EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: SArepairman on April 06, 2014, 05:19:58 am
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I was wondering what the 1/f noise will look like for a PWM signal that is sent through a RC filter.
Does the 1/f noise of the resistor dominate the circuit? Does the capacitor cause anything?
how about a really big LC filter?
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I ask this because I need a null voltage circuit, I have a PWM output that I don't need... I was wondering if I used a big transformer and a capacitor could I get a nice "nuller"
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Nulling what, just a low bandwidth (a few Hz) DC signal to read off a galvanometer?
I'd be way more concerned about the supply stability (your PWM digital output buffer is being used as a 1-bit DAC) and switching speed (and stability) than 1/f noise from a resistor. Which, if this is for nulling...will carry zero DC current anyway, so what's the 1/f noise coming from?
If this is more about, like, nulling on a scope, then you'll probably want more attenuation than just a few stages of RC or LC filtering can provide. Just to get the ripple down, nothing to do with noise.
Noise is more of a microvolts thing, what are you doing that you need that kind of precision? Is there a better way to do it? If it's that precise, it's probably best done in analog in the first place, leave the MCU out of it. Is that an option?
Tim
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well im 99.9% likely to implement this using a DAC, I am just wondering what the filtered signal would look like on a FFT in a DSA.
its not a practical solution considering that highly stable dacs are available.
its just difficult to imagine for me what it would look like.
I imagine that a LC filter changing would just cause the ripple voltage to drift ( as when the cut off frequency changes the attenuation @ ripple frequency will change by some miniscule amount).,.
but what about 1/f?
like, if you took the output signal of the RC/LC filter and put it into a 1Hz cutoff preamplifier filter connected to a digital signal analyzer and nulled out the error from the amplifier to get accurate 1/f information about the DUT.
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I was wondering what the 1/f noise will look like for a PWM signal that is sent through a RC filter.
Does the 1/f noise of the resistor dominate the circuit? Does the capacitor cause anything?
how about a really big LC filter?
I couldn't imagine the thermal noise voltage of the resistor being anywhere near the amplitude of the raw PWM, let alone any flicker (1/f) noise that the resistor may have.