ROBUST RECEIVER FOR RFI MITIGATION IN RADIO ASTRONOMY Rodolphe Weber (1) , Vincent Clerc (2) , Laurent Denis (3) , Carlo Rosolen (4) (1) Laboratoire d’Electronique, Signaux, Images, Université d’Orléans, 12 Rue de Blois BP 6744,Orleans, Cedex 2 F-45067, France, E-mail : rodolphe.weber@univ-orleans.fr (2) Observatoire de Paris-Meudon, 5, Place Jules Janssen,92195 Meudon cedex, France, E-mail : vincent.clerc@obspm.fr (3) Station de radioastronomie de Nançay, 18330 Nançay, France, E-mail : laurent.denis@obs-nancay.fr (4) As (2) above, but E-mail : carlo.rosolen@obspm.fr ABSTRACT In radio astronomy, it appears that the design of receivers, which is based on the hypothesis of non-corruptive environment, is not adapted. Indeed, coarse quantization is usually applied to the signal. Unfortunately, when the interference power increases, the non-linearities induced become progressively too important and make spectral estimation completely unusable. That is why new generation of robust receivers must be designed. Two objectives have to be reached : 1) receivers must be linear enough to prevent any spread of strong interferences, 2) receivers must reach sufficient spectral resolution to extract radio astronomical information between interference lines. INTRODUCTION As a consequence of the development of telecommunications radio astronomers have to deal with an increasing number of unworkable observations polluted by man-made radio interferences. This issue gets worse in the decameter wavelengths where ionospheric effects are sensitive. In this framework, many efforts are currently put into improving RFI mitigation algorithms. Time properties, frequency properties and/or spatial properties are considered in order to find efficient excision processing techniques (see for example [2], [3]). However, it appears that the design of current receivers, which is based on the hypothesis of non-corruptive environment, is not adapted. Indeed, coarse quantization (a few bits) is usually applied to the signal. Unfortunately, when the interference power increases, the non-linearities induced become progressively too important and make spectral estimation completely unusable [1]. That is why new generation of robust receivers must be designed [4]. Two objectives have to be reached : 1) receivers must be linear enough to prevent any spread of strong interferences, 2) receivers must reach sufficient spectral resolution to extract radio astronomical information between inter- ference lines. These two points have been corroborate by experimental measurements on site [5]. DESCRIPTION In our design (see Fig. 1), height radio telescopes or antennas can be connected to the robust receiver (RR). Optical fibers links are used to send analog signal from antennas to the RR. A non-blocking matrix is used to configure the RR and to share, if needed, its computing power. Then, analog down conversion is applied to shift a 100 MHz bandwidth from 300 MHz to 70 MHz. A final anti aliasing filter limits the useful bandwidth to 14 MHz. At this point, each signal is digitized with a 14 bits ADC. The digital signal processing is based on PCI boards supporting HERON modules (HEPC 9 from HUNT Engineering). The down conversion of the 14 MHz bandwidth at 70 MHz (IF) to base band is achieved digitally in two steps (see Fig. 2). First, an undersampling is applied with a 56 MHz sampling frequency. Then, a direct digital synthesizer (DDS) following by successive decimation filters selects the band of interest. The decimation filters have been optimized to both minimize the hardware and maximize the frequency selectivity. Thus, five half-band filters have been implemented to process the decimation. A final selective FIR filter with 83 coefficients (17 bits) ends the processing (see Fig. 3 and Fig. 4). This design has been fitted in a VIRTEX II 1000.