How can I calculate the 3db Frequency for a given filter?

I have a plot of the voltage vs. frequency from 100Hz to 100MHz.

I know the formula for gain is:

G(db) = 20* log(Vo/Vi) where Vo is Vout and Vi is Vin.

I know that the frequency of the carrier wave is: c(t) = C*sin(wt + k) where C is the amplitude, w is the angular frequency and k is a phase constant.

My input ac sine wave has an amplitude of 1 V and a DC Offset of 0.

I know the message wave is of the form: m(t) = M*cos(wt + k) where M is the amplitude...etc

The wave sent over the air is of the form: S(t) = [1+m(t)]*c(t)

The 3db frequency is when the Vo = 1/sqrt(2) * Vin. But how to I get frequency from this calculation?

So confused. The project is due tomorrow night and I put off this part until right now. I thought it would be intuitive but its really confusing.

Help???

Methinks thats a mad scientists related topic, being more physics than chemistry. Lab Tips is generally more chemistry related, specifically organic, more specifically droogs :D

I imagine a friendly mod will move it over to Mad Scientists soon enough, I'm afraid my grasp of physics was woefully poor in general, and I'm godawful at maths, so I can't help. I'll bet a few chaps in here could help you though, but I think they frequent Mad Scientists, so they'd see it whether its moved or not.

This is not a double post.

I posted it in two forums so i could get the best answer. Please delete one kthxbye

You should be able to get it from your plot. Just find the point where the output voltage is 0.707 of the input.

It's a basic concept, the -3db frequency just occurs where the output power is half the input, or in terms of your gain function with voltage:

20*log(1/sqrt(2)) = -3.010

If you have your amplitudes and phase angles for the carrier wave c(t) and baseband signal m(t) then just plug all those numbers in and formulate s(t) which is just a standard AM signal. Plug in the input and output voltages and solve for frequency: omega = 2pi*f