RE: differential probes

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

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.

3.  There is some offset and I think we can use the Vocm pin to fix that.

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.

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...

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.
.....................