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nanaVNA TDR distance resolution


Dave Brown 2021/07/07 09:36

What sort of distance resolution using the TDR function can one get with
any of these nanaVNAs?
I don't have one yet and reading the docs its not clear(to me) what the
figure is.
DaveB
NZ

gary miller 2021/07/06 20:07

In time domain bandpass the time resolution in seconds is 1/(frequency span). Distance resolution is given by multiplying by the velocity of light in the media being measured.

John Gord 2021/07/06 22:55

Dave,
The resolution is highest with a wide frequency sweep, but beyond that it depends on just what you mean by resolution.  If you have a single large discontinuity, like an open, you can resolve very small changes in round-trip time to that open.  (I can easily see 70pS changes with 3GHz sweep width on my V2.)  If, however, you are trying to resolve two discontinuities close to one another as separate "things", the resolution is more like 300ps, at least to my eye.   (I'm sure there are formulas for this, but I am too lazy to search them out right now.)
For long cables, you will need to use lower frequency spans to get unambiguous TDR results, and will therefore have coarser resolution.  If you can stand some ambiguity in the TDR plots (due to time aliasing), you can keep the frequency sweep wide even on a long cable.  The results can be hard to interpret, though.
--John Gord

On Tue, Jul 6, 2021 at 07:33 PM, Dave Brown wrote:

Dave Brown 2021/07/07 22:42

Thanks John.

So that looks like - using the 3GHz sweep- I should be able to see somewhat better than a 1cm difference in an airline for a single discontinuity. (using 70 pS for out and back time- and 1nS/foot for c) That’s the sort of ballpark figure I was wondering about,

given my old Tek 545/1S2 setup is starting to show its age, plus being rather large.

DaveB, NZ







From: NanoVNAV2@groups.io [mailto:NanoVNAV2@groups.io] On Behalf Of John Gord via groups.io
Sent: Wednesday, July 07, 2021 17:55
To: NanoVNAV2@groups.io
Subject: Re: [nanovnav2] nanaVNA TDR distance resolution



Dave,
The resolution is highest with a wide frequency sweep, but beyond that it depends on just what you mean by resolution. If you have a single large discontinuity, like an open, you can resolve very small changes in round-trip time to that open. (I can easily see 70pS changes with 3GHz sweep width on my V2.) If, however, you are trying to resolve two discontinuities close to one another as separate "things", the resolution is more like 300ps, at least to my eye. (I'm sure there are formulas for this, but I am too lazy to search them out right now.)
For long cables, you will need to use lower frequency spans to get unambiguous TDR results, and will therefore have coarser resolution. If you can stand some ambiguity in the TDR plots (due to time aliasing), you can keep the frequency sweep wide even on a long cable. The results can be hard to interpret, though.
--John Gord


On Tue, Jul 6, 2021 at 07:33 PM, Dave Brown wrote:

What sort of distance resolution using the TDR function can one get with
any of these nanaVNAs?
I don't have one yet and reading the docs its not clear(to me) what the
figure is.
DaveB
NZ

Joe Smith 2021/07/07 04:15

On Wed, Jul 7, 2021 at 10:33 AM, Dave Brown wrote:

>
> What sort of distance resolution using the TDR function can one get with
> any of these nanaVNAs?

I made a review for the V2Plus4 some time ago.  At 1:10:00  a linear slide with a custom made air line attached to it is used to demonstrate the TDR measurement.  OWO has been talking about a new VNA which would certainly improve this measurement.

https://www.youtube.com/watch?v=XaYBpPCo1qk

Jim Lux 2021/07/07 06:24

On 7/6/21 10:55 PM, John Gord via groups.io wrote:
> Dave,
> The resolution is highest with a wide frequency sweep, but beyond that
> it depends on just what you mean by resolution.  If you have a single
> large discontinuity, like an open, you can resolve very small changes
> in round-trip time to that open.  (I can easily see 70pS changes with
> 3GHz sweep width on my V2.)  If, however, you are trying to resolve
> two discontinuities close to one another as separate "things", the
> resolution is more like 300ps, at least to my eye.   (I'm sure there
> are formulas for this, but I am too lazy to search them out right now.)
> For long cables, you will need to use lower frequency spans to get
> unambiguous TDR results, and will therefore have coarser resolution. 
> If you can stand some ambiguity in the TDR plots (due to time
> aliasing), you can keep the frequency sweep wide even on a long
> cable.  The results can be hard to interpret, though.
> --John Gord
>
Or use more points in the sweep (e.g. NanoVNA-Saver can do arbitrary
numbers of points, in segments) -

As mentioned - the total bandwidth of the sweep sets the resolution, the
spacing between frequencies sets the "unambiguous range".

If you want some wild ideas, there's nothing that says that the sweep
has to be linear, or even continuous - but none of the tools support
this - you can't just plug it into a FFT to get the transform if you're
doing something other than linear equally spaced frequencies.


John AE5X 2021/07/07 11:06

On Wed, Jul 7, 2021 at 02:24 AM, gary miller wrote:

>
> In time domain bandpass the time resolution in seconds is 1/(frequency
> span). Distance resolution is given by multiplying by the velocity of
> light in the media being measured.

Also, velocity factor is not a constant - it varies with frequency.

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