Data Acquisition From Data receivers to correlator Acquisition

Data Acquisition /From / Data receivers to correlator Acquisition System / Recorder / Station Unit – DAS 2 / 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. /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)

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.

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

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

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 18 Mbps /Manchester encoded for clock/data recovery.

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.

SU Modules /Ck. RM: clock and data recovery from bitstream /TRM: track recovery module – reform frames /CRM: channel-recovery module (demux, barrel) / Phase /DM: calibration delay module provides signal delays /SUIM: SU interface module provides sampler statistics, pulsar gating, correlator

Reference /See “Mark IIIA/IV/VLBA Tape Formats, Recording Modes and Compatibility” by A. R. Whitney. / ftp: //dopey. haystack. edu/pub/mark 4/memos/230. 3. pdf /Next talk: VLBI Correlators