with length- to-width ratios of 10: 20, are connected in parallel, that is, with gates 112 and 114 connected together, drains 112 and 114 connected together, and sources 112 and 114 t connected together. Sources 112 and 114 are additionally connected to supply node 104. The connection of gates 112 and 114 forms the pixel-control-voltage input, and the connection of drains 112 and 114 forms a converter output through. which the converter outputs pixel-control current Icontrol to current router 120.
Current router 120 exemplarily includes three p-channel mosfets 122,124, and 126 and a transmission gate 128. Mosfets 122,124, and 126, all of which have : channels with an exemplary length-to-width ratio of 60: 5, constitute a two-to-one current mirror, with sources 122,124, and 126 connected to positive supply'node 102, and with gates 122 and 124 connected together and to drains 122 and 124.
(Though the exemplary embodiment uses a two-to-one current mirror, other embodiments use current mirror with lower or higher gains.) Gate 126 connects to drain 124, and drain 126 connects to an input 128a of transmission gate 128.
Transmission gate 128 has complementary control nodes 128b and 128c which are coupled respectively to the negative column-select input and the positive column- select input and an output 128d, which with application of appropriate signals at the column-select inputs, communicates the scaled version of current Icontrol---current Icontrol'---to pixel 130.
Pixel 130 includes n-channel mosfets 131,132,133, and 134, a capacitor 135, and an emitter resistor 136. Mosfets 131 and 133 have channels with an exemplary ratio of 10: 2; mosfets 132 and 134 have channels with respective ratios of 10: 20 and 7: 5. Drain 131, which serves as an input of pixel 130, connects to output 128d of transmission gate 128 to receive current Icontrol'. Gate 131, connects to the row-select input, and source 131 connects to gate 132, to drame32, and to drain 133. Source 132 connects to ground node 104; gate 133connectsitothcr row-select input via gate 131. Drain 133 is connected to gate 134 and to a first capacitor node 135a of capacitor 135, which is a one-pico-farad capacitor im-the X exemplary embodiment. Mosfet 134 has its source connected to a second capacitor node 135b, and its drain connected to positive supply node 102 via emittr resistor 136.
In specific operation, pixel 130 receives current Icontrol'at drain 131. An ! active or high voltage at the row-select input, which is connected to gates 131 and 132, turns on mosfets 131 and 133, which function as switches. In response, mosfet 131 conducts current Icontrol'to mosfet'132, a diode-connected transistor, 2and mosfet 133 couples drain 132 to capacitor node 135a and to gate 134. Mosfet 132 conducts current Icontrol'and, produces a corresponding control voltage across. capacitor 135. Mosfet 132 and 134sform, a current mirror. Thus, mosfet 13t he pixel-control transistor, draws current Icontrol'through emitter resistor 136 which' produces a corresponding thermal emission, which is independent of the ground voltage at the pixel. The control voltage across capacitor 135 charges the capacitor, enabling mosfet 134 to continue conducting and resistor 136 to continue emitting for a period of time after deasserting the row-select input.' Notably, the exemplary embodiment includes two current mirror circuits, one within current router 120 and the other within pixel 130. Some other embodiments, however, may use one current mirror circuit, with the input transistor of the current mirror located outside the array and the output transistor, that is, the pixel control transistor located within the array. This alternative arrangement is less desirable than the exemplary embodiment, because it not only entails the lower- precision operation of routing voltages rather than current into the pixel, but also produces thermal emissions that are still dependent on local ground voltage, since the routed voltage : is still. referenced to ground. Additionally, splitting the input and output transistors of thelcurrent rnirror introduces a greater degree of mismatching.
':.Figure2:shQWS)anex'emplaryihn'aredscene'projector 200. Projector 200 includes a.se:ene-gcnera.tipniunit,"a:thermalpixel-array 220, and an optic unit 230.
Scene-generation unit 220 includes, for example, one or more conventional processors, scene-generation ! programs,'scene-definition iiles, analog-to-digital converters, and interface and control logic, for producing pixel-array control data 212...:Inthe.exemplary embodiment, control data 212 includes one or more pixel addresses and pixel-control voltages for controlling operation of thermal pixel array <BR> <BR> <BR> <BR> 220.
Array 220 incorporates one of more teachings of exemplary pixel array 100.
In the exemplary, ernbodiment,-array 220 : includes one or more 128x128 arrays, with . each array including eight, voltage-to-current converters 110, eight current routers .120,and:assQciated'multiplexers to address. and route current to 16,384 pixel circuits . 130.. However,. Qther. embodiments include other numbers of smaller or larger arrays. In response to control data 212, array 220 generates a first thermal image in tthe form of thermal, emissions 222. Thermal emisstons 222 are transformed via optics unit 230 to an output thermal image in the form of thermal emissions 232.
Conclusion In furtherance of the art, the inventors have presented not only innovative infrared pixel arrays and related control methods, but also an infrared scene projector incorporating them. The exemplary infrared pixel array includes two or more pixels, with each pixel including a resistor and its own current-mirror circuit.
A related control method entails receiving a pixel-control voltage, converting the pixel-control voltage to a current, and then coupling the current to the current-mirror circuit in one of the pixels. The current mirror applies an appropriate control current to the resistor and thus enables operation of the pixel independent of ground-voltage variations.
The embodiments described above are intended only to, illustrate and teach one or more ways of practicing or implementing the present invention, not to restrict its breadth or scope. The scope of the invention intended to encompass all ways of practicing or implementing the principles of the invention, is defined only by the following claims and their equivalents.
