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Dave,

Maybe I am missing something but where in wiki are you referring to? I
have been looking all through wiki but I can’t find anything that answers
my questions.

One thing I noticed is is that you are using the logarithmic scale for the
frequency axis. The default mode and what is used in most conventional
plots you see is linear.

Roger

On the Nanovna Saver, there are markers that you can assign values to like
SWR, Return Loss, etc. Could someone in simple terms explain some of the
others like:

Quality factor
S11 Phase
S11 Group Delay
S21 Phase
S21 Group Delay

Hello GM7,

I hope the following suggestions will help.
Please send a photo of your test fixture with out the filter on top of it
and some photos of how you did the SOIT terminations. This may help us
figure out what your hands are doing and spot ways to help you.

I would advise you use the NanoVNA to characterize each of your components
and learn the "parasitics" each have.
Parasitic components are those "free" extra elements each part we buy (or
make) comes with.

Resistors can be the most ideal but all resistors come with some parasitic
capacitance and inductance.
Capacitors have series resistance and inductance.
Worst of all are inductors which are "blessed" with generous amounts of
resistance and capacitance. They are so "blessed" that you will want to get

an estimate of the size of the parasitic elements and include them in the
filter simulation to decide if they can be ignored or if you tweak another
component to compensate if you can.

You approximate the parasitic elements by finding the peaks and valleys of
the series and parallel measurements of the components. If you need help
understanding peaks and valleys of the series and parallel measurements of
the components that would be a good conversation too.

I was in my early 20s when a mentor showed me how to measure coil impedance

on a network analyzer and was so baffled by what I saw. We were measuring a

common mode choke for an AC line input filter and I assumed since the FCC
requirements went (working from memory here) something like 450KHz to 30 MHz

that the choke would be inductive over that range. Again IIRC, the choke
stopped blocking way lower than 30 MHz and then had several saw tooth like
shapes in the impedance versus frequency plot. It was a learning moment.

Hope this furthers the conversation.

F. Lee Erickson



-----Original Message-----
From: GM7
Sent: Thursday, June 04, 2020 1:17 PM
To: [email protected]
Subject: [nanovna-users] Help Testing a Low Pass Filter

Hi All,

I'm trying to test a set of Low Pass Filters for 40m, 30m, 20m & 10m. I'm
getting consistent results for each of them but not the results I expected.

I'm struggling to establish whether the problem is with the filters or with

my test setup / calibration or even with the NanoVNA.

I've repeated this with and without a test fixture and got essentially the
same results. I attach the test fixture (see photo) and then calibrate
(reset, followed by open, short, isolation, thru). I then check the smith
chart with short, open and 50ohm loads to sanity check the calibration.

The attached image shows the 30m LPF. It doesn't have the passband I would

expect or the roll-off I would expect. Instead it's almost a linear
roll-off over the range under test.

Can anyone help? For example by either correcting my test methodology or
offering any advice. Thanks in advance.

Hi All,

I'm trying to test a set of Low Pass Filters for 40m, 30m, 20m & 10m. I'm getting consistent results for each of them but not the results I expected. I'm struggling to establish whether the problem is with the filters or with my test setup / calibration or even with the NanoVNA.

I've repeated this with and without a test fixture and got essentially the same results. I attach the test fixture (see photo) and then calibrate (reset, followed by open, short, isolation, thru). I then check the smith chart with short, open and 50ohm loads to sanity check the calibration.

The attached image shows the 30m LPF. It doesn't have the passband I would expect or the roll-off I would expect. Instead it's almost a linear roll-off over the range under test.

Can anyone help? For example by either correcting my test methodology or offering any advice. Thanks in advance.

I haven't measured the parts yet. I have checked the capacitors are
correct based on the markings and that the torroids have the correct number
of turns.

All 4 filters that I built show the same response so I suspected that my
test setup or methodology was likely to be wrong.

I have another kit so I could build it measuring the components as I go.

Is there a standard setup for measuring filter response with the NanoVNA?

Thanks Dave,

Your simulation is the response I expected. Unfortunately i's not the
response I'm measuring. I re-calibrated and then used the Smith Chart as a
sanity check. I got the expected results for open, short and 50 ohm load.

When I built the fixture I connected the ground permanently so that the
ground is connected between CH0 and CH1 even when the LPF is not in place.
I've re-tested without this link and got the same results. In general
terms I'm not sure if this link is a good idea or if I should remove the
link.