[mmaimcal] Meeting tomorrow

Al Wootten awootten at nrao.edu
Mon May 21 11:18:43 EDT 2007


Folks,

Let's have an imcal meeting tomorrow.  We need to finish discussion on
the B7 change requests.  These are 170, 171 and 173.  CRE 172 is on the
prolongation of ATF activity, also of interest. We should also discuss 
the progress at the ATF.

CRE 171: Sideband rejection.
At the Board meeting, Vigroux and van Dishoeck inquired about the B7
CREs.  Ewine worried about the effect on sensitivity.  Here is how I
responded to Ewine, for discussion.  Comments please?

As you may recall, no one from the science ipt was invited to the B7
CDR.  We did, however, have contact and some discussions with 
the review committee members.  It seems to be a characteristic of the
B7 cartridges that toward the upper end of the USB side there is
decreased performance.  A close look at the plots Lazareff supplied with
the CRE illustrates this well.  He illustrates a non-compliant assembly,
No. 9, which shows excellent noise temperature at many places in the band,
but suffers at the high end.  He notes that the noise spec is relaxed
for 370-373 GHz, that it applies to the integrated spec over 4-8 GHz,
and that there is a relaxed spec over 20% of the band.  There is no
'relaxed spec' for any percentage of the band in image rejection.
Thus, a cartridge Lazareff illustrates shows rejection out of spec in only
2.5% of points.  In one case, 'improvement' of the cartridge image rejection, so
that all points met the spec, degraded noise performance by about 20K.
Since that spec is easily met, and is met by the retuned cartridge, this
cartridge is in spec but in a way which might not be preferred by an astronomer.

We worried about the sideband rejection ratio spec enough to remove it
from the document approved by the Board, and put it at a lower level in
the hierarchy.  When Stephane wrote Memo 505, on band calibration, the
spec was (not yet agreed by all) 14 dB.  This was lowered to 10 dB in one
of our negotiations--I think Tony Kerr suggested the number.  It wasn't
really scientifically driven so much as negotiated.  Should it have
specified 10dB over 95% of the band?  In retrospect, I think it probably
should have.  

For B6 we had this discussion also, and I expect it to arise shortly for
B3.  For B6, the builders chose to build a cartridge with a much wider
IF than necessary--in the end, they just unilaterally chose to provide
ALMA with a narrower IF, promising to meet specs over 5.5 GHz rther than
8 GHz (the receiver will have a few points with 9 dB rejection over the
whole 8 GHz).  There was some negotiation here--at first the offered
4 GHz but we countered with 5.5, suggesting that being able to do all the
CO isotopes at once to full spec was an important science driver.  We
discussed using waivers with Webber, but he did not like the paperwork
involved.  He chose a solution which required neither waivers nor
change request in the end, but one which can really only apply to B6.

How should one approach errant receivers--with CRE or with waivers?
We had a discussion with Webber on this for the SB gain ration for B6.
We had suggested waivers but he disliked the paperwork.  Darrel and I prefer
waivers as then errant receivers are identified and flagged, for possible
upgrade to spec.  If we have a CRE, the spec is changed and nothing
gets improved until the next generation.  But if a large fraction of
receivers might be waivered, then a CRE becomes better, particularly from
the supplier's point of view.

I think that perhaps for the B7 sideband gain ratio CRE, a modified spec
over some percentage of the band might be allowable without deterioration
in the ALMA science goals.  I think that this could meet everyone's goals,
from scientist to engineer.

Clear skies,
Al



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