Signal Hound BB API
Real-Time Spectrum Analysis

The API provides methods for performing real-time spectrum analysis up to 160MHz in bandwidth.

Real-time spectrum analysis is accomplished for the SM using 50% overlapping FFTs with zero-padding to accomplish arbitrary RBWs. Spans above 40MHz utilize the FPGA to perform this processing which limits the RBW to 30kHz. Spans 40MHz and below are processed on the PC and lower RBWs can be set. See the Real-Time RBW Restrictions for more information.

RBW directly affects the 100% POI of signals in real-time mode.

Real-time measurements are performed over short consecutive time frames and returned to the user as frame and sweeps representing spectrum activity over these time periods. The duration of these time periods is ~30ms.

Real-Time Frame

Real-time spectrum analysis returns the sweep, frame, and alphaFrame.

The frame is a 2-dimensional grid representing frequency on the x-axis and amplitude levels on the y-axis. Each index in the grid is the percentage of time the signal persisted at this frequency and amplitude. If a signal existed at this location for the full duration of the frame, the percentage will be close to 1.0. An index which contains the value 0.0 infers that no spectrum activity occurred at that location during the frame acquisition.

The alphaFrame is the same size as the frame and each index correlates to the same index in the frame. The alphaFrame values represent activity in the frame. When activity occurs in the frame, the index correlating to that activity is set to 1. As time passes and no further activity occurs in that bin, the alphaFrame exponentially decays from 1 to 0. The alpha frame is useful to determine how recent the activity in the frame is and useful for plotting the frames.

The sweep size is always an integer multiple of the frame width, which means the bin size of the frame is easily calculated. The vertical spacing can be calculated using the frame height, reference level, and frame scale (specified by the user in dB).

An example of a frame plotted as a gray scale image, mapping the density values between [0.0,1.0] to gray scale values between [0,255]. The frame shows a persistent CW signal near the center frequency and a short-lived CW signal.
The same frame above as is plotted in Spike, where density values are mapped onto a color spectrum.

Real-Time Sweep

The sweeps returned in real-time spectrum analysis are the result of applying the detector to all FFTs that occur during the real-time frame period. A min/max detector will hold the min and maximum amplitudes seen during the frame period. The average detector will average all sweeps together during this period.