During GBT spectral line, frequency switching observations at L-band it was noticed that one polarization (linear Y) had significant baseline structure. There appeared to be structure which was roughly periodic with a wavelength of ~8 MHz. Initial tests were performed to investigate this problem. By swapping cables along the signal path it appeared that this structure was coming from the RF filter component. Baseline variations with smaller amplitudes were also observed as a function of subreflector position in both polarizations. The standard +/- wavelength/8 defocussing appeared to cancel most of this smaller amplitude structure. The receiver was taken down for inspection. No major problems were discovered although an isolator which had a dip in its passband was replaced.

When the receiver was re-installed on the telescope the baseline structure still existed. In fact with longer integration times there appeared to be ~10 MHz structure in both polarizations and a ~1.5 MHz ripple in the linear Y polarization. This structure was observed with both the spectrometer and the spectral processor.