Wow Bryan! Thanks for that in-depth piece of work!
I use a Blackman window, which is created by putting in "np.blackman()" ;-)
So I can use whatever window function is the most relevant. These ones are
available:
I've had suggested that I should pad the input data with as many zeroes as
I have data values, I think in order to increase either resolution or
range? I haven't done it yet, but I was thinking about having some
experiments.
If the second half the values shown are useless, I might as well just
discard them! :-)
You're right that the graph can currently only exist in one place. It's not
particularly difficult to fix, but it involves re-doing a tiny bit of the
application architecture; which I really should do at some point anyway. I
don't promise when I will get it done, though. ;-)
Thank you very much for your feedback, and I hope you can help me select
things such as:
1) The window function to use,
2) Whether to pad the input,
3) How many points to use for the FFT,
4) What part of the output should be displayed (half, like you suggested?)
--
Rune / 5Q5R
On Thu, 3 Oct 2019 at 17:16, bryburns via Groups.Io <bryburns=
[email protected]> wrote:
Rune,
I am taking a deeper look at your TDR plot. Thanks for implementing this.
I think you are plotting all of the samples of the IFFT of the S11 data.
If so, I would recommend that you chop off the last half of the plot. With
5 segments (505 total samples) from 50kHz to 900 MHz I am seeing the right
side of the plot indicating 55.4 m as the longest distance one could
measure. I set the the cable type to RG58/U with a velocity factor of
0.66. I think 55.4 m is too large by a factor of 2. The last half of the
bins in the IFFT are time delays that we won't measure in this way with
real cables. As it is, you give the impression to someone that they could
measure a 55.4 m cable for the settings indicated above which is not
correct.
Are you applying a frequency domain window function to the data prior to
doing the IFFT? If so, it would be useful to let folks know what window
you are applying when it comes time for documentation. If not, there are
an infinite number of possible windows. I would suggest something fairly
easy to implement such as a Hanning window or a Hamming window. For cable
length measurement, what you have is great.
I am interested in using the TDR to look for impedance deviations along
the transmission line which can get easily obscured by time-domain
sidelobes if no window is applied. I am using the TDR capability to look
for coax damage or breaks in the protective jacket caused by gardening. My
coax cables run through a garden area to my antennas which are above the
garden. Outright breaks in the coax are fairly easy to find; however, some
types of damage are a bit more difficult to observe. The TDR is especially
helpful for this application.
I do like the separate plotting capability you have implemented for the
TDR data which is in the same window as the coax definition. I have
noticed that once you put it into the main window as one of the graphs, you
cannot ever see the TDR data again as a separate plot unless you restart
the program. It seems like it would be a good idea to restore the plot to
the TDR window if it is removed from the plots in the main window or just
permanently leave it in the separate window that the user can bring up as
desired?
Another alternative is to only plot the TDR data in the main window. When
the TDR plot is a graph in the main window you should show at least the
velocity factor you have assumed on the TDR plot and perhaps the cable type
the user selected. This makes the plot self-contained when it is saved as
an image. I prefer a separate window where I can make the TDR plot as big
as my screen; however, that is not in any way essential and probably makes
more work for you.
Thanks, again, for your great work on this software. I greatly appreciate
it.
73,
Bryan, WA5VAH
