Re: [PSES] [SI-LIST] Re: Measurement Dilemma - THE EXPLANATION!

[Ken Javor]

Loop antennas used for long and medium wave reception are based on the
principle of a large loop whose inductance is tuned out with an air variable
capacitor or varactor in series with loop. It does just what Doug describes:
allows a lot of current to flow at a very sharply tuned resonant frequency.

Ken Javor
Phone: (256) 650-5261




From: Douglas Smith 
Reply-To: Douglas Smith 
Date: Mon, 29 Jan 2018 22:55:06 -0800
To: 
Subject: Re: [PSES] [SI-LIST] Re: Measurement Dilema - THE EXPLANATION!


 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 
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 

Re: [PSES] [SI-LIST] Re: Measurement Dilema - THE EXPLANATION!

[Douglas Smith]

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  
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 

[PSES] Answer to Measurement Dilemma

[Doug Smith]

Hi Everyone,

I gave the answer in a reply to a few people but realized others may not have 
seen the answer to the Measurement Dilemma.

First, for those who did not see the 3 1/2 minute video yet, here is the link:
https://youtu.be/qj-HBFMEJiY


The answer is:

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.

Every time one connects an unbalanced probe to a PCB, you get this effect which 
is an error in your measurement. The best way to see what the error is, is to 
short the probe to its ground lead and touch the tip to PCB ground, and you 
will see the error displayed. I have seen a lot of probes register 50 mV which 
can be a problem for small signal measuerments but I have seen errors of 4 
Volts due to switching power supply currents flowing in the system (not 
magnetic field emissions, that is a separate issue) and tens of Volts or more 
due to ESD in the room. If there is so much system noise that you can't make 
your measurement, your probe is telling you something, like maybe an EMC 
problem is lurking in the system.

Doug Smith
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
--

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Re: [PSES] Measurement Dilema - THE EXPLANATION!

[Doug Smith]

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
 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

Re: [PSES] Measurement dilemma

[Ralph McDiarmid]

The ground leads of the two probes are “dressed” very differently.  Have a 
close look at the video.

Long ground leads on probes are a problem at RF frequencies or when trying to 
measure high dv/dt waveforms.  I used to removed probe clip, remove the ground 
lead, and then wrap #24 buss wire around the ground collar at the probe tip and 
keep that lead <1 cm (or a short as practicable) 

That’s how we used to characterize the peak-peak voltage “noise” at the output 
of bench power supplies which used HF switch-mode technology.  Measurements 
where done right at the buss bar or output connector with just such a probe, 
modified to keep probe parasitic inductance to a minimum.

At least that’s what I think is going on with Doug’s demonstration.


Ralph McDiarmid
Solar Business
Schneider Electric


From: Doug Smith [mailto:d...@emcesd.com] 
Sent: Friday, January 26, 2018 2:50 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] 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

Doug 


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[PSES] Eval and Demonstration Units

[Schmidt, Mark]

Hello Group,

What is the procedure for Evaluation/Demonstration Units in China. This is a 
hand-held Wi-Fi device, FCC/IC/CE/AusNZ/SAR testing has been completed and I do 
not anticipate completion for China until end of March. Is this just a matter 
of Labeling the device "Engineering prototype for evaluation only" or is more 
complex than that.

Best Regards,
Mark
Please be advised that this email may contain confidential information. If you 
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