13 kHz FM-bandwidth Roofing Filter, Factory Installed*
*Required for FM. AM and FM bandwidth filters are required on the main K3S TX/RX board for TX in AM and FM modes. (Only one AM filter is required for AM TX/RX, or one FM filter for FM and AM TX/RX.) All filters can be used for RX in any mode.
There are 5 filter positions per K3S and per KRX3A. The 2.7 (or 2.8) kHz SSB filter uses one of these positions