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Unformatted text preview: Notes #7, ECE594I, Fall 2009, E.R. Brown 108 THz Receiver Architectures and the Process of Detection As discussed in the Introduction, an important aspect of all THz sensor design, and all RF systems in general, is receiver architecture. There are two types, incoherent and coherent, both shown schematically in Fig. 1. The incoherent architecture is as old as RF technology itself and the coherent came soon thereafter, dating back to the early part of the 20 th century. Like many other system architectures, they persist largely by the ability of engineers to continually improve performance by perfecting the components. In this set of notes, we will address these canonical receiver architectures, Then we will look more closely at the concept of detection, which is so important to understanding how RF systems perform. Canonical Receiver Architectures A. Direct-Detection The block diagram of a generic direct receiver is shown in Fig. 1(a). The incoming radiation from the target, be it thermal emission or transmitted power from the sensor itself, is collected by the receiver where it is rectified from RF (THz) to baseband by a direct detector. In most practical cases the baseband is defined by amplitude or frequency modulation of the incoming signal to reduce the effect of gain drifts and 1/f noise that occurs in the THz electronics. The rectified THz signal is then amplified and demodulated down to DC using synchronous detection. For AM modulation the synchronous detection is often carried out using a lock-in amplifier. In the THz region the direct detector is almost always a power-to-voltage or power-to-current converting device. That is, it is a device that puts out a voltage or current in proportion to the incoming power. There are many examples of such devices, but the most popular are field detectors and bolometers. Field detectors, such as Schottky diodes, respond directly to the THz electric field and generate an output current or voltage through a quadratic term in their current-voltage characteristic. Bolometers are composite devices consisting of a THz absorber and a thermistor. The THz absorber is generally isolated thermally from the environment so that the absorbed THz power raises the temperature both of the absorbing layer and an attached thermistor. The thermistor is, by definition, a device that displays a large change of resistance to a small change of temperature. In some bolometers, such as the composite type, the absorber and thermistor are separate elements. In other bolometers, such as the hot electron type, they are integrated into the same device. Notes #7, ECE594I, Fall 2009, E.R. Brown 109 A key factor in all direct detectors is spectral bandwidth. As in most analyses of signal processing, we assume here that this is band limited between o and o + . This can be a real bandwidth defined by a THz bandpass filter, or it might be an approximation to a real spectrum....
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This note was uploaded on 12/02/2009 for the course ECE 000 taught by Professor O during the Spring '09 term at UCSB.
- Spring '09