Croaker wrote on Jan 19th, 2007, 8:06am:Yes, measuring a signal at different fres values will change the PSD. For thermal or white noise, the PSD remains flat for different fres values because as fres changes so does the measured power. However, if you look at a pure sinewave at freq f1 and increase fres, the PSD value for f1 will drop because the PSD is a constant power divided by an increasing fres.
I don't think I'm mixing up energy and power. Don't you get more measured power if you use a wider bandwidth to measure thermal noise? This is what I'm talking about in the paragraph above.
Actually, you raise an interesting point. 1/f noise does not have infinite noise power at f=0, however, as you get lower in frequency you need to decrease fres to make the measurement, and so the PSD does go to infinity at f=0! This is what my original post is kind of about. I'm confused about how the actual noise power of flicker noise differs from PSD. What is the actual power of flicker noise at f=0 for instance?
Marc
Dear Marc,
I do think that you are mixing something up. You are right about the white noise part. But true PSD is defined for convenience as the power
within a 1 Hz bandwidth. Obviously, you cannot use a 1 Hz measurement bandwidth to measure 1/f noise at say 0.0001 Hz. So, some sort
of renormalization is in order, and this must be and is performed by whoever does the measurement. Think of it as the scaling that you would do
when you keep on increasing the number of FFT points for a given set of data points. You get progressively lower noise floor, but the actual noise
is not going down. All you are seeing is the processing gain.
Regarding the 1/f noise going to infinity at f=0, I don't see why that presents a problem. You would have to make a measurement for an infinite amount of time before that affects you.
Regards
Vivek