1 / 13

VLBI Correlators

VLBI Correlators. Radically different correlators: Mark 4 and VLBA; varieties of Mark 4! Mark 4 is a (traditional) X–F correlator VLBA is a F–X correlator X = cross correlation, F = Fourier transform Need the F to achieve a significant field of view Concentrate mainly on Mark 4. Why F?.

liesel
Download Presentation

VLBI Correlators

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. VLBI Correlators • Radically different correlators: Mark 4 and VLBA; varieties of Mark 4! • Mark 4 is a (traditional) X–F correlator • VLBA is a F–X correlator • X = cross correlation, F = Fourier transform • Need the F to achieve a significant field of view • Concentrate mainly on Mark 4

  2. Why F? • Field of view! 20 arcsec = 10^-4 rads => 4uS delay spread for baseline length 40mS. For less than 1% decorrelation, bandwidth < 20KHz! Hence need multiple spectral channels/lags. Also helps search for those 10 arcsec errors.

  3. References • Theory of Correlation in VLBI by Jon Romney, ASP Conf. Series, Vol. 82, 1995 • The EVN-Mark 4 VLBI Data Processor by Schilizzi et al, Experimental Astronomy, in press • The VLBA Correlator by John Benson, ASP Conf. Series, Vol 82, 1995 • Numerous Mark 4, JIVE and VLBA memos

  4. Complications • Delay compensation • Fringe-rate correction because delay correction is not done at RF • Correction of phase jumps because delay compensation is done in steps • Handling data known to be bad because of parity or data reframing errors

  5. Delay Compensation • Delay compensation for white-light fringes. Up to 40 mS required for terrestrial baselines • Step-wise correction by skipping or repeating samples and by tape-speed changes • Amplitude modulation of fringes down to 90% at delay rate, 2.5% loss of sensitivity • Phase jump by 90 degrees at delay step

  6. Mark 4 Correlator Chip

  7. Mark 4 Chip Continued • Delay-step and phase information decoded from serial data header • On-board phase rotator compensates both phase rate and phase jumps. Needs DSP processor help • Input data includes a “validity” bit (from parity or framing errors). Sample only used if data are valid • Need to know how many samples are valid • Validity counted lag-by-lag, globally or not at all

  8. Additional Features • Data Distributor between SU and correlator allows: • Flexible reconfiguration • Data Recirculation. By storing data in RAM, the same data can be correlated many times if correlator has the time, i.e. if bandwidth less than maximum. Each pass has different lag range • Recirculation increases spectral resolution

  9. VLBA Does Things Differently • VLBA compensates delay and phase on a station basis thus is inherently closure-phase friendly • Delay correction is a phase gradient across the spectrum • Phase correction can be applied with delay correction • Only whole frames of data can be invalid

  10. Is F-X Better Than X-F? • F-X needs fewer multiply-accumulates (MAC) • Each F-X MAC is more complicated. Needs floating point representation (11 bits) • Effects in F-X are less well understood • F-X needs zero-padding and data to be transformed in blocks • Because data are blocked, the F-X process does not recover proper cross-correlation coefficients

More Related