RE: differential probes

["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 Terry,

I am using the neutral for the MOV ground right now,
is there anything wrong with that?

more questions/comments below

--- Tesla list <tesla-at-pupman-dot-com> wrote:
 > Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
 >
 > Hi Jimmy,
 >
 > I got the boards today and was working on them
 > tonight.  I had to chop
 > things up a "little" :o)) but I think they will work
 > very well.  Here are
 > the changes the boards could use so far.  I had
 > better write them down
 > before I forget ;-)
 >
 > 1.  Lets take the MOVs off the board.  It is best if
 > the board just has
 > solder eyes for the HV wires.  We can just solder
 > the MOVs to the fuse
 > holders and ground terminals rather than clog the
 > board with them.
 >
 > 2.  The 1K feed back resistors need to be 2K.

I noticed today that it is a 2000x probe. If I want to
make my other channel a 10x probe, then I would change
it to 200k right?
 >
 > 3.  There is some offset and I think we can use the
 > Vocm pin to fix that.
 >

Grounding that pin worked perfectly for me.

 > 4.  We need the expensive 10nF caps after all.
 >
 > 5.  The 2K resistors need ~~50pF of compensation.
 >
 > 6.  To go to a BNC cable, we need to put 50 ohm in
 > series with the output
 > to match the cable.
 >
 > 7.  The feedback path was very noisy,  I had to chop
 > it all out.  Need to
 > get ride of the ground plane near the input and
 > reposition the 2K resistors
 > right on the input.

Can you explain this one? Does it have something to do
with capacitive coupling to the ground plane?

 > 8.  I replaced the 1 Meg 1/2 resistors with low
 > voltage surface mount
 > ones.  The big resistors were very noisy.  Hard to
 > wait for DigiKey to get
 > the right ones...

What makes the resistors noisy?

 > Our input capacitance is about 0.2pF!!  That is
 > extremely low and
 > great.  Tek uses a parallel capacitor with the input
 > which gives smoother
 > response but sure can load down high frequency
 > stuff.  Our response is not
 > quite as perfect but we don't need that very
 > expensive cap either.
 >
 > Running the board next to the Tek 5205 looked very
 > good.  Our rise time is
 > the same as theirs.  they compare well.  I was able
 > to get a good 10MHz
 > response.  Seem to have a resonance around 13.8MHz.
 >
 > I still need to do a lot of other tests but all is
 > looking well.
 >
 > Cheers,
 >
 >          Terry
 >
 >
 >
 >
 >
 > At 02:18 PM 7/25/2003 -0700, you wrote:
 > >Hi Terry,
 > >
 > >I sent the boards to you yesterday, but I forgot to
 > send you the fuses. Do
 > >you have any, or do you want me to send them to
 > you. The banana jacks I
 > >got were "little", I didn't know there is more than
 > one size. I don't
 > >think that's the kind you wanted, but I threw them
 > on one of the boards
 > >anyway, because they're so easy to take off.
 > >
 > >Here are some pictures of my board
 >
 ><http://www.hot-streamer-dot-com/chunkyboy86/diffprobe.jpg>www.hot-streamer-dot-com/chunkyboy86/diffprobe.jpg
 >
 > >www.hot-streamer-dot-com/chunkyboy86/diffprobe2.jpg
 > >
 > >If the voltage of the batteries powering it aren't
 > equal, there is a DC
 > >bias, which makes it tough to read small voltages
 > on my old scope. Why
 > >wasn't the VOCM pin grounded? Don't say cost ;-))
 > >
 > >When I hooked it up to my function generator, there
 > were some spikes at
 > >the leading edge of the square wave (200khz). The
 > spike doesn't appear
 > >with my other probes, so I think that it is fake.
 > .....................
 >
 >


=====
Jimmy

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