Method and apparatus for a superspeed USB bus powered real-time spectrum analyzer
View Patent ↗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.
1. A method, comprising the steps of:
feeding a first local oscillator signal with a fixed frequency into a first mixer;
receiving a leakage of the first local oscillator signal at a second mixer;
feeding a second local oscillator signal into the second mixer and combining with the leakage of the first local oscillator signal to generate an IF signal;
sweeping the second local oscillator signal across a range of frequencies selected so that the IF signal is sweeping across a nominal passband of a filter downstream of the second mixer;
acquiring filter data downstream of the filter during the sweeping of the IF signal across the nominal passband of the filter; and
processing the filter data to obtain frequency response and amplitude corrections for the filter.
2. The method of claim 1 , further comprising the steps of:
applying the amplitude corrections to spectrum data collected during analysis of an RF signal.
3. The method of claim 1 , further comprising the steps of:
finding 3 dB points nearest each side of a peak amplitude in the filter data;
determining an actual center frequency for the filter based on the 3 dB points;
determining a maximum bandwidth available for the filter based on the 3 dB points; and
adjusting frequency sweeps of the second local oscillator during analysis of an RF signal to maximize an amount of useable filter bandwidth based on the determined actual center frequency and maximum bandwidth available for the filter.