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.

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 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

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.

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.

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

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

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