VLBI Correlators

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VLBI Correlators

• Radically different correlators Mark 4 and VLBA
  varieties of Mark 4!
• Mark 4 is a (traditional) XF correlator
• VLBA is a FX correlator
• X cross correlation, F Fourier transform
• Need the F to achieve a significant field of view
• Concentrate mainly on Mark 4

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Why F?

• Field of view!
• 20 arcsec 10-4 rads gt 4uS delay spread for
  baseline length 40mS.
• For less than 1 decorrelation, bandwidth lt
  20KHz!
• Hence need multiple spectral channels/lags.
• Also helps search for those 10 arcsec errors.

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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

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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

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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

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Mark 4 Correlator Chip
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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

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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

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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

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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

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