... NRAO^1.1

The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation

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... VLA^1.2

URL address: http://www.nrao.edu/

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... pair.^2.1

where the two IF pairs overlap, no reduction of the noise by $\sqrt 2$ is achieved because the creation of the two IF pairs occurs after the first amplification and they will therefore be essentially the same and have identical noise.

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... MHz^2.2

Typically, one would leave this to the default; a narrower filter, however, can be used to avoid strong interference which might cause gain compression in the waveguide system. Use of narrower filters will restrict the allowed settings of the local oscillators chain.

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... 1D)^2.3

the number of a correlator mode indicates how many IFs are involved, the characters indicate which IFs.

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... phase^2.4

the round-trip phase continuously monitors the length of the path between the local oscillator transmitter in the D-rack in the control building equipment room and the LO receiver in the B-rack in the antenna vertex room; it is used to correct phases for variations in this path.

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... pair^2.5

because of the way the LO chain was designed, when using a 50 MHz bandwidth there is no freedom to move this wide bandwidth about in frequency.

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... D.^2.6

Note that the restriction that the IFs be tuned within the same observing band does not apply when observing in 4P or LP mode, in which case the AC IFs are tuned to 4-meter or L-band while the BD IFs are at P-band. This is possible as the 4- and P-band feeds are at the prime focus whereas the feeds for all other frequencies are at the Cassegrain focus. However, in LP mode the subreflector is set at the L-band focus which implies that the P-band data will be degraded.

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... versa^2.7

P- and 4-band signals take a different route through the IF system and avoid the unit which contains the transfer switch; consequently this option is not available at the prime focus bands.

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... components^3.1

see chapters 5 and 17 in SIRA.

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... do.^3.2

In the past, autocorrelation normalization has been used. However, there are several drawbacks. Firstly, this correction is done on-line and cannot easily be undone at a later stage. Secondly, because the cross-correlated spectra are divided by the geometric mean of the autocorrelated spectra of the antennas involved, only the amplitudes will be corrected (phase information is lost when doing an autocorrelation). Lastly, if there are features in the spectrum, real or due to radio frequency interference (RFI), which are strong enough to show up in the autocorrelated spectra, they will be folded into the data, corrupting the cross-correlated spectra. This occurs, for example, when observing strong masers, strong absorption, Galactic H I emission, or planetary radar. For all these reasons, the use of autocorrelation normalization is strongly discouraged.

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