Background noise

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Leo
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Background noise

Post by Leo » Wed Dec 19, 2018 2:57 am

Dear all,

I bought a Picoscope 6403C some months ago. When I connect nothing to the probe,there are serious background noises.I don't know why it happened and how to solve it.

I have attached a screen capture of one of this events.

Any ideas?

Regards,

Leo.
Attachments
capture.png

Gerry
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Joined: Mon Aug 11, 2014 11:14 am

Re: Background noise

Post by Gerry » Wed Dec 19, 2018 2:17 pm

Hi Leo,

If you look at what you call noise you will see that it is actually repeating at the same rate with the same shape. This is an indication that you are picking up some kind of signal interference. The interval between repeats is 20ms, which represents a frequency of 50Hz, which is the frequency of the mains electricity supply.

Common causes of picking up mains interference are, from running the Probe/USB cables too close to a power supply, and from bad earthing (typically an earth-loop, or poor measurement point for the probe earth) causing a voltage drop along the earth return.

If you post some photographic images of how everything is connected up and being measured, along with a diagram of your connections (if it isn't immediately obvious from the images) then we can look for what might be causing the problem.

regards,

Gerry
Gerry
Technical Specialist

Leo
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Re: Background noise

Post by Leo » Thu Dec 20, 2018 1:05 pm

Hi Gerry,
Thanks for your reply. According to your advice, i put the Probe and USB cables far away from the power sypply, then the signal interference from the mains electricity supply went down.However it still maintain in 4mv.Is it normal?

In addition, when i use PS6403C measured an 1MHz 1V standard sine wave, the result was only 200mv. I wonder what the problem is.

I have attached somephotographic images of these events.

Regards,

Leo.
Attachments
4.1.jpg
connection diagram
3.2.jpg
signal generator
222.jpg
measurement result
111.jpg
signal interference

Gerry
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Posts: 305
Joined: Mon Aug 11, 2014 11:14 am

Re: Background noise

Post by Gerry » Fri Dec 21, 2018 11:01 am

Hi Leo,

It's still possible that you could have inuduced signals, but without images of all of your cabling (i.e. what we can't see from the image you sent) it's difficult to tell. You may be able to reduce the mains interference more by moving the USB lead for the PicoScope further away from the Tektronix Function Generator. Alternatively, it may be better to connect the PicoScope probe directly to the Function Generator output, as the long the signal lead is the more prone it is to picking up interference (or better still get yourself a short BNC to BNC connector). If you have a laptop then you can check whether or not the remaining interference is through the mains earth, by running the PicoScope from the laptop while battery powered.

Regarding the 200mV level displayed on the PicoScope, this could be because you haven't set your measurement equipment up correctly. Do you have the Probe switched to x10 (if so, then you need to either switch it to x1 or, if you don't want a more restricted bandwidth, go to the Channel A Options window by clicking on the icon near the top of the screen labeled 'A', and select x10 from the drop-down list of Probes, otherwise your measured signal level will be divided by 10) and is the Signal Generator creating a full scale output for a 50 ohm load (if so then you need to select 50 ohms from the drop-down list for the Channel input coupling, which is the window just to the right of the Channel input range window, otherwise your measured signal level will be double what it should be).

Regards,

Gerry
Gerry
Technical Specialist

Leo
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Joined: Wed Dec 19, 2018 2:18 am

Re: Background noise

Post by Leo » Mon Dec 24, 2018 8:49 am

Hi Gerry,
Thanks for your reply. By moving the USB lead for the PicoScope further away from the Generator as you said, the mains interference always keep the average around 5mv. By running the PicoScope from the laptop while battery powered, the interference signals are more serious. I have attached some images of the connection and the results of the measurements.

The probe I used is the original probe TA150. I couldn't find how to switch it to x1 or x10. but I found the attenuation ratio of the TA150 is 10:1 from the user's guide. Is it the reason why the results of the measurements reduced by ten times? I attach the printscreen when I clicked on the icon ‘A’ and the measurements of the 1V standard sine.

Regards,

Leo.
Attachments
1-3.jpg
1-1
1-1.jpg
1-2
1-2.jpg
1-3
2-2(battery powered).png
2-1(battery powered)
3-2.png
3-1
3-4.png
3-2

Leo
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Posts: 4
Joined: Wed Dec 19, 2018 2:18 am

Re: Background noise

Post by Leo » Mon Jan 07, 2019 1:37 am

Hi Gerry,
Thanks for your reply. By moving the USB lead for the PicoScope further away from the Generator as you said, the mains interference always keep the average around 5mv. By running the PicoScope from the laptop while battery powered, the interference signals are more serious. I have attached some images of the connection and the results of the measurements.

The probe I used is the original probe TA150. I couldn't find how to switch it to x1 or x10. but I found the attenuation ratio of the TA150 is 10:1 from the user's guide. Is it the reason why the results of the measurements reduced by ten times? I attach the printscreen when I clicked on the icon ‘A’ and the measurements of the 1V standard sine.

Regards,

Leo.

Gerry
PICO STAFF
PICO STAFF
Posts: 305
Joined: Mon Aug 11, 2014 11:14 am

Re: Background noise

Post by Gerry » Wed Jan 09, 2019 10:59 am

Hi Leo,

So, there is clearly strong Electromagnetic interference at 660 Hz that is being mostly shielded by the grounded chassis and leads when you power the computer from the mains power source. Could you answer the following:
What is the building used for (what industry)?
What is it that you are doing (what are you measuring)?
What devices are in the room that are running from the mains power supply of the building (including type of lighting)?
Is there any device in the building that requires a lot of power from the mains supply, and if so where is it located?

Regarding the reduced level signal, yes, the 10:1 ratio of the probe will be reducing the signal by a factor of 10. As I mentioned you can set the channel to a x10 probe setting so that the display will then be corrected for the 10:1 ratio of the probe (you can do this by clicking on the icon near the top of the screen labeled 'A', and select x10 from the drop-down list of Probes).
What happens to the signal if you click on the arrow in the coupling window (just below the TimeBase window) and select '50Ω DC'?

Regards,

Gerry
Gerry
Technical Specialist

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