Re: signal leads that pick up less ambient noise?

On Wed, 19 Feb 2025 14:36:17 -0900, Christopher Howard
<christopher@librehacker.com> wrote:

John Larkin <jjlarkin@highlandtechnology.com> writes:
>

I assume that your "leads" are a coax.

>
Yes, the loads something my co-worker threw together from old cable
laying around. One one side is BNC that plugs into the signal generator.
After than is a thin coax about 3mm thick. At the other end, he split
the inner conductor and the shield into two leads, one with a pin at the
end, and one with an alligator lead.

You can see the ground lead contribution by shortin the probe tip to
the grounding lead: common mode is visible with shortest gnd lead
and probe tip connected to same zero-voltage point of DUT, when
ON/OFF, with sig gen connected/not connected, With sig gen ON/OFF.

This can be repeated iteratively, as the larger contributors are
identified.

Commercial scope probes are typically supplied with a probe tip
clip that allows probe grounding near the measurement point with
minimal physical loop.

>
How are you measuring that noise? It's more likely to be ground loop
noise than coax shield leakage.

>
I have a project I'm doing building my own analog computer — which
currently does not have any filtering installed on it, other than some
100 nF bypass caps. I can see this heavy noise — about .6 mV p-p — on
all signal output, regardless of what op amp I tap into. If I remove the
analog computer and just tie the signal generator to the scope, I see
the same noise.
>
If I turn the computers off around my workbench, the noise becomes less,
proportional to the percentage of computers I turn off. If I take the
signal generator and the computer into another room with no computers,
the noise almost vanishes.

Don't forget other sources of noise - ccfl light bulbs and ballasts -
printers, modems, smart phones, AC motors and fans and simple mains
wiring(re-radiatioon).

Don't forget possible scope contribution - it is also following your
measurement setup. There are some self-noisy scopes out there.
Battery-powered scopes offer some interesting noise test setup
options, in extreme situations.

>

>
Common-mode chokes, ferrites or toroids, can help. Just plugging the
generator and the scope into the same outlet may help.
>

>
I am planning to go that route, with the chokes and such, once some
parts come in. But I was also wondering about the single leads
themselves, which feed from the signal generator into analog computer
inputs.

Sig Gen output in coax as well. Same or similar precautions.
Try different sig gen at some time.

Simplest interconnection and most familiar test environment is best.

I also recommend Morrison's book. Earlier editions prefered.

RL