Editing Submillimeter Array (section)

== Receivers ==
[[File:SMA230GHzInsert.jpg|thumb|An SMA receiver insert, covering frequencies from 194 to 240&nbsp;GHz.   The large cryostat in each antenna can house up to eight inserts.]]
[[File:SMACryostatDiagram.png|thumb|Cutaway diagram of an SMA receiver cryostat showing the signal path]]
The SMA uses cryogenic [[Superconducting tunnel junction|SIS]] [[Superheterodyne receiver|heterodyne receivers]], at a bent [[Nasmyth telescope|Nasmyth]] focus.<ref>{{cite book |last1=Blundell |first1=Raymond |title=The Submillimeter Array – Antennas and Receivers |date=2004 |publisher=15th International Symposium on Space Terahertz Technology |location=Northhampton, MA |url=https://www.nrao.edu/meetings/isstt/papers/2004/2004003015.pdf |access-date=12 November 2020}}</ref>  All receivers are mounted in a single large [[cryostat]] within the antenna cabin.   The cryostat can accommodate up to eight receiver inserts, each of which holds a single receiver.   A rotating wire grid [[beam splitter]] followed by a rotating mirror directs the two linear polarizations of the incoming radiation to two of the receiver inserts.   This allows the array to observe either a single polarization of two different frequency bands simultaneously, or both polarizations of a single band simultaneously to improve sensitivity and measure [[Stokes parameters]].

Receivers are available to cover frequencies from 194 to 408&nbsp;GHz, without gaps.  However full polarization measurements can only be made around 230 and 345&nbsp;GHz, where pairs of receivers can be tuned to the same frequency, and [[Waveplate#Quarter-wave plate|quarter wave plates]] optimized for those frequencies can be inserted into the optical path.

The receivers are sensitive to both sidebands produced by the heterodyne mixing.   The sidebands are separated by introducing a [[Walsh function|Walsh pattern]] of 90 degree phase changes in the [[local oscillator|LO signal]], and demodulating that pattern within the correlator. A Walsh pattern of 180 degree phase changes, unique to each antenna, is also introduced to the LO, in order to suppress cross talk between the [[intermediate frequency|IFs]] arriving at the correlator from different antennas.

Thanks to the recent wideband update of the SMA receivers, with two receivers tuned to frequencies offset by 12&nbsp;GHz, the array can observe a 44&nbsp;GHz wide interval of sky frequencies without gaps.