Above is a Bode plot of signal strength vs. frequency captured by my oscilloscope for a simple low-pass circuit I built. The low-pass consisted of a 10k resistor and a 10 nF capacitor. If you do the math, and your resistor and capacitor values are exact, that gives you an f3dB* point of 1/(2*pi*R*C) = 1/(2*pi*.0001) = 1500 Hz (approximately).
The Velleman oscilloscope will automatically do a sweep of a frequency range you select and plot a graph as it collects data. When it's finished, you can get horizontal and vertical cursors to show you exact data values on the graph. In the graph above (click on it to get a larger version), the starting plateau is at about 5.4 dB. So 3db down from that is about 2.4 db, which is where the horizontal blue line is at. The vertical blue line shows where the horizontal line intercepts the curve: tracing this back to the frequency (x) axis gives you f3db. When you do that, it corresponds to a frequency of about 1200 Hz, which is pretty close to the calculated f3db. (You can see the values for these lines in blue at the center bottom of the graph. The y axis is Voltage or V, the x axis is frequency or f.)
*Note: f3dB is where the signal is attenuated by 3 decibels from its full strength or equivalently when its amplitude is approximately .7 of its maximum (.7 = 1/sqrt(2)). It's used as a conventional measurement point in frequency-dependent circuits. In general, you want to make sure any signals you're interested in preserving are well to the good side of f3db in the circuit fragment you're designing. Also, the f3db point corresponds to the power/intensity of the signal being halved, which may explain its significance better.
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