An aerospace client observed some anomalous signals in an unexpected frequency band. They contacted S&ST to provide some additional consulting expertise. A follow-on sub-contract was put into place to provide an analysis of the Harmonics and Intermodulation Distortion effects of the transmitting and receiving devices. An in-depth simulation was made using a Volterra series to replicate the instantaneous output vs. input power. Coefficients up to the 43rd power were determined for a faithful modeling of the input/output.
Literature searches at local universities by S&ST provided a head start and avoided "re-inventing the wheel." Building on these results and on a preliminary MATLAB simulation program, a sophisticated software tool was produced. Interim reports and color documentation in both hardcopy and magnetic media were produced. Some of the specific items addressed included:
AM/PM Conversion
Anti-aliasing of signals with formidable harmonics
Basebanding and sub-band isolation
Cascading of the Intermodulation Distortion effects
Frequency Interpolation
Gaussian Elimination
Histograms
Multirate Signal Processing
Nelder-Meade Simplex Minimization
Nonlinear Systems
Phase Preservation
Piecewise Linear methods
Power Spectral Density (PSD) and other spectrum estimation methods
RMS/dB interchangeability
Saturation and Clipping analyses and Compression Point studies
Smoothing
Spectral Leakage
System Simulation and Modeling
TWTA/Klystron & GaAsFET operation
Volterra Series
The software is user-friendly (including examples with tutorials), structured, and modular for interchangeability. To our knowledge, nothing of this magnitude has ever been built to correctly simulate the intermodulation distortions. The high order capabilities proved most helpful to our client because of the surprising strength of the IMD/harmonics in frequency bands where they were not expected to occur. Their representatives have expressed a desire for future teamings as appropriate opportunities arise. With prior approval, they may be contacted regarding our services.
