Why Are Digital Scopes Noisy (Or what appears to be Noisy)

Sorry for digging an old topic back up, I found it interesting ... let me start off by saying, I'm not a high performance scope expert - so please do correct me (nicely) if I'm wrong ...

Well, part of it might be the better bandwidth of modern DSOs as well, thus being able to let through and measure high frequency noise which would have been filtered by less bandwidth capable scopes. The phosphors of old CROs didn't shine that brightly for "random" signals, so any noise would be a very dim, possibly hardly perceptible blur.

The inherent noise contribution is likely the thermal noise (Johnson–Nyquist noise - Wikipedia) and shot noise (Shot noise - Wikipedia) which is a matter of physics and can't be eliminated, all mixed in with other sources of noise already discussed (i.e. A/D, RF, SMPS). I suspect a wider bandwidth means that more thermal noise is admitted which increases the apparent noise floor of your channel. If you have a bandwidth limiter, maybe you can see its effect on the readings (at the cost of bandwidth, of course).

I suspect the method of scaling the horizontal axis also has something to do with it - as scopes are likely to show "bars" which span the lowest to highest value in that pixel bin, rather than the "most common value" or intensity coded value. This is just the way it's "combined" several A/D values together at that particular time interval, and it's not the same as the phosphor of CROs which will have an intensity gradient representing how often the signal is at a particular reading, thus the noise "fades" around the signal and is often not seen.

That's what old topics are for so no worries

My main concern was that on the lowest settings, it is over a full division of noise, with a 8 bit ADC less a bit for overshoot management, you have say 250 count, so 30 count per division , that's well over 10% noise on the range

The thermal noise from a  1k resistor over a 1GHz bandwidth is 127uV, from 1 1M resistor it's about 4mV which are about 0.762mV and 24mV pk-pk respectively. If you are using  a x10 probe it is the 1M case that applies, regardless of the actual source impedance but the scope will multiply the reading by 10 so the indicated noise will be 240mV pk-pk.

I set my scope to 200MHz bandwidth and with a shorted =10 probe I got 9.5mV pk-pk (1Gs, LeCroy 610zi) -  the expected value is 10.8 mV pk-pk.

So I conclude that the scope is not noisy but as Gough suggested is just showing what physics says should be there.

Of course this was only one quick experiment with one scope.

And watch out for those x10 probes - handy but noisy !

(thermal noise data from Thermal Noise Calculator one of many such pages on the web).

MK

The thermal noise from a  1k resistor over a 1GHz bandwidth is 127uV, from 1 1M resistor it's about 4mV which are about 0.762mV and 24mV pk-pk respectively. If you are using  a x10 probe it is the 1M case that applies, regardless of the actual source impedance but the scope will multiply the reading by 10 so the indicated noise will be 240mV pk-pk.

I set my scope to 200MHz bandwidth and with a shorted =10 probe I got 9.5mV pk-pk (1Gs, LeCroy 610zi) -  the expected value is 10.8 mV pk-pk.

So I conclude that the scope is not noisy but as Gough suggested is just showing what physics says should be there.

Of course this was only one quick experiment with one scope.

And watch out for those x10 probes - handy but noisy !

(thermal noise data from Thermal Noise Calculator one of many such pages on the web).

MK

Thanks Michael for that handy link ... and the calculations. Very enlightening!

To clarify, a 10x probe has a 9Mohm in it, the scope typically has a 1Mohm (Irrespective of X10 or X1) and in some better ones the option of 50ohm

The thermal noise from resistors while very real, very much depends on the type of resistor, some of the newer kinds have extremely low thermal and galvanic noise figures (The bonding of the lead wire to the resistor element can cause low galvanic reactions)

If they have bulk metal film for instance the noise figures are way, way less.

I do agree in principal though that all these factors add up to what appears to be a noisy scope

Either way, it still sucks no matter the cause, and if the probes came with the scope and the scope has a low value range, I expect it to be usable otherwise why have it there in the first place.

A great discussion, im happy to keep this alive

Peter

Made a mistake here, the resistor type I meant to refer to was metal Foil, not metal Film

Even ideal resistors have thermal noise and my estimates are based on perfect resistors. Real resistors are worse and the amount they are worse varies with the type of resistor. I simplified my calculations a bit - the real source impedance seen at the scope connector of a 10x probe is 1M//9M = 0.9M so the anticipated thermal noise for my scope test would be 10.26mV pk-pk rather than the 10.8 from my rougher estimate. The reason that my measurements look better than ideal is that the RMS * 6 for the pk-pk is a rough estimate.

Certainly the lower ranges are useable on my scope - because you can constrain the bandwidth by using analogue and/or digital filters. It would be a bit mad to use x10 probes to measure tiny signals.

I'll try to find time to check the scope on some different settings - and even check the Rigol which make some claims to be low noise.

MK