The detected RF signal is processed an direct-mixed to DC level. Fortunately this direct-conversion principle already provides a large amount of filtering against off frequency noise and interfering signals. But additionally the DC level is further heavily filtered to even increase the extremly narrow-band measurement. The only reason to not make the bandwidth even smaller is the resulting group-delay, not to destroy measurements at the highest sample rate of 3kHz. Measurement bandwith is only 5Hz/-3dB. Signals outside the bandpass don't interfere the measurement results - a great asset, should you ever try to measure the impedance of a 40m high 160m vertical... with this VNA you can even do it at nighttimem when the antenna picks up a lot of RF-energy...