Impact of scope noise when measuring signals

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

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.

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?

1. 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?
2. 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:

Scope noise with absolutely nothing attached to the input connectors mostly comes from the front end amplifier and whatever little can be coupled into the input connectors. Terminating the inputs reduces the coupled components. This is the noise "floor" - there is no reliable way the oscilloscope can usually resolve below this, although with reduction in bandwidth through averaging, this may be possible. The noise will have both relatively "white" AC components and possibly a DC "offset" component depending on how well it is calibrated, and some of these may also drift over time with changes in operating temperature and ageing of components.

Connecting a probe, even with nothing on the other end of it, is expected to increase the noise, sometimes significantly as sources of RF, EMI, ESD are coupled into the oscilloscope inputs via the probe lead which can act a bit like an antenna. The values will change as you move the probe around because of the way the lead may pick up noise will change depending on the geometry in which the lead/probe makes to the source of the noise. Some DC offset may even be measurable due to thermoelectric voltages being built up at junctions between dissimilar metals. If you use the flying ground clip lead, that would also be a good source of noise. Other potential sources include using the front USB connection and rear LAN/USB connections on big-box units, or poor grounding.

If you use a 10x probe, you're bound to have even more noise as the signal is divided down in return for the full bandwidth. Using 1:1 coupling is usually best if you're trying to resolve very small signals, but bandwidth will often be limited ~<10MHz.

Spiky noise could be a result of many possibilities - it could exist because the probe contact has shifted very slightly during measurement (e.g. contact crackle), it could be "radiated" into the setup (e.g. Wi-Fi router, mobile phone), it could be coupled ESD static, it could even be buggy oscilloscope ADC firmware especially when interleaving multiple ADCs where they are not perfectly matched. It can even be from within the oscilloscope - some cheaper units aren't always internally well shielded or have the cleanest power supplies. Other times, you could be measuring actual artifacts in amplifier circuits - e.g. popcorn noise which is a real signal.

As for whether you can take the RMS values and subtract them - yes and no. The RMS average calculated across the whole buffer acts as an averaging mechanism. Noise (if purely white) averages out leaving the signal component alone. But if there is any "bias" in your noise, then averaging that would cause a bias in the output. Averaging the signal over a longer period to eliminate white noise contribution may seem like a good idea, but then you become vulnerable to instability and drift of the actual intended reading. Finally, if the signal is below the floor, you just don't know whether you're reading the oscilloscope's own internal biases and non-linearities as you're really only using a few LSB of the relatively "low" precision converter (as compared to using an appropriate DMM/SMU). In the end, your resulting signal-to-noise ratio may not be large enough to have confidence you are measuring an actual signal, rather than the oscilloscope front-end drift!

Generally speaking, what you are trying to do doesn't seem sensible to do with an oscilloscope input "bare" - I wouldn't trust the measurement results even from a higher-end scope for signals that don't form at least 10mV peak-to-peak purely because the noise will affect the computed measurements. What some people do is to use an active probe with a sensitive preamplifier front-end to measure such small signals using an oscilloscope - something like https://www.alphalabinc.com/product/lna10/ although as expected, because of the characteristic of white thermal noise, what you gain in measurement stability comes at a cost of bandwidth. This is also where some rather expensive power-rail probes come into play as well if measuring the power supply ripple-and-noise is your thing - e.g. R&S RT-ZPR40 but everything comes at a price ...

- Gough

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 the noise level rise, but for question 2, it's not a maths addition so subtracting won't provide the exact answer, because the RMS noise of the 'scope plus the RMS noise of the supply isn't a direct sum, you'll see a lower value than the actual value. You'll get a more accurate reading when your signal is much larger.

Regarding point 3, I think this is occurring because your device under test may be unscreened. Biscuit tins are good for enclosing circuits : ) The stuff you're picking up, could even be the fan in test equipment perhaps.

Incidentally, coax on it's own almost always rings alarm bells with a 'scope (even though BNC-to-banana or BNC-to-alligator cables are available), unless it's a low frequency measurement like audio, or unless 50 ohm termination exists. If your 'scope doesn't have this, it's possible to use a BNC T-piece and a 50 ohm BNC terminator (ideally not the absolute cheapest ones, because those have just a physically large resistor in them, intended for ancient LAN).

However, as you may expect, 50 ohm load isn't always desirable or safe. A solution is to have a resistor in series at the probing end, say 950 ohms (could just be a 1 kohm resistor measured from a batch to be closest to 950 ohm) and it plus the 50 ohm termination makes a potential divider, and then set your 'scope to a 20X setting. (This is an extremely high level description and not a general rule either, for want of just typing a paragraph hehe). But as Gough suggests, with very low level signals, an amplifier is helpful. I tried an experiment recently, described here: Building a Measurement Amplifier perhaps that can give some ideas too.

By coincidence I spent last night making such probes up, because I too wanted to use coax pigtail type thing, just so I can solder the probe in place. I'll write it up sometime, waiting on some bits and pieces to finish it off.

It's not a general-purpose solution, and I don't think will help for your current problem anyway, the real solutions may be to shield everything, and use an amplifier.

By the way I wasn't sure where you wished to apply the ferrites (to the supply of the device under test?) but it could be worth keeping a load of ferrites at home just to experiment in situations like this, to rule in/out sources of noise at different locations. Sometimes a single ferrite isn't enough, so really keeping a dozen or so at hand could be better (like clip-on ones, as well as the toroid/tubular ones. It could get pricey, so might be worth keeping an eye out for ferrite kits occasionally from ebay, not that I've ever seen a decent one come up. There is one kit there for £35 currently, but I'm not sure I'd go for that personally, it's not ideal. There's a very nice Wurth kitvery nice Wurth kit but it is £134. I'm tempted to buy it one day. Currently I keep a mixture, from an old kit, and some ferrites purchased individually. If you want me to recommend some 'problem-solver' ferrites (huge ones just for testing : ) then I can look up the part codes for the ones I have.

If you are looking for a practical figure of the noise in your setup: handle the test leads as "an integral part of the oscilloscope".

Attach a probe to the channel you want to test. Shortcut the ends.

Then watch the noise. This is a decent representation of the oscilloscope's noise floor in your setup.

If you want to have an indication of noise floor without probes (why?), then using a 50 Ohm or a plain shortcut at the input is better than an open input connector. Not ideal, but better. Set the channel to X1.

A springy little piece of u-shape bent wire with one end into the BNC connector input and the other end touching the inside of the BNC shield part can do the job.

Thanks Gough, some very useful information in this.  I've responded to everyone in one post below.

Thanks everyone - I think it’s easier to answer everyone in one response than try to do so individually: you’ve all provided some great information to get my head around.

To contextualise a bit more, what I'm trying to do is measure how noisy a 5V output rail is.  I have to be careful with attaching the probe into the circuit to get what looks like a clean signal (image 2) vs something that swings around and just looks like the probe isn't connecting properly.  When I was measuring what I captured in image 2, it sort of felt too good to be true (as yet, I have a lack of empirical experience to draw upon)!  Ignoring the spikes, using cursors I measure the signal at 330uV pk-pk.  This led me to wondering if maybe I wasn't properly probing and just viewing noise generated by the scope itself: reading around the topic led me to the above approach and questions to fill in some blanks.  So my questions weren’t in relation to a specific problem or issue, they were more to do with getting a better understanding of what may be happening in general.

Having said that…

“In the end, your resulting signal-to-noise ratio may not be large enough to have confidence you are measuring an actual signal, rather than the oscilloscope front-end drift!”

“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”

“…with very low level signals, an amplifier is helpful”

My understanding from this is that any signal that may be captured in Image 2  is buried in the noise floor, or is so small that it can’t be captured, and to actually see it I’d need to amplify it by some means?  From a practical perspective, the fact that any noise the output does have is measured in uV is fine for my purposes - to some extent it’s experimentation.

“If you want to have an indication of noise floor without probes (why?)…”

TBH Jan, only because that’s what I read!  Normally, I go to multiple sources to get answers/ideas to make sure I’m not just reading what some loony has decided to post up.  In all cases, the approach to measure the noise floor was as I described above: without probes (or anything) attached to the scope inputs.  In fact, every source I read specifically said to remove connections.

I redid the tests:

[with 50Ohm terminator attached to Ch4 input - not necessarily a great one as it’s what I have and it was fairly cheap]

[with the probe ends shortcut]

And measuring the circuit with a tin over it:

And zoomed in:

Thanks Shabaz - I responded to everyone in a post below.

If I understand what everyone is saying correctly, it may well be worth building that measurement amplifier for more experimentation.  It's a great build so thanks for sharing. 

I've vacillated on the kits (not just ferrites, but resistors and capacitors as well) but they're so expensive and I'm really reticent to buy off eBay and Amazon.  If you could recommend some ferrites it may be cheaper to buy a few individually than as a whole kit so that would be great.  I had wondered about putting a bead on the supply to the DUT - it's a wall wart and currently doesn't have one and I thought it may help.  I can't turn everything off in my workroom as I need the router and phone running (they are about 15ft away from my bench), but as the wife is out I can turn off everything else in the house without complaint: it doesn't make a difference unfortunately.  The biscuit tin trick made a slight difference by reducing the peaks of the spikes by 0.5mV or so.

Thanks Jan, I responded to everyone in one post below.

Hi Andrew,

Ideally the biscuit tin needs connecting to 0V, but won't make any difference if the noise is already part of the DUT or can enter on the wires. For the supply, if you have a bench linear supply, it could be good to try that too, just to eliminate the plug-in switching supplies, or try battery operation if that's feasible.

Here are some useful ferrites:

Fair-Rite 2675102002 2675102002 - 12.8mm inner diameter, so coax with SMA connectors on the end will pass right through. Excellent to have on hand.

Fair-Rite 2631625102 2631625102 - 7.9mm inner diameter, so usually only suitable if the wires do not have any connector on them, e.g. wires from a bench supply etc.

Fair-Rite 2675821502 2675821502 - 19mm inner diameter, so many different connectors like BNC will pass through fine if needed. Can also loop through several times if just using normal wires. Would be worth having half a dozen of these on hand.

Fair-Rite 0475164181 0475164181 - plastic halves snap-on type, with 12.7mm inner diameter. This is ideal for almost any cable with or without connector, and also allows for attaching without disconnecting the cable of course. However, it's pricey. I only have two of these.. but at least they are reusable and not a consumable!