To see what might be causing the problem, I used a version of the filter noise test matlab code that Matt had in the elog.

To see if it was a single precision problem, I just recast the input data:   x = single(x)

This is not strictly correct, since some of the rest of the operations are as double precision, but I think that attached plot shows that a casting from double to single is close to the right amount of noise to explain our excess noise problem in the 0.1-1 Hz region.

Den is going to interview Alex to find out if we have some kind of issue like this. My understanding was that all of our filter module calculations were being done in double precision (64 bit), but its possible that some single stuff has crept back in. Currently the FIR filtering code IS single precision and in the past, the SUS code which didn't carry the LSC signals (meaning ASC and damping) were done in single precision.