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The Cycle 0 Technical Handbook has some gaps in its discussion of ALMA receivers (SSB, 2SB, DSB). What else can you tell me about them? - Knowledgebase / Early Science - Cycle 0 - ALMA Science

The Cycle 0 Technical Handbook has some gaps in its discussion of ALMA receivers (SSB, 2SB, DSB). What else can you tell me about them?

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The ALMA Cycle 0 Technical Handbook discusses the bands and their receivers (Single Sideband; SSB, Dual Sideband; 2SB, and Double Sideband; DSB). This knowledgebase article assumes this has already been read, and is intended to answer additional questions one might have about the receivers associated with ALMA bands.

1. Why will Bands 1 and 2 be SSB?

Answer: The issue of why different types of receivers are built for different bands is largely a question of engineering rather than a science-based decision. The frequency range of bands 1 and 2 is such that the range of each band will be fully covered by an SSB receiver.

2. The Handbook says that for the 2SB receivers "both the upper and lower sidebands are provided separately and simultaneously". Later in the paragraph, however, it talks about an "unwanted sideband rejection". Are both sidebands kept or is one rejected?

Answer: For the 2SB receivers in bands 3 through 8, signal from either sideband can be kept or rejected as desired for each spectral window. For each spectral window, the signal from one sideband will be kept while the signal from the other sideband will be rejected. However, spectral windows can be independently tuned in the two sidebands: a spectral window in the lower sideband can be tuned to pick up a frequency that was rejected from a different spectral window in the upper sideband. This is different from an SSB system, where the signal from only one sideband is retained by the receiver hardware, and from a DSB system, where the lower and upper sidebands are super-imposed. (In one sense, a 2SB system is similar to two SSBs with sidebands separated by a fixed amount: 8 GHz in the case of ALMA bands 3, 7, 8 and 10 GHz in the case of ALMA band 6.)

3. When discussing the band 9 receivers, the Handbook states: "In the future, suitable phase switching will be introduced in the correlator, and both sidebands can be correlated and processed independently, thus doubling the effective system bandwidth." How far away is that future? Will both sidebands be available for Cycle 1 observations? When this future arrives, how will this system be different, from an observer's point of view, from a 2SB system? Will both sidebands will be available separately for both systems?

Answer: It hasn't yet been decided whether or not phase-switching will be offered in Cycle 1. The status of phase-switching depends on the outcome of tests that are still underway. Please stay tuned; the full Cycle 1 capabilities will be announced at the time of the Cycle 1 Call for Proposals. However, both sidebands will be available for Cycle 1 in a similar manner as described above for the 2SB receivers. Like those bands, band 9 will have independently tunable spectral windows. What is different currently for DSB and 2SB observing with ALMA is that the noise from the "unwanted" sideband is not suppressed, and for single-dish observing the signal from the opposite sideband is not suppressed either. The latter will always be true for single-dish mode. When phase-switching becomes available, both band 9 sidebands will still be available separately. However, the signal from the opposite sideband will also be available "for free", which is where the effective bandwidth doubling comes in. The system with phase-switching will thus provide twice as many spectral windows, but the spectral windows will no longer be completely independently tunable. (e.g., 4 independent tunings of 8 spectral windows in DSB, instead of 4 independent tunings of 4 spectral windows in 2SB.)