• Referee3

Grade: 5.0

Title: ALMA band 6 Upgrade

Alignment with NA ALMA Partnership strategic goals

As stated in the proposal, Band 6 receiver is currently the workhorse of ALMA, producing significant amount of science. However, it can be argued that as ALMA becomes more established, more of the work will be moving to other bands, notably above 350 GHz, where no other interferometers operate. From that perspective, waiting too long to improve Band 6 may be the wrong way, but providing quicker incremental improvement to enhance its performance may be more relevant.

Strength of the scientific case for the proposed ALMA upgrade concept

One science case that stands out is that the present Band 6 receivers cannot cover the 3 isotopic CO (J=2-1) lines with a single tuning. This can be solved by going to a wider IF bandwidth. However, ALMA’s current correlator can only handle a certain bandwidth. So, any bandwidth improvement can only be materialized after the correlator upgrade, which is being proposed elsewhere.

Quality of the upgrade conceptual design

To achieve the wider IF bandwidth, the PI proposes to explore balanced LNA. Low noise factory has produced very good amplifiers, based on the MMIC approach, versus the discrete amplifiers that NRAO has relied on in the past. However, a look at the LNA website shows that a 4-16 GHz amplifier requires a bias of 0.7 V 14 mA, which translates into a power consumption of 10 mW. For a balanced amplifier, this means a power consumption of 20 mW per polarization, and for a balanced mixer followed by a balanced amplifier, the whole cartridge would consume 80 mW for all amplifiers. This number appears to be quite high. In many ALMA bands, cryogenic isolators are used. It is hard to understand if the isolators reduced the sensitivities that much, then the other ALMA would not be meeting the specifications. On the other hand, the use of isolators flatten out the receiver’s IF pass-band, and remove the problems that the PI wants to solve with the balanced amplifiers.

The PI also wants to explore balanced mixer to reduce LO induced noise. The effect of LO noise is common to all receiver bands. So, it seems that solving at the warm cartridge level would be the better way to do, rather than introducing more complexity. A balanced mixer requires twice as much supporting electronics. This may be the view of an outsider, but some rigorous engineering improvement of the warm cartridge would solve the problem for all bands. For example, a somewhat low-Q electrically or electro-mechanically tunable filtering after the last stage power amplifier would be able to remove the excess noise from the LO chain. Bearing in mind that the ALMA IF starts at 4 GHz, the filter bandwidth has to be less than 8 GHz wide, which seems not too difficult a task, except for the lack of determination to solve this issue at the warm cartridge level.

Readiness for production in the context of the ALMA Development Plan

The project is to produce 2 prototype cartridges. There is no doubt that they can do it within the proposed time scale. But that falls short of an upgrade path to improve Band 6 operation.

Strength of the consortium organization

Have no problem with the organization of CDL to handle this project.

Qualifications of the key personnel of the Study The people involved are well qualified. Technical expertise, past experience

The personnel certainly have the right expertise, but the reason why Band 6 falls short of expectation is that they make not so correct decision along the way, like the mixer-preamp combo Vs isolator-based system. So, we have to trust that they make conservative decision.

Assessment of the level of risk inherent in the design

The PI states that there are significant risks in pushing some of the new ideas. The notion of balanced mixer + balanced amplifier may be too avant-garde. They may have to fall back on more conservative technology. The OMT they propose, for example, has stringent alignment problems already at 95 GHz. It will be hard push to meet stringent specifications for Band 6.

Strength of the Scientific Team supporting the Study

Have no problem with the Scientific team working with the engineers on the team.

Level of support guaranteed by the Institutes Seems to be OK. Budgeted cost of the Study The cost appears to be high given that prototype cartridges are produced at the end of period. In summary, it is good to explore the various technologies related to heterodyne mixing. However, it is not clear that these explorations would necessarily translate into better performance for ALMA, while there exists simple fixes, which can be used to provide cost effective update. If the issues with Band 6 are that troubling, it should be upgraded on a shorter time scale with such cost effective measures.

  • Referee4
Grade: 5.0

Title ALMA Band 6 Upgrade (Kerr)

1. Alignment with NA ALMA Partnership strategic goals;

Well aligned. The project directly or indirectly addresses a number of goals including increasing (effective) receiver bandwidth, increasing sensitivity, and, arguably most important: decreasing ALMA overhead by avoiding unnecessary retuning.

2. Strength of the scientific case for the proposed ALMA upgrade concept; Comment on the relevance to the ‘ALMA 2030’ development documents.

The authors have very clear ideas about the types of science that is generally attempted with the highly utilized Band 6 receiver system. They focused mostly on star formation science, but their arguments regarding the need to simultaneously observe pairs or groups of spectral lines extend to many other subfields.

There were a few things that could have improved the proposal: (1) throughout the proposal both 220 GHz and 230 GHz were used as the frequency of the resonance; this was particularly confusing, (2) some of the figure number references were incorrect, (3) at least one caption was unfinished, and (4) at least one plot lacked labels.

3. Quality of the upgrade conceptual design;

Excellent. A number of the upgrades are exciting, in particular moving away from lapping and dicing the SIS devices. I encourage the authors to exercise particular caution with the split-block OMT; previous telescopes (e.g., CARMA) have had major issues with exactly the block-misalignment issue that the authors raise.

4. Readiness for production in the context of the ALMA Development Plan (the aim is to support a range of upgrades including both those which can be implemented rapidly and those requiring longer-term research and development);

All of the machinery is in place, as the entire team in Charlottesville has already successfully executed a similar Band 6 program.

5. Strength of the consortium organization (if applicable);

Strong NRAO+UVa connections.

6. Qualifications of the key personnel of the Study;

Highly qualified.

7. Technical expertise, past experience (also in series production, if relevant) and technical facilities in the Institutes taking part in the Study;

The authors worked on the first-generation Band 6 system, and are thus have the expertise to make the upgrades.

8. Assessment of the level of risk inherent in the design;

Low/medium, as outline in the contingency plan.

9. Strength of the Scientific Team supporting the Study;

Unclear.

10. Level of support guaranteed by the Institutes;

None.

11. Budgeted cost of the Study;

Expensive: only marginally within the $1.5M/yr cap, when averaged over 3 years.

  • Referee5
Grade: 3

489 - ALMA Band 6 Upgrade

1. Alignment with NA ALMA Partnership strategic goals;

This project would improve the performance of ALMA band 6 and support development of technologies that may apply to many bands. It is aligned with the strategic goals.

2. Strength of the scientific case for the proposed ALMA upgrade concept; Comment on the relevance to the ‘ALMA 2030’ development documents.

The scientific case for this development is clear enough. It supports all band 6 science, which is a workhorse band. Considered against the current correlator, these improvements are perhaps not entirely necessary despite improving B6 in many ways. Over a longer period, these improvements will significantly improve the utility of B6 as ALMA moves to wider bandwidth. Since this is only a prototyping exercise, the timescale for full implementation brings it closer to the timescale for widening the correlator. Perhaps the greatest downside is that a major B6 change sits behind completion of bands 1 and 2 in priority, and even once all bands are complete, it is not at all clear that B6 is the one that needs to be addressed first. The high frequency bands (9, 10) could be significantly improved and the other workhorse bands (3, 7) have flaws on the same level as band 6.

3. Quality of the upgrade conceptual design;

The proposal includes many improvements over the current B6. Technologies that would be used (e.g., OMT, SC hybrids, couplers) can likely be adopted in other bands as well in future upgrades, so there are many positive aspects here.

4. Readiness for production in the context of the ALMA Development Plan (the aim is to support a range of upgrades including both those which can be implemented rapidly and those requiring longer-term research and development);

This would deliver prototypes, so it is longer term.

5. Strength of the consortium organization (if applicable);

The team subcontracts lithographic fabrication to their long-term collaborators at UVa.

6. Qualifications of the key personnel of the Study;

The team is eminently qualified to do this work.

7. Technical expertise, past experience (also in series production, if relevant) and technical facilities in the Institutes taking part in the Study;

Unassailable.

8. Assessment of the level of risk inherent in the design;

The risk of device fabrication problems did not get much attention, but probably should have. It seems that most device fabrication sites (UVML included) experience problems in delivering new wafers of devices, even ones that they have made before.

9. Strength of the Scientific Team supporting the Study;

The scientific input has been thorough.

10. Level of support guaranteed by the Institutes;

N/A

11. Budgeted cost of the Study;

This is an expensive program, but many developments are tied together and it would result in a production-ready band.

  • Referee6

Grade: 4

Band 6 Upgrade

1. Alignment with NA ALMA Partnership strategic goals;

Increases the efficiency of Band 6.

2. Strength of the scientific case for the proposed ALMA upgrade concept; Comment on the relevance to the ‘ALMA 2030’ development documents.

The increase in efficiency is produced by making the entire IF range available with good noise properties. A number of specific cases where observing multiple lines at the same time would be easier are shown (however it is not entirely clear that the same signal to noise is needed for these typical line pairs as often one line is significantly stronger than another and thus the multiplexing advantage is less than advertised).

3. Quality of the upgrade conceptual design;

The plan looks well thought out and detailed. Very specific.

4. Readiness for production in the context of the ALMA Development Plan (the aim is to support a range of upgrades including both those which can be implemented rapidly and those requiring longer-term research and development);

This proposal only covers the development cost of two prototypes and it is unclear what the final cost of a full production run would cost.

5. Strength of the consortium organization (if applicable);

The project is led by an excellent team from NRAO and UVa.

6. Qualifications of the key personnel of the Study;

The team is very well qualified.

7. Technical expertise, past experience (also in series production, if relevant) and technical facilities in the Institutes taking part in the Study;

Excellent.

8. Assessment of the level of risk inherent in the design;

The biggest risk would appear to be in the balancing of the mixers but they do discuss how this would be dealt with.

9. Strength of the Scientific Team supporting the Study;

Good.

10. Level of support guaranteed by the Institutes;

N/A

11. Budgeted cost of the Study;

This is an expensive proposal, resulting in only two prototype cartridges by the end of the funding period.

  • Referee8

Grade: 5.0 because this gives a modest improvement in science throughput

Title: ALMA band 6 upgrade PI: Kerr

1. Alignment with NA ALMA Partnership strategic goals;

The proposal is well aligned with the strategic goal of improving the performance of ALMA, in this case by improving the noise temperature, polarization and sidelobe level of band 6. The higher sensitivity will improve the overall efficiency of ALMA.

2. Strength of the scientific case for the proposed ALMA upgrade concept; Comment on the relevance to the ‘ALMA 2030’ development documents.

The science case in the proposal is based on improving observing efficiency by reducing the receiver noise temperature. The upgrade will allow simultaneous observations of interesting lines that are currently not possible because of high LO and IF noise. The intent is to achieve baseline performance for band 6, which is a key part of the original ALMA science case, and one of the most heavily used bands. I am generally supportive of fixing the band 6 problems, but I would like to see a quantitative estimate of the improvement in efficiency before starting a $5M development. The proposal directly addresses the ALMA 2030 recommended development path of improving speed by increasing the receiver sensitivity.

3. Quality of the upgrade conceptual design;

The conceptual design of the upgrade is sound and the plans are well developed. I do not see any technical problems with the proposed approach.

4. Readiness for production in the context of the ALMA Development Plan (the aim is to support a range of upgrades including both those which can be implemented rapidly and those requiring longer-term research and development);

The proposal is for 2 prototype band 6 cartridges. Production of a full upgrade for ALMA could start in 3-4 years.

5. Strength of the consortium organization (if applicable);

This is a strong consortium. NRAO and UVa have worked successfully together for many years on similar developments.

6. Qualifications of the key personnel of the Study;

The key personnel are well qualified to do the work.

7. Technical expertise, past experience (also in series production, if relevant) and technical facilities in the Institutes taking part in the Study;

NRAO and UVa have an excellent track record in this field. Their groups have substantial expertise and experience, and good technical facilities.

8. Assessment of the level of risk inherent in the design;

This is a relatively low risk development, based largely on design approaches that have already been tested at other wavelengths. The greatest technical risk is the balanced SIS mixer chips, but switching to unbalanced chips is a reasonable fall back. The project risk assessment includes CDL resources being over allocated. This does not make much sense to me. Doesn’t the CDL know its resource allocation?

9. Strength of the Scientific Team supporting the Study;

The science case for band 6 is part of the ALMA baseline science case.

10. Level of support guaranteed by the Institutes;

Appropriate.

11. Budgeted cost of the Study;

$5M seems a bit high to me for 2 prototype cartridges, but the proposal does include a lot of design effort and new test setups. The proposal only makes sense if the projected cost of the full band 6 upgrade fits into ALMA cost planning, so I would insist on a preliminary estimate of the total cost before funding the proposal.

-- AlWootten - 2017-05-09
Topic revision: r2 - 2017-06-27, AlWootten
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