RE: DRSSTC -- EMI scope problems

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

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

Hi Steve,

The scope wouldn't trigger on the noise, which is more
evidence that the magnetic field from the primary
directly steers the beam.

I read about the impedence matching thing, and built a
100x probe with the TV cable. It looks like it works
better than the other probes :-)

  I now get to look at the microprocessor level. It has
absolutely no 60khz on it, but has high frequency
"chirps" at every half cycle, adding a choke had
absolutly no effect on it. I am not sure what this is
coming from, because it seems like it would be hard
for high frequency junk to get in, because I put an RC
filter in with 33 ohms, and 1000uf (plus a 10uf
ceramic). The wires coming from the power supply are
also twisted pretty well. Whatever it is, I don't
think its a problem.

The voltage across the IGBT brick may be a problem
though. It looks like a 60 khz sinewave that goes from
0 to twice the normal voltage. I can tell by the shape
that it is not a resistance problem, it is either an
inductance problem, or a scope problem. For the
inductance to be causing the problem, the reactance
must be about .67 ohms (20 volt sinewave, 30 amps). At
52 khz ( I think thats the real switching freq. right
now), it takes a three foot diameter turn to get
enough inductance. There is no way I have that much
area, I twisted my wires pretty well. The scope probes
checked out OK on the micro, but I hope they have a
problem here. Again, adding a choke didn't help. I
think my choke might add too little inductance. The
coax just passes through, and the thing was just taken
from a video camera. Maybe I should wind my own wit
~10 turns?
--- Tesla list <tesla-at-pupman-dot-com> wrote:
 > 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.
 >
 >
 >
 >



=====
Jimmy

_