Actually the oscillator is producing about the same current it would into a short circuit, 40 mA) because it is driving a very low impedance of a series resonant circuit! Istvan, let’s talk on the phone to save s lot of typing.
Doug Smith Sent from my iPhone IPhone: 408-858-4528 Office: 702-570-6108 Email: d...@dsmith.org Website: http://dsmith.org On Thu, Apr 5, 2018 at 21:07, Istvan Novak <istvan.no...@verizon.net> wrote: Doug, Thank you for sharing your interesting experiments. These always trigger my curiosity to look a little further. In my basement lab I came up with a small series of experiments to separate the possible different coupling mechanisms from the oscillator to the oscilloscope input. Though I did not intend to replicate your exact setup, I believe it was conceptually and essentially similar enough that the results may apply to a large number of setups, including yours. From all of the different tests my conclusion is that the oscillator is not supplying a significant amount of current to produce the effect in question. I have not had the good fortune yet to see your full live demonstration, plus as you say you have much more data than what you publish, so it very well could be that you already have done similar experiments and came to similar conclusions. Therefore I would like to hear the thoughts of those list members who did not attend your demonstrations, what kind of experiments would they suggest to prove or disprove my conclusion. Regards, Istvan Novak On 1/30/2018 12:06 PM, Douglas Smith wrote: > 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 >> Website: http://dsmith.org >> >> On Mon, Jan 29, 2018 at 22:24, Istvan Novak >> <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 >> > Web: 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 >> > >> > Teraspeed Labs >> > 9999 SW Wilshire Street >> > Suite 102 >> > Portland, OR 97225 >> > >> > >> > -----Original Message----- >> > From: si-list-bou...@freelists.org >> [mailto:si-list-bou...@freelists.org] On >> > 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 >> > >> > 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 >> [mailto: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 >> > >> > 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. >> > >> > Doug >> > >> > >> > >> > >> > >> > >> > >> > >> > >> > >> ------------------------------------------------------------------ >> > To unsubscribe from si-list: >> > si-list-requ...@freelists.org with 'unsubscribe' in the >> Subject field >> > >> > or to administer your membership from a web page, go to: >> > http://www.freelists.org/webpage/si-list >> > >> > For help: >> > si-list-requ...@freelists.org with 'help' in the Subject field >> > >> > >> > List forum is accessible at: >> > http://tech.groups.yahoo.com/group/si-list >> > >> > List archives are viewable at: >> > http://www.freelists.org/archives/si-list >> > >> > Old (prior to June 6, 2001) list archives are viewable at: >> > http://www.qsl.net/wb6tpu >> > >> > >> > >> ------------------------------------------------------------------ >> > To unsubscribe from si-list: >> > si-list-requ...@freelists.org with 'unsubscribe' in the >> Subject field >> > >> > or to administer your membership from a web page, go to: >> > http://www.freelists.org/webpage/si-list >> > >> > For help: >> > si-list-requ...@freelists.org with 'help' in the Subject field >> > >> > >> > List forum is accessible at: >> > http://tech.groups.yahoo.com/group/si-list >> > >> > List archives are viewable at: >> > http://www.freelists.org/archives/si-list >> > >> > Old (prior to June 6, 2001) list archives are viewable at: >> > http://www.qsl.net/wb6tpu >> > >> ------------------------------------------------------------------ >> > To unsubscribe from si-list: >> > si-list-requ...@freelists.org with 'unsubscribe' in the >> Subject field >> > >> > or to administer your membership from a web page, go to: >> > http://www.freelists.org/webpage/si-list >> > >> > For help: >> > si-list-requ...@freelists.org with 'help' in the Subject field >> > >> > >> > List forum is accessible at: >> > http://tech.groups.yahoo.com/group/si-list >> > >> > List archives are viewable at: >> > http://www.freelists.org/archives/si-list >> > >> > Old (prior to June 6, 2001) list archives are viewable at: >> > http://www.qsl.net/wb6tpu >> > >> > >> > >> >> ------------------------------------------------------------------ >> To unsubscribe from si-list: >> si-list-requ...@freelists.org with 'unsubscribe' in the >> Subject field >> >> or to administer your membership from a web page, go to: >> http://www.freelists.org/webpage/si-list >> >> For help: >> si-list-requ...@freelists.org with 'help' in the Subject field >> >> >> List forum is accessible at: >> http://tech.groups.yahoo.com/group/si-list >> >> List archives are viewable at: >> http://www.freelists.org/archives/si-list >> >> Old (prior to June 6, 2001) list archives are viewable at: >> http://www.qsl.net/wb6tpu >> >> > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-requ...@freelists.org with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: http://www.freelists.org/webpage/si-list For help: si-list-requ...@freelists.org with 'help' in the Subject field List forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: http://www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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