Re: signal leads that pick up less ambient noise?

Christopher Howard <christopher@librehacker.com> wrote:

liz@poppyrecords.invalid.invalid (Liz Tuddenham) writes:
 
Checking just now, continuity from sig-gen earth to croc-clip appears to
be good.
 

The next test is to unclip the croc-clip and measure the resistance
from the chassis of the device-on-test (or the 'scope) to the sig-gen
earth. If it is low, that means there is an alternative D,C. signal
path in parallel with the screen of the co-ax, which could be picking
up interference.

 
I'm a little fuzzy on how to do this test, as the DUT does not have a
chassis (all on breadboards) and the scope is fully encased in plastic,
except for the probe ports and such.
>
But if I connect just the signal pin to the DUT, leave the alligator
clip disconnected, and then measure from DUT gnd bar to sig-gen earth,
it is infinite resistance.

That shows you haven't got a parallel D.C. earthing path, which rules
out one possible noise-injection pathway.

 If the D.C. test appears satisfactory, there may be an A.C. path
through mains suppression capacitors, which passes the interference
but doesn't showwith the D.C. of a continuity test. Does the signal
drop and the interference get worse when the croc-clip is unclipped?
If the signal doesn't drop, you have an A.C. path in parallel with the
earth.
>

 
For this test, I put the signal generator leads in as input to DUT and
scope as output from DUT. When I remove the crop clip from DUT gnd, the
sinusoidal signal is still visible on the scope at the same amplitude,
but the noise surrounding it increases in amplitude significantly.
 

Do similar checks on the 'scope earth lead. .

 
Once again, the sine signal is still visible on the scope at the same
amplitude, but the noise surrounding it increases — even more
dramatically.

Those tests suggest that there is some degree of capacitive coupling
between the sig-gen, the DUT and the 'scope, so that could be how the
noise is getting in.  As others have mentioned, plugging all three ito
adjacent sockets on a distribution strip should reduce this noise path
and putting a filter between the strip and the mains supply should
reduce it still further.

Some cheap filters, especially those built into socket strips, only have
capacitors, with no inductors.  These will decrease the noise voltage at
the expense of increasing the noise current;  depending on how the noise
is getting in, that may make things worse, rather than better.


--
~ Liz Tuddenham ~
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www.poppyrecords.co.uk