Determining the frequency of a signal is only one of MANY things you can do with an oscilloscope. Determining the passband of a filter is another.
The cutoff frequency of a lowpass filter is usually defined as the frequency where the output falls off by 3dB for a constant amplitude input. You can use an oscilloscope to determine this point by watching the vertical deflection of the waveform on the screen. You could also use an AC voltmeter, if it can handle the frequency being used.
The formula will give you the THEORETICAL 3dB point. Actually sweeping the circuit with a signal generator and a scope will find the ACTUAL cutoff point, which can differ from the theoretical one due to component tolerances, lead inductances, etc.
[This message has been edited by NJwirenut (edited 06-16-2004).]
