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

[Bricolo]

Here are some "measurements" I made with my scope, O think there's much noise on it, is this normal?

1st picture, nothing connected to the scope (no probe)

 

[Bricolo]

2nd picture, only a 10:1 probe connected on the scope, but not connected to any circuit.

The probe's tip is directly connected to the ground wire of the probe
You can see some 50Hz and HF

Note1: the amplitude of the 50Hz varries when I place the probe elsewhere

Note2: all the measurements have been made in storage mode, otherwise the camera would only take the spot and not the full screen

 

[Bricolo]

3rd picture, the same configuration as above (10:1 probe, with the tip connected to ground) but touching a metal ruler (20cm)

The HF is much more important

 

[Bricolo]

4th picture: the probe, and the way it was "connected to ground" during the tests

 

[Damon Hill]

RFI?

Do you have any high pass filters that can be set?
It looks like the scope's sensitivity is set to maximum?

I have a 100 Mhz Hewlett Packard 'scope that does
have a visible internal noise floor with the input set
to the lowest range. I think that's normal.

 

[Bricolo]

What do you mean by RFI?

Yes the scope is set to its maximum sensitivity.
No I don't have any HP here, but I could build one it needed.

I didn't manage to measure the noise's frequency, since I can't get my scope to trigger on the signal

 

[phase_accurate]

First of all: You won't hear this noise anyway, so why do you care ?

2nd (and serious) try:

1.) The loop that you form with the ground wire will be able to pick up some noise and RF. If you a curious enough you might connect a shorted BNC to the input and see if there is an improvement.

2.) This small amount of noise isn't that bad at all with the scope set to its maximum sensitivity.

Regards

Charles

 

[Bricolo]

This small amount of noise isn't that bad at all with the scope set to its maximum sensitivity

It makes my scope totally uneffective to measure power supply noise

 

[Bricolo]

Compared to the 150MHz Tek probe, my selfmade Kwak Probe (I hope elso won't mint if I call it this way ) that is made of a 1m RG58U coax cable with a 50R at the scope side, is dead quiet

 

[peranders]

It makes my scope totally uneffective to measure power supply noise

You must probably make yourself a preamp with gain of 100 or 1000.

AD797, LT1028 etc with batteries. Don't forget to a LP-filter so you only measure < 20 kHz.

Your GND cable is an antenna! You can twist it around the probe in order to get a smaller loop (=antenna).

 

[Bricolo]

things are getting better

 

[Bricolo]

This was this the probe's tip grounded this way

 

[Bricolo]

And the result is nearly the same (just a little more HF)

 

[Bricolo]

Last test has been done this way, with the probe's ground wire twisted

 

[Bricolo]

Thank you for your help, guys

No I know how to get rid of this HF noise. But the 50Hz is still here, with the same amplitude

P-A: can you tell me more about this preamp? What's the advantage of using this?

 

[peranders]

Just check the datasheets for the opamps. It's really simple and you can do a P2P or vero-board design.

The thing is that you have an environment with RFI noise and you cable picks it up. What happens if your short the inputs of the oscilloscope? Completely silent or still noise?

Don't forget that your scope also has digital noise with can be rather high. I used an old Tek 100 MSp scope with noise of 2-5 levels (of 256)!

Do you have your scope plugged in in an earthed mains outlet?

 

[Bricolo]

Here it is with the input shorted

No noise, the same as when no probe is connected to the input.


But it's very sensitive. If the wire isn't very closely twisted, there's 50Hz+HF

Yes, the scope is earthed

 

[SY]

Borrow someone else's 10x and see if the results are the same. I wonder if the shield in your probe has gone funny. This is a LOT more noise than I see with mine- on the 1 mV scale with the ground clip attached to the probe tip, I get an absolutely flat line.

 

[peranders]

My Hitachi DSO is also dead silent- with probes. My guess is also bad probes.

 

[Bricolo]

That's strange

I've got 2 of those probes

TekP6109

150MHz
11.8pF
10X
1.5M


By testing them as on pictire in post#4

Probe A gives: