Electronics > RF, Microwave, Ham Radio

when earth is not earth

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Simon:
I was messing with my least favourite project in the office today. Something designed by a subby that I now need to get through EMC, well its power supply actually.

So the cabling we use has a shield, it's that industrial grade mains cable, it's steel. So I have taken advantage of this and connected it to the metal box the power supply is in. I made a mock up of the actual machine with a die cast box with resistors in it to load the supply with a "non reactive" load.

I used a near field probe and was surprised to find stuff coming off the shield at various frequencies, around 110MHz and various spots I had not noticed before as it is usually around 70MHz (I've been messing with it for a while). Then as I was doing some narrower band measurements my boss was changing the lights and I could see the signal literally going up and down to the banging he was doing (getting those horrible grills off office ceiling lights). Then I pointed the probe in the direction of the light fitting above me, I noticed some very slight bumps in exactly the same place.....

So it seems that i am picking up noise from the lights from cable screening on a power lead plugged into a socket.

This leads me to wonder as to what is the best way to separate myself from such earth interferences. Should I have a dedicated earth, or put some sort of filter in place? the spectrum analyser and the power supply were plugged into the same socket bar, so I seem pretty doomed.

Geoff-AU:
A LISN will help isolate power-borne noise, they're used for making conducted EMC measurements.  But normally you're only doing conducted up to 30MHz and radiated above that.  You might need some lighting that doesn't come from Alibaba :)  If it's radiating in then it'll be a pain in the butt.

Simon:
I looked at LISN's but they only ever show a live and neutral, I would expect then that the earth just passes through. Just the same as all CMC you get off the shelf.

I'll probably have to go back up to my semi faraday cage area where at least there are no lights and I am on a completely separate circuit.

jonpaul:
Suggest you read the classic Noise Reduction by Henry Ott
See SYNAUD COM for many audio interference and shieldiing, earthing practices.

Bon courage

Jon


PS said in 1971...

"there is NO such thing as a ground"

By The late and great Dick Sequerra,
RF and hifi pioneer, ex Marantz

Jon

tggzzz:

--- Quote from: jonpaul on November 01, 2022, 08:40:06 am ---PS said in 1971...

"there is NO such thing as a ground"

By The late and great Dick Sequerra,
RF and hifi pioneer, ex Marantz

--- End quote ---

Precisely.

"Ground" or "earth" is a fiction, one that is convenient in some circumstances.

Common example: cheap mains checkers verify that the three connections are correct and that the "earth resistance" is OK. Daughter's house shows green, my house shows orange, parent's house showed red (despite, according to the current owner, it having a three phase supply!).

Extreme example: when caught out in a thunderstorm. One of the recommendations is not to stand with feet apart, but keep your feet together. The reason is that if there is a nearby strike to the ground, there will be a lot of current going through the ground; you would prefer that the easiest path is through the ground between your feet, not through your body between your feet.

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