David, Yes, you need the 6kHz filter for ESSB. The 2.8kHz filter will strip it down to 2.8kHz. 73, Don W3FPR On 1/29/2016 8:07 PM, David Ahrendts wrote: > Am I correct that you do not achieve 4.0 kHz ESSB (widest TX with the K3S) without selecting the 6.0 kHz crystal filter for SSB TX? Just want your affirmation. :—) >