John wrote:
# I noticed an interesting effect: If you set the Start frequency at half of the step size
# (say 15MHz to 3000MHz, 101 points), the impedance steps at the first (and every odd)
# "delay overflow" are inverted!! Probably makes perfect sense to someone (but not to me).
That's very curious indeed!
Perhaps with some head scratching, one might use this effect to distinguish between
ambiguous regions on long runs of cables.
Though of course, far better to figure out how to process more data points.
Perhaps with nanovna-saver, or a nanovna-SAA2
At the top of Bryan's document, Step 2 states:
"I changed the nanoVNA stop frequency to 130 MHz which gives a maximum length that can be observed in the nanoVNA of about 31.5 m. "
Speed down the cable is 3e8*0.66 meters/second, assuming a velocity factor in his cable of 0.66.
So Bryan's 31.5m might represent a delay of 31.5/(3e8*0.66) = 160 ns.
If we assume Bryan did not take the velocity factor into account, that's 31.5/3e8 = 105 ns.
Neil's formula gives a delay of tmax = 39/fmax = 39/130e6 = 300 ns
John's formula gives us a delay of tmax = 100/fmax = 100/130e6 = 769 ns
I prefer the looks of John's formula, the others appear to have been arrived at empirically.
But that's a rather large spread.
Jerry, KE7ER
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On Mon, Sep 21, 2020 at 10:33 PM, John Gord wrote:
Jerry,
I noticed an interesting effect: If you set the Start frequency at half of the
step size (say 15MHz to 3000MHz, 101 points), the impedance steps at the first
(and every odd) "delay overflow" are inverted!! Probably makes perfect sense
to someone (but not to me).
--John Gord
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On Mon, Sep 21, 2020 at 09:39 PM, John Gord wrote:
Jerry,
I think the discrepancy in maximum delay range is confusion between the
range
shown on one screen and the range available when including Electrical
Delay.
As you surmised, with 3GHz and 101 points, a discontinuity with a delay of
1.5ns also shows up at (1.5ns +33.3ns) and (1.5ns + 66.6ns), etc.
--John Gord
On Mon, Sep 21, 2020 at 08:20 PM, Jerry Gaffke wrote:
John,
Good stuff!
Thanks for the write-up.
# 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.
...
# "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
So if we do at 3000mhz/100=30mhz steps, then window into the region around
66.6ns,
I believe you are saying that our reading will be confused by stuff that
happens at 33.3ns.
Likewise, if we are looking at 33.3ns but our cable is greater than 66.6ns
long, we could get
confused by stuff that is happening at 66.6ns. Correct?
John seems to be saying that tmax (the maxi delay through the coax in
seconds)
is 1/fstep,
where fstep is the step size in Hz. With 100 steps, that is the fmax (the
max
frequency)
divided by 100. tmax=1/(fmax/100)=100/fmax.
With 100 steps and a minimum frequency of close to zero, the step size is
fmax/100,
and tmax=1/(fmax/100) = 100/fmax
In post 17561, Neil said that he found the relationship between max
frequency
and
the max delay through the coax was tmax=39/fmax.
Curious that there is a greater than 2:1 discrepancy.
I may have to play with it and see.
Jerry, KE7ER
