How we want to calculate a noise from our scope?I mean,i want to know the frequency of the strength exist.For example, i attached this scope.So, from the scope how we could measure the noise?
I also recommend searching around the Tektronix site for application notes that talk about noise measurement.
If you just want an estimate, then you can find a rule of thumb for typical white noise crest factor. The crest factor is the Peak-to-average ratio. It is easy to see the peak value on the scope, but what you really want is the average noise voltage so that you can calculate the average noise power. The rule of thumb I use for white noise (ie. flat broadband in the frequency domain) is 10. So, if you measure the peak value on the scope (over a one second sweep, for example) then just divide by 10 to determine the average noise voltage, which is what you would use for signal-to-noise ratio and other things.
I recall reading about a noise measurement technique many years ago for random signals using a dual-trace oscilloscope. You put the noise signal into both inputs set at the same sensitivity to get a noise trace of several divisions. You then use the vertical position controls to move the traces until the overlap of the two traces blends together and it looks like one trace. The vertical separation of the two traces then roughly equals the RMS value of the noise.
This technique probably works best with an analog scope since it depends upon the adding of intensity levels between two traces, and digital scopes have a constant intensity when you overlap the traces.
I can make no sense of what you want.
BUT, you appear to be using a Tektronix oscilloscope in the TDS2xxx series. If so, you have every tool you need at your fingertips. You have over a dozen automated measurements to select from, and math functions, which includes three forms of FFT.