How to detect Noise in a Signal using Oscilloscope?

[IndiJones]

Say I have a noise free square wave signal of 1 khz frequency and 5 v amplitude. The signal passes through some optical receiver and the shape of the signal becomes slightly distorted with some noise in it. I have a digital oscilloscope that has FFT. How can I use this FFT to detect the frequency of the noise so that I can separate the noise from the signal. I don't want to go through complex maths. Just tell me how to use the oscilloscope FFT function in determining the noise. Any help would be highly appreciated. Thanks in advance.

<font size=-1>[ This Message was edited by: IndiJones on 2001-10-05 05:43 ]</font>

 

[borut]

This is very easy, you set the time-base in the mode that you see the noise so you have to "speed-up" the time-base and in this way you may "read" the frequency of the noise signal. If the noise is constructed with many different frequencies than you have to detect the lowest noise frequency that is in your signal and than you simple do one low-pass filter to filter and clean your main signal. That`s it.

By

 

[johnyaya]

The formula for a square wave is the infinite series:
Pi=3.14159
t=time
f=frequency
w=2*Pi*f
y=cos(wt) - cos(3wt)/3 + cos(5wt)/5 - cos(7wt)/7 ...
In other words, a square wave is made up of odd harmonics. As the number of odd harmonics increase the waveform will look more like a square wave. Look for any signal (noise) that doesn't fit into this.

<font size=-1>[ This Message was edited by: johnyaya on 2001-10-05 17:03 ]</font>

<font size=-1>[ This Message was edited by: johnyaya on 2001-10-05 17:06 ]</font>