Editing Submillimeter Array (section)

== Correlator ==
The original SMA correlator was designed to correlate 2&nbsp;GHz of IF bandwidth per sideband from each of two active receivers in eight antennas, producing spectral data for 28 baselines.    Because the [[analog-to-digital converter]]s sampled at 208&nbsp;MHz, the IF was [[Heterodyne|downconverted]] into 24 partially overlapping "chunks", each 104&nbsp;MHz wide, before sampling.  After sampling, the data were sent to 90 large PC boards, each of which held 32 [[Application-specific integrated circuit|ASIC]] correlator chips.   The correlator was an XF design; in the default configuration 6144 lags were calculated for each of two receivers on 28 baselines, before an [[Fast Fourier transform|FFT]] was applied to convert the lag data to spectra.<ref name="ho" />   In the default configuration the spectral resolution was 812.5 kHz per channel, but the correlator could be reconfigured to increase the spectral resolution on certain chunks, at the expense of lower resolution elsewhere in the spectrum.  The correlator chips were designed at MIT Haystack, and funded by five institutions: SMA, [[United States Naval Observatory|USNO]], [[NASA]], [[Netherlands Foundation for Radio Astronomy|NRFA]] and [[Joint Institute for VLBI ERIC|JIVE]].<ref name="peck" />   The correlator could also be configured to correlate all 45 baselines produced by adding the CSO and JCMT to the array, but only for a single receiver per antenna.

[[File:SMASWARMCorrelatorOutput.png|thumb|A spectrum produced by the SWARM correlator when the SMA observed Orion BN/KL in 2016. This spectrum was produced when only four quadrants of SWARM were available. Six quadrants are available now.]]
In 2016 a new correlator called SWARM was brought online, allowing more total IF bandwidth to be correlated, increasing the array's sensitivity to continuum sources as well as its instantaneous spectral coverage.   The new correlator, an FX design, uses 4.576 GHz analog-to-digital converters<ref>{{cite journal |last1=Jiang |first1=H. |last2=Liu |first2=H. |last3=Guzzino |first3=K. |last4=Kubo |first4=Derek |title=A 5 Giga Samples Per Second 8-Bit Analog to Digital Printed Circuit Board for Radio Astronomy |journal=Publications of the Astronomical Society of the Pacific |date=July 2014 |volume=126 |issue=942 |pages=761–768 |doi=10.1086/677799 |bibcode=2014PASP..126..761J |s2cid=120387426 |url=https://www.researchgate.net/publication/271651990 |access-date=9 November 2020|doi-access=free }}</ref> and Xilinx Virtex-6 SX475T FPGAs rather than purpose-built correlator chips.   The FPGAs are housed with additional electronics on ROACH2 boards produced by the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER).   The new correlator operates at only one spectral configuration, uniform 140&nbsp;kHz per channel resolution across the entire bandwidth.   The data are stored at this high spectral resolution even for projects that require only low resolution, so that the highest resolution will be retained in the observatory's data archive for use in later research.   Each quadrant of the correlator can process 2&nbsp;GHz of IF bandwidth per sideband for two active receivers in all eight antennas.   When the two receivers are tuned to the same frequency, full [[stokes parameters|Stokes]] polarization parameters are calculated.<ref>{{cite journal |last1=Primiani |first1=Rurik A. |last2=Young |first2=Kenneth H. |last3=Young |first3=Andre |last4=Patel |first4=Nimesh |last5=Wilson |first5=Robert W. |last6=Vertatschitsch |first6=Laura |last7=Chitwood |first7=Billie B. |last8=Srinivasan |first8=Ranjani |last9=MacMahon |first9=David |last10=Weintroub |first10=Jonathan |title=SWARM: A 32&nbsp;GHz Correlator and VLBI Beamformer for the Submillimeter Array |journal=Journal of Astronomical Instrumentation |date=2016 |volume=5 |issue=4 |pages=1641006–810 |doi=10.1142/S2251171716410063|arxiv=1611.02596 |bibcode=2016JAI.....541006P |s2cid=114780818 }}</ref>   Somewhat confusingly, there are now six SWARM "quadrants" in the full correlator, allowing 12&nbsp;GHz of bandwidth to be correlated for each sideband of two receivers on all baselines, allowing a 48&nbsp;GHz total sky frequency coverage.

SWARM can also operate as a phased array summer, making the SMA appear to be a single antenna for [[Very-long-baseline interferometry|VLBI]] operations.