Re: signal leads that pick up less ambient noise?

On 21/02/2025 4:07 am, Edward Rawde wrote:

"Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vp6drd$2mqf1$1@dont-email.me...

On 20/02/2025 5:35 am, Christopher Howard wrote:

Hi, I have a very noisy workbench (lots of digital computers and
computer monitors nearby) and it seems like I pick up a lot of noise on
the long leads coming out of the signal generator BNC output - around
600 mV p-p. I am wondering if there are any particular leads I could buy
that would somehow pick up less ambient noise.

>
You might think about double shielded coax.The standard woven braid outer offers about 98% shielding and adding a wrap of
aluminised Mylar underneath it get you closer to 100% shielding.
>
As other posts have pointed out, your problem is probably going to be earth loops, and wrapping a short length of the coax around
a ferrite toroid can help with that. Ralph Morrison wrote the book on the subject back in 1967. I read the first edition back
then, and I've had access to most of the subsequent editions. I've got the fourth edition from 1998 when I finally had to buy my
own copy.
>
https://books.google.com.au/books/about/Grounding_and_Shielding_Techniques_in_In.html?id=IxUjAAAAMAAJ&redir_esc=y\

 The sixth edition (2016) appears to be called
Grounding and Shielding: Circuits and Interference.
And is not hard to obtain.
But unless you want to wade through a book which touches on the calculus of electromagnetic theory (and it's perfectly fine if you
do), you're likely better off focussing on the practical side of things.

Wrong. Ralph Morrison's book is sublimely practical; it includes enough electromagnetic theory to let you understand what is going on.
Cook-book style texts don't, and should be avoided.

Decades ago before anything like mobile phones existed I worked at a newly constructed facility which had a powerful radio
transmitter.
The gatehouse was outside the facility and had a PA (Public Address) system installed for communication with people inside the
facility.
When keying the microphone you could hear nothing inside the facility other than the transmitter modulation.
It took many weeks to solve the problem and more than a few people (possibly with PhD level qualifications) demonstrated their lack
of knowledge by proposing solutions such as ferrite toroids in the speaker wiring (100V line system).
Eventually the problem was solved as follows.
Remove the preamplifier board from the all in one pre amp/power amp unit in the gatehouse.
Obtain one metal box of suitable size and drill holes for suitable feedthrough capacitors for all connections.
https://www.google.com/search?&q=feedthrough+capacitor&udm=2
Mount the board in the box and connect to the feedthrough capacitors as necessary. Close the box.
Glue the box to the back of the pre amp/power amp unit.
Wire from the feedthrough capacitors to the appropriate points in the unit.
The PA was now totally quiet, except for the announcement being made.

Ralph Morrison talks quite a lot about double-screened transformers, which help when where enough current is being injected into the screen on one side to produce the significant voltage drops across the screen.
Powerful radio transmitters do produce quite intense electric fields. You have to put Faraday cages around fluorescent lights to stop them lighting up even when they are switched off.
Back in the days when you'd build a preamplifier board on two layer printed card without any kind of ground plane, you would have had to screen it carefully.
When I was working at Cambridge Instruments a number of old two layer boards were re-worked as four and six layer boards with buried ground planes, and we could leave out the aluminium screening plates we'd had to fit to stop adjacent boards in our racks from messing up their neighbours.

The 40kHz periodicity suggests that old cheap switching power supplies are the source of the noise.

--
Bill Sloman, Sydney