TDS 220 Shielding?


Jim Adney
 

Among others, I have a TDS 220 which is nothing special except that it's small, so that's the one that I use if I need to move it around. One of the things I do with it is test, repair, and burn in, automotive Capacitive Discharge Ignitions (CDIs.) Testing requires that I drive an automotive spark coil and spark plug, which makes for a LOT of RF noise. When I do this, the TDS 220 goes crazy, losing settings, changing gains, changing triggering, etc. Annoying!

I have done lots of things to eliminate conducted noise on the wiring in my system, but that has made very little difference. On a whim last week, I placed a piece of alum foil in front of the control knobs of the scope. Suddenly, almost all of the interference was gone. I'm now thinking of opening up the scope to see if I could glue a layer of alum to the backside of the plastic front panel. It seems like this is something Tek should have done themselves.

Does anyone know if I'm going to find that there is already shielding inside that plastic housing?
Does anyone have suggestions for a suitable adhesive to use?
Any reason that doing this might be a bad idea?

Yes, if I do this, I'll be careful to make sure my foil doesn't short to anything inside, other than the internal ground of the scope.


frank carcia <carcia@...>
 

Hi,I have seen guys who do lightning tests install the scope in a metal box open only on 1 side and short the shield to ground. Lightning generators generate a lot of radiated emissions causing the same problem you have. Maybe you could get a short rack cabinet and put a plate over the back of it as a shield?? Frank, WA1GFZ On Friday, August 27, 2021, 01:08:00 PM EDT, Jim Adney <jadney@vwtype3.org> wrote:

Among others, I have a TDS 220 which is nothing special except that it's small, so that's the one that I use if I need to move it around. One of the things I do with it is test, repair, and burn in, automotive Capacitive Discharge Ignitions (CDIs.) Testing requires that I drive an automotive spark coil and spark plug, which makes for a LOT of RF noise. When I do this, the TDS 220 goes crazy, losing settings, changing gains, changing triggering, etc. Annoying!

I have done lots of things to eliminate conducted noise on the wiring in my system, but that has made very little difference. On a whim last  week, I placed a piece of alum foil in front of the control knobs of the scope. Suddenly, almost all of the interference was gone. I'm now thinking of opening up the scope to see if I could glue a layer of alum to the backside of the plastic front panel. It seems like this is something Tek should have done themselves.

Does anyone know if I'm going to find that there is already shielding inside that plastic housing?
Does anyone have suggestions for a suitable adhesive to use?
Any reason that doing this might be a bad idea?

Yes, if I do this, I'll be careful to make sure my foil doesn't short to anything inside, other than the internal ground of the scope.


Ed Breya
 

I doubt there's much internal shielding at the front, and also doubt it would be practical or effective to add some inside. Before wrapping everything in Faraday cage, first check out and fix the ground loop situation that you've made by connecting the scope to the system under test. You should explain what the test setup is doing, and what equipment is involved, and how it's hooked up - then we can get some idea of what's going on.

The most important thing is to control the ground currents within the test setup and between the equipment. I suspect that a lot of high speed edge junk is going through your probes or cables into the scope's ground, then finally to earth ground through the power cord, making the scope itself fairly "hot" RF-wise. The front panel controls are likely in a high impedance environment (CMOS logic), and apparently susceptible to the interference. When you put foil in front, you change the distribution of the interference signals. It would be first better to reduce the interference as much as possible.

In the test setup, be sure that everything besides the scope has good low impedance returns to earth ground - that alone may fix the problem.

Ed


frank carcia <carcia@...>
 

I have snapped a string of ferrite beads on the scope probe leads to block any ground currents to the scope chassis.gfz

On Friday, August 27, 2021, 04:16:49 PM EDT, Ed Breya via groups.io <edbreya=yahoo.com@groups.io> wrote:

I doubt there's much internal shielding at the front, and also doubt it would be practical or effective to add some inside. Before wrapping everything in Faraday cage, first check out and fix the ground loop situation that you've made by connecting the scope to the system under test. You should explain what the test setup is doing, and what equipment is involved, and how it's hooked up - then we can get some idea of what's going on.

The most important thing is to control the ground currents within the test setup and between the equipment. I suspect that a lot of high speed edge junk is going through your probes or cables into the scope's ground, then finally to earth ground through the power cord, making the scope itself fairly "hot" RF-wise. The front panel controls are likely in a high impedance environment (CMOS logic), and apparently susceptible to the interference. When you put foil in front, you change the distribution of the interference signals. It would be first better to reduce the interference as much as possible.

In the test setup, be sure that everything besides the scope has good low impedance returns to earth ground - that alone may fix the problem.

Ed


Jim Adney
 

I should add that the interference to the scope occurs even if there are no probes connected to anything.

While running my burn-ins, I typically have only 2 cables connected to the scope: a coax that carries a square wave from a signal generator to a transistor that triggers the CDI (so I can monitor the trigger signal) and the scope's power cord. The coax has a ferrite snubber clamped around it. Under these conditions the scope is completely unusable if the spark plug is sparking.

I've not done anything with the power cord. I should try that.

So far, however, the important point is that a piece of alum foil in front of the control knobs made a BIG difference. I think that points the way to a useful solution, but I'll be surprised if I find that Tek made no attempt to provide any internal shielding for their circuits.


Ed Breya
 

Yup, that sounds like a pretty noisy environment. Since the interference is there without even making connections, I'd suspect it's from the high voltage end of things. Controlling the currents and ground loops can be improved later, once the radiated RF is reduced. How far away from the DUT setup is the scope? Does it need to face the setup? Can you shield the source? If you put the DUT setup in a Faraday cage of sorts, it would quiet things down a lot. Also of course, applying the usual rules for HV and RF can help - minimizing loop areas in wiring, making the shortest possible trips for all connections, and good ground returns inside the loops. Can you say, just put the coil and spark plug circuit in a metal box (with some venting or a fan to clear the O3 and NOXes from the spark)? If you use the same parts for all testing, it could be kind of a standard, fixed unit, like a dummy load for RF.

Ed