Let's look at a little bit more about the characteristics,

starting with a Bandpass Filter.

So let's define the passband as the region during which we pass through frequencies,

now where does these defined?

These are defined, again, as three decimals below the passband gain.

So this is the pass band gain, G sub DB here, this is plotted on a plot and

so this point right here is three decimals below that.

And again, over here, this is three decimals below that,

then this is the passband.

Sometimes we use this, for example, in communications.

We're interested in certain frequency bands.

We want to pass through those frequency bands and attenuate everything else.

Now, a Notch Filter looks like this.

This is what we'll call the stopband.

That's the region of interest, that's where we stop frequencies.

Again, we have the passband gain.

Sometimes this is different on one side verses the other side, but

we'll call that the passband gain and this is stopband region.

Where we might us a stopband is when we have a particular frequency that

bothers us, so we want to get rid of it.

60 hertz comes to mind, because a lot of times a measurement signals 60 hertz

shows up as a measurement noise.

And in other countries, 50 hertz might be, because that's the line voltage.

Now if I want to implement this with an RLC circuit,

I would take the voltage across resistor and this being a bandpass filter.

The center frequency of that band is 1 over the square root of LC and

the width of that bandpass is R over L.

So I can use these variables,

these parameters to try to select a particular frequency band that I want.

The width and the center frequency of my design.