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
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
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......
Sent from my iPhone
On Mon, Jan 29, 2018 at 22:24, Istvan Novak
I see your point, but I think we would need a little more
argument for the amount of current you assume in this example.
I agree with the approximate calculation of the L*dI/dt
where the ground-lead inductance is assumed to be 100nH and I
measured the rise/fall times of the HC240 to be around 2ns.
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
the same) that the oscillator's return (its 'ground') is left
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
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
(we can call it common-mode impedance) and eventually it has
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
capacitive coupling to not limit to current to lower values.
crudely from the video, the 'stray' capacitance of the
very well be much lower. However, here is another possible
to the waveforms on the oscilloscope screen: the oscillator
common-mode voltage onto the scope probes, which creates
current in the
shields and these being passive probes, through the finite
transfer impedance of the cable braid, voltage is induced at
connections. To prove/disprove the existence and amount of this
potential contributor, you could redo the test by eliminating
ground-lead loop (for instance by using coax receptacles for
tips), leaving everything else the same. If the displayed
significantly, I would consider as a proof that in fact the
source of the waveform is related to the ground-lead
inductance. If the
displayed waveforms would not change considerably, it still
prove that the finite surface transfer impedance is the man
it would prove that there should be another major contributor
ground-lead inductance. In this letter case further tests
devised to nail down the major contributor.
What do you think?
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
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.
University of Oxford, Course Tutor
Department for Continuing Education
Oxford, Oxfordshire, United Kingdom
P.O. Box 60941
Boulder City, NV 89006-0941
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
9999 SW Wilshire Street
Portland, OR 97225
Behalf Of Austin Mack
Sent: Monday, January 29, 2018 2:28 PM
To: d...@emcesd.com; 'si-list'
Subject: [SI-LIST] Re: ***UNCHECKED*** Measurement Dilema
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
is tied to GND at the probes, it and the floating power
pack will be
radiating like crazy at 40MHz and electromagnetically and
coupling to the CH2 shield which BTW is close to a quarter
Behalf Of Doug Smith
Sent: Friday, January 26, 2018 2:53 PM
Subject: [SI-LIST] ***UNCHECKED*** Measurement Dilema
Can you explain the result in this video I just made? Scope
plots of the
same two nodes are completely different. Probes and scope
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
Hint 2: There are no active components inside the box.
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 on March 13-16.