I did that experiment a long time ago, almost 30 years ago, when developing
this experiment, which is described in my book.
Reducing ground lead length to near zero eliminates the effect almost
completely with no other changes in the experimental setup. You still see a
little effect about 1/50 of before, due to the shield transfer impedance of
the probe cables. A tiny ground lead always swamps shield transfer
impedance of practical shielded cables. I do that experiment for my classes
as an extension of this experiment.
My live experiments are always more complete than the versions I publish
both to keep published versions reasonably short and to provide extra value
to live experiments. I usually have ten times the data I actually publish!
Doug Smith Sent from my iPhone IPhone: 408-858-4528 Office: 702-570-6108
Email: d...@dsmith.org Website: http://dsmith.org
On Tue, Jan 30, 2018 at 6:42, Istvan Novak <istvan.no...@verizon.net>
wrote:
Doug,
I see what you mean, but to me this does not seem to prove either way
whether the mechanism bringing the signal into the signal paths is the
inductance of the ground leads or is it happening through finite surface
transfer impedance. The convincing argument/experiment would be to show
that by eliminating the suspect element, the signal pickup goes away.
Regards,
Istvan Novak
Oracle
On 1/29/2018 10:55 PM, Douglas Smith wrote:
Hi Istvan, If I connect the oscillator directly to the scope chassis, I get
similar readings at different frequencies. I generally get less amplitude
as well.
The small capacitance between the scope and oscillator is capable of tuning
the whole system to a low input impedance on the probe end. I have seen
this in many scenarios that ended up loading the oscillator to a near short
circuit! The whole system is resonant. I only get enough current to get
large probe voltages at discrete frequencies. I had to tune around to get
the results you saw. These is a lot more information when I do the demo
live for a class that are not practical to put in a video (would last way
too long).
I think amateur radio operators can identify with the amazing
characteristics of resonant systems.
A loop is not nearly dangerous as a loop terminated in a small capacitance!
Tesla coils resonate using stray capacitance. I built a 600 Watt coherent
300 kHz RF driven Tesla Coil in the 9th grade, but that is another
story......
Doug K4OAP
Doug Smith Sent from my iPhone IPhone: 408-858-4528 Office: 702-570-6108
Email: d...@dsmith.org [d...@dsmith.org] Website: http://dsmith.org
[http://dsmith.org]
On Mon, Jan 29, 2018 at 22:24, Istvan Novak <istvan.no...@verizon.net>
[istvan.no...@verizon.net] wrote:
Doug,
I see your point, but I think we would need a little more convincing
argument for the amount of current you assume in this example.
I agree with the approximate calculation of the L*dI/dt voltage drop,
where the ground-lead inductance is assumed to be 100nH and I assume you
measured the rise/fall times of the HC240 to be around 2ns. If we
assume 40mA current delta, we approximately get the voltages you see on
the screen. I am just not convinced that the current is really in the
order of 40mA. If I am not mistaken, you say (and the video suggests
the same) that the oscillator's return (its 'ground') is left floating.
In this case it matters much less what is the inductance of the
ground-lead loop (you say it is 100nH, not disputed), but for current to
flow through the oscillator output, it has to find its way back to its
return by going through the parallel of the two probes' braid impedance
(we can call it common-mode impedance) and eventually it has to close
back to the floating return of the oscillator. This last portion of the
current loop, closing back from the oscilloscope chassis/ground to the
oscillator return would need to be in the order of 50 pF or higher in
capacitive coupling to not limit to current to lower values. Judging
crudely from the video, the 'stray' capacitance of the oscillator might
very well be much lower. However, here is another possible contributor
to the waveforms on the oscilloscope screen: the oscillator imposes a
common-mode voltage onto the scope probes, which creates current in the
shields and these being passive probes, through the finite surface
transfer impedance of the cable braid, voltage is induced at the probe
connections. To prove/disprove the existence and amount of this
potential contributor, you could redo the test by eliminating the
ground-lead loop (for instance by using coax receptacles for the probe
tips), leaving everything else the same. If the displayed voltage drops
significantly, I would consider as a proof that in fact the dominant
source of the waveform is related to the ground-lead inductance. If the
displayed waveforms would not change considerably, it still would not
prove that the finite surface transfer impedance is the man cause, but
it would prove that there should be another major contributor beyond the
ground-lead inductance. In this letter case further tests could be
devised to nail down the major contributor.
What do you think?
Regards,
Istvan Novak
Oracle
On 1/29/2018 3:13 PM, Doug Smith wrote:
> OK Everyone, here is the explanation:
>
> The box is superfluous, it just hides what is actually going on. The box
contains two heavy gauge wires. One shorts the two tips sig and gnd
together and the other connects the center point of the first wire to the
center pin of the BNC connector. This is equivalent to removing the box and
putting a stiff wire into the BNC center pin on the generator and
connecting both probe tips and both ground leads to this stiff wire.
>
> Doing so causes the generator to push a current out the prob e ground
leads creating a voltage across the inductance of the leads that create a
loop at the front of the probe. The current flowing through the ground
leads creates magnetic fields that for the most part are captured by the
loop formed by the ground leads and probe delivering the induced voltage to
the probes.
>
> The probes only respond to voltage between their tips and ground lead
attachment point, and the ground lead induced voltage is delivered to each
probe by loop they form.
>
> Since the probes are lying apart from each other, one on the plastic
table with a loop in its cable, and the other over a highly conducting
(center layer) ESD mat, their common mode impedance will be different,
varying at different frequencies. That results in different common mode
currents on the probes and the ground lead induced voltages are therefore
different and that is what is displayed on the scope. It is interesting
that the current output of an HC240 Octal Inverting Buffer can induce volts
across the ground leads when its output is only 5 V P_P, but entirely
understandable if you calculate e = Ldi/dt = 100 nH*.040 A/2ns) as an
approximation.
>
> For the generator to push a current onto the probe cables, it must form
an image current somewhere and that is displacement current to the nearby
scope chassis and some radiation from the oscillator itself into the
"ether," although it is on the small side to radiate itself efficiently at
40 MHz.
>
> I suspect the total current being pushed onto the probes is about 40 mA,
the short circuit current of the HC240 IC being used. At some frequencies,
each probe cable will resonate with the capacitance back to the scope from
the oscillator. By changing the frequency, I can make either probe register
a larger signal than the other probe.
>
> I love experiments like this. I have been doing this one for over 20
years for my classes and have have tons more demos to challenge engineering
minds. Most of my demos have an unexpected result and the discussion that
follows elucidates some engineering principle or addresses a myth.
>
> Doug
> University of Oxford, Course Tutor
> Department for Continuing Education
> Oxford, Oxfordshire, United Kingdom
> --------------------------------------------------
> Doug Smith
> P.O. Box 60941
> Boulder City, NV 89006-0941
> TEL/FAX: 702-570-6108/570-6013
> Mobile: 408-858-4528
> Email: d...@dsmith.org [d...@dsmith.org]
> Web: http://www.dsmith.org [http://www.dsmith.org]
> --------------------------------------------------
>
>
>
> On Mon, 29 Jan 2018 14:40:17 -0800, "Tom Dagostino" wrote:
>
> Why is everybody looking for tricks or complex answers. This is really a
> very simple, basic issue. If you look closely you will see the
difference.
>
> Tom Dagostino
> 971-279-5325
> t...@teraspeedlabs.com [t...@teraspeedlabs.com]
>
> Teraspeed Labs
> 9999 SW Wilshire Street
> Suite 102
> Portland, OR 97225
>
>
> -----Original Message-----
> From: si-list-bou...@freelists.org [si-list-bou...@freelists.org] [
mailto:si-list-bou...@freelists.org [si-list-bou...@freelists.org] ] On
> Behalf Of Austin Mack
> Sent: Monday, January 29, 2018 2:28 PM
> To: d...@emcesd.com [d...@emcesd.com] ; 'si-list'
> Subject: [SI-LIST] Re: ***UNCHECKED*** Measurement Dilema
>
> Hi All,
>
> It looks to me like the oscillator and its power pack (with very long
leads)
> are in close proximity to the CH2 probe cable. Since the oscillator
output
> is tied to GND at the probes, it and the floating power pack will be
> radiating like crazy at 40MHz and electromagnetically and capacitively
> coupling to the CH2 shield which BTW is close to a quarter wavelength.
>
> Austin
>
> -----Original Message-----
> From: si-list-bou...@freelists.org [si-list-bou...@freelists.org] [
mailto:si-list-bou...@freelists.org [si-list-bou...@freelists.org] ] On
> Behalf Of Doug Smith
> Sent: Friday, January 26, 2018 2:53 PM
> To: si-list
> Subject: [SI-LIST] ***UNCHECKED*** Measurement Dilema
>
>
>
>
> Hi All,
>
> Can you explain the result in this video I just made? Scope plots of the
> same two nodes are completely different. Probes and scope are operating
> normally, no problem with the equipment itself.
>
> If you have been to my seminars you know the answer, please do not post
the
> answer unless you have not seen this experiment until now.
>
> Hint 1: There are no EM fields radiating from the shielded box affecting
the
> probes.
> Hint 2: There are no active components inside the box.
>
> https://youtu.be/qj-HBFMEJiY [https://youtu.be/qj-HBFMEJiY]
>
> I do a lot of experiments in my classes that give surprising results.
Each
> one addresses a design or troubleshooting problem that engineers do not
> realize their troubleshooting efforts. Next
> one: http://emcesd.com/bcsem_hfmeas.htm
[http://emcesd.com/bcsem_hfmeas.htm] on March 13-16.
>
> Doug
>
>
>
>
>
>
>
>
>
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