====================================================================== NOTES ON PROPOSED EXTENSION OF BAND 3 TO LOWER FREQUENCIES L. D'Addario, 2000-11-07 BACKGROUND In the report of its March 2000 meeting, the ASAC recommended a priority ordering for the 10 ALMA receiving bands. They placed Band 3 (89-116 GHz) in the first priority group and Band 2 (67-90 GHz) in the second priority group. However, they also recommended a study of the feasibility of extending the low end of Band 3 to 86 GHz to cover the SiO line at 86.2 GHz; if that is done, they said, Band 2 would drop to third priority. To date, the recommended study has not been carried out, although informal notes (emails) giving opinions about the feasibility of some components have been circulated. Nevertheless, the JRDG has proposed that the official specification for Band 3 be changed so that it covers 84-116 GHz (with no explanation of the further extension beyond that suggested by the ASAC). It is reported (in an email from J. Payne on Oct-18) that the AEC would like the Systems Engineering Division to comment on the system design implications before approving the change, but the Systems Group has not received any direct request from the AEC. In the report of its October 2000 meeting, the ASAC assumes that Band 3 has been extended to 84 GHz, and it changes the priorities for several bands from those in its March report, all without explanation. This is premature and unjustified. The following notes are presented on behalf of Systems Engineering. It should be understood that this is done in somewhat of a vacuum, there being no other available analysis and no explanation of the proposal beyond two sentences in the ASAC's March report. ANALYSIS The present frequency range for ALMA Band 3 is 89.0 to 116.0 GHz. This is a frequency ratio of 1.303, which is quite similar to the frequency ratio for most other ALMA bands. For reference, these are: 1 31.3-45 1.438 (HFET band, compromise optics) 2 67-90 1.343 (HFET band, compromise optics) 3 89-116 1.303 (SIS or HFET, TBD) 4 125-163 1.304 5 163-211 1.294 6 211-275 1.303 7 275-370 1.345 8 385-500 1.299 9 602-720 1.196 10 787-950 1.207 Possible extensions would result in 3+ 86-116 1.349 Includes SiO at 86.2 GHz 3++ 84-116 1.381 Includes methanol at 84.4 GHz 3+++ 80-116 1.450 Includes VLBA coverage of 80-96 GHz Thus, it would become the widest band except perhaps for band 1, where compromises have already been made to accomodate this. Difficulties caused by having to operate over a wide fractional bandwidth include: -- Windows: Coverage of Band 6 already requires a 5-layer design, and satisfactory performance has been demonstrated only in simulations. The current Band 3 could be scaled from this, but Bands 3+ or wider would be difficult to accomodate without poor performance (>5% loss?) in the outermost ~20%. -- SIS mixer fixed tuning leads to degraded performance -- LO range (for DSB or sideband-separation mixing) 101-104 currently 1.029 96-104 for 3++ with 4-12 IF 1.083 92-108 for 3++ with 4-8 IF 1.174 84-108 for 3+++ with 4-8 IF 1.286 It appears that most of these can be supported within the baseline LO architecture, except perhaps the last (worst) case (which would require multiplier bandwidth beyond the SOA for tunerless devices). -- LO range (for SSB conversion, HFET option) 1st conversion to 14-22 GHz IF: 75-94 currently 1.253 WR10 70-94 for 3++ 1.343 WR10- 66-94 for 3+++ 1.424 WR12+ The extension would be hard to accomodate within the present architecture, especially for Band 3+++. Of all the >50GHz frequency multipliers required for the array, the one for the present band 3 must support the largest frequency ratio, and no power amplifier for this range is currently planned. For any of the extended ranges, the feasibility of the required components is in doubt. Even if suitable components were available, the need to span two standard waveguide bands makes implementation messy. Thus, the extension may preclude the SSB HFET option. (An HFET front end with DSB conversion is still feasible, however.) -- Optics The relatively long wavelenth of the extended low end would require larger optical components, including lenses, mirrors, window, and horn. The window for 89 GHz was already large enough to force the use of room-temperature refocusing optics. (Band 7 may also be too wide for similar reasons.) CONCLUSIONS The system architecture does not contain any "show stoppers" for the band extension. Even the extreme case of "band 3+++", 80-116 GHz, might be accomodated. However: - Some options are excluded, especially that of having SSB conversion following an HFET front end (with low side LO and an extra frequency conversion), due to the required LO range. An HFET amplifier followed by a DSB or sideband-separating mixer would be feasible, since it would require the same LO range as an SIS receiver. - Performance will suffer in several ways: . SIS mixer noise temperature will be relatively poor over a significant portion of the extended band. It could be optimized for the upper, middle, or lower part of the range. . SIS mixer performance may be poor over the upper part of the 4-12 GHz IF band, for all input frequencies. . Receiver temperature and aperture efficiency may suffer over a signifcant part of the band due to performance of windows and other optics. Again, performance can be optimized over a pre-selected part of the extended range. - For an HFET amplifier, performance is not significantly compromised by the extended frequency range. Thus, requiring the extended range favors selection of the HFET option. Every one of the proposed extensions, and more, is covered by Band 2. There is thus no scientific advantage of any extension to Band 3 unless Band 2 is deleted. Furthermore, the Band 2 and Band 1 front end assemblies ("cartridges") are the simplest and least expensive of any band (since they use HFET amplifiers, simplified optics, and cooling only to 15K). Including Band 2 in the initial complement of front end assemblies incurs only marginal cost, since all the necessary infrastructure (dewar, most of LO, support electronics) will already be available. It is therefore recommended that: a. No change should be made in the range of Band 3, keeping it at 89-116 GHz, so as to achieve the best receiver performance and aperture efficiency. b. Band 2 should be retained as no lower than second priority, and its construction (along with Band 1) should be undertaken relatively early. Building the Band 2 and Band 1 cartridges should be among the first uses of any available contingency funds or cost reductions relative to present estimates. c. Should it be decided that the lower part of Band 2 (67-84 GHz) is of so little interest that it can be permanently deleted from the array (not just reduced in priority), and only in that case, the above recommendations might be reversed. Band 3 should then be extended by the minimum amount necessary (since performance degradation increases monotonically), and Band 2 should be deleted. Elimination of Band 2 has advantages in optics simplification, allocation of focal plane space, dewar space, crogenics load, and cost. Only if these advantages can be realized are the design compromises of the extension to Band 3 justified.