2 Replies Latest reply on Oct 22, 2017 7:26 PM by NoriakiT_91

    Why PWM Generating side frequencies ?


      Hello Every One,


      I am generating 25 kHz frequency from PWM and passing it through a low pass filter with a cut off frequency 50 kHz. If I see the FFT of this signal I am getting many frequency peaks (e.g. 1kHz, 3kHz, 5kHz and so on). My PSOC IMO Clock is 48MHz. PWM clock is 1MHz. I don't !@ know the reason why this is happening.

      I will Look forward to your reply!



        • 1. Re: Why PWM Generating side frequencies ?

          Can you share your circuit diagram, a picture of how you build it (PCB, breadboard?), and scope shots (wave form before and after the filter, and the FFT result)? And maybe your project as well.

          • 2. Re: Why PWM Generating side frequencies ?

            Please let me know...


            1) Did you observe side band spurious around the 25kHz peak signal?  In other words, are there 20kHz, 22kHz, 24kHz, 25kHz, 26kHz, 28kHz, 30kHz spectra?


            2) Or did you observe lower frequency spectra like 1kHz, 3kHz, 5kHz?


            3) What is the sampling rate of the FFT and the number of samples used for the FFT?



            If 1) is true, it is assumed that the PWM output amplitude is modulated by a 1kHz square wave.  The possible noise source is power supply.  Are there any switching devices using a power supply for the PSoC's VDDIO?


            If 2) is true, it is assumed that the ground level is affected by a noise source. Please connect the ground line of the FFT probe to a ground pattern close to the PSoC.