Calibration Errors and Artifacts from Imperfect van Vleck Correction
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ALMA Memo # 591
Calibration Errors and Artifacts from Imperfect van Vleck Correction
D. Emerson (NRAO)
2011-04-28
Keywords: Van Vleck correction, calibration, digitization, XF correlatorIn modern radio astronomy systems, the signal from the receiver is usually converted to an IF band and then digitized using a small number of bits. In an XF correlator, this results in a non-linearity of the resultant correlation function, which is compensated for in a way usually referred to as the van Vleck correction. For a given but small number of quantization levels, the shape of the van Vleck correction function is itself dependent on the level of the input signal, compared to the size of the discrete steps of the digitizer. In typical radio astronomical observations, especially at mm- and submm-wavelengths, changing atmospheric conditions may produce relatively rapid change in the total noise power presented to the receiver from the antenna. For highest ultimate dynamic range of observations, either the level of the signal needs to be held constant in terms of the digitization steps, or else the specific van Vleck correction needs to be recalculated sufficiently often, based on the measured input level, so that the detailed correction function remains appropriate. Imperfect linearity correction will introduce errors in the derived amplitude and phase of an interferometer, which vary with frequency across the digitized pass-band. Harmonic distortion will appear within the pass-band, which may limit the spectral dynamic range. The potential magnitude of such errors is examined.
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