[mmaimcal] Re: ALMA rx specs (draft version)

[Al Wootten]

Folks:  Any comments on the following response to the receiver spec? -Al

Comments on Wild/Payne proposed receiver specifications:
http://www.cv.nrao.edu/~awootten/mmaimcal/receiverspecs.html
--------------------------------------------------------------------
1.3 Definitions
* 4 GHz IF bandwidth as a fall back position for the initial bands
This is too vague.  Does it apply to 900 GHz, not an intial band?
I believe that any specific proposal for a design with less than 8 GHz
bandwidth should be approved by others in the project whom it might affect.
See below.

*band 3 starts tentatively at 86 GHz (before 89 GHz)
See below.  
The ASAC actually wrote
(http://www.cv.nrao.edu/~awootten/mmaimcal/asacreport/node3.html):
'We strongly urge that the JRDG study the possibility of extending the 
lower frequency range of Band 3 to include the SiO maser transition near 
86 GHz. If this is possible, Band 2 would drop to third priority. '
See comments below.

* a water vapor monitoring radiometer operating simultaneously with all
frequency bands except band 1

I don't think this is worded correctly.
The ASAC said:
The ASAC does note that the simultaneous operation at 183 GHz and Band 1
receivers is not a scientific requirement, so it is straightforward to 
locate these systems in the same Dewar if that makes sense. 

In the best of all possible worlds, the WVR would work with band 1 as
well as the other bands--this will be necessary under a wide range of
conditions at Chajnantor for which ALMA may be operable at band 1 with WVR
but ALMA would be shut down otherwise.

However, if the cost of operating both simultaneously is very high one
possible sacrifice might be simultaneous operation of WVR and band 1.
This is very different from designing a receiver system which will not
accommodate simultaneous operation of both from the outset, in my opinion.
--------------------------------------------------------------------
Table 1

Band 3 should conform to the original recommendation, with a footnote as
written.

The ASAC actually said:
(http://www.cv.nrao.edu/~awootten/mmaimcal/asacreport/node3.html)
'We recommend study of extending the lower end of Band 3 to include 86 GHz.'
Actually, since the VLBA receivers at 3mm will go down to 84 GHz, perhaps
some attention should be paid to the actual number of the lower limit.
I would suggest 84 GHz.
--------------------------------------------------------------------
Reference Documents
I would include the CDR, PDR reports at
http://www.alma.nrao.edu/administration/index.html
--------------------------------------------------------------------
3.1
I'm not sure what available atmospheric frequency windows means.  Change this
to The ALMA receiver subsystem will cover frequencies between 30 GHz and
950 GHz as given in Table 1.
--------------------------------------------------------------------
3.2 Polarization
The ASAC report at
http://www.cv.nrao.edu/~awootten/mmaimcal/asacreport/node12.html
needs to be made more specific.  Steve Myers has put work into this, and
his draft recommendations are located at:
http://www.aoc.nrao.edu/~smyers/alma/polspecs-imcal.txt
We invite comments on this document.
--------------------------------------------------------------------
3.4 Receiver noise performance.
I think that this should be more specific.  Stephane proposed:
'1)  Receiver noise temperature should not exceed (NUMBERS GIVEN FOR Freq.
below 370 GHz) for Single Side Band receivers, 6 h.nu/k averaged over the 
best 80 % of its nominal bandwidth, and 10 h.nu/k at any place in the 
nominal bandwidth for Double Side Band receivers, these numbers
should be halved
...
 What about sub-mm frequencies. Would 10 h.nu/k be acceptable, conservative
or too difficult ?'
I agree.  In our memo on sensitivity goals, Bryan and I used an equation:
TrxSSB= A * (hnu/k) + 4 K
And for current technology, based on the review at the URSI 99 meeting:
with A = 3 below 500 GHz
     A = 6 for band 9 (602-720 GHz)
     A = 12 for band 10 (787-950 GHz)

We suggested goals for these future receivers of:
     A = 3 below 500 GHz
     A = 4 for band 9 (602-720 GHz)
     A = 8 for band 10 (787-950 GHz)

I believe that there should be specifications and goals, with achieved receiver
performance to fall between the two.  I think Stephane's proposals are
acceptable for specifications, though I feel fairly confident that better
results may be achieved.
--------------------------------------------------------------------
Sidebands, IF bandwidths and Simultaneous operation of bands

This seems too unspecific to me.  The Munich PDR
(http://www.cv.nrao.edu/~awootten/mmaimcal/ALMA-DesRevRec4.html) said:
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.

Of course, I guess this is only a recommendation, but it might be a
starting point.  Larry made more specific recommendations toward a decision.
--------------------------------------------------------------------
3.7  This states that band 1 can operate without the WVR.  As above, I think
the system should only be designed this way if no alternative seems possible.
--------------------------------------------------------------------
3.8 Stability
The ASAC report suggested 1e-4 over 1 sec.  A memo from Wright 
http://www.alma.nrao.edu/memos/html-memos/abstracts/abs289.html
suggested 1e-4 over 0.1s, which begins to define the spectrum of stability.
--------------------------------------------------------------------
4.3 Selection of a new observing band

'Selecting and tuning a new band shall require no more than 15 min.'
I'm not sure where this came from, but it should be better defined.
This time should be measured in seconds, along with the others.  It shouldn't
be several orders of magnitude different, as it is in this draft.

The System PDR (ref above) said:
' Changing the band should be limited to intervals of the order of 15
    minutes. '
but I never heard this discussed in Munich (neither did anyone I have
discussed this with) so I remain unclear what it means.
--------------------------------------------------------------------
Stephane also noted that:
'The receiver temperatures should be measured in a reference plane outside
the dewar.'
I agree with what he proposed for this.

Further, he noted that coupling to the telescope remained poorly defined and
should be made more definite.  I agree.