Introduction

The GBT IF system was know to have a non-linear response in the presence of strong continuum sources when data was obtained with the spectrometer. This non-linearity is responsible for the strong continuum induced baseline structures. (For more information see the reports of the GBT Spectral Baseline Investigation Group.)

In the course of checking out the GBT01A_004 proposal the non-linear response was isolated to be between the LO1 mix with the RF signal in the X-band receiver and the LO2 mix with the IF signal in the Converter Rack. (See Toney Minter's document GBT01A_004 Observation Checkout Report -- March 31, 2004.)

Test observations were performed on April 4 and April 8, 2004 in an attempt to further isolate where the non-linear response occurs in the IF system. We also wanted to determine if the non-linearity could be seen by all GBT backends. We report on the results of these observations in this note.