How do you know it's filtering the harmonics out? Feeding in a square wave (into a non-resistive load) might not have the harmonic structure you expect.
It's really hard to interpret the wiggles after the transition in any meaningful manner, short of dumping them into some software and running an FFT.
And with a sine excitation, you need to pick a couple frequencies to test at - say 30 and 50 MHz.
I'd believe the VNA.
As an experiment you could download ELSIE and build the circuit with the inductance from the leads and see what it gives you.
If you want something simpler, just use the first pi section - shunt C, series L, shunt C.
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-----Original Message-----
From: <nanovna-users@groups.io>
Sent: Mar 12, 2025 9:48 PM
To: <nanovna-users@groups.io>
Subject: Re: [nanovna-users] Help with LPF Measurement
Hi Jim,
Thank you for your suggestions. I will shorten leads and properly dress the components tomorrow. BTW I had no 270 caps but had 50s and 220s, hence the two caps in parallel.
You make a really good point in saying "the filter's not working right...." But what I don't understand is why the results from the sig generator/oscilloscope show the filter is filtering out high frequency harmonics pretty well, but the NanoVNA shows a very weird S21 result. It improved slightly when I added the ground leads between the board and the test rig (my bad for missing that!!), but it's not even close to what it should look like. Onwards....Thanks again. Best, Mitch NK3H
