The BPF board contains individual bandpass filters for each ham-radio band (9 filters) and the control circuit that allow to switch filters using the two wire serial line.
The filters are 3 tank circuits with inductive coupling between tanks and capacitive with source and load. The coils are wound on the cylindrical frames (with 14mm diameter) made from the teflon. The BPF design is taken from the famous RA3AO high performance transceiver.
The BU2090F IC is used to switch the filters. It is a 12 bit shift register with output latch and high voltage (25V) open drain outputs. It is controlled by two wire serial interface.
A separate interface for the bandpass filter, attenuator, lowpass and antenna switching is used because this interface is active only during band/attenuator/antenna switching and will not be the source of the noise during normal operation. (While the SPI bus used to interconnect DSP, synth and control processor is very noisy, but is isolated from the signal paths.)
The schematics of the BPF board is here.