RE: DRSSTC -- EMI scope problems

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Steve Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

Original poster: "jimmy hynes by way of Terry Fritz <teslalist-at-qwest-dot-net>"
<chunkyboy86-at-yahoo-dot-com>

 >The magnetic field from the primary is freakin' out the scope when the
 >supply voltage is only 20 volts!

 >Is there any way to make good measurements with the 60khz magnetic field,
 >or do I just have to hope it doesn't blow up?

I had similar problems when doing the OLTC stuff. I have seen a trace that
should be a sine wave bent into a series of loops.

I believe what is happening is, the magnetic field is inducing a current in
a ground loop, either composed of your scope probes, scope, and AC power
wiring. Or it could be acting on a ground loop internal to the scope itself.
Or the field could actually be strong enough to deflect the beam directly 8-at-
Given that your scope showed the noise with no probes connected, and the
voltage dial didn't affect it, I suspect one of the latter two.

Things you could try are:

*Get some longer probes and move the scope further away
*Turn the scope around, on its side, etc, till you find a position with
lowest interference
*Put iron sheets around the scope :)
*Get another scope that doesn't suffer so badly :))

To reduce ground loops in your probe system:

*Put ferrite rings on your probe leads
*When using two probes, twist the two probe leads together and only use the
ground clip on one of them
*Use both channels, clip one of the channels to the ground reference, and
use the ADD/INVERT function on the scope to measure the difference between
them

Steve C.