# High pass filter - detailed explanation

Discussion in 'Homework Help' started by grushka, Feb 3, 2009.

1. ### grushka Thread Starter New Member

Feb 3, 2009
2
0
So I have this filter:

and I plot ratio of Vout/Vin versus frequency:
(there are 2 curves, as one represents simulation and the other one measurements obtained during the laboratory exercise)

Could some please explain to me what is exactly happening in this filter step by step? I know there are 2 resonances at f=1300 Hz and f=2400 Hz, I know what a high pass filter is, what it does and now I need very detailed explanation for this circuit, like for a complete noob. Feel free to include some complicated formulas, I'm not afraid of them The most important thing for me is to know why it happens and when, not what happens as this is quite clear.

2. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,177
407

Not all filters operate at resonance (although some do, most notably the Cauer Elliptical filter and its derivations).

It's best to think of a filter as a reactive voltage divider, with two reactive elements. Let's say you have a C and an L in series. As you increase the frequency, by the reactance formula, Xc=1/(2pi*fC), as the frequency increases, the reactance (impedance) decreases. At the same time, the Inductive reactance (L=2pi*fL) INCREASES, with corresponding increase in impedance. It's simple to see that the voltage at the junction of these two points will increase with increasing frequency.

You don't have to know ANYTHING about the phase to explain it on this simple level.

Hope that helps.

eric

3. ### grushka Thread Starter New Member

Feb 3, 2009
2
0
well,I'm afraid it's not enough, I basically said the same thing to teacher and I heard "these are just a basic information, you're analyzing it on a surface level, students need to know more complex explanation". So no general rules which are obvious, but more deeper conclusions are needed this time...

4. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,581
464
Do you know how to derive the transfer function, and then plot the poles and zeros on the complex plane?

Having done that, you can draw some conclusions about the filter response from the pole-zero plot.

Maybe that's what your instructor wants.