[asac] [almanews] ALMA Memo 357 and Memo 358 Released

[Carolyn White]

ALMA Memo 357

Sideband Calibration of Millimeter-Wave Receivers 

A. R. Kerr, S.-K. Pan and J. E. Effland

2001-Mar-27

Unless the sideband ratio of a heterodyne receiver is close to the ideal
value (less than ~ 0.1 dB for a double-sideband receiver, greater than ~
20 dB for a sideband-separating receiver), a significant correction is
required to the single-sideband receiver noise temperature deduced from
the Y-factor measured using broadband hot and cold loads.  If, in
addition, there is significant conversion gain from one or more of the
higher harmonic sidebands, nfLO +/- fIF (n = 2, 3,...), that must also be
taken into account when evaluating the sideband ratio and the
single-sideband receiver noise temperature.

In principle, the sideband ratio of a receiver can be measured by
injecting CW signals of known relative amplitudes into the upper and lower
sidebands and measuring the IF response to each.  At millimeter
wavelengths, however, it is difficult to determine with sufficient
accuracy the relative amplitudes of two low level RF signals separated in
frequency by twice the IF (2fIF = 8–24 GHz in the case of ALMA
receivers).

This note shows that the image rejection of a sideband-separating mixer
can be measured accurately using CW test signals in the upper and lower
sidebands, even when the relative power levels of the test signals are not
known.  This allows accurate determination of the upper- and
lower-sideband gains and the single-sideband noise temperature of a
sideband-separating receiver, even if it has poor image rejection.  In
contrast, there is no simple and accurate way to determine the sideband
ratio and SSB sensitivity of a DSB receiver.  This has implications for
ALMA’s single-dish mode of operation, in which sideband separation using
LO phase switching is not possible, but high SSB measurement accuracy is
required.


View a PDF version of ALMA Memo #357
     http://www.alma.nrao.edu/memos/html-memos/alma357/memo357.pdf

Download a postscript version of ALMA Memo #357
     http://www.alma.nrao.edu/memos/html-memos/alma357/memo357.ps


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ALMA Memo 358

OBSERVING


Melvyn Wright (Radio Astronomy Laboratory, University of California,
Berkeley)

2001-Mar-23

This memo proposes an entity which provides an integrated description for
dynamically scheduled observations.  The ``observing object'' is created
from the observing proposal and used throughout the observing and data
reduction process. It is archived and provides a self contained and
complete record of the data.

View a PDF version of ALMA Memo #358
     http://www.alma.nrao.edu/memos/html-memos/alma358/memo358.pdf

Download a postscript version of ALMA Memo #358
     http://www.alma.nrao.edu/memos/html-memos/alma358/memo358.ps
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