Wideband Noise Envelope-Transient Simulation in Radiometers operating with multiple time-scales J. P. PASCUAL (1) ,F. J. CASAS (2) , M. L. DE LA FUENTE (1) , B. AJA (1) , E. ARTAL (1) (1) Dpto. de Ingeniería de Comunicaciones. Universidad de Cantabria. Avenida los Castros s/n, 39005, Santander, Spain. (2) Instituto de Física de Cantabria, CSIC-Universidad de Cantabria, Avenida los Castros s/n, 39005, Santander, Spain. http://www.unican.es/ Abstract: - A radiometer simulation procedure with special emphasis on high frequencies and switched system performance is discussed. The whole system simulation is implemented in Envelope-Transient, a combination of frequency domain and time domain, on the same software platform which is used to design separate microwave components, allowing the use of measured S parameters and measured current-voltage characteristics of the detectors. 1/f gain fluctuations are introduced in the simulations and switched differential operation is implemented and validated as a way to minimize their impact. Low frequencies are scaled-up to allow the simulator a reasonable CPU time and memory handling without losing generality. Coupling between a base-band equivalent solution and a complete mm-wave solution is provided by the proposed simulation, providing a step beyond typical separate analysis of RF parts and base-band equivalent in this kind of systems. Key-Words: - Radiometers, Envelope-Transient, 1/f noise, wideband circuits and systems 1 Introduction Radiometers employ RF and microwave technology extensively to provide remote sensing data like content of the soil, salinity of the oceans and astronomical data for scientific models of the universe. The level of signals received by radiometers (basically broadband noise) is extremely low, which makes low noise operation and stability of the radiometric system basic requirements which must be properly simulated. 1.1 30 GHz PLANCK radiometer Simulation procedures for radiometers will be presented, with application to the 30 GHz Planck Mission radiometer. There are different types of radiometers for radio-astronomy which respond to a common principle of operation [1]. The basic one is called total power radiometer and most other types are variations of it to improve some features. This work focuses on the referred 30 GHz radiometers of the Planck Mission, which is a mission of the European Space Agency (ESA) Science Program [2], to perform astronomical investigations in the sub millimeter and millimeter wave range. The mission will produce calibrated maps of the whole sky with high sensitivity. Planck mission will characterize small fluctuations in the Cosmic Microwave Background by mapping its anisotropies. The Planck Low Frequency Instrument (LFI) receiver is a form of differential radiometer that minimizes 1/f noise due to low frequency gain and noise fluctuations. The Planck 30 GHz radiometers consist of Front End Modules (FEM) formed by cryogenic amplifiers cooled at 20 K and Back End Modules (BEM) with amplifiers, filters and detectors at room temperature (300 K) (figure 1). Detector diodes Cold LNAs Ref. Load Warm LNAs Phase switches Hybrid Hybrid φ Sky φ Bandpass filters DC Amp FEM BEM Out1 Out2 Fig. 1. Block diagram of a Planck radiometer The radiometers operate as direct detectors in which the signal is first sufficiently amplified in the band of interest, band-pass filtered with 20% effective bandwidth (around 6 GHz), not -3 dB bandwidth, and then detected with a diode. Stability of the amplification chain (in gain and noise temperature) is desired. Differential operation is required for a good suppression of systematic errors. Also BEM amplifiers behavior in terms of 1/f noise is important for this suppression. One input is connected to a 4 K reference load; the other input comes from a feed horn receiving the sky signal (2.7 K). The hybrid couplers act as a divider and a combiner respectively, in a balanced structure containing two branches with 8th WSEAS International Conference on SIMULATION, MODELLING and OPTIMIZATION (SMO '08) Santander, Cantabria, Spain, September 23-25, 2008 ISSN: 1790-2769 346 ISBN: 978-960-474-007-9