Why does the bandpass filter act as highpass

filterpassive-networks

I have designed a bandpass filter. Here is the circuit and spectral characteristic:

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As you can see from the response curve, it has cutoff with the peak ranging from 260 kHz to 290 kHz.

Here is the actual circuit :

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However, the filter works more like a High pass, passing frequencies all above 250 kHz, without any distortion or attenuation.

Here is one of the outputs :

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(I observe outputs at all frequencies above 250 kHz. This is just one example)

Where am I going wrong, and what corrections do I need to make?

Update :

I decided to simulate my circuit , and it was clear why my circuit acted as Highpass. (The last freq is graph in 1 MHz). Dunno why the filter design tool showed that (incorrect) graph then.

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Best Answer

With the photos, I'd venture that you have stray capacitances all over your circuit because it is on a solderless breadboard. You shouldn't be doing high frequency work like this on one of those. Of course, this doesn't rule out parasitic effects addressed in the comments to your question, but there is a gorilla in the room, and it's the breadboard.

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