NanoVNA V2 / Forum

Note: this page is a mirror of https://groups.io/g/NanoVNAV2.
Click here to join and see most recent posts.

TDR range and resolution


John Galbreath 2021/08/30 08:31

HPAK has an informative application note on TDR.

See pages 27 - 30 of link:

https://www.testunlimited.com/pdf/an/5989-5723en.pdf

for discussion of TDR range and resolution.

Please read page 11, paragraph 4.1

"The measurement frequencies must be set so that the stop frequency
is equal to the product of the start frequency and the number of points (which would
yield harmonically related frequencies). The VNA has a function that performs this
automatically. From this, the rise time is determined by the maximum slope of the
highest frequency measured . . . "

Does the LabView NanoVNA system perform the automatic function ? [probably automatic only for 60G, $ 60K VNA]

Joe Smith 2021/08/30 15:48

On Mon, Aug 30, 2021 at 11:31 PM, John Galbreath wrote:

>
>
> Does the LabView NanoVNA system perform the automatic function ?

No.

Joe Smith 2021/08/30 16:02

While I implemented TDR for the 8754A, I had made some improvements based on the following article:

https://dspguru.com/dsp/howtos/how-to-interpolate-in-time-domain-by-zero-padding-in-frequency-domain/

Of course, if you want to see it in action:

https://youtu.be/XaYBpPCo1qk?t=4258

Joe Smith 2021/08/30 16:08

>From the same V2Plus4 review, showing my poor man's Beatty standard (very loose use of the word)

https://youtu.be/XaYBpPCo1qk?t=3122

John Galbreath 2021/09/14 12:14

Attached is the XL spreadsheet I use to help me find optimal freqency span and # of points to use for T D R with LabView NanoVNA.  Don't know what the "Samples/Dec" does (same row as # of points).  The TDR step plot is a "home-made" Beatty standard consisting of 4-foot RG-58 coax cables:  1 in series with 2 in parallel ( mismatch ) in series with 1 open at end.

Joe Smith 2021/09/14 17:23

On Wed, Sep 15, 2021 at 03:14 AM, John Galbreath wrote:

>
> Don't know what the "Samples/Dec" does (same row as # of points).

Covered in the manual.

John Galbreath 2021/09/14 17:33

Paragraph 15.1.1 (page 70) covers this, however I still do not understand it and I think it would be beneficial if I could understand it.
I am also not sure about the decimal point in the resolution calculation in my spreadsheet - please verify.

Joe Smith 2021/09/15 18:58

Also pg 142, section 23.7.  I suspect though you are not understanding what a log sweep or maybe what a decade is.  These terms are very common in the industry and you have the internet if you would like to learn more about them.

John Galbreath 2021/09/15 19:10

No, I do know what a log or linear sweep is, and I know about octaves and decades, but I do not understand what effect the number of samples per decade has on the inverse FFT to convert the frequency domain operation of the VNA to the time domain operation of the resulting reflectometry information.

I did not look at section 23 in detail since I am not yet interested in PDN.  The TDR display is linear in time/distance so I doubt if I would ever need to utilize the log sweep for TDR.

Actually it was very clever for you to augment a logarithmic sweep when the NanoVNA lacks this capability.

Joe Smith 2021/09/16 10:04

On Thu, Sep 16, 2021 at 10:10 AM, John Galbreath wrote:

>
> No, I do know what a log or linear sweep is, and I know about octaves and
> decades, but I do not understand what effect the number of samples per
> decade has on the inverse FFT to convert the frequency domain operation of
> the VNA to the time domain operation of the resulting reflectometry
> information.

>From your original comment of "...however I still do not understand it and I think it would be beneficial if I could understand it."  you leave it up to readers to guess at what you are not understanding.  I suggest in the future, think about what it is you are wanting to ask, then provide the details up front.

When I came up with the idea of a quasi log and segmented sweeps, I was very limited with what I could do with the old HP8754A.  You have no remote control over the frequency range.  The HP8501A nomalizer had a very limited number of points it could collect.  The only way to make narrow band measurement was to use an external RF generator.   I learned a lot of the basics from that system.

Compared with my first VNA, the original NanoVNA offers some advantages when trying to control it with a PC.  Still, it had a lot of the same problems.  A very limited number of data points and no log sweep.  Something I had worked around 15 plus years ago, still hunting me...  Then again, it's only $50.

PDNs are also feature rich at lower frequencies and having a log sweep can really improve the measurement by reducing the time and processing required.

You won't find any mention of log or linear segmented sweeps in section 18 where the manual covers the TDR measurements.   While the paper you linked and I assume read, mentions the bandpass mode not requiring frequencies that are harmonically related.  I'm assuming there's something you are wanting to exploit.  Just keep in mind as I mentioned earlier, the log isn't really a log but rather a sequence of segments just like with the linear mode.  The difference being that each segments start frequency is based on a log rather than a linear function.

Kit 2021/09/16 17:50

Joe,

Appreciate a link to the "manual" commented on in this thread.  There's nothing I could locate in the Wiki or Files (just Nano User manuals) that had the number of pages or chapters being referred to !
Thanks,

Kit

Joe Smith 2021/09/16 18:25

The manual may be found on my Github account.   Just download the latest PDF.    The software as well as links to various videos I have made are also there.  Start with the README.

https://github.com/joeqsmith/NanoVNA_Software

To reply to this topic, join https://groups.io/g/NanoVNAV2

View this thread on groups.io