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EZ-KIT 21489, sample rate, harmonics

Question asked by PGibb on Aug 7, 2013
Latest reply on Sep 2, 2013 by MaheshN

I'm running the talk-through demo with input sampling of 48kHz.

My input is a sine wave at 1000Hz.

Because my algorithm runs better at a lower sample rate and because I need more available processing power I want to down sample to 12kHz.

I'm aware that I will need a low pass filter to take out higher frequencies but for right now all I'm feeding in is 1000 Hz. When I change to code to only output every second sample (24kHz output sampling rate) I notice a harmonic (if that's what it is) at 21500Hz. So far this wouldn't be a problem.

Now when I change the code to output every 4th sample (12kHz sample rate) I get this harmonic at 13000Hz. There is also a "mirror harmonic" at 11000Hz. I say mirror harmonic because as opposed to the other kind, this one goes down in frequency as I increase my input frequency.

For example if I change the input tone to 1400Hz, the harmonic goes to 13400Hz and the mirror harmonic goes to 10600Hz.

 

I would be happy with a bandwidth up to 5000Hz, a tone at this frequency causes the mirror harmonic to be at 7000 Hz. Interestingly enough the actual tone and mirror harmonic converge at 6000Hz. At this point the previously mentioned harmonic will be at 18000Hz, where it has also converged with a mirror harmonic of itself.

 

I would really like to understand what's going on here, I thought that everything would be fine as long as I used at least twice the sampling rate of my highest frequency component.

 

On a side note: Looking at a 750Hz tone on the oscilloscope at this 12kHz sampling rate, I can see very clear steps from the DAC. These steps create very high frequency components and I assume the EZ Kit was designed with the lowest sampling rate being 48kHz. I feel comfortable that I can filter these out with a low pass RC filter, at least it makes the curve look very smooth on the scope. My problem are these lower frequencies.

 

I have attached a sweep from 1000 to 7000 Hz. Thanks in advance for any help.

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