Xcube Linear2Circular Chris Phillips 15 October 2013 CASS Xcube Wideband VLBI Backend X-­‐cube Data Logger • 100% COTS hardware recorder • Proprietary so<ware • 16 Gbps capability • External replaceable storage modules – Up to 32 TB packs • Management of slow or broken disks • Standard file-­‐systems Back View of Chassis SAS Disk Cables (16 x 300MB/s) Ethernet (2 x 10Gb/s) © 2010 XCube R&D Self Contained Disk Pack Cooling and Power Xcube Internals • “Mix and Match” approach to plug in devices • 1 or 2 dual 10 Gbps Ethernet – Intel SPP+ Op\cal module needed • Can support up to 4 disk modules • GPU for number crunching • Other op\ons available – However may run out of slots…. 5 | DIFX KOREA 2013 -­‐ Chris Phillips Data access • Data cannot be accessed directly • Cannot directly use standard transfer so<ware such as <p, grid<p, tsunami-­‐udp etc • Supplied with easy to use library to read data from diskpack • Can copy “raw” files and decode on a different system • Easy to write code to copy off disk • Time filtering etc available not tested 6 | DIFX KOREA 2013 -­‐ Chris Phillips Xcube Access • Xcube supply (closed source) libraries to access data files • Distributed as part of DIFX • utilities/trunk/xcube
• Has concept of “packet reader” • Read each recorded packet in series • Each packet is (system) \me stamped • Time filtering available 7 | DIFX KOREA 2013 -­‐ Chris Phillips Xcube Terminology • Recording split into “Projects” • We would use one project per experiment • Cannot name project! • Projects split into “Streams” • Usually one stream per start/stop • E.g. Set of scans with no gap 8 | DIFX KOREA 2013 -­‐ Chris Phillips Xcube UFliFes • x3projects List projects on disk • lf1list • List first few packets of project/stream • lf1vdifcheck • Check VDIF recorded data is OK • lf1extract • Extract VDIF recorded data to file (with filtering) • hr<2vdif, hr<Stats • Xcube HRFT sampler specific 9 | DIFX KOREA 2013 -­‐ Chris Phillips Xcube sampler (HRFT) • 4 Channel 1 Gsample/sec • 4x512 MHz IFs • For each IF 16 channel polyphase filterbank • 32 MHz channels, complex “double sideband” sampling • Re-­‐quan\za\on to 2 bit • 8 Gbps VLBI 10 | DIFX KOREA 2013 -­‐ Chris Phillips HRFT data • HRFT data is not VLBI formaped • Mark5B bit encoding • No explicit \me encoding • Channel order not similar to any VLBI format • Synchronized to 1PPS • To convert to VDIF need to • Flip bits • Reorder channels • Add VDIF header • DIFX integra\on? 11 | DIFX KOREA 2013 -­‐ Chris Phillips Linear 2 Circular Linear PolarizaFons • While VLBI standard is to use circular polariza\ons, linear polariza\ons are “beper”: • Easier to make wideband receivers • Beper separa\on between polariza\ons • Need to convert linear to circular for compa\bility with other antennas and calibra\on and analysis so<ware • RF and IF hybrids typically narrow band 13 | VLBI TECHNOLOGY KOREA -­‐ Chris Phillips Digital PolarizaFon Conversion • Digital polariza\on conversion can overcome wideband problem • Has worked very well with ATCA for 20 years • Conceptually easy: • Align amplitude, phase and delay between two polariza\ons • Rotate phase of one polariza\on by 90 degrees • Simply add and subtract two linear polariza\ons • Essen\ally same as hybrids • Ideally added to VLBI DAS firmware but never implemented 14 | VLBI TECHNOLOGY KOREA -­‐ Chris Phillips L2C in DiFX • Can we do circular conversion within DIFX • Very “cheap” to do conversion a<er FFT in correlator • Biggest DIFX change is to get calibra\on values into correlator • Side benefit is polariza\on dependent delay offset 15 | VLBI TECHNOLOGY KOREA -­‐ Chris Phillips FFT FFT 16 | VLBI TECHNOLOGY KOREA -­‐ Chris Phillips Xmult Amplitude & Phase Correc\on Add and subtract Xmult Vex2difx Changes • clockOffset, clockRate, deltaClock & deltaClockRate simple arithme\c parsing
clockOffset = -45.9-0.0043 • freqClockOffs encodes polariza\on specific delay and phase offsets IFdelay:Polarisa\onDelay:Polarisa\onPhase freqClockOff = 0:0.023:10, 0.003:0.045:-30.3
17 | DIFX KOREA 2013 -­‐ Chris Phillips Enabling Linear2Circular • Available in branches/linear2circular
• Set “PROCESSING METHOD=LINEAR2CIRCULAR” in .input file • Minor changes in mode.cpp to do conversion a<er FFT • Need slight further code change as does not support kurtosis mode 18 | DIFX KOREA 2013 -­‐ Chris Phillips Status • Delay and phase offsets seem to work OK • Not exhaus\vely tested • Could be moved to trunk • Tes\ng of L2C not promising – no apparent change in cross polarisa\on amplitudes • Could be calibra\on of offsets is wrong • Could be coding error • More tes\ng needed 19 | DIFX KOREA 2013 -­‐ Chris Phillips Complex Sampling Complex Sampling • Polyphase filterbanks naturally produce complex sampled data • Can do “direct” complex sampling also • There are, at least, 2 types of complex sampling schemes: Single Sideband and Double Sideband • Single Sideband complex support in DIFX for awhile, Double sideband in complex_dsb branch • Set encoding in .v2d file: sampling = COMPLEX_DSB
21 | DIFX KOREA 2013 -­‐ Chris Phillips Double Sideband 22 | DIFX KOREA 2013 -­‐ Chris Phillips Thank you Astronomy and Space Science Chris Phillips t +61 2 9123 4567 E [email protected] w www.csiro.au/atnf ASTRONOMY AND SPACE SCIENCE