60Hz Noise with Tektronix p6245 and 1103 Probe Supply

[MarkL]

The other 1103 I have I believe is older, and is a little different internally.  It has a different front panel PCB which is further from the transformer, and it's also in a different orientation which may make a difference in picking up stray EM fields from the transformer.  I did not experiment with this unit as much, but the noise is clearly less severe.

  older_1103_photo.jpg
  older_1103_scope.png

I decided to do the "alternate ground" from the previous post on the older 1103.  But this time, I did it as a permanent mod and with a very short wire.  The factory ground was disconnected.  The results were very good.

  older_1103_with_ground_mod_photo_anno.jpg
  older_1103_with_ground_mod_scope.png

Results were so good, I went back and did the permanent ground mod on the 1103 from the previous post.  While there is still a little bit of noise, results were pretty good here too.

  1103_with_ground_mod_photo_anno.jpg
  1103_with_ground_mod_scope.png

The noise still remaining are the tiny spikes that were more prominent before, and are related to the crest of the incoming sine wave as the capacitors charge up each cycle.  After swapping transformers between the two units, I found that the tiny spikes followed the transformer, which I didn't expect.  The transformer manufacturer is different between the units.


All this was interesting and I had a chance to improve low signal performance of my two 1103 units.  I don't think it's quite as perfect as remoting the transformer like you did, but I didn't really have a need to go that far.  The ground mods were really easy.

EDIT: Fix minor typo.

[Tdeball]

Great work MarkL!

Your solution packs much more bang for the buck compared to the effort of adding another enclosure. I plan to add that grounding wire to mine for peace of mind. I am happy how mine turned out, but this is probably not required for the average user.

I think you’re spot on with the metal chassis is acting as an additional half turn of the transformer. I wonder if electrically isolating the front panel from the metal chassis would help.

Now that you mention it, when I was playing around with shielding, I think I had better performance when my shielding was protecting the powered BNC connector compared to protecting the front panel circuit board.

Thanks again! 

[MarkL]

The ground jumper approach might also come in handy for some who don't have a 3D printer (like me!).

My first thought was also, "Aha!  The chassis is forcing current through the front panel and ruining it as a ground!"  But alas, unbolting the front panel did not fix the issue, although it did reduce it by about half, as I recall.  Being a dead end I didn't post any screen captures of it (although it was mentioned).

You're welcome.  Sometimes I get caught up trying to figure out these weird little design problems.  Thanks for pointing it out.  I would not have expected such an issue from the Tek designers.