Why am I picking up so much noise on my scope?

[Bricolo]

Probe B (on the same channel)

 

[Bricolo]

Probe A was used for all the tests before

 

[peranders]

1 set the scope in GND mode - noise hum?

2 Short the input like SY has done - hum

3 Connect the probe and set it in GND if the probe has that switch - hum

4 Take a wire and short the probe tip - hum

5 Switch off your lamps and other stuff (PC's etc) - hum

Do you get my idea? You must rule out things

 

[Bricolo]

1 set the scope in GND mode - noise hum?

2 Short the input like SY has done - hum

3 Connect the probe and set it in GND if the probe has that switch - hum

4 Take a wire and short the probe tip - hum

5 Switch off your lamps and other stuff (PC's etc) - hum

Do you get my idea? You must rule out things

1: done, no hum

2: done, no hum

3: no GND switch on the probe

4: look at posts 11 and 12

5: the scope in in the same room as a computer, the room aside is the kitchen with the frige running. I'll try to switch them off later in the afternoon. I have to go now.

 

[rif]

Check out Tek's website -- they have lots of very good primers on noise, probes, grounding, etc. They're meant for beginners, but have very detailed and address very complex issues.

 

[tiroth]

I vote for computer for the HF. You can't imagine how much crap the supplies throw out.

The 50Hz is odd though...I've never seen that in any scope...unless there was a ground loop somewhere.

 

[till]

Tek7904; 7A16A 225Mhz Plugin,
shorted like #12

Computer on same desk

 

[till]

shorted like #4

Probe 1:1 setting on a 1:1 / 1:10 probe

 

[DRC]

Interesting difference between the two probes, Bricolo. Do both probes look OK on a calibration signal ? Seems to be something strange about your probes ...

By comparison I see more noise on my analogue scope, but only on a 10uV or 20uV / div setting !!

 

[Bricolo]

Interesting difference between the two probes, Bricolo. Do both probes look OK on a calibration signal ? Seems to be something strange about your probes ...

By comparison I see more noise on my analogue scope, but only on a 10uV or 20uV / div setting !!

Calibration signal is ok (no ringing or damping) on the 2 probes, even if there's some small HF on it

 

[DRC]

Did you check continuity of the shield ?

dave

 

[Bricolo]

how?
just with an ohm meter?

 

[Christer]

I just checked with grounded probes on my 25+ year old 15MHz
Iwatsu scope. With 1x probe I get approx 200uV pp noise and
with 10x probe I get 2mV pp. That is I get the same scale
reading in both cases, so it is clearly the internal noise floor
of my scope. Maybe you have a noisier environment?

 

[DRC]

how?
just with an ohm meter?

Sure. I was thinking if the shield is not grounded it will not be so effective.

dave

 

[Bricolo]

Sure. I was thinking if the shield is not grounded it will not be so effective.

dave

both have 0.7 Ohm on the DMM (the DMM shows 0.7 Ohm when I connect its wires)

 

[Christer]

both have 0.7 Ohm on the DMM (the DMM shows 0.7 Ohm when I connect its wires)

That's probably just the contact resistance of the DMM probes,
so the shield should be OK.

Have you tried a shorted BNC directly at the scope as
someone suggested?

 

[Bricolo]

That's probably just the contact resistance of the DMM probes,
so the shield should be OK.

Have you tried a shorted BNC directly at the scope as
someone suggested?

done (post 17)

 

[Christer]

done (post 17)

OK, I missed that. Seems to be the small ground lead picking
up some background noise then. Try to switch off all electric
stuff you can, especially the computer. Also try in the middle
of the night or very early morning, when the electric background
noise is usually at its lowest.

 

[Frazzled]

Hmmmmm...

Here is my TEK 465B with a 10X probe grounded with a 12inch ground wire, scope set for 50mv/div and .1us.

I had to do this same test to see what I would get. Not bad considering my ground wire used was a foot long.

An externally hosted image should be here but it was not working when we last tested it.

Here is where things get a little more interesting...

An externally hosted image should be here but it was not working when we last tested it.

This is with the probe tip touching the ground post on the scope.

An externally hosted image should be here but it was not working when we last tested it.

And this is with the probe sitting on the bench ungrounded....
Hmmmmmm!!!!

 

[Sch3mat1c]

What's the ringing signal? The second picture just shows a double image due to faulty triggering. I'd zoom out on the first picture to see a few cycles of whatever that signal is.

My Tek 475 is absolutely quiet. With the 10x probe laying around it picks up a good bit of 60Hz and HF noise as is understandable. On the 2mV scale with BW set at 200MHz, the trace does get pretty thick though. Goes thin again with BW reduced to 20MHz. I have little use for anything so high so I usually keep it in this range anyway.

Tim