[mmaimcal] WVR memo

[Larry D'Addario]

Richard,

Your document on WVRs is very helpful.  I have just a few comments...

1.  In "Introduction" the purpose is called "phase correction" but in
    par 1 of "Status..." it is called "phase calibration."  I think
    the right word is "correction."

2.  In "Design Considerations..." par 5, you refer to my note on phase
    stability specifications, and particularly to the requirement on
    systematic errors.  You say that "this is not relevant because
    [it] ... will be taken out by ... calibration."  On the contrary,
    the spec refers to the residual error *after* all calibration,
    just as did the predecessor spec by Woody (MMA Memo 144).  The
    uncalibrated phase error will be drastically worse.  If the
    astronomical calibration is not to be degraded by use of the WVR,
    then the systematic error in the WVR's estimate of the atmospheric
    phase must change by much less than the spec over the interval
    between astronomical calibrations.  Thus, the spec is relevant.

3.  In par 9 of "Design Considerations..." (bottom of p 3), I agree
    that the LO should be locked to the system and that you can use
    any of the fixed references.  It is now important to specify
    exactly which reference(s) are needed, so that the distribution
    circuitry can be designed.  Similarly, all interfaces to the WVR
    need to be finalized now; provided that the interfaces are fixed,
    the internal design of the WVR can be allowed to evolve.  Also, it
    should be made clear that the "requirement that the LO be shifted
    by a small amount" is imposed on the WVR's internal circuitry, not
    on the externally supplied references.  Note that the WVR does not
    involve any phase measurements, so its internal LO synthesis is
    allowed to be phase-ambiguous (unlike the LOs of the astronomical
    receivers).

4.  Under "5a) SIS," you suggest keeping options "open for the
    present."  This is no longer practical.  If an SIS receiver is to
    be used for the WVR, then we must explicitly design for it.  This
    would have significant effects on the receiver layout.  Please
    note that the "slots" for the various bands are not
    interchangeable; the inserts are unlikely all to be the same size,
    and each will have band-specific optics.  So you cannot later
    decide to delete an unpopular band and replace it with the 183 GHz
    WVR.  The alternative of using the already-planned band 5 or band
    7 receiver as the WVR is also rather problematical.  It is not
    just a matter of adding an independent LO and dedicated back end;
    considerable switching would be needed to change the receiver
    between the two modes, and there would be major constraints on the
    optics design if either of these receivers must be operated
    simultaneously with others.  Unless it can be decided right away
    that one of the ideas in this section will be implemented, they
    will soon be designed out.

5.  Under "5b) Cooled Schottky," the first sentence mentions a
    separate dewar as an "advantage."  This is actually no advantage
    at all, and a requirement to provide another dewar and cryocooler
    would only be an unnecessary complication.  The comment that "a
    new development to obtain suitable mixers" is needed is
    questionable; commercial mixers for this band, although intended
    for room temperature operation, are likely to be coolable.

    You also mention the possibility of cooling to 70K using available
    cheap cryocoolers.  This should be considered; but the main dewar
    will already have a 70K stage, so it is still questionable whether
    a separate cooler is reasonable, no matter how cheap.

6.  Under "Form of switching," par 3, you say that for a 2-mixer
    correlating radiometer "with appropriate switching we can
    ... separate the sidebands."  I don't think so, since this is just
    a radiometer, not an interferometer.  Do you know of some sideband
    separating technique of which I'm unaware?

7.  Also under "Form of switching," you discuss the optics for
    implementing the pointing correction feature.  To cover only half
    the subreflector requires an f/16 beam (as opposed to the f/8 beam
    for the whole subreflector), and consequently double the aperture
    diameter in the focal plane (to about 110 mm at 5w).  It's very hard
    to fit this in, especially if it needs to be close to the other
    feeds so as to minimize the beam offset.  When this is combined
    with the increased sensitivity requirement and with the need for
    multiple-temperature calibration loads, the technical difficulties
    of this receiver are getting substantial.  In view of the fact
    that the pointing correction scheme is completely unproven, and
    that the main points of the receiver design need to be frozen very
    soon, I don't see how we can plan to support this feature.  Unless
    it can be established within the next month or two that it is both
    essential and likely to work, I expect it to be designed out.


--Larry