Re: 453 calibrator

Fabio Trevisan
 

Hello Mike,

Never need to excuse yourself, the gold in exchanging with civilized people like it's usually the case in this group is that not knowing or misunderstanding is no sin (although I`m myself a caveman).
Be careful indeed not to getting shocked when playing with those toys, but no care needed when expressing yourself.
We learn from our mistakes and from the others too!

The current loop of the 453's calibrator is not meant to be "voltage-probed"... the voltage there is essentially zero, because the current is so small and the loop is a bare ground wire (barring possible defects, like false contacts to ground or loose tightening nuts). The voltage output is at J579 only.
Now, while seeing the extents you took into trying to measure it at the ground loop, it just occurred to me that this calibrator is unlike the one from the 475 itself, and from others of the same family (465, 464, 466 etc...). So there's maybe a possible misunderstanding here.
On your 475, the current sensing loop is, AT THE SAME TIME, the calibrator's output voltage too. (there's no Calibrator output connector like on the 453).
On the 475 the bottom resistor of the voltage divider (a 10 ohm resistor, across which the 300mV voltage develops) is between the current loop and ground so you can either surround the current loop with a current sensing clamp, or probe the loop wire directly for calibrator voltage. It's a more economic approach.
This is not the case for the 453, in which the calibrator's current loop is in the bottom of the voltage divider and it's a ground wire, not meant to be probed for voltage.
You shouldn't see even a 2.5mV there like you managed and, if you do see something there's a chance there's false contacts at play. You may be needing to tighten the nuts that hold the current loop and connect it to ground.

On your background... Wow! It's not that common to see someone from ecology / biology / botany to have interest in electronics and as such, you can consider yourself already a champion.
I for myself, apart from "Avatar" pseudo-science, didn't think there was electrical activity on plants (apart from the electrical exchange that is intrinsic to the chemical reactions). So maybe Avatar is not so SyFy as it seems (BTW, I loved the base concept... It's indeed conceivable that a world covered by forests could actually develop a "mind" of its own, from the possible interconnections there could exist).

Rgrds,
Fabio

On Fri, May 11, 2018 at 10:22 am, Mike Merigliano wrote:


Hello Fabio,

Thanks for the response!  It's clear now that I need to be more clear in
my messages.

Despite contrary evidence, I know what a current probe is, and I have a
bigger clamp-type one for dealing with house wiring etc., but I really
appreciate your succinct explanation of the ones involved with scopes; I
mentioned the probe model number so readers would know which voltage
probe I was using.  Also, I don't mind if people assume I don't know
something.

You are correct about my mistake about the voltage divider -- I mis-read
it on the schematic.  I need more practice reading these things quickly
without checking with actual measurements. That's the main reason I have
been responding to posts lately. I'll be more careful next time.

After sending my previous message to you, I was a little curious and
looked for the 1kHz signal in all of the noise on the current probe
loop. I know this is weird place to measure the 1kHz voltage, but wanted
to check what I could see anyway.  I could see it much more clearly via
cascading the inputs on a Tek 475, and cutting the bandwidth to 20 MHz.
Still quite a bit of high frequency noise but the 1kHz square wave
shows, and my best guess is p-p voltage of 2.5 mV at the current probe
using the 1x voltage probe (P6028).  I did the same cascading on the 453
-- not much worse than what I can see on the 475.

My background is in ecology -- especially plants. They have some common
aspects to electrical currents and potentials in their vascular systems,
and plant ecologists use various sensors to measure activity from the
leaf scale to much broader ones via satellites.  I read the Tekscopes
forum quite often, just to expand my horizons and think out of the plant
box a little bit. I also fix up audio gear, mainly to save money, but
find it an interesting hobby. Of the many electronics forums out there,
I think the Tekscopes one has mostly experienced people, and some of the
wide-range of problems are quite difficult.

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