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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.

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Last modified: March 27, 2001

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