The A-B-C board combination can be re-configured via the SMA connectors to act as a simple spectrum analyzer with a tracking generator to measure filter responses. That configuration is shown below.
The figure below is the response of a home made single section coaxial cavity filter. The gold line is a memory capture of the C Aux output directly into B RF input. Not sure whats going on with the base response on the lower end (its stable and consistently like this - my guess is there is some kind of coupling going on between the boards in this frequency range). The blue line is with the C board Aux connected to the filter.
The unit constructed was an initial test version to explore and understand the construction techniques outlined in "Quarter Wave Cavity Filters Using Copper Pipe, Mike Suhar, WB8GXB" and "Coaxial Cavity Filter for Modularized Spectrum Analyzer, Scotty Sprowls". The most significant difference is trying to use board edge SMA connectors rather than semi-rigid coax connectors and tubing (current price and availability). The pictures are prior to tuning (center tube not placed yet). A single section is constructed for simplicity and learning.