Problem with RC band pass filter

Thread Starter

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?

Thanks for your help in advance.
 

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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
 

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.
 

Thread Starter

jn2

Joined Jan 3, 2017
18
Hello,

Thank you for your help and your answers.

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
 

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.
 

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
 
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