[sdiy] 5mV of noise ...

[harrybissell]

Another non-obvious noise source can be pickup in the
ground lead of the scope probe.   Today I picked up some
5V distortions in a 5V signal from the loop made by grounding
the probe with about a 3" long lead.

The really saavy people wrap a bare wire around the probe
end and solder it to ground right near the measurement point.
This cuts the pickup by making the loop VERY small.

I might add that I was passing some 2000A pulses with
a 10us rise time near that probe.  So I just ignored the noise
cause I know it ain't real !!!   :^P

H^) harry     (it will pick up switchers as well...)

Seb Francis wrote:

> JH. wrote:
>
> >I don't know how it was measured - it might just be the scope's own noise.
> >Not uncommon even on expensive scopes, because of the wide bandwidth.
> >
> >
> >
>
> especially digital scopes ..
>
> also, if the typical signal level is 10V p.t.p then this amount of noise
> isn't bad at all ...
>
> .. and another thing: you may get more noise when you breadboard
> (compared to PCB or even veroboard) because of all the extra tracks and
> long wires, etc.
>
> seb
>
> >----- Original Message -----
> >From: "Magnus Danielson" <cfmd at bredband.net>
> >To: <bert at percussa.com>
> >Cc: <synth-diy at dropmix.xs4all.nl>
> >Sent: Thursday, June 16, 2005 11:40 PM
> >Subject: Re: [sdiy] 5mV of noise ...
> >
> >
> >
> >
> >>From: "Bert Schiettecatte" <bert at percussa.com>
> >>Subject: [sdiy] 5mV of noise ...
> >>Date: Thu, 16 Jun 2005 23:15:53 +0200
> >>Message-ID: <004701c572b8$941266b0$4101a8c0 at laptop>
> >>
> >>
> >>
> >>>Hi all,
> >>>
> >>>I have a silly question so don't shoot me. I'm still learning on analog
> >>>stuff.. I have been breadboarding some audio circuitry and I measured
> >>>between 4mV and 5mV of noise. Exactly how bad is this? Of course I could
> >>>convert this into dB I suppose but even then I'm not sure how bad this
> >>>is once you start passing sound through it.
> >>>
> >>>
> >>If your referens level is 775 mV, a noise of 5 mV(RMS) equals -43.8 dBm.
> >>
> >>
> >This
> >
> >
> >>is bad. That's only 43.8 dB of dynamics. OK, let's say you have 6 dB of
> >>
> >>
> >head-
> >
> >
> >>room but that only gives you 50 dB of dynamics. OK, if you like the
> >>
> >>
> >dynamics
> >
> >
> >>of tape-recorders it may fit your needs, but you can do alot better.
> >>
> >>Cheers,
> >>Magnus
> >>
> >>
> >>
> >
> >
> >
> >
> >