Re: Help with LPF Measurement
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Here's a plot of the expected response from Elsie
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-----Original Message-----
From: <[email protected]> Sent: Mar 12, 2025 4:11 PM To: <[email protected]> Subject: [nanovna-users] Help with LPF Measurement I've searched through the message list looking for symptoms similar to mine and didn't find anything. I'm somewhat new to this and would appreciate any help you can offer. I've been trying to get my NanoVNA-F (firmware 1.0.5) to show the results of a 40-meter low pass filter I've built. I know I'm doing something wrong here but can't figure it out. The VNA image is calibrated (including pass-thru), set for the range 6.5 to 29 MHz and logmag S21. Shouldn't this give a flat area leading up to the design frequency followed by a significant roll off through the remainder of the frequency range? My setup is very similar to W2AEW's YouTube demonstration of the NanoVNA with a LPF but gives entirely different results. The image is entirely different, with very large insertion loss and a steep area of suppression peaking at 11 MHz followed by a steep recovery. I worried that my filter wasn't what I thought it was so I tested it with a 7 MHz square wave, looking at input and output with the FFT function of my 'scope. The before and after images show several things: (1) the square wave (yellow trace) is hardly a square wave. It wasn't much of one before attaching to the input of the filter but it's definitely loaded and distorted by the filter. (I can live with the distortion in the test, since the filter is supposed to reduce the high freq distortion anyway, right?) (2) The purple trace is the filter output--pretty nice sine wave. (3) The FFT shows a good 40+ db suppression of the harmonics, especially the odd numbers. So the filter seems to be doing its job. There may be some things not right about my testing of the filter using signal generator and oscilloscope -- I'm new at this. For example, the shoulder on the filter output at about 10 MHz is troublesome, but still the logmag S21 output shouldn't be affected, should it? Any help in understanding why my results look as they do would be greatly appreciated. Best & 73, Mitch NK3H |
