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Walsh Function Demodulation in Presence of Timing Errors... Abstract

by Dong-Chan Kim last modified Apr 25, 2011 by Pat Murphy

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ALMA Memo # 537

Walsh Function Demodulation in the Presence of Timing Errors, leading to Signal Loss and Crosstalk
D.T. Emerson


In the ALMA system, switched phase offsets of pi/2 radians are added to the first LO signal at each antenna as a means to separate upper and lower sidebands; these offsets are put on the LO phase following a Walsh switching cycle, and are decoded following correlation. Additional phase offsests of pi radians are applied to the first LO at each antenna, as a means of reducing spurious correlations between unwanted coherent signals that find their way into the IF at the antenna; these pi phase steps are demodulated at the antenna by corresponding sign change at the digitizer.

If there is a timing offset between the modulating and demodulating Walsh waveforms, then in general

there are 2 unwanted effects: (1) A loss of amplitude of the wanted signal, and
(2) Unwanted crosstalk introduced between otherwise orthogonal modulations.

This memo investigates the magnitudes of these effects in the ALMA context, and how the magnitudes are dependent on the specific choice of Walsh function. A strategy for choosing the optimum Walsh functions is suggested.

View a pdf version of ALMA Memo #537.