Grounding and 50Hz noise

[PeterAS]

Can anyone recommend a good technical book on grounding of highly sensitive instrumentation and elimination of 50 Hz noise?
We are using CCD cameras in conjunctipn with a Scanning Electron Microscope. Any 50Hz can produce a pulsing effect in our high resolution images.
Peter

 

[nbucska]

The 50Hz magnetic field can directly interact with the E-beam, induce into interconnecting wires and circuits, shift
GND potentials or modulate the gain of amplifiers -- and each needs different measures to diagnose and eliminate.

It is definitelly not a trivial task.


<nbucska@pcperipherals.com>

 

[JTreglio]

I've worked a lot with switching power supplies and pulsed, high voltage systems, and electrical noise is a very serious problem. You should check to make sure your building has a solid earth ground. Many do not. A common mistake is to connect the shielding on shielded cables on both ends, setting up a ground loop. That's a quick way to pick up 50 Hz (or 60 Hz, in the US) noise.

Jim Treglio
Molecular Metallurgy, Inc.

 

[a31ford]

I aggree with JTreglio, most &quot;electricians&quot; will connect both ends of a sheild, therefore inducing a ground loop right ON the cables that one is trying to sheild!!

If one looks to the design of older &quot;Tube type&quot; amplifiers (audio) there was only ONE common ground point, and ALL (AC) power wires where twisted (twist rate DOES affect how effective this method is) most AC cables made today, are of the &quot;Drawn style&quot; (where the wires are simply drawn over a mandrel and the outer sheath of the bundle is applied.) Control or comunication cables however are generally &quot;twisted pair(s)&quot; first, then drawn 2nd.

Another option to ground loops is optical interface between different parts of equipment (like the audio portion of a 5.1 DVD player to home stereo connection), the Siginal to noise floor on this type of equipment is exceptionally high, compaired to the actual amplifiers.

In the world of audio sound recording, the standard is what is known as &quot;600 Ohm Balanced Line&quot; if you are simply hooking a microphone to the other end of the cable then the ground wire IS used and connected to both ends, HOWEVER if you are connecting to say, a keyboard or another amplifier, the common method is to use a &quot;DI&quot; box (Direct Injection) these's boxes generally have a &quot;ground lift&quot; switch on them, for easy elimination of that ever humming &quot;ground loop&quot; as well as the boxes other function of impedance matching.

(YES, I know you are talking about instrmentation, BUT, all of these practises are based on &quot;NO ground loops&quot;, and therefore still apply).

My two cents worth

Greg



Regards,
Greg,

Called &quot;Mr. Fusion&quot; by the Locals

 

[nbucska]

You can use the SEM itself so sense the field.

<nbucska@pcperipherals.com>