Memo Review

Memo: 423 -  The Vane Calibration System Revisited
      Guilloteau, 2002May27

Reviewer: Jesus-Martin Pintado

Date Received: 2003Feb07


This memo shows the implications of the expected saturation of the ALMA 
receivers on the accuracy of the semi-transparent (S/T) vane calibration 
scheme. It is shown that achieving 1% accuracy in the calibration 
requires the use of two different vanes. One optimized for 86 GHz and 
the other for 280 GHz. The memo also shows the advantages of adding an 
ambient load which allows to estimate the degree of saturation of the 
receivers and to correct for the effects of possible saturation on the 
S/T vane.

General comment

The memo clearly shows that achieving 1% accuracy is very challenging. 
The memo addresses the effects of mixer saturation and its implications. 
For completeness, it would be useful to include other possible effects 
like the gain compression in the IF amplifier chain. What is the present 
specification for the IF gain compression?.

Section 1
I am a bit confused with Fig. 1. Trec SSB is a factor of 2 larger than 
DSB, but (in Fig. 1) Tan (Trec+Jsky) in SSB is only 1.4 time larger than 
in DSB.

Section 2
In page 4, it is motioned that compression in the IF chain is not 
considered. As mentioned before, it might be quite useful to include at 
least the baseline specification for the IF gain compression.

Section 3

After the equations of the semitransparent vane it would be useful to 
define 1-f as the transmission coefficient of the vane.
I suggest to suppress Fig 3 (referred in the text as Fig. 3.1 ) since it 
is clearly explained in the text.


It is unclear to me what on-time fraction means in Figs 4 and 5 bottom.

All the calculations are made for a S/T vane absorption coefficient 
proportional to the frequency. However, most of the materials measured, 
so far, seem to show an absorption coefficient proportional to the 
frequency to the 1.5. It might be useful to consider also this case to 
specify the optimum number of S/T vanes the their transmission 


End of page 12. This is the first time that it is mentioned that the 
nature of the losses in the S/T vane calibration scheme might be of 
importance to achieve the 1% calibration accuracy. It might be 
interesting to mention this assumption at the beginning of section 1.

It is also suggested that 1% calibration accuracy in Band 7 is difficult 
to achieve. Since this has important implications it might also appears 
in the Abstract.


Last line in Page 2 say "measure by the correlator" -> "measure on the 
 source by the correlator"

Fig2. figure caption "function for frequency" -> "function of frequency"