1) Please post a schematic (already requested).
2) Did you measure the individual components before constructing the
filter with them?
3) We assume the filter is designed for 50-ohms in and out? (already
assumed)
4) Is the dielectric of the capacitors appropriate for the frequency range?
5) Instead of using the o'scope and FFT, please give us a plot of S21
using the NANOVNA. Please use SAVER (or equivalent) to accomplish your
sweep with more points than are available on the native instrument.
6) If your signal generator can sweep, you can also measure the filter
response with that. A flat output noise generator can also be used instead
of the swept sig. gen.
Dave - W?LEV
On Wed, Mar 12, 2025 at 11:11?PM Mitch NK3H via groups.io <mitch=
[email protected]> wrote:
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
--
*Dave - W?LEV*
--
Dave - W?LEV
