An RF processing module that when coupled to a computer having appropriate signal processing software will function as a spectrum analyzer. The RF processing module has a first stage mixer and a switch that allows either port of the first stage mixer to be selected as the input and receive the RF signal. The RF processing module has a sample packing buffer that packs digitized signal samples into a fewer number of longer words and loads the words into a circular buffer and from then to one of two DMA channels on a USB link. The circular buffer can be emptied faster than it can be filled, allowing the sample packing buffer to create time intervals for a USB controller to switch DMA channels. LO leakage through the mixer-to-IF path is used to self-calibrate the filter's frequency response as temperature and other environmental factors change.