Harmonic cancellation in switched mode Linear Frequency Modulated (LFM) excitation of ultrasound arrays

Switched excitation is a convenient alternative to high frequency linear power amplifiers for multi-channel array systems. Unfortunately, switched mode excitation signals often contain harmonics that are undesirable in applications such as coded harmonic imaging. Harmonics can be reduced in switched excitation with the addition of intermediary voltage levels and careful control of switching timing. This work discusses two, three, five and nine level switched mode excitations suitable for Linear Frequency Modulated (LFM) coded excitation. Simulations in MATLAB compare two LFM signals with a central frequency 0.5 MHz, duration of 100 μs and bandwidths of 25% and 100% to an ideal `analogue´ case. Simulated results shows harmonic cancellation in the switched mode signals. For the nine level case, all harmonics below the 11th are cancelled. Experimental measurements are also obtained using the UARP system loaded with a 0.5 MHz transducer. Experimental results show cancellation of harmonics to -40 dB below the fundamental, however an increase in the third harmonic is apparent when comparing nine levels to five levels. To further verify harmonic reduction, calculations of the Total Harmonic Distortion are provided for each switched mode excitation up to the ninth harmonic.