RE: DRSSTC -- EMI scope problems

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

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

At 11:34 23/06/03 -0600, you wrote:
>Original poster: "jimmy hynes by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
><chunkyboy86-at-yahoo-dot-com>
>
>
>I thought it might be directly affecting the beam, it
>got better when on its side, and no probes attached,
>but now with additional input it shows the beam is
>just deflected the other way (reletive to the
>scope)the sinewaves look like they're almost loops.

That's what I saw on my rig too! It's got to be a sure sign that the 
magnetic field is affecting the CRT directly.


>I was thinking about making an extension with TV
>cable, because I already tried changing its position,
>and it just changed the axis of interference.

TV cable (I mean the 75 ohm coaxial kind, not 300 ohm ribbon) will screw up 
the response of 10x probes completely. You could probably make your own 
low-Z 1x probes out of it, but they would present a very heavy load when 
probing gate drive signals, logic circuits, and the like.

However TV cable would be ideal for a back-terminated probe. It has 75 ohm 
terminations: one at the scope end, and a 75 ohm resistor at the probe end. 
One end of this resistor connects to the coax core and you probe the 
circuit with the other end. It has an impedance of 150 ohms and a 2x 
attenuation. The cable can be as long as you like because it's matched :)) 
By using two resistors at the probe end, a 75 ohm to ground and a 3712 ohm 
to the signal you're probing, you could make a back-terminated 100x probe 
that would be more suitable for measuring 100s of volts at high frequencies.


>How would the ferrite rings help? The stuff I want out
>is the same frequency I want to measure!


You put the whole coax through the ferrite ring. It lets through the 
desired normal-mode signal, since the current flows in one direction in the 
core and the opposite direction in the screen. But any common-mode signal 
(current flowing in the screen only) gets choked out. In theory that is ;) 
But we already know you have a magnetic field problem that only some 
serious iron shielding, or moving the scope further away, would fix. Or you 
could pester your parents for one of those $1000 Tek scopes with an LCD 
screen :))

Steve C.