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

[Czech Martin]

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