może doświadczyłeś efektu tzw "ringing" gdy filtr jest za wąski.
https://books.google.pl/books?id=EcbjKvhgPLsC&pg=SA8-PA22&lpg=SA8-PA22&dq=to+narrow+filter+ringing&source=bl&ots=0K1IS6wXCl&sig=ACfU3U2bAQkMFq1PzxFoSQBLj8DOFs3qFg&hl=pl&sa=X&ved=2ahUKEwjWnqOO3ujoAhXRk4sKHYdEDIEQ6AEwAnoECAwQMg#v=onepage&q=to%20narrow%20filter%20ringing&f=false
https://books.google.pl/books?id=EcbjKvhgPLsC&pg=SA8-PA22&lpg=SA8-PA22&dq=to+narrow+filter+ringing&source=bl&ots=0K1IS6wXCl&sig=ACfU3U2bAQkMFq1PzxFoSQBLj8DOFs3qFg&hl=pl&sa=X&ved=2ahUKEwjWnqOO3ujoAhXRk4sKHYdEDIEQ6AEwAnoECAwQMg#v=onepage&q=to%20narrow%20filter%20ringing&f=false