50Hz magnetic field and 50Hz noise

[fala]

50 hz noise

hello, alternating magnetic field produces electric field that can be induced as a noise voltage and noise current.
I want to know if I connect sheild of my cables to ground of a board(as shown in the attachment image), where does the induced current noise go? current always move in a close circuit, does it go back to transformer? if yes how? should it pass the rectifier diodes?
another question why when we touch probe of osciloscope we induce powerful 50Hz noise? even when we measure high impedance signals our body proximity can induce 50Hz noise.
Regards.

 

[mwmmboy]

50 hz noise pickup scope

Hi, I don't know exacly your application but i think the coupled filed through the cable has a very little effect on your circuit, smaller than other radiatig phoenomema.

Bye

 

[throwaway18]

50hz noise harmonics

>why when we touch probe of osciloscope we induce powerful 50Hz noise?

Capacitance between building electrical wiring and your body.

 

[fala]

50hz noise

Hello,

...I don't know exacly your application but i think the coupled filed through the cable has a very little effect on your circuit, smaller than other radiatig phoenomema.

Well, actually It does. have you seen the osciloscope snap shots in precesion amplifiers datasheet. I attach one here(it has 0.1uv / vertical division resolution). I always wanted to have one of those things.I don't want them just for fun I really need them for work, so I started experimenting. I could reach as low as 80-90uv p-p noise at best, the noise wasn't from amplifier and you can guess what was the frequency of noise. it was 50 hz and although all wires were shielded, the shield had little or no effect on 50Hz noise later I read in one article that 50hz magnetic field can penetrate very deeply into most metals(in aluminium for 9db attenuation it should be thicker than 1Cm, can you believe that?) the only way I could reduce noise was to distance transformer from all wires, otherwise noise was above 2mV p-p. so I need to understand where the induced current goes( does it get back to transformer?), if I don't understand the theory I can't go further. I also appreciate if someone introduce some materials/articles that can explain this.

Capacitance between building electrical wiring and your body.

thanks alot throwaway, can you explain a little more please. when our body distance with walls is in matter of a meter this capacitance should be too low, shouldn't it? and if it is too low it should be open to low frequencies shouldn't it ?
is it the magnetic field from wires that are induced, then there should be no induction when all lamps and electric staff are off, if that's so it seems easier for me to comprehend. if that's so where the induced current goes does it go back to main?

 

[throwaway18]

reduce 50hz noise

when our body distance with walls is in matter of a meter this capacitance should be too low, shouldn't it? and if it is too low it should be open to low frequencies shouldn't it ?

Do the mathematics.

If you have 230V mains electricity with the neutral at/near earth potential you can model it as a 240Vrms supply across an unknown capacitance and 1Mohm resistance (oscilloscope input)

What value of capacitance gives you 1Volt across the oscilloscope input? (It's very small)

If you have 110V mains with a centre tap of the distribution transformer connected to earth then
the problem is probably not as significant (anyone? I have 240V mains here, touching the probe of my scope gives lots of volts of 50Hz) because the voltages on the live and neutral should cancel out to some extent and there is only 55V to earth.

 

[VSMVDD]

50hz capacitance

if you attach an earth wire to your body the noise will reduce
this noise and 50hz pickup is fine

and your equipement via an opto or direct to a better earth

what this shows to me is there is a poor electrical earth in the place your measuring from

appart from this some pickup of rf is normal and dont forget 50hz harmonics also

things like overhead striplights can be the source of all the noise

wiring in the house will be less

the most noise comes from a tv or other test gear

noise is a measure of a voltage being amplified by the scope's front end

it is a very small voltage around .1-50uv max

em radiation in the form of magnetic radiation wont show as a voltage till its induced somewhere

i dont think this is an issue
as it will eventualy find its self decoupled
or the scope wouldnt be able to measure it