The lack of a normative framework to measure the electrical grid disturbances in the 9-150 kHz range leads to the need of assessing the accuracy of candidate methods against different types of disturbances. For that, a wide set of test waveforms with known amplitude levels are needed. This work proposes a methodology to generate synthetic test waveforms, with known theoretical values of the power spectral density along the entire spectrum. The synthetic test waveforms can be combined to form compound signals, containing complex patterns in time and frequency domains, with characteristics similar to those observed in the distribution grid. The synthetic test waveforms developed with this methodology allow a further analysis of the performance of the measurement methods, as they provide a study framework similar to the end-use of the measurement methods in the LV grid.