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 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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