STO FFT Spectrometer and Data System ● Eight 1 GHz FFT spectrometers on four 3U boards, needing 80 watts DC power total ● 1024 channels each (0.3 km/s res at 1.9 THz) STO Craig Kulesa

Spectrometer boards ● Atmel dual 1 Gsps 8-bit digitizers ● Xilinx Virtex 4 FPGA with swappable microcode ● Integrated synthesizer and VCO ● Eurocard backplane for data transfer to embedded ARM computer ● 100 Mbit ethernet interface to outside world

End-to-End Test Spectra Difference spectrum of Timoc noise source on spectrometer channel 1 Difference spectrum of modified DStar mixer looking at harmonic mixer through CIT IF processor on spectrometer channel 2

Allan Variance Tests ● For a sample 4-hour period, we split up Allan variance analysis across 1-hour intervals while: ● IF processor is warming up ● Timoc noise source is warming up ● Spectrometer & LO source have been running ● Next 4 panels show continuum Allan variances w/ the Timoc noise source; that is, we are following the raw power fluctuations in a “typical” spectral channel (#256)

1 st hour2 nd hour 3 rd hour4 th hour textbook example of drift noise! 10s100s1000s10s100s1000s

Continuum vs. Spectroscopic Allan Variance ● spectral channels are independent.... sort of ● amplifier gain variations and other sources of drift noise may impact all channels ● define a spectroscopic Allan variance: we remove a zeroth-order baseline (i.e. a mean DC offset) from the spectrometer counts before computing the variance

continuum variance, t = 1-4 hours [Timoc noise source] spectroscopic variance, t = 1-4 hours [Timoc noise source]

Channel-Averaged Allan Variance Timoc noise source, spectroscopic variation 10% deviation from radiometer noise at 500 sec 30% deviation from radiometer noise at >2000s

Channel-Averaged Allan Variance CIT IF processor, spectroscopic variation 10% deviation from radiometer noise at ~200 sec 30% deviation from radiometer noise at 450 sec traditional Allan time is 650 sec

Data System Summary ● Principal job is to accumulate spectra to solid state memory and/or pressurized hard drives, with appropriate header information from the telescope systems. Dedicated data computer independent of instrument control computer. ● Background thread on data computer performs preliminary baseline subtraction and regridding, returns single OTF spectrum every N dumps and a preprocessed integrated intensity map ~hourly. ● By computing T RMS continuously, data computer can determine if a scan needs to be re-observed. ● All of this is part of the Supercam FFT spectrometer development in 2008 and will be ready for STO.