Pulse Cal Detection in the RDBE: Initial Thoughts

Pulse Cal Detection in the RDBE:Initial Thoughts East Coast VLBI Meeting Feb 23-24, 2011, Haystack Observatory Bill Petrachenko

RDBE PCAL Options • Types of PCAL detection • Pulse averaging (recommended for VLBI2010 RDBE) • Detects all tones in a band at the same time. • Very efficient at high (wrt pulse rep rate) sample rates • Tone extraction • Works well for random tones • Very efficient at low sample rates • Places in RDBE signal path for detection • At input: 8-bit data, 1x 1024-Msps (recommended for VLBI2010 RDBE) • After PFB: 25-bit data, 16x 64-Msps • After requantizer: 2-bit data, 16x 64-Msp

PCAL pulse Reflections Pulse period Advantage of pulse averaging at input: PCAL reflections are more easily identified In the past Amp vs. Phase

PCAL pulse Reflections Pulse period Advantage of pulse averaging at input: PCAL reflections are more easily removed • Use pulse windowing to remove the reflections • Not possible with tone extraction • Highest resolution for pulse windowing requires all tones to be detected • Guaranteed in the RDBE but not in the correlator • Windowing of the pulse also results in noise reduction.

PCAL pulse Reflections Pulse period Possible problem: windowing reduces frequency resolution for phase corrections • Solution: Pre-correct for known filter non-linearities • reduces need to track fast phase variation with frequency.

High SNR results in precise group delay as well as precise phase delay * Assumes: 1-s integration, 512-MHz BW, and one tone per 5-MHz

Precice PCAL group delay means system delays can be separated from LO phases, i.e. • PCAL group delay depends on cable and electronic delays only. • PCAL phase delay depends on cable and electronic delays and LO phase. • Hence, PCAL (phase delay – group delay) depends on LO phase only.

Possible operational/troubleshooting PCAL displays • For selected seconds, plots of: • Reconstructed PCAL pulse • PCAL amp & phase across the band • For selected tones, plots of: • Amp vs phase • For time records of data, plots of: • PCAL group delay time series • PCAL group delay Allen Standard Deviation • PCAL (phase delay - group delay) time series • PCAL (phase delay - group delay) Allen Standard Deviation • PCAL amplitude time series • Plots of PCAL differences between bands and polarizations

Possible operational alarms • PCAL amplitude below threshold • PCAL time series has too large scatter or drift rate: • PCAL group delay • PCAL (phase delay-group delay) • PCAL amplitude • PCAL scatter across the band is too large: • PCAL phase • PCAL amplitude • Secondary (reflected?) pulses too large in pulse record

Data to store (per 512-MHz band)[Could be up to 16 of them] • Every second • PCAL group delay (33.75-Kbytes/day) • PCAL phase delay (33.75-Kbytes/day) • PCAL amplitude (33.75-Kbytes/day) • PCAL phase scatter across band (33.75-K/d) • Total storage (135-Kbytes/day ~ 0) • Every “on-source” second • Full vector PCAL pulse (8-kbytes/s * ~5/30*86400-s = 115.2-Mbytes/day • Total storage for 16-bands • 1.85-Gbytes/day

Don’t forget stable uplink Cable Feed Front-end PCAL Antenna Fibre Stable cable H Maser Back-end Control Room

Question? • Do we want to accumulate into different bins for calon/coloff.

Pulse Cal Detection in the RDBE: Initial Thoughts