The Instruments for Noise Spectroscopy

Abstract

The article describes basic study of broadband noise signal application in the investigation of materials. The aim is flnd a metrology method utilizable for the research on metamaterials in the frequency range of about 100MHz to 10GHz. The instrumental equipment and other requirements are presented. This research report provides an overview of the current potentialities in the described fleld and summarizes the aspects necessary for noise spectroscopy. In the complex investigation of material structures for the micro-wave application (tensor and composite character), the properties of materials are studied by means of the classic single-frequency methods, which bring about certain di-culties in the process (1). In boundary changes with a size close to the wave-length there can occur wrong information concerning the examined objects (2,3). One of the possible ways of suppressing the negative sources of signals consists in the use of wide- band signals like white noise, and in researching into the problem of absorption in the examined material (4). These methods require a source of noise, a receiving and a transmitting antenna, and A/D conversion featuring a large bandwidth; for our purposes, the bandwidth ranged between 0Hz and 10GHz. Until recently it had not been possible to realize an A/D converter of the described speed, or devices with the above-mentioned bandwidth. Currently, high-end oscilloscopes are available with a sampling frequency of tens of Gsa/s. 2. NOISE SOURCE For UWB systems, several methods of the generation of short pulses with large bandwidth have been developed to date (5). However, these singly-iterative processes are not applicable for noise spectroscopy; in this respect, there is a need of a continuous source of noise signal (ideally of white noise) with the given bandwidth. The type of source referred to is currently being produced by certain manufacturers specialized in this fleld. Importantly, for the noise spectroscopy application we require a comparatively large output power of up to 0dB/mW; the assumed bandwidth char- acteristics range up to 10GHz. Nevertheless, at this point it is appropriate to mention the fact that there occurs the fundamental problem of flnding active devices capable of performing signal ampliflcation at this kind of high frequencies. As a matter of fact, our requirements are thus limited by the current status of technology used in the production of commercially available devices; the highest-ranking solution for the bandwidth of up to 10GHz can be found only up to the maximum of 0dB/mW. Our response to the above-discussed problem consisted in an attempt to produce a noise gener- ator in laboratory conditions as, in principle, this type of generator can be considered as su-cient for testing and basic measurement. In view of the price and availability of noise diodes we decided to apply thermal noise on electrical resistance as the basic source of noise. The speciflc connection is shown in Figure 1. The flrst transistor is in the CC conflguration, where we require mainly a high input impedance of the amplifler. The thermal noise at the input is given by its input parameters. The generator could operate even without a resistor at the transistor input, yet the unconnected input would cause a substantial deterioration of the stability. The second and the third transis- tors form a cascade voltage amplifler in the CE conflguration. The output impedance of the third amplifler is 50› for its matching to coaxial line. Figure 2 shows the realization of the tested noise generator; the BFP620 vf transistors were applied. This type of transistor features the characteristic of ft = 65GHz and the maximum stable ampliflcation of 11dB at the frequency of 6GHz. The overall ampliflcation of the two CE ampliflers in cascade for the output power of 0dB/mW would have to approximate the value of 10000, and there is no hf transistor available for this kind of stable ampliflcation. Therefore, for the stable noise generator we have to accept a lower output power.