BNC Test leads

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Technician
TwoWaves
TwoWaves
Posts: 706
Joined: Thu Apr 24, 2014 8:32 pm

BNC Test leads

Post by Technician »

I have been experiencing noise in the operation of the scope, I tried the scope at home on the laptop and found some noise from channels B and C when no leads connected but channel A and D seemed OK. Back at work on initial test no leads plugged into scope I had no noise, however after connecting all BNC test leads I get noise on all four channels, and I'm sure this noise must be affecting the scope traces in use. looking on each lead they are labelled at 50 Ohm leads. In the leads there are black connectors indicating a filter of some sort, however after pulling back the cover the leads are soldered together and seem to just pass through unfiltered. I'm assuming the leads are the cause, if the attachment works I'll upload a view, please advise of your opinions.
Attachments
All channels leads plugged into scope.psdata
(22.02 MiB) Downloaded 455 times

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Tronic
TwoWaves
TwoWaves
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Joined: Fri Oct 23, 2015 2:43 pm
Location: Sweden

Re: BNC Test leads

Post by Tronic »

Hi Technician!

When nothing is connected to either of channel A to B - as seen in my picture with just a slight DC offset because I just turned my 4425 on - there should be no more than, with a 50 mV range setting, a couple of mV of noise.

In the other trace of mine you can see the 50Hz (20mS between the peaks) picked up by the cable with ungrounded cable screen (shield).

What you see in your trace is simply 50 cycle (50 Hz) picked up by the the cable. If you got a 50 Hz wave or lots of noise on your scope unconnected, then something is wrong.
Attachments
CH A 50 Hz.jpg
4-ch 50 mV.jpg

Technician
TwoWaves
TwoWaves
Posts: 706
Joined: Thu Apr 24, 2014 8:32 pm

Re: BNC Test leads

Post by Technician »

Hi Tronic,

So to be clear, in your first trace set at 50 mV you have some noise present. Is this noise present with the BNC cable plugged into the scope channel and not grounded!

In your second trace with minimal ripple I get that when no BNC leads are plugged into the scope, but when leads plugged in I get the examples I uploaded. My BNC leads at that time are not grounded, they are just open circuit leads.

I am also wondering if the fault I am experiencing is also the cause of my 'over range' warning (RED) that I always get, no matter what voltage settings per division I use!

User avatar
Tronic
TwoWaves
TwoWaves
Posts: 90
Joined: Fri Oct 23, 2015 2:43 pm
Location: Sweden

Re: BNC Test leads

Post by Tronic »

Hi again technician!

First trace - the uggly 50Hz wave on channel A (blue trace) with cable ground hanging in the air - channel B-D BNC's is unconnected. On the second trace, all channels are unplugged and that is why the noise level is that low and as it should be unless there is something wrong with the front end of your scope. This scope trigger the over range even on very fast events so it may not originate by picked up 50Hz. In my first reply I wrote "to either of channel A to B" but meant A to D. :oops:

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