Hi Dana, The 1500 Hz hole in SSB mode is an artifact of Weaver Demodulation. Looking at the KX3 Firmware Information document, MCU 00.77, DSP 00.74, 3-6-2012: IF the KXFL3 is installed, Weaver demodulation is enabled when HI-CUT is below 2.4 kHz. John KN5L