In her latest video Sarah from the SignalsEverywhere YouTube channel describes how it is possible to use an RTL-SDR to measure RF filters when combined with a noise source and the Spektrum SDR software. In the video Sarah comprehensively explains how to set all the various parameters in Spektrum, before demonstrating a filter measurement with a noise source. Sarah explains how the power measurements may not be entirely accurate, however it is enough to get some idea about the shape of a filter.
