# Problem with RC band pass filter

#### jn2

Joined Jan 3, 2017
18
Dear all,

I am working on the design of an active band pass filter with a high quality factor, and I have encountered a problem with my circuit in which I solicit your help.

In the literature, it is said that when the quality factor of a filter is very large there is a possibility that the circuit oscillates, ie, the structure does not function as a filter but rather as an oscillator. Therefore, to check this, we must attack the structure by a square signal and see the nature of the output signal: is there always a square signal or we have oscillations.

I tested this principle on my circuit but I do not find the good outcome. Therefore, I tried to check it on a simple RC circuit that works like a bandpass filter, but also I cannot find a square signal in the output, I have rather a distorted signal (attached figure).

Therefore, I would like to know if this method is correct. If so, why the RC bandpass filter not follow this principle? Moreover, is there another method to check the operation of a bandpass filter with a high quality factor?

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#### ericgibbs

Joined Jan 29, 2010
14,707
hi jn2,
I assume you are familiar with Fourier analysis of a square signal regarding its sinusoidal structure.?

Your pass band filter is modifying some of the sine wave components, so the wave will no longer be 'square'.

E

#### jn2

Joined Jan 3, 2017
18
So what is the solution from your experience???

#### ericgibbs

Joined Jan 29, 2010
14,707
hi,
E

#### OBW0549

Joined Mar 2, 2015
3,566
I tested this principle on my circuit but I do not find the good outcome. Therefore, I tried to check it on a simple RC circuit that works like a bandpass filter, but also I cannot find a square signal in the output, I have rather a distorted signal (attached figure).
The output of your RC circuit looks perfectly normal to me, and is exactly what I'd expect to see; given that it's a bandpass filter, I would NOT expect to see a square wave on the output. The highpass portion of the bandpass characteristic is evident from the fact that the tops and bottoms of the waveform are sloping, rather than level; and the lowpass portion is evident from the low-to-high and high-to-low transitions, which are smoothed and rounded rather than abrupt.

In the literature, it is said that when the quality factor of a filter is very large there is a possibility that the circuit oscillates, ie, the structure does not function as a filter but rather as an oscillator. Therefore, to check this, we must attack the structure by a square signal and see the nature of the output signal: is there always a square signal or we have oscillations.
That is incorrect; what you'd expect to see is a filtered version of the square wave, or oscillations. You will not see a perfect square wave coming out of a bandpass filter.

#### jn2

Joined Jan 3, 2017
18
Hello,

So, you say that it is impossible to see a square signal at the output of a bandpass filter, and that the signal I found is correct.

#### ericgibbs

Joined Jan 29, 2010
14,707
Hello,
So, you say that it is impossible to see a square signal at the output of a bandpass filter, and that the signal I found is correct.
hi,
The output will appear as shown in your image.
If you post your full filter circuit I will run a LTSpice simulation.
E

#### jn2

Joined Jan 3, 2017
18
Here is the diagram of the filter RC.

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#### OBW0549

Joined Mar 2, 2015
3,566
So, you say that it is impossible to see a square signal at the output of a bandpass filter, and that the signal I found is correct.
Yes. Your circuit is operating normally, and its output is exactly what you should expect to see.

#### bertus

Joined Apr 5, 2008
21,764
Hello,

You made a filter like in figure 5 of the attached PDF.
(only high and low pass are exchanged).

Bertus

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#### ericgibbs

Joined Jan 29, 2010
14,707
hi jn2,
This is from LTSpice.
I have superimposed your image, its a close match.
E

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#### AnalogKid

Joined Aug 1, 2013
9,639
We still do not know anything about your bandpass circuit requirements or design.
Filter order
Gain
corner frequencies
Topology (filter type)
Q
The first thing to do when a circuit does not work as expected is to LOOK AT THE SCHEMATIC, but we cant.. Do you have one?

ak