Wednesday, April 13, 2016

Noise Generator

After making the MMM, I was thinking about making a low pass filter to clean it up.  But having no toroids kicking around the garage at the moment, I was thinking about building all air core wound on forms.

But then I started thinking how to measure the LPF.  I started thinking about noise generators - feed broadband noise into the LPF, and look at the output on the FFT of the oscilloscope.  In theory, anyway.

I came across this circuit and decided to build it.  

Had several missteps (wrong resistor value, forgot to cut a trace, etc).  But after all that it still wasn't working.  I noticed in their writeup they mention that they got it from a different site, and when I looked at that one, I noticed they were using slightly different values (including a 12v source).  On a whim, I tried the higher voltage, and got noise!

I'm not sure if I'm getting as much noise as I want, though.  The output of this seems to be maybe 2v P-P.  If I'm reading the FFT right, it looks like the noise is up 30dBVrms.  But the FFT doesn't look to be going out as far as I would think.  If I'm reading it right, it is only showing me the fft out to around 600 KHz.  Strange.

After asking around and playing with the scope, I found that you need to adjust the horizontal time scale *before* you go into the Math function.  I also adjusted to use the maximum memory (under the horizontal menu) - not sure if that helped.

I think the next step here is to look at the output signal level and see if I can figure out how to go about increasing that - feels like that will be needed to really see signals when I try to measure filters.

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