Re: signal leads that pick up less ambient noise?

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.

That sounds like the usual setup and doesn't normally cause any trouble,
(although the lead to the croc-clip, described in another post as 8",
would be a bit long if you were working at R.F.).  Have you checked the
continuity of the earth connection from sig-gen earth to croc-clip with
an ohm-meter?  (The earth leads sometimes fracture under the sleeving
where they enter the back of the croc-clip)

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.

 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.

Do similar checks on the 'scope earth lead.
.

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~ Liz Tuddenham ~
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www.poppyrecords.co.uk