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Signal noise?

D

davidd31415

Jan 1, 1970
0
Hi,

I am trying to troubleshoot a problem with an electro-dynamic shaker.
There is a coaxial output from a computer to an amplifier and outputs
from the amplifier to the shaker. I am sending a 15-150Hz 0-1V signal
to the shaker.

There was a "ground loop isolator" between the computer and the amp
(coax converted to rca, then back to coax), which had been used to
remove 60Hz line noise in the past. I know 60Hz is the AC line
frequency but have not yet understood why this particular frequency
manages to cause interference through ground loops so often; an
explanation of that would be great here as well.

What I am ultimately hoping to understand is what might be going on
with the signal:

The shaker is intermittently knocking and does not seem to be working
properly (with THIS amp only). If the ground loop isolator is hooked
up and I touch the shielding around the connectors with my hand, the
knocking goes away (otherwise the ground loop isolator makes the
knocking occur on a regular basis). I'm curious how my touching the
connector could have anything to do with this? I would initially guess
ground it has something to do with me being grounded, but to such a low
voltage does my being grounded (through shoes) make a difference to the
signal?

The knocking occurs intermittently when the ground loop isolator is not
connected. I've hooked a Y up to the output of the computer and
watched the signal with an oscilloscope (no ground loop isolator) and
have not noticed anything strange there, but when the scope is hooked
up and the input to the scope is disconnected (but the coax remains
connected to the Y), the shaker starts knocking more often than in any
other situation. I've also noticed that if I use the other (red) set
of rca leads on the ground loop isolator (rather than the white), there
is no knocking.

I'm thinking there may be something wrong with the ground loop isolator
since only one set of its leads causes regular knocking, but seeing as
the knocking occurs even without it and my touching it effects it, I've
though perhaps electromagnetic waves or something else may be involved
here as well.

Any input would be appreciated! Thanks,

Dave
 
J

John Popelish

Jan 1, 1970
0
davidd31415 said:
Hi,

I am trying to troubleshoot a problem with an electro-dynamic shaker.
There is a coaxial output from a computer to an amplifier and outputs
from the amplifier to the shaker. I am sending a 15-150Hz 0-1V signal
to the shaker.

There was a "ground loop isolator" between the computer and the amp
(coax converted to rca, then back to coax), which had been used to
remove 60Hz line noise in the past. I know 60Hz is the AC line
frequency but have not yet understood why this particular frequency
manages to cause interference through ground loops so often; an
explanation of that would be great here as well.

Line powered equipment often leaks significant current into ground
through either resistive paths or capacitive ones. This current
alters the local potential of ground at line frequency. If you
amplify a voltage compared to local ground, and it was generated with
respect to a different local ground potential, the difference between
the two local ground voltages gets added to the signal.
What I am ultimately hoping to understand is what might be going on
with the signal:

The shaker is intermittently knocking and does not seem to be working
properly (with THIS amp only). If the ground loop isolator is hooked
up and I touch the shielding around the connectors with my hand, the
knocking goes away (otherwise the ground loop isolator makes the
knocking occur on a regular basis). I'm curious how my touching the
connector could have anything to do with this? I would initially guess
ground it has something to do with me being grounded, but to such a low
voltage does my being grounded (through shoes) make a difference to the
signal?

There is probably something in the vicinity that is radiating strong
electric fields that the cable shield is picking up as capacitive
current (probably where multiple cables run parallel). Your body adds
a lumped capacitive load to that shield when you touch it. How is the
cable shield on each side of the ground isolator grounded?
 
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