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I assume it was properly terminated at both the load and the source. Otherwise you will not get the proper filter response curve. You shouldn't feed the output of the filter into a high impedance scope input. It should include a 50 ohm termination at the filter output before it goes into the scope. You can improve the source termination by using a 50 ohm 3dB or 6dB attenuator at the input, rated for the power used. Depending on the LPF design, it is possible to get a peak. A Butterworth design should give a smooth fall-off with no peak. Other designs may have a slight peak near cutoff or a rising response in the bandpass.
------ Original Message ------
From: "Peter GM0EUL" <gm0eul@...>
Sent: 9/13/2020 9:28:14 AM
Subject: Re: [QRPLabs] Low pass filter advice #40m #80m #vfo
Thanks Alan- that puzzled me, I wasn't expecting a peak and don't know what to make of it or what sort of artefact may have caused it. I want to try and understand it before I do anything to change it.