Im designing microcontroller based dc to ac inverter based on unipolar PWM switching. The switching frequency used is 20khz and the sinusoidal frequency that I want to produce is 50hz with an amplitude of about 10V. I have an LC low pass filter at the output of my full bridge transistor circuit. The dc bus voltage is stiff and is around 40V. The sine wave reference given in the micontroller is based on a table lookup and contains only a sine wave of 50hz [made of 400 points].
When observing the output voltage im not able to get an exact 50hz signal. The signal produced contains additional 2 low frequency components at about 45 and 55 Hz however with smaller amplitudes (other than the 50Hz).
Im unable to trace the reason of why im getting such a signal, though the amplitude that im targeting (10V) can be observed. However this 10V is not directly seen due to the superposition of the 45 and 55hz components.
When removing the LC filter and observing the frequency spectrum of the voltage signal, more frequency components around the 50hz were observed. All the components were symmetrical to the 50hz and had smaller amplitudes as moving to the left or to the right of the 50hz component.
Has anyone experienced something similar before?