ALMA System- Preliminary Design Review
Held at ESO, Garching – 2000-02-28
Review Committee Composition:
US Division Heads: Brian Glendenning, Peter Napier , John Payne, Richard Sramek, John Webber, Al Wootten.
EU Team Leaders: Torben Andersen, Alain Baudry, Stephane Guilloteau, Gianni Raffi, Wolfgang Wild.
Outside Reviewers: Bryan Anderson, Yoshihiro Chikada, Richard Hills, Dave Woody.
The following recommendations are listed in the order in which the subject appeared on the Agenda of the meeting. The Agenda is attached for completeness.
Requirements. It is established that there exists no unanimity about the required number of dishes, which must be equipped for total power observation to support mosaicking at sufficient SNR. The Science Group, in consultation with the receiver group, should consider this point and decide on a binding number. The design of the necessary special equipment must be included in the project plan.
A maximum of 25 km transmission distance (optical fiber length) from the central array building is considered acceptable.
The operation of ALMA with maximally 4 Subarrays is recommended.
Baseline Design. In addition to the baseline IF bands of 8 GHz Upper and Lower Sideband, receiver designers are free to select any of the following alternatives: 8 GHz Single-Side-Band, Upper or Lower, 8 GHz Double-Side-Band or 4 GHz Upper and Lower Sideband. In all cases, dual polarization for a total of 16 GHz IF band width. Sideband separation in DSB-mode will be possible for integration times in multiples of 1 sec. Depending on the choice, and maintaining the currently proposed LO coverage, this might lead to some loss of frequency coverage. The impact of this should be evaluated by the Science Group.
The system design is based on an array of 64 antennas. An increase in this number has significant consequences for the entire system. This should be considered in the definition of the Japanese contribution to ALMA.
Switching between any preselected pair of receivers within 1.5 second is acceptable. Changing the band should be limited to intervals of the order of 15 minutes. Within a single band, switching at 10 seconds is required, while frequency switching for f/f< 0.03 at <10 msec (1 msec goal) is required.
Frontends. The inclusion of the 31.3-45 GHz receiver has a big impact on the array design, including LO-layout, receiver layout and optics in the cabin. The Science Group should issue a definitive statement as to the need of this frequency band. The choice between SIS and HFET for the 3mm band should be deferred until comparative tests have been performed, considering in particular TP performance, sensitivity, reliability and cost.
Photonic Components. Option 2 should be adopted as the current baseline for ALMA. It is recommended to pursue Option 3, the direct photonic LO, although it is not part of the baseline design. To this end, a laser for every antenna in the array should be seriously considered in the central building; this would also greatly simplify the electronics at the antenna.
It is recommended to continue development of the photonic phase calibration system, although it is not part of the baseline plan. In particular the system implications should be worked out.
In addition, it was noted that there is no baseline plan for amplitude calibration. Such a plan should be devised.
Digitizers and FIR filters. We recommend that the FIR filter be located at the central array building, in line with the baseline plan and allowing evolution to future correlator concepts. The digitizers are identified as a “single point failure” in the system. New developments are required and a European plan was presented to this end, but with still uncertain scheduling. A Japanese development plan was also presented at the meeting. The following recommendation emerged: bandpass sampling (2-4 GHz) should remain the baseline design and 1 or 2 bit sampling is considered as an acceptable fall-back solution for interim operation until the desired full 3 bit stage is realized. The European and Japanese “digitizer teams” are encouraged to cooperate in their respective development and test plans.
Timing, Fringe tracking and Phase switching. It should be assured that the DDS initializes the phase correctly during frequency changes. It was established that the baseline system provides hardware doppler tracking over 10 MHz, which is considered adequate. The possible mutual influence of Monitor and Control (50 ms) versus Correlator (16 ms) timing should be examined to avoid operational problems. More work is required on the details of the baseline system in this area.
Packaging and Power Distribution. NRAO presented some ideas in the areas of electronics packaging and power distribution. The two partners need to reach an agreement soon on these issues so that the chosen standards can be used for the test interferometer equipment.
Procedures. It is recommended that the ALMA Project adopt the existing review guidelines, as used by the US partner.
Addendum
There exists confusion, and possibly differences of opinion, among
the scientists and engineers of the ALMA Project as to the final
decision regarding the number of sub-arrays, which would be supported
in ALMA. After discussions in the ASAC and among the Project
Scientists and System Engineers, we have agreed that the following
definition is appropriate.
We distinguish three types of sub-array levels:
2000-02-28, Garching
09:00 Welcome ...................................................... Cesarsky
09:10 Overview; Requirements; Standards ............................... Napier, Emerson, Baars
09:20 Operations Plan ............................................... Brown, Guilloteau
09:35 Block Diagram, Baseline Design ................................... D’Addario
10:15 Coffee Break
10:45 Major Design Decisions Taken ...................................... D’Addario
11:00 Photonic Components of the baseline LO design .................. Payne
11:10 Correlator Specifications and Interfaces ............ Webber
11:20 Technological Risk Areas Affecting System-level Design .....D’Addario, Webber
11:40 Discussion
12:00 Lunch Break
12:45 Open Questions and TBD Parameters .................................... D’Addario
Alternative Designs:
13:15 – Direct photonic LO ..................................................... Payne
13:35 – Different IF and baseband bandwidths .................................D’Addario
13.55 Interfaces and ICDs .................................................... Emerson, Napier
14:15 Digitizer Development Plan .............................................. Baudry
14:30 Timing and Synchronization .......................................... D’Addario
14:45 Realtime Software .................................................. Glendenning
15:00 Fringe Rotation, Phase Switching, and Related Issues ............D’Addario
15:15 Tea Break
15:40 Power Distribution .................................................... D’Addario
15:50 Packaging ............................................................. Moorey
16:05 Schedule ...................................................... Emerson & Baars
16:15 Discussion
17:00 Committee Closed Session
18:00 Adjourn
2000-02-29, Garching
08:30 System Design Committee, Closed Session
08:45 System Design Committee Report
09:00 End of System Design Review