[sdiy] 100 MHz EMI, what can it be?

[harrybissell]

Hi Martin....

Didn't your scope probes come with a little spring clip that attaches
right to the ground barrel of the probe...with a little needle point
to stab into a (hopefully) nearby ground ???

I always wondered what this was for (now I know).

I had to take some measurement in an industrial cabinet... looking for a
millivolt
signal in an ambient of 700VDC, switched at 1200Hz, slew of 500V/us.

With the ground lead (you described) the noise was about 100x the signal. I
could not
believe the circuit could work.  After spiraling the ground lead (you
described) it was
10X. After using the probe ground adapter (I described)... it was about
2X... with a direct
soldered ground it was about 1X.... so I figure if I had a perfect probe it
would be just
perfect.  :^P

Czech Martin wrote:

> It's actually not so much the circuit,
> (it did not fool me too long)
> the scope allone will catch the RF by
> it's long ground clip cable.
>
> As long as I'm in the 1V/cm range
> I can not see any disturbance,
> but the 10mV/cm range is useless
> until I bend my own ground "spiral"
> to keep wires very short.
> I.e. you have to unmount the probe
> parts, clamp, isolation tube,
> ground lead, it is all plugged on the probe.
>
> The RF is much higher than the scope
> input amplifier noise.
> I think there was a Pease Porridge
> about "how can you have so clean screenshots
> when mine are so noisy".
> The older Tek skopes had a "virus"
> clamp, i.e. something that looks
> like a virus or spider where you could
> poke the scope shield in, so only few mm
> of the "hot" tip was exposed.
> You could also solder them into a PCB
> in order to poke the naked probe metal
> right in.
>
> For mV measurments the ground clips
> of scopes are pretty ueseless.
> But I havn't seen anything better,
> it is a mechanical problem also.
> Of course, this depends on bandwidth, a lot.
> A 20MHz scope simply won't see much of the
> 100 MHz RF stuff.
>
> Anyway, I'm a good candidate for a
> "banana, XLR, telephone plug, shielded or not
> shielded, balanced or not balanced measurement comparison".
> The air is full of RF, the railway will add
> 16 2/3 Hz power line noise and magnetic fields
> to that. Could you ask for more for such a
> test? ;->
>
> I think I will get to that on easter,
> at least I hope so.
>
> m.c.
>
> -----Original Message-----
> From: jhaible [mailto:jhaible at debitel.net]
> Sent: Montag, 14. April 2003 12:41
> To: Czech Martin; Ren Schmitz
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] 100 MHz EMI, what can it be?
>
> >107,0MHz or 107,7MHz FM , 500W
> >
> >directly in my neighbourhood.
> >
> >Do you see something like this with your local
> >FM stations?
> >
>
> I have once traced some "oscillation" in an Emu filter
> for hours, until I noticed it came "from the air".
> Why?
> It was my first time to use LM318 opamps, so I didn't
> trust them. And the circuit was so clean that I saw
> a periodical signal where in other circuits this would
> have been below noise.
>
> JH.
>
> -----Original Message-----
> From: René Schmitz [mailto:uzs159 at uni-bonn.de]
> Sent: Montag, 14. April 2003 12:01
> To: Czech Martin
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] 100 MHz EMI, what can it be?
>
> Could that be be a FM radio station? I would check with a
> receiver what that signal is.
>
> Cheers,
>   René
>
> Czech Martin wrote:
> > If I connect the (long) ground clip of the oscilloscope
> > probe to the tip, I get a nice loop.
> > I can see 10 mVpp, around 100MHz. No matter how I orientate the loop,
> > it will not disappear.
> >
> > What can this be? Computer is of, cell phone is much higher,
> > wireless phone should also be higher.
> >
> > Of course, connecting the probe tip to shield via 5mm wire
> > will make the noise disappear, so it is clearly something
> > from outside the scope.
> >
> > I live near a railway track, is this railway communication radio?
> >
> > m.c.
> >
> >
>
> --
> uzs159 at uni-bonn.de
> http://www.uni-bonn.de/~uzs159