LibreVNA is an open source USB based vector network analyser with 2 ports. The VNA is an RF frontend with some processing power but everything else is handled in the PC application. The PCB has 3 ADC's and a FPGA for all the signal signal processing. It is able to do 10000 point dual-port measurements within one second and has an effective dynamic range of 100 dB.    
Thanks to two measurement ports this vector network analyser can be used to measure the complete set of reflection and transmission coefficients (S11/S12/S21/S22). Although the LibreVNA hardware is designed to be a VNA, the general hardware architecture is so similar to that of a spectrum analyser that simple two-channel spectrum measurements can also be performed.