- Joined May 15, 2010
There is no trough.I have a microcontroller generating a 1kHz sine. The FFT screenshot is below. There's a peak at 1kHz, of course. And there's a trough at 500Hz for some reason. And there's large content below 500Hz? I'm confused.
I sure am glad that you can see that detail on the image. Wish I could.No. The value at the “trough” is higher than the background noise above 1kHz.
Look at the 0Hz level. Now extrapolate as if there were no 1kHz signal. You will see there is no trough.
Your resolution is very poor. You need to sample for a longer period.
One cycle of 1kHz is 1ms. You have only captured half a cycle. Hence the FFT is showing the large DC component.
Reduce your sampling rate to about 10ksps and sample for at least 100ms.
A DAC is not actually a DAC without the LPF, which is an essential part of the conversion. DACs are just fancy switches (digital), and the output of an unfiltered DAC is a stair-step signal. DACs cannot produce smooth curves without a LPF. Each step transition in the output contains very high frequencies that don't exist in the original signal, they are artifacts of the conversion process.It's a DAC, not PWM. I know I need an LPF, but I wanted to understand the FFT analysis.
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by Steve Arar
by Steve Arar