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Data Acquisition

Data Acquisition. From receivers to correlator Data Acquisition System Recorder Station Unit – DAS2 Correlator Key Features: Digital, KISS recording Longitudinal, X-track/pass/head, mux, barrel-roll. Samplers/ADCs. Sample at Nyquist rate: at twice maximum frequency in the band.

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Data Acquisition

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  1. Data Acquisition • From receivers to correlator • Data Acquisition System • Recorder • Station Unit – DAS2 • Correlator • Key Features: • Digital, KISS recording • Longitudinal, X-track/pass/head, mux, barrel-roll

  2. Samplers/ADCs • Sample at Nyquist rate: at twice maximum frequency in the band. • Digitise samples to 1-bit or 2-bit precision • Accept 36% or 13% loss of sensitivity. • (Be prepared to accept more losses when data are resampled after phase rotation)

  3. Data Multiplexing • Until VLBA, one channel = one track • Can multiplex (mux) bitstreams N to 1 (fan in) • Or demux 1 to N bitstreams (fan out) • For faster or slower playback with • Increased efficiency or • Enhanced spectral resolution respectively. • NOTE: N = 1, 2 or 4, maybe.

  4. Barrel-Rolling • Tracks can be swapped repetitively • Principle is one of shared misery • All suffer, not just one • But have to keep “track” of tracks and • Keep inverse process time aligned: • Use AUX data field and • Swap between data frames

  5. Data Modulation • Reproduction is via Faraday’s Law of magnetic induction – no magneto-resistive replay heads. • Frequency response is poor at low frequencies. • Nearly all ‘1’s and nearly all ‘0’s are bad. • Optionally, can modulate data in each frame with a fixed pseudo-random sequence. Makes long runs of constant data unlikely. • This aids the recovery of the clock from bit stream.

  6. Data Frames – Each Track (64!) • 22,500 bits per frame, frame rates 800, 400, .. Per second. • 64-bit AUX data (e.g. track label) • 36-bit SYNC pattern • 64-bit time code and CRC • 19840-bit astronomical data • 2496-bit odd parity (1 for 8) except over SYNC • NOTE: parity changes rate from 16 to 18Mbps • Manchester encoded for clock/data recovery.

  7. Station Unit • Philosophy: SSIK • Has to be undo correctly all that has been done -as if data had not been recorded. Ultimate aim: to produce channel bitstreams as if they were coming from the samplers/ADCs. • Has to do some extra jobs as well and has to interface gracefully to the correlator.

  8. SU Modules • CkRM: clock and data recovery from bitstream • TRM: track recovery module – reform frames • CRM: channel-recovery module (demux, barrel) • Phase calibration • DM: delay module provides signal delays • SUIM: SU interface module provides sampler statistics, pulsar gating, correlator headers with delay step and phase information for correlator, and reserialises the data

  9. Reference • See “Mark IIIA/IV/VLBA Tape Formats, Recording Modes and Compatibility” by A.R. Whitney. • ftp://dopey.haystack.edu/pub/mark4/memos/230.3.pdf • Next talk: VLBI Correlators

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