Jerry, something is wrong here. The displayed values should show the
one-way delay, and directly read the cable length if you set the velocity
factor via the menu. How did you calibrate for this measurement, and what
frequency start/stop are you using?
On Wed, Sep 23, 2020, 12:58 PM Jerry Gaffke via groups.io <jgaffke=
[email protected]> wrote:
The post included below from John on the procedure for using stock
nanovna-H firmware
as a TDR is my gold standard, it answered almost all my questions.
One point it does not explicitly state: The times reported indicate round
trip times,
out to that part of the cable and back to the VNA.
I'm sure that's obvious to some. Not so obvious to others such as myself.
I have a 100 foot coil of RG8X, a web source said RG8X had a velocity
factor of 0.66.
I measured 240ns on the display where the reflection at the end was
displayed.
But 240e-9 * 3e8*.66 * 39.37/12 = 155 feet. What?
(The 3e8 term is the speed of light in meters per second, the 39.37/12
converts meters to inches to feet)
Reading the fine print on the cable jacket, in addition to "RG8X", it says
"218XATC".
Googling that, I find the velocity factor is actually 0.84, not 0.66.
240e-9 * 3e8*.84 * 39.37/12 = 198.4 feet
Ahh, 100 feet of cable, and the nanovna tells me it's almost 200 feet long.
Makes sense the nanovna is telling me the round trip time.
The nanovna is just doing a Fourier Transform, and doesn't know that I am
looking at a length of cable.
Jerry, KE7ER
On Mon, Sep 21, 2020 at 03:29 PM, John Gord wrote:
Neil,
Here is some more information on how the TDR setup works. It is a bit
"hand
wavy", but its been a long time since I really worked with this kind of
calculation.
First, the Fourier transform provides a connection between the time
response
and frequency response of a system. The time response is often described
by
the impulse response, which is the response of the system to a very
narrow
pulse. In theory the pulse is infinitely narrow, infinitely high, and has
finite area under the amplitude/time curve. The theoretical impulse has
infinitely high frequency components. In a practical system, we can use a
reasonably narrow pulse as an input to a system, but for say, a 1GHz
response,
you need something like a 350ps wide pulse to stimulate it. Similarly,
you
need a 1GHz bandwidth system to look at a 350ps pulse. A 1GHz system
(filter
or whatever) can be described by how it passes a 350ps pulse or by how it
passes (amplitude and phase) a range of sine waves from 0 to 1GHz. The
Fourier
transform relates the time description and the frequency description. If
you
know one, you can compute the other with the Fourier transform. In
practice,
we use something called the Discrete Fourier Transform (DFT) because we
have
discrete samples spaced out in frequency or time. Further, the DFT is
implemented as the Fast Fourier Transform (FFT) because it is
computationally
efficient. (The FFT was figured out when I was in high school, so it is a
recent development, although some folks say it was anticipated by Gauss.)
Conventional TDR:
In a conventional TDR system, an impulse (Tek 1503) or step (Tek 1502) is
transmitted into one end of a cable, and the system looks for
reflections. A
perfectly terminated cable has no reflection, a cable open at the end
reflects
a positive signal, and a shorted end reflects a negative signal. An
improperly
terminated cable reflects something between the short and open signals;
the
termination value can be determined by the sign and amplitude. A piece
of 75
ohm cable spliced into the middle of a 50 ohm cable produces
reflections, and
in the case of step input (Tek 1502) the reflections can be viewed as
showing
the impedance versus distance along the cable. For impulse input (Tek
1503)
the interpretation is not so simple, but it does clearly show where
discontinuities are located. Integrating the reflected impulses can
produce a
display equivalent to the reflections you would get with a step input.
The
distance resolution is something like the step risetime or impulse width
times
the speed of light. If you know what you are looking at, a cut cable
end, say,
you can get much better resolution, but it is hard to figure out
arbitrary
faults with dimensions less than risetime*(speed-of-light).
VNA TDR:
With VNA TDR, a series of sine waves are applied to the cable end and the
amplitude and phase of the reflected signals are measured. Higher
frequency
applied signals get bigger changes in phase for a given distance to a
reflecting fault and thereby allow better resolution. Wide frequency
spacing
of signals shortens the maximum unambiguous measurement range. Signals
spaced,
say, every 30MHz (3000MHz/100 steps) cannot distinguish reflections at
33.3ns
and 66.6ns. For VNA TDR, the FFT converts the amplitude and phase of the
reflected frequency signals into the impulse response of the reflecting
cable.
Integrating that impulse response converts the display to the reflection
response to a step input, which is the Tek 1502 style of display.
Setup reasoning:
"Set stimulus for a wide sweep, say 50kHz - 900MHz or more."
High frequency gives good resolution, but shortens maximum length
"Do an SOLT calibration at the desired measurement point (if not already
done)"
VNA works best if calibrated
"Set TRACE 0 for Real format"
We want linear (not log) input to the FFT. But "LINEAR" format looses
polarity
(short vs open) info
"Turn TRACE 1 off"
Reduce clutter
"Set TRACE 2 for Smith "
Smith markers let us read off cable impedances directly
"Turn TRACE 3 off"
Reduce clutter
"Turn TRANSFORM ON"
Convert from frequency to time representation
"Select Transform LOW PASS STEP"
I like this format, gives cable impedance directly, similar to Tektronix
1502
TDR
"Adjust ELECTRICAL DELAY to move the displayed window to the desired
location
along the cable"
This extends the good resolution to greater lengths, still subject to
the ( 1
/ (frequency-step-size)) limitation
The resulting display should be similar to that on a step-type TDR like
the
Tek 1502. With 900MHz max, the display width is about 43ns, with 3GHz it
is
about
8ns.
Impedance along any connected cable can be read by moving the marker to
the
desired time (distance) and looking at the Smith chart marker values.
You need
to mentally add the
ELECTRICAL DELAY to the marker time to get the actual delay.
" Save via the CAL menu when you have it all set up."
Saving is a good idea
