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Jerry,
I think the -spacing- between the test frequencies is what sets the maximum
length.
--John Gord

On Mon, Sep 21, 2020 at 09:59 AM, Jerry Gaffke wrote:

I would normally think that Neil's upper frequency limit only sets the
resolution
of the time domain response that gets displayed.
And that it is the lower frequency limit that determines the maximum length
of the cable that can be analyzed.

However, the nanovna only stores 100 measurement points in stand-alone mode.
So to get higher resolution we must reduce the length of the transmission

line

to be inspected.

Well, that's my guess as to what's going on here.



From

#########################################
Time domain operation
NanoVNA can simulate time domain measurements by signal processing frequency
domain data.

Select DISPLAY TRANSOFRM TRANSFORM ON to convert measurement data to the

time

domain. TRANSFORM ON is enabled, the measurement data is immediately

converted

to the time domain and displayed.

The relationship between the time domain and the frequency domain is as
follows.

Increasing the maximum frequency increases the time resolution
The shorter the measurement frequency interval (ie, the lower the maximum
frequency), the longer the maximum time length
For this reason, the maximum time length and time resolution are in a
trade-off relationship.

In other words, the time length is the distance.

If you want to increase the maximum measurement distance, you need to lower
the maximum frequency.
If you want to specify the distance accurately, you need to increase the
maximum frequency.
##############################



Never quite says it, but that sounds like a Fourier Transform to me.

Below is a brief excerpt from the above passage.
I suspect the first two statements are only true only because we can store a
max of 100 points of data when standalone.
And I'm not convinced the final statement logically follows from the

previous

statements, though it is true
since it takes time for a signal to go down a length of transmission line.

# The shorter the measurement frequency interval (ie, the lower the

maximum

frequency), the longer the maximum time length
# For this reason, the maximum time length and time resolution are in a
trade-off relationship.
# In other words, the time length is the distance.


Would be great if somebody who fully understands this stuff could expand on
the material
posted on nanovna.com regarding "Time Domain Operation".

Jerry, KE7ER




On Mon, Sep 21, 2020 at 09:29 AM, Jerry Gaffke wrote:

Here's a simplification of Neil's formula for stop frequency.
Length is in meters or in feet, velocity factor is a decimal number such

as

0.66, result is in mhz.

fmhz = 38376*vf/feet
fmhz = 11697*vf/meters

From Neil's example where he somewhat arbitrarily increased the scan

beyond

50

feet
by a factor of 100ns/77ns:

feet = 50*100/77
vf = 0.66
fmhz = 38376*vf/feet

That gives a result of 390.05 MHz, which agrees with Neil's example.

I haven't looked into it yet, but I assume John's "Transform, Low Pass

Step"

involves a Fourier Transform or similar, and thus some rather nasty math.
So exactly how this works and where Neil's magic value of 39 comes from
may never be terribly obvious for most of us.

Jerry, KE7ER