Medichats 14 October 2008 ADC bit number and input power needed, in new radio-astronomical applications View of the Medicina Radiotelescopes - Italy Eng. Bianchi Germano IRA – RadioAstronomy Institute INAF – National Institute for AstroPhysics

THE SKA PROJECT (Square Kilometer Array) 5 Km 700 mt

THE SKA PROJECT: features High sensitivity High resolution Total area = 1Km2 (sensitivity > 100 time the VLA) Large FOV (Field Of View) Multiuser and Multitasking Frequency coverage = 0.1-25 GHz 4 GHz instantaneous band Total cost = 1.5 billion €

RF spectrum scenario SKA band We have studied a procedure to estimate the required number of bit (resolution and dynamic) and the ADC input power level in radio astronomical applications.

(Basic Elements for SKA Training) BEST PROJECT (Basic Elements for SKA Training) The experiences gained with the BEST demonstrator is very suitable for the SKA community: in the final configuration it will have about 8000 m2 of collective area, a value comparable with the area of a proposed SKA station (about 10000 m2). A- BEST-1: One single N/S antenna - Ag=176 m2  Aeff=125 m2 - Band: 16MHz @ 408MHz - 4 RX installed BEST-2 BEST-1 B- BEST-2: 8 N/S antennas - Ag=1410 m2  Aeff=1000 m2 - Band: 16MHz @ 408MHz - 32 RX installed C- BEST-3: 14 N/S antennas + 6 focal lines on the E/W arm. - Ag=7300 m2  Aeff=5100 m2 - Band: 16MHz @ 408MHz - 80 Rx installed

RF spectrum scenario at the Medicina site Radio relay stations Radio relay stations 16 MHz Scientific stratospheric balloon 4 MHz astronomical protected band

BEST PROJECT ARCHITECTURE Front end (16 MHz @ 408 MHz + Optical Tx) Which ADC is more suitable? Optical RX A DIGITAL BACK-END A D Fiber Optic Cable LO Receiver room 7

RFI Measurement campaign 22 mt

Dynamic range estimation 0dBi the power level referred to an isotropic antenna (unitary gain in all directions). The equivalent system input noise power for 16 MHz bandwidth, in a single N-S antenna, is: The maximum dynamic range result:

Pd = 37.8 dB  How many bit correspond? Number of bit Pd = 37.8 dB  How many bit correspond? Since an ADC converts voltage into bit and not power into bit, we need a relationship between the power and the voltage at the input of the A/D converter. If we consider the simplest possible situation, where there is only a monochromatic tone at the input of the ADC, we can easily find the relationship: VP = voltage peak (Volt) P = input power (dBm) From this equation, each bit corresponds to an increment of 3dB in voltage and 6dB in power. If we divide the dynamic range Pd by 6, we can obtain the required ADC number of bits. 7 bit The more the RF scenario is dominated by a strong signal, the more accurate the previous relationship is.

10 bit 7 bit for RFIs 3 bit for the astronomical signal P AD6645 -69.4 dBm Strongest RFIs level 37.8 dB (7 bit) -107.2 dBm Input noise level (KTsysB) 3 bit AD6645 14 bit (ENOB = 12 bit @ 100MSPS) 107.2 dBm ? WHICH GAIN? ADC 50 

G = 54.2 dB P P - 53 dBm 107.2 dBm -69.4 dBm -53 dBm -69.4 dBm Strongest RFIs level - 53 dBm -53 dBm Input ADC level -69.4 dBm Strongest RFIs level G = 54.2 dB -107.2 dBm Input noise level (KTsysB) 3 bit 107.2 dBm ADC AD6645 50  ENOB = 12 bit G = 54.2 dB VIN-PP = 2.2 V

Measurement bank POWER METER SPECTRUM ANALYSER ANTENNA RECEIVERS POWER SUPPLY LOGIC ANALYSER AND ADC CONFIGURATION PROGRAM LABVIEW PROGRAM ADC 14 bit, 100 MSPS

Results scientific stratospheric balloon radio relay stations

Results 6 bits 3 bit for RFIs 3 bit for the astronomical signal This measurement phase ran for few weeks to achieve data with the antenna pointed in all the directions. From the measures we performed, only 3 bit seem to be necessary to sustain the man made radio signals, so the ADC total bit required is 6: 3 bit for RFIs 3 bit for the astronomical signal 6 bits Following these considerations, an 8 bit A/D converter could work properly.

BEST Back End ibob Bee 2 4 x A/D @ 8 bit 5x Xilinx Virtex-2 Pro 70 (Serializer) RX1 RX2 RX n-1 RXn Dual 1GS/sec @ 8 bit Infiniband CX4 Cables FPGA 1 FPGA 2 FPGA-3 FPGA-4 FPGA-5 Bee 2 5x Xilinx Virtex-2 Pro 70 4 x A/D @ 8 bit (1.0 GS/sec) ibob Finally the signal arrives to the back end. As back end we use the BEE2 system developed by the Berkeley University for the Allen Telescope Array, the US SKA demonstrator based on small dish with single wide band feed.

Estimated number of bit = 10 Number of bit measured = 6 Conclusion Estimated number of bit = 10 Number of bit measured = 6 We attribute the difference between the estimated bit number and the measured one to the different antenna systems: Yagi antennas pointed towards the horizon and working in the max-hold mode. VERSUS A half wavelength dipole focal line inside a cylindrical reflector pointed towards the sky. We have concluded that our estimation method to valuate the number of bits is conservative, but applicable to radio astronomical scenario. Further investigations should be performed to reduce the difference between the estimated and actual requested number of bit.