A method for evaluate your oscilloscopes ground noise? - Page 1
Forum section: electronics / testgear

author: lemon
author_rank: Frequent Contributor
timestamp: October 03, 2013, 02:30:25 pm
content: I would like to anyone that have last version of any oscilloscope to give us the result from the following test.Procedure for measurent of Ground Noise--------------------------------------------------------Place the scope to one position with a less interference from various environmental RF & EMCompensate the probe as your manual described.Change switch probe to 10x.Fix the probe cable like the attachment photo.Turn the CH1 on (other channels to off)Do the same to the Probe Scope Menu (10X).Set the BW of your scope to maximum.Connect probe to Probe Compensation Area Testwith both Probe Tip and Ground clip wire to GND.Set the CH1 to DC, 50mV/Div.Trigger 50% or similar.Set the Memory Length at least 1M or max if you have lower depth of memory (note what length choosed)Mode to Peak or similar.Set the Horizontal speed to 100usSet the measurement to noise Voltage peak to peak.Take a capture of noise and upload to this thread with note the Model of Scope, the Serial Number without the last digits, what type of probe used and how mV noise is appeared.

author: lemon
author_rank: Frequent Contributor
timestamp: October 03, 2013, 02:38:01 pm
content: For example.Owon SDS7102, Serial Number=sds710213193XX,28mV

author: sync
author_rank: Frequent Contributor
timestamp: October 03, 2013, 04:15:07 pm
content: What do you want to achieve?You can't compare scopes this way. You need controlled conditions. Look at my attachments. The same scope with the same setting can have very different results.

author: lemon
author_rank: Frequent Contributor
timestamp: October 03, 2013, 05:48:14 pm
content: Sync, thanks for the reply.The difference between the two captures is with the probe cable fix as I described to the procedure description or the one is with the straight cable and the other is like the attachment photo?A long time ago, we have a talk about this to the Owon SDS7102 thread and we have reliable repetitive results with the same procedure.

author: sync
author_rank: Frequent Contributor
timestamp: October 03, 2013, 06:22:26 pm
content: Quote from: lemon on October 03, 2013, 05:48:14 pmNo. Both are like in your photo. But they are two different probes. The one with less noise is a Tektronix. The other a cheap Chinese.QuoteHow? I see FM radio stations with my scopes. With your procedure the probe is an antenna. So the noise you see will depends how strong the RF fields are. These vary with different locations and times. You need at least a RF shielded room/ box for reliable repetitive results and for measuring the scope noise, not radio.

author: Unknown
author_rank: Unknown
timestamp: October 04, 2013, 02:31:53 am
content: Or put a BNC shorting cap directly on the input connector, assuming the scope itself is well shielded.

author: lemon
author_rank: Frequent Contributor
timestamp: October 04, 2013, 10:47:24 am
content: Quote from: sync on October 03, 2013, 06:22:26 pmsync, I understand your thought and I agree that this is not stable variable to this procedure.It is a kind of testing for one oscilloscope to the same position each time and with the same probe, but probably the suggestion of member alm is a solution to this.Quote from: alm on October 04, 2013, 02:31:53 am

author: TomC
author_rank: Frequent Contributor
timestamp: October 06, 2013, 03:14:00 pm
content: Quote from: alm on October 04, 2013, 02:31:53 amI posted my views on this subjecthere.

author: Unknown
author_rank: Unknown
timestamp: October 06, 2013, 04:09:41 pm
content: If the noise is not present with a shorting cap, how do you know that it's not:Radiated noise from the front panel picked up by the loop antenna formed by theshorted scope probe. The old Tek TDS-200 series would radiate noise from the CCFL inverter from the LCD.Noise conducted from the probe calibration output (not exactly the most critical circuit of the scope).I really don't see what measuring either of these would accomplish. Neither would matter for measurements in a real DUT.A ferrite would help: it takes out common mode signals, like the noise you're complaining about. There should be no differential mode signals if the probe is well shorted and shielded.

author: Carrington
author_rank: Super Contributor
timestamp: October 06, 2013, 07:29:36 pm
content: You are responding yourself: "Radiated noise from the front panel picked up by the loop antenna formed by the shorted scope probe."I.e. with this "test" (probe loop) we are catching the noise radiated by the oscilloscope and maybe by other instruments, FM station, etc...IMHO this "test" may be worth, because it can give you clues, and perhaps you discover that you have to shield the lab room, or that one of your instruments is not properly shielded (even poorly designed). Is true this is not the better way to do this, but is fast and cheap.As TomC says: "We are always advocated to do the test in an environment" with more or less interference, but this does not mean that we do not consider the noise radiated by the oscilloscope.The noise present with a shorting cap, "just show the baseline noise plus a minimal amount of GND noise produced by the subtle unbalance of the shorting cap connections".A shorting cap is very well to see the baseline noise, but I don´t use a shorting cap to measure, generally I use the probe. What if a similar loop is formed when I make a measurement?

author: sync
author_rank: Frequent Contributor
timestamp: October 06, 2013, 07:38:22 pm
content: Quote from: Carrington on October 06, 2013, 07:29:36 pmBut you can't compare scopes with this method. That's the point.

author: Carrington
author_rank: Super Contributor
timestamp: October 06, 2013, 07:42:27 pm
content: Quote from: sync on October 06, 2013, 07:38:22 pmRight, but for the same/similar reason we can not compare it with the shorting cap method.Only we can compare if all are tested under the same conditions/circumstances.

author: lemon
author_rank: Frequent Contributor
timestamp: October 06, 2013, 07:49:05 pm
content: Sure this test needs a good position with minimal of external interference.If we accept that all cheap chinesse probes are almost same, then the different results on various scopes (with the same chinesse probes) will be from scopes and no from probes (sorry my english is not perfect to describe it better).For example, in the case of member sync, he uses two different probes, one cheap (Chinesse) and another expensive (Tektronics). With expensive Tek probe the very good shielding "filtering" the RF environment effect and the result was low gnd_noise. The other cheap Chinesse Probe, haven't any good shielding and catch like a good antenna a lot of RF and appears moderate results.But in case of Owon SDS7102 scope with the almost identical cheap probe the noise was low, why? There was better enviroment with minimal RF, the Owon has better internal shielding (I don't think so, all these cheap scopes has the same internal no almost shielding) or the specific Owon has a lower gnd_noise.Mine scope with the BNC shorting cap demonstrates a little more noise than baseline, i.e 6-10mV (baseline, 10X) to 12-16mV (BNC shorting, 10X)Sure this theme has a lot of theory and needs good documentation to understand it.

author: Carrington
author_rank: Super Contributor
timestamp: October 06, 2013, 07:59:16 pm
content: But all this tests are very interesting.

author: lemon
author_rank: Frequent Contributor
timestamp: October 06, 2013, 08:07:44 pm
content: Quote from: sync on October 06, 2013, 07:38:22 pmBut, if you compare scopes to the same environment and same probe, then it isn't good test to see the differente noise level of scopes?

author: sync
author_rank: Frequent Contributor
timestamp: October 06, 2013, 08:29:22 pm
content: Quote from: lemon on October 06, 2013, 08:07:44 pm

author: Unknown
author_rank: Unknown
timestamp: October 06, 2013, 09:36:12 pm
content: Quote from: Carrington on October 06, 2013, 07:29:36 pmHow close is your DUT usually to the front panel? In the case of the radiating LCD and a calibration output close to the screen, the distance may be a few centimeters, while it might easily be a meter from scope to DUT in a normal bench setup. Field strength will decrease with the square of the distance for simple point sources (inverse square law), so changing the distance from 5 cm to 1 m could reduce the field strength by a factor 400. Even from 5 cm to 10 cm would change it by a factor four. You would also generally use a coaxial ground connection (e.g. the ground springs that fit around the ground sleeve near the probe tip) for critical measurements in the presence of external fields.The results of this measurement will depend on the area of the loop formed by the ground lead, the orientation of this loop relative to the noise source, the EM fields produced by the scope and any external fields if you're not working in a shielded lab. Any scope probe will pick up noise in this configuration, that has very little to do with the scope or the probe. If you want to measure the radiated noise from the front panel, then you need a defined shape and area of the ground lead (longer ground leads will pick up more noise) and a null measurement with the probe at the same location but with the scope removed to measure any external fields. I don't see the point of clipping the probe calibration point, and I wouldn't call noise radiated from the front panel 'ground noise' either.If the coaxial cable is poorly shielded as you imply, then the orientation of the cable becomes an additional variable. Try wrapping it around the power cord, draping it across the scope or some other noisy instrument, or coiling it up.Quote from: lemon on October 06, 2013, 08:07:44 pmDefine 'noise level of scopes'. You would generally measure the noise produced by the front-end and ADC by shorting the inputs. But of course you can also measure noise under all kinds of different conditions. You could also put the loop antenna near the power supply, or near the fan. How meaningful this number is for daily use is a different matter.

author: Carrington
author_rank: Super Contributor
timestamp: October 06, 2013, 11:17:17 pm
content: Quote from: alm on October 06, 2013, 09:36:12 pmAnd even so the manufacturers shield the LCD, well Owon does not.Quote from: alm on October 06, 2013, 09:36:12 pmSo? The name of the topic may not be the most appropriate, but I repeat, all this tests are very interesting. And mate call it as you want.

author: onlooker
author_rank: Frequent Contributor
timestamp: October 07, 2013, 01:54:32 am
content: I guess call it ground noise is ok since it is a RF radiation particularly detectable from the scope ground and it behaves differently from other external RF noise. I think the attached picture may help to explain what is happening and what is to be measured.The problem is caused by the PSU creating a RF source (up to 100Mhz) and made the whole scope riding on the emitting antenna side of this RF source. I guess all scopes with switch mode PSU would have the potential to encounter such problem. But, apparently Owon is particularly bad in this regard.This attached picture explains multiple things (as many may have mentioned in scattered places):1). A short cap on BNC is not a way to measure this (since it is in common mode).2). This RF noise may not show up when probing DUT using short ground tip (again due to being common mode).3). This RF noise will show up when using the 6" ground lead even if the DUT is far away from the scope. This is actually the interesting part. The reason is the combination of both conductive and radiative passages for the noise. First, the probe cable as a whole (both conductors) becomes part of the emitting antenna to transmit the RF noise; then the radiation at the probe tip is picked up by the tip loop acting as a receiving antenna to covert a common mode noise into a differential one and gets send back to the scope. This particular way of noise passing and conversion does not seem to exist for other typical external RF interferences.The goal here IMO is to measure the inference of this particular RF noise in a typical 6" tip loop setup.

author: lemon
author_rank: Frequent Contributor
timestamp: October 07, 2013, 08:47:08 am
content: Quote from: onlooker on October 07, 2013, 01:54:32 amYes, you correctly guessing. In the situation of those scopes that have a SMPS psu the whole scope emit like antenna but it isn't only Owon problem. Although still use SMPS, the whole gnd_noise is very low at the SDS7102 for example, at the older versions it has a lot of such noise.OK guys the title isn't perfect and not decribed precisely the test but it is a test with some limits. For test to a specific scope with some kind of such noise, it is a method to see what made ??the modifications.

author: lemon
author_rank: Frequent Contributor
timestamp: October 07, 2013, 11:15:38 am
content: Quote from: alm on October 06, 2013, 09:36:12 pmSorry alm but I can't understand well the underline text, I think that explains the meaning of this test better. The area of the looped formed by the ground lead is fixed like the photo, not straight not to any position, there are 2 or 3 rounds of cable from the right to left (it depends to length of probe cable). The orienation of this loop from any source of noise is fixed again and the last about any external fields is the only not stable variable but we could try to eliminate.You said that "to that has very little do with the scope" but if the scope uses a bad SMPS PSU design this radiates everywhere and the probe it'll catch it.In some older versions of Owon SDS7102 there was a bad circuit design (they used SMPS design) and the gnd_noise with this method was very high, at the same time the measures with the low Vpp signal was terrible by artifacts.Today the company corrects all these faults, still uses the same SMPS topology but the gnd_noise is very low and all low Vpp measures are fine. The same results have done owners with their mods at the same circuits like Owon. In this situation the issue wasn't the external fields but the internal issues.Am I wrong, I don't think so.To another case a member try to an gnd_noise scope (due bad psu design) to put an expensive probe (like sync doing to a Rigol, probalbly DS1052, at the third message), but with no results to the gnd_noise. Why in this test, the well made probe dosen't the difference -> becouse the problem was internal and not external.So, this test (sure not to all cases) have the fan or the value for evaluate our scopes for my opinion.Any way, everyone can try this test (exactly as the directions of the first message says) and tell us what he is found and what is thinking. Aspects about various probes or specific lab areas are welcome.Not as comparison as said the title but as reference for our talk (maybe I must changed the title...how is the "A method for evaluate your oscilloscopes ground noise?")

author: Unknown
author_rank: Unknown
timestamp: October 07, 2013, 02:46:16 pm
content: Some scopes have the calibration output next to the LCD, so if the LCD panel is radiating noise, then this will make a huge difference to the coupling. For example look atthis picture of the probe calibration output of an Agilent DSOX2000 scope. The probe would be much closer to the LCD, and the loop would be oriented horizontally. If your explanation of the probe cable acting as an antenna is indeed correct, then I would expect the proximity of the probe tip to the coiled probe cable to be important. Something that is again hard to keep constant with different scopes, probes and people performing the measurements. Ask three techs to perform this measurement could very well give three different results for some scopes.You seem to assume that the common mode noise from the power supply is the only noise source. This is not the case in sync's measurement. This method might work to compare Owon scopes with and without the fix, but I wouldn't use it to compare different scopes.

author: TomC
author_rank: Frequent Contributor
timestamp: October 07, 2013, 04:03:47 pm
content: Quote from: alm on October 07, 2013, 02:46:16 pmI agree with your objections regarding the accuracy and repeatability of the test. However, I think that the goal here is not undisputable lab accuracy or near perfect repeatability. This would be out of reach for most of our members that want some idea of where different scopes stack up regarding GND noise.I think that, in the absence of a better idea, the method outlined can reasonably achieve that goal. To improve noise immunity to external sources and near field radiation, I would suggest adding ferrites to the probe cables. This should concentrate the results around the conductive part of the GND noise for a more fair comparison.

author: Unknown
author_rank: Unknown
timestamp: October 07, 2013, 05:13:56 pm
content: Wouldn't wrapping the probe cable around a ferrite core attenuate the common-mode noise that caused the problem in the first place?The most straightforward way would probably be to measure the signal between ground and 'Owon ground' with a second scope, but I guess that few low-end scope owners will possess a second scope.

author: TomC
author_rank: Frequent Contributor
timestamp: October 07, 2013, 05:33:50 pm
content: Quote from: alm on October 07, 2013, 05:13:56 pmThere would be some attenuation, but a large portion of the conductive common mode noise converted to differential noise by the inductive imbalance caused by the long ground lead remains intact in my experience. In contrast, the near field emissions from the screen or nearby RF sources which is typically picked up by the probe cable acting as an antenna will be largely attenuated. Granted, the probe tip itself and the long ground lead still have some antenna effect, but it is minimal compared to the effect of the longer scope probes.For some time now I've been using ferrites on my scope's probe cables, one near the BNC and another one close to the probe itself. The ferrites are the type typically used for power cables, this allows me to loop the probe cable at each end for double the inductance. This has reduced the near field emissions from the screen and external noise sources considerably. So they are now an integral part of my Owon probes.

author: lemon
author_rank: Frequent Contributor
timestamp: October 21, 2013, 08:08:14 am
content: Here some results fromOwon SDS7102,Siglent SDS1102CMLandHantek DSO5102B.The complete test is heldhere, but I copied a comparative image and transfer it here.Left is Hantek, middle is Owon and right is SiglentAt the first serie is noise capture (base line) without any probe connectedAt the second serie is noise with probe connected to scope and tip and ground joined.At the third is the ground noiseAt the fourth serie is a detail zoom of test square waveform.

author: akis
author_rank: Frequent Contributor
timestamp: October 21, 2013, 02:56:07 pm
content: If you connect the probe and then move the cable about, even half an inch movement makes a difference. Since all scopes emit some noise, it would be better, in my opinion, to move the cable so as to pick the maximum noise from each scope under test. For example on the Owon the worst location is with the cable under the screen. For Rigol the maximum noise may be picked up with the cable in a different position. But in all cases what we want to see is the worst noise those scopes are putting out and which noise can be picked up during other measurements.Another test would be to test for maximum noise at a minimum distance of, say, one foot away from the scope. The assumption here is that you will be testing equipment on the bench and the scope will be somewhere at the back of the desk, say 1 foot away.I just ran those two tests, and here are the pictures. As you can see, when the probe and cable are tucked under the screen we have a lot of noise. But when we maintain 1 foot distance (cable leaving the scope at 90 degrees for at least one foot) there is no noise at all (almost).

author: TomC
author_rank: Frequent Contributor
timestamp: October 25, 2013, 02:26:06 pm
content: I view this a little different. I find that the radiated noise can be avoided, as a result I find it of less importance. In contrast, the conductive noise is insidious, and you can't get away from it just by moving further away. In my opinion, this affects the performance of the scope on real life conditions and every day use quite a bit more. So, if I was to use comparisons between scopes as a means of judging performance, I would prefer to concentrate on the conductive noise.