An unusual Oscilloscope phenomenon

[David L. Jones]

Check out a rather unusual phenomenon you can see on your oscilloscope in my
latest blog (#14):
http://www.alternatezone.com/eevblog/
See if you can get it to happen on your scope!
I'll offer a free subscription to my blog for the best screen shot posted
:->

Oh, and the probe doesn't have to be shorted either.

Have fun.
Dave.

 

[Dirk Bruere at NeoPax]

Check out a rather unusual phenomenon you can see on your oscilloscope in my
latest blog (#14):
http://www.alternatezone.com/eevblog/
See if you can get it to happen on your scope!
I'll offer a free subscription to my blog for the best screen shot posted
:->

Oh, and the probe doesn't have to be shorted either.

Have fun.
Dave.

Got it immediately, because I once worked in lab that had cheap plastic
chairs that generated 1cm sparks when you stood up!

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.theconsensus.org/ - A UK political party
http://www.onetribe.me.uk/wordpress/?cat=5 - Our podcasts on weird stuff

 

[David L. Jones]

Hey, is that Chinese take-out I spy?

No. Where?

Dave.

 

[David L. Jones]

On the right. Eh, probably an open box.

Yep, an open Altronics box!
Curiously though the contents were made in Australia and not China - how
novel!

Dave.

 

[TonyS]

That's usually the best way to ground shields.

John

IIRC head signals mostly went via twisted pair. If they had a shield as well we
would certainly not ground them on both sides.

Tony

 

[Spehro Pefhany]

Got it immediately, because I once worked in lab that had cheap plastic
chairs that generated 1cm sparks when you stood up!

Back in the old days, they used to generate 1/2" sparks.

 

[Dennis]

Back in the old days, they used to generate 1/2" sparks.

Nasty stuff that Bakelite.

 

[Spehro Pefhany]

Nasty stuff that Bakelite.

Goes nicely with wood cases:

http://ip.co.za/shop/images/wheatstone.jpg
http://www.surplussales.com/Images/...Northrup/eqp-wbm-119828/eqp-wbm-119828_lg.jpg

 

[amdx]

Look at a piece of blank paper and you will see what i see on that site.
Therefore the mashed and flattened tree wins.

If you set the time base correctly and crunch down on a carrot
you will see a complex waveform on the scope.
Very difficult to store on a digital scope though.
Mike

 

[Dennis]

Goes nicely with wood cases:

http://ip.co.za/shop/images/wheatstone.jpg
http://www.surplussales.com/Images/...Northrup/eqp-wbm-119828/eqp-wbm-119828_lg.jpg

When I started out with a government run telco and some of the gear was
still in nice wooden cases. Over the years I saw pallet loads of test gear
consigned to scrap. Loads of bridges, wood cased multimeters and bakelite
AVO meters. These things sell for quite a few dollars nowadays - aint
hindsight nice!

 

[terryc]

If you set the time base correctly and crunch down on a carrot

Lunch pictures?

 

[Nico Coesel]

Check out a rather unusual phenomenon you can see on your oscilloscope in my
latest blog (#14):
http://www.alternatezone.com/eevblog/
See if you can get it to happen on your scope!
I'll offer a free subscription to my blog for the best screen shot posted
:->

I'm afraid someone already got a patent on electrostatic discharge.

 

[Fred Bartoli]

John Larkin a écrit :

You *want* large amounts of high-frequency common-mode noise?

Probably. Lots of people don't understand this and I've seen this kind
of mistake (opened shield) recommended in lots of highly regarded EMC books.

But that's really nice for us. (Joerg?)

 

[Rich Grise]

That's usually the best way to ground shields.

When I was a kid, my brother built a Heathkit TOOB amp; in the low level
circuits, they only ever grounded one end of the shields.

Cheers!
Rich

 

[Joerg]

John Larkin a écrit :

Probably. Lots of people don't understand this and I've seen this kind
of mistake (opened shield) recommended in lots of highly regarded EMC
books.

But that's really nice for us. (Joerg?)

Yeah! But shhht, don't spill the beans here, at least not until I am
retired )

 

[Rich Grise]

IIRC head signals mostly went via twisted pair. If they had a shield as well we
would certainly not ground them on both sides.

You *want* large amounts of high-frequency common-mode noise?
[/QUOTE]
No, which is precisely the reason to ground only one end of the shield.

Cheers!
Rich

 

[Joerg]

At the receive (instrument) end, the ungrounded end of the coax shield
becomes the hot side of a resonant "vertical" antenna. Any voltage
there becomes 100% common-mode signal on the inner twisted pair.

What's wrong with a little current in the shield? The signal pair
isn't affected. But volts, or tens of volts, of common-mode RF can be
nasty.

It could become really, really nasty if the cable length happens to be
close to 1/4, 3/4 and so on wavelength of the offending RF signal. If
the shield current is of concern it can be broken by a wee ferrite.

Probably the litmus test for RF stuff like this is when people schlepp
it to Otis Street and see if it can survive Sutro Tower.

 

[Joerg]

Inside a steel chassis pan, it wouldn't matter. But then Heathkit was
hardly an authority on signal management.

Now wait a minute, they engineered darn good stuff. The old HW100/101
transceivers were among the best ever made back in the tube days. Still
got one. No noises or buzzes whatsoever and its performance in the
vicinity of strong signals is stellar. The only not so good thing was
the two rubber pulleys because they cooked out too fast, but you could
keep a stash of them in the drawer.

 

[Fred Bartoli]

Joerg a écrit :

Yeah! But shhht, don't spill the beans here, at least not until I am
retired )

I think I'll retired after you, but this kind of misconception is
sufficiently widely spread that we don't have to worry

 

[Dirk Bruere at NeoPax]

Back in the old days, they used to generate 1/2" sparks.

They had more volts in them as well

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.theconsensus.org/ - A UK political party
http://www.onetribe.me.uk/wordpress/?cat=5 - Our podcasts on weird stuff