2008-09-15
ALMA system adopts Walsh functions of 128 patterns in length as 180-degree and 90-degree phase switching patterns. However, ACA can not freely choose patterns from them, because switching base time is required to be a multiple of FFT operation interval of ACA Correlator. In addition, the pattern length of 128 is not necessary, because there are only 16 antennas in the ACA. Thus, from the Walsh functions of 128 patterns, we searched ones which were available in the ACA. We also estimated sensitivity leakage, sensitivity loss and crosstalk power with 1% time shift of the Walsh functions. As a result of this study, sixteen Walsh functions with smaller sensitivity leakage and loss have been chosen as candidates for phase switching patterns for the ACA. With the selected functions, the sensitivity leakage and loss can achieve 2.5 % or less and 0 % in the case of 1 % time shift between modulation and demodulation of an identical Walsh function, respectively. The averaged sensitivity leakage and loss over the combination of two Walsh functions are 1.3 % or less and 1.3 % or less in the same case, respectively. The RSS crosstalk power can achieve 1.2 % or less in the case of 1% time shift between two different Walsh functions, respectively.
View a pdf version of ALMA Memo #581.
alma-memos@nrao.edu