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Forum Impact of scope noise when measuring signals
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Impact of scope noise when measuring signals

Andrew J
Andrew J over 4 years ago

I'm currently taking noise measurements of a working circuit and I don't really understand how my scope's noise actually impacts what I see or measure so I'm hoping someone can explain it to me in non-technical (or at least not deep technical!) language.  I've measured the scope's noise floor(??) on channel 4:

 

image

Image 1: no connections

 

Scope has no probes connected, is set to maximise capture memory and is at the smallest V div possible.  I've shown RMS and pk-pk measurements (I believe RMS is more relevant to noise, is that right?)

 

Here I'm taking a signal measurement on channel 4, with the probe set at x1 using a pig-tail and the scope configured as: AC coupled, 20MHz bandwidth, peak detect acquisition, no other probes connected or channels turned on, measurements reset to zero first:

 

image

Image 2: measured signal (core signal is 0.37mV pk-pk)

 

What I'm interested in understanding:

  1. How does the scope noise measured when the channel is disconnected from a probe - noise floor? - impact on the measured signal?
  2. How do I take that into account when interpreting the signal?  For example, could I take the mean RMS value shown in image 1, from the mean RMS value in image 2?  Does it mean the scope just isn't capable of measuring signals around the min-max RMS values?
  3. I don't touch the probe once I've started taking measurements - it's connected on test points - but even a fractional movement can change the measured signal to the extent I can't trust that I've made a proper probe connection, if that makes sense.  Specifically, the amplitude of the 'spikes' can grow or shrink although the 'core' signal remains pretty much as-is; by 'core' I mean the bright green portion of what is shown.  Does that imply anything about the spikes not related to my circuit - environment noise?  Probing issue?
    I've turned off other devices and lights in the room with no impact.  I get the same signal measuring directly across an output cap (albeit a 10uF electrolytic.)  I've zoomed in to the signal in the image below - although regular, it doesn't look like switching noise:

 

image

Image 3: measured signal in image 2 zoomed in.

 

Please correct me where I've mistyped anything in the above - it helps me make sure I research the right thing and not the thing I think is right!! 

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  • Gough Lui
    Gough Lui over 4 years ago +6 verified
    Can't say I'm an expert, but here goes at my understanding: How does the scope noise measured when the channel is disconnected from a probe - noise floor? - impact on the measured signal? How do I take…
  • shabaz
    shabaz over 4 years ago in reply to Andrew J +3 suggested
    Hi Andrew, The 'scope noise can be higher than the power supply you're measuring, so on it's own the 'scope isn't going to provide the measurement you need I think. Regarding your question 1, you'll see…
  • DAB
    DAB over 4 years ago +2 suggested
    You raise some good issues. Without a probe attached you are getting noise through the input connectors. first thing to do is at least put a terminating load on the input so you are not picking up noise…
Parents
  • DAB
    0 DAB over 4 years ago

    You raise some good issues.

     

    Without a probe attached you are getting noise through the input connectors.

    first thing to do is at least put a terminating load on the input so you are not picking up noise like an antenna.

    Most Oscopes use a 50 ohm termination to establish the noise floor for the amplifier input.

    You should see no difference in the noise when you attach the probe with the appropriate terminating resistor.

    You can check with the manufacturer, but termination is a basic test setting to establish the basic operation of the scope.

     

    Grounding the probe input with termination should get you a nice clean floor signal or hopefully the lack there of.

     

    An open probe enables the scope to act as a noise antenna, so any source of RF signal, even your cell phone, will give you some very interesting waveforms.

    If you have any CFL lights nearby, they are also a good source of RF noise.

    Mains power can produce a large EMF if the source is under a heavy load.

     

    DAB

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  • Andrew J
    0 Andrew J over 4 years ago in reply to DAB

    My scope - a Siglent SDS1000x-e - doesn't have a 50Ohm input path, they are all 1MOhm so I did the test for image 1 with the same settings I would use for probing.  Reading around how to measure the noise I think I could summarise instructions as:

    1. Turn on the 50Ohm input path (I can't, so it has to be done at 1MOhm)
    2. Disconnect all probes from inputs
    3. Configure for a large amount of acquisition memory and a large sample rate
    4. Measure the Vrms AC.
    5. Repeat for all vertical settings and channels - I just did the vertical setting and channel I was using for probing (image 1)

     

    I think the right starting point is to make sure I measure the noise floor correctly.  When you say " You should see no difference in the noise when you attach the probe with the appropriate terminating resistor." do you mean attach a probe and a 50Ohm terminator to a T-connector then the T-connector to the scope.  If I leave the probe flapping in the breeze at this point I should see no change from Image 1 or perhaps a better result - by which I would mean 'more accurate'?  As you say, connecting a probe but leaving it open gives some very interesting waveforms!

     

    I do have a couple of CFL bulbs in the room but these were turned off with no impact.  In fact, all lights / devices were turned off without impact.

     

    Would it be worth putting a ferrite bead onto the input power lead - I don't have any to hand but I could get some?  The load is 5V @ 22mA so I wouldn't classify it as heavy.

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  • Jan Cumps
    0 Jan Cumps over 4 years ago in reply to Andrew J

    Andrew J  wrote:

     

    ... I also tried with a different bench supply and got a similar capture but without the spikes so it does point to the supply I need to use. ...

     

    Can you temporarily try probing with batteries as power source? They have virtually no own ripple ...

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  • Andrew J
    0 Andrew J over 4 years ago in reply to Jan Cumps

    I can join a couple of 9V in series and give that a go for a test.

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  • Andrew J
    0 Andrew J over 4 years ago in reply to shabaz

    shabaz - some of these ferrites are US delivery only so not very environmentally friendly to order!  How are you selecting them so I can find alternatives?  A mix of inner diameters, frequency range and impdances?  Material?

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  • Jan Cumps
    0 Jan Cumps over 4 years ago in reply to Andrew J

    Andrew, on the carbon footprint of shipping these from US to UK (ducking for cover because on macro level I am wrong):

    No one will put an extra airplane in the sky or boat on the ocean for your package. Planes are flying empty these days.

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  • Andrew J
    0 Andrew J over 4 years ago in reply to Jan Cumps

    imageimage It's the principle of it really (also £15.95 delivery charge) plus I'd rather source locally if I can.

     

    Useful article here: https://palomar-engineers.com/ferrite-products/ferrite-cores/ferrite-mix-selection

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  • DAB
    0 DAB over 4 years ago in reply to Andrew J

    Yes, you attach the 50 ohm terminating resistor with a T connector with the scope probe or BNC cable to the source device.

    Yes, the input impedance of the scope is probably in the 10 M ohm range, which is why it is susceptible to noise without termination.

     

    There are a lot of subtleties involved with getting down to a true noise floor.

     

    The scope manufacturer probably has a application note about how they establish the noise floor for testing their devices.

     

    DAB

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  • Andrew J
    0 Andrew J over 4 years ago in reply to Andrew J

    It's a bit off-topic for the original post but I think I've got it, so for anyone else who reads this.  By design or by accident, Shabaz has a range of ferrites of varying inner diameters and two materials: Nickel-Zinc (Ni-Zn) and Manganese-Zinc (Mn-Zn).  These materials are useful for different frequency ranges (see image below.)  Manufacturers use different codes to represent these materials: in terms of the parts that Shabaz references, Fair-Rite use 31 and 75 and Kemet use 700L and 5H; there are other codes as well to cover different ranges and the link I posted above gives a good description with reference to Fair-Rite codes.  I couldn't work out what other manufacturers such as Wurth or TDK use.  A good representation of this is in this image from a Kemet datasheet:

    image

    So one could obtain a variety of ferrites of different materials and different inner diameters to cover a range of frequencies.  As a reference, here is a x-ref of Fair-Rite products and Kemet equivalents:

    image

     

    Hope this is useful.

     

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  • shabaz
    0 shabaz over 4 years ago in reply to Andrew J

    Hi Andrew,

     

    Unfortunately I don't know of a UK source, as you say that cost can be prohibitive, I waited until there were several things I needed so that the £16 charge didn't feel as bad as it does!

    Also, as you mention in your comment, the material type determines its usable frequency, sometimes there's no need to know the type but depends on the datasheet quality - Wurth and others have a plot, whereas other manufacturers might just indicate the impedance at a few frequencies, both of which can be usable, but others may not have even that. Down at the low frequencies (say 100kHz or 1 MHz) none of them offer as much impedance as might be desired, so it can be useful to loop the wire through a few times, or stack several ferrites on the wire, if you're interested in suppressing noise at these frequencies. Once you're into the tens of MHz or beyond, then there's a bit more choice, e.g. type 43 material is fine for tens of MHz to VHF, type 61 for VHF to 1 GHz etc. But for your noise measurements you're more interested in the lower range.

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  • shabaz
    0 shabaz over 4 years ago in reply to Andrew J

    Hi Andrew,

     

    Very useful information and table! I had not realized Kemet also offered these.

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  • Andrew J
    0 Andrew J over 4 years ago in reply to shabaz

    I think you'd still need to check the impedance as they aren't exactly the same from that specification but you'd need to use multiple turns, e.g. the clamp ferrite in that table, at 200kHz the Fair-Rite has 20Ohms impedance but the Kemet only 5Ohms.

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  • Andrew J
    0 Andrew J over 4 years ago in reply to shabaz

    I think you'd still need to check the impedance as they aren't exactly the same from that specification but you'd need to use multiple turns, e.g. the clamp ferrite in that table, at 200kHz the Fair-Rite has 20Ohms impedance but the Kemet only 5Ohms.

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