Rigol DS2102 - grounding the probe doesn't flatline the trace

[agdr]

When I ground the probe tip on my Rigol DS2102 on a high vertical gain setting I don't get the flatline trace I would expect.  Instead I get a fast, complex, waveform.  Any input on what might be going on here is appreciated!

This is a DS2102 100 MHz scope with the 200MHz software upgrade.  The probe is a 200MHz probe set to 10x and compensated.  Not only does this happen with the probe tip grounded,  it happens with the probe entirely disconnected and even with a BNC shorting cap applied to the input.  The only thing that properly flatlines the trace is setting the scope input coupling to ground.

The scope is set to AC coupling, 10x ratio to match the 10x probe, and BW limit off which should give the full 200MHz.

The first photo shows the problem.  Probe tip grounded.  This is on channel 2. Moving the probe to channel 1 results in exactly the same thing.  10mV/div vertical and 2nS time base.  Trigger is set to single sweep to freeze the waveform.

The second photo shows the probe removed entirely!    I have another photo (won't fit here, my photos are too big) with a shorting cap attached that results in exactly the same waveform.  But setting the input coupling to "GND" will finally flatline the display.

On lower vertical gain settings the scope works just fine, such as viewing a 2Vp-p waveform at 500mV/div, or the probe compensation square wave from the scope's compensation ring.

[technogeeky]

Is the frontend busted?

Check the resistance of the center pin to the outer ring; when the channel is on and off. Is it 1.000M in both cases?

[agdr]

Looks like 1.2 meg on both channels, the entire scope is powered off here.

About the only thing I can think of - other than what must be operator error on my part here, lol! - is just a noisy vertical amplifier in the thing.  But i can't imagine a company would design a scope such that the highest couple of gain settings show up significant noise in their amplifier.  That "gnd" coupling setting that flatlines the display must either be after the vertical amplifier (given that a BNC shorting cap didn't flatline it) or simply a software trick that draws a flat line on the display. 

[agdr]

Here is the BNC shorting cap...

5mV/div and 2nS time base

[agdr]

...and the flat line (finally!) with the coupling set to ground.

[rs20]

The real question is, does anyone else have the same scope who can confirm/deny whether this is working as intended? Given that you're horizontally zooming right into the shortest timescale where all the high-frequency noise would be evident, I'm not 100% convinced that this is actually a problem? If you zoom out to a more normal timescale it'll just appear as the classic fuzziness which is inevitable when dealing with a 200 MHz scope collecting 200 MHz BW worth of noise.  I'll admit though, it does look a bit more than I'd have expected.

Good on you for trying with the shorting cap, that was going to be my next question! Do you have probe compensation still set to 10x when using the grounding cap? Because then 5mV/div is actually 0.5mV/div on the frontend, which would be indicated correctly if you configured your scope to read 1x.

Also, sorry to be pedantic, but nS is nanosiemens. ns is nanoseconds.

[technogeeky]

Other things to try:

• Turn on the 20 MHz bandwidth limiter. Does the noise go away?
• Make sure you are testing on calibrated timescales, not the vernier timescales. This shouldn't make that much of a difference, but at least we can rule things out.
• Use a probe-tip wire (the little coil spring that fits on the ground on the tip of the probe) and then connect that to the tip. This is a much shorter wire, so any real high frequency noise (via inductive pickup) will be eliminated.
• Turn off any nearby switching power supplies; especially LED ones.
• Similarly, turn on AC triggering and see if the signal correlates with that trigger or not.

[A Hellene]

This is something to contemplate on:
Rigol DS1052E nasty surprise!

In a few words, "I guess that we have both been, amongst others, the lucky winners of this specific (non-)QC market..."


-George

[agdr]

A Hellene - thanks for the pointer!  I figured there must be a thread on this already by now. I tried searching for "grounded probe noise" and even just "Rigol" but I must not have gone back far enough in dates.  Well wow!  Apparently the Rigol is "noise challenged".    Skimming through that thread I see you even tried the shorting BNC caps and the issue of coupling "gnd" working came up.

I've had the scope for a year or two, bought new from Tequipment.com.  I had noticed the trace wasn't particularly sharp, but hadn't much need to go down to super-low voltage readings until recently.  Then I became aware that "fuzz" was actually a small waveform.  Then tried grounding the probe... and still there.

Interesting about your 95MHz measurements.  I hadn't measured the period yet, but the frequency must be around there since it is showing up as a few cycles at 2ns (rs20 - you are right! - thanks).  Must be a clock signal in the thing somewhere for the digital logic that is leaking.

I'll read through your thread in more detail tomorrow.  I'm wondering if the higher priced 4000 series 500MHz Rigol scopes have the problem.   I've been thinking of dropping some money into an upgraded scope this summer.  I'm *really* glad I've found out about this problem now!  It may already be in your thread that I haven't seen yet, but if anyone out there reading this has a Rigol 4000 series, please try grounding the probe tip (to probe ground / bnc ground / calibration ground), turning the vertical sensitivity and time base to maximum, and post what you get.  I would be most curious.

[tautech]

There's some history with design problems, search the forum for Project Yaigol.

[A Hellene]

You 're welcome, agdr!

By the way, do not miss the reverse engineered DS1000E schematics that I've drawn and shared with the community while I was investigating the problem in my device.


-George

[agdr]

tautech - thanks!  Found it:

https://www.eevblog.com/forum/projects/project-yaigol-fixing-rigol-scope-design-problems/

Well geezzzz.....    Just that first paragraph doesn't look encouraging in terms of scope quality.  Some great detective work by the forum folks here.  I'll read through this week.

[agdr]

A Hellene - I just scanned through all 6.  I can see a tremendous amount of work went into ringing out the circuitry and transcribing it all.  What a great resource.  Thank you for the contribution!

[rs20]

Then I became aware that "fuzz" was actually a small waveform.  Then tried grounding the probe... and still there.

What did you think it was?

Anyway, my DS2202 produces a similar-looking waveform with a grounding cap and all the dials turned to the extremes like you have (that's 500uV/div actually, not 5mv/div.) I don't want to be "that guy" desperately defending his purchase, but the set of problems that can't be solved by a) setting bandwidth limit, b) using higher level signals, or c) using more gentle timescales appears to be a small one; certainly one I haven't encountered after using the scope for many years now.

One thing that is disappointing, though, is that Rigol don't seem to specify a noise value or figure in their datasheet. Not sure if it's standard practice to omit such a figure from the specs, but it certainly makes it harder to defend the performance of a product when they don't make a specific claim.

Noise is an inevitability of using wide bandwidths, although my attempts to calculate a figure have given an unexpectedly high result. A plain old resistor, such as the termination resistor in the scope, of 1 megaohm at 20 degrees C produces 1.8mV RMS white noise when observing over a 200 MHz bandwidth. This is over 3 divisions on your scale, RMS, which would mean that your scope is performing better than a theoretically perfect scope, which is clearly wrong. I'm not sure how the 16pF capacitance of the frontend factors in, or whether I'm missing something else?

TLDR; I suspect you should watch the EEVBlog episode debunking the claim that digital scopes are noisier than analog. It does a good job of conveying that noise is an intrinsic part of life, not some design mistake that can be arbitrarily quashed. Clearly there is some sort of design imperfection in the Rigol scope, but still cruicially important to understand what a theoretically perfect scope looks like -- because it's not dead flat, ever.

But i can't imagine a company would design a scope such that the highest couple of gain settings show up significant noise in their amplifier.

The high gain settings are 100% useful on more sensible timescales, when using acquisition averaging, or when using the bandwidth limit. So your expectation is unreasonable here.

[rs20]

Correcting myself: turns out the noise (across the entire bandwidth) of a parallel RC circuit is dependent on the C alone, so we can calculate that we should expect a theoretically perfect scope to have 16uV, or about 1/3 of a division. So this confirms what has already been pointed out that this scope is far from theoretically perfect (let alone, apparently, a top-of-the-line modern scope), but that the "flatline" that the OP was hoping for was also unreasonable.

[tautech]

Correcting myself: turns out the noise (across the entire bandwidth) of a parallel RC circuit is dependent on the C alone, so we can calculate that we should expect a theoretically perfect scope to have 16uV, or about 1/3 of a division. So this confirms what has already been pointed out that this scope is far from theoretically perfect (let alone, apparently, a top-of-the-line modern scope), but that the "flatline" that the OP was hoping for was also unreasonable.

Yep, noise on a DSO is one thing and can be managed in a # of ways as Dave outlined in his 2 vids but periodic noise is entirely another thing, it originates from the immediate environment or from within the DSO itself.
On a cramped bench it is not unusual to pick up the noise from the display.....dangle an unterminated probe around a DSO and you'll see. 

[DimitriP]

With the input shorted at the BNC jack , expecting a flat line is not unreasonable.

I'll accept it as a posssible side effect of a performance vs price design compromise but that's as far as I wll go

Someone reading this with a megakilobucks scope should be able to confirm if there is a flat line displayed with the input shorted ....

[ebastler]

With the input shorted at the BNC jack , expecting a flat line is not unreasonable.

I beg to differ. Of course it is unreasonable to expect a flat line (if you crank the vertical amplification up to the maximum). I am flabbergasted by this whole discussion, in a TME forum! The only valid questions one should ask here are: "What level of noise should one expect? What level of noise do other scopes produce, under comparable circumstances -- same bandwidth, same vertical amplification?"

[rs20]

Someone reading this with a megakilobucks scope should be able to confirm if there is a flat line displayed with the input shorted ....

Wasn't aware that the laws of physics could be bribed with millions of dollars. Vrms = sqrt(k_BT/C) = 16uV in this case. End of story, even before the noise figure of the frontend is considered.

(Unless I'm misapplying the formula)

[tggzzz]

When I ground the probe tip on my Rigol DS2102 on a high vertical gain setting I don't get the flatline trace I would expect.  Instead I get a fast, complex, waveform.  Any input on what might be going on here is appreciated!

Do some experiments to characterise the signal...

What's the frequency? (Looks like ~100MHz - that ought to remind you of something )

Is it constant with time, or pulsed?

What makes it change? Loop orientation, position relative to animate and inanimate objects...

What happens if you get rid of that 100nH inductor (the ground lead) and directly connect the tip to ground?

Welcome to the world of EMI/EMC.

[rs20]

...

I think you missed the post where the OP used a BNC shorting cap, and ended up with essentially the same outcome. This is partly an imperfection with the Rigol, as explored in other threads.

[DimitriP]

Someone reading this with a megakilobucks scope should be able to confirm if there is a flat line displayed with the input shorted ....

Wasn't aware that the laws of physics could be bribed with millions of dollars. Vrms = sqrt(k_BT/C) = 16uV in this case. End of story, even before the noise figure of the frontend is considered.

(Unless I'm misapplying the formula)

More expensive scopes have better lawyers.

[capt bullshot]

That's just noise, as some other pointed out.
But look at my "good 'ol" TDS224 here: this waveform is result of some defect (first picture) no matter if the input is open or shorted

second picture: same input settings, just maximum speed - see the noise

third picture: this scope's also cheating on you when you set the input coupling to GND

[David Hess]

The first photo shows the problem.  Probe tip grounded.  This is on channel 2. Moving the probe to channel 1 results in exactly the same thing.  10mV/div vertical and 2nS time base.  Trigger is set to single sweep to freeze the waveform.

This is a common problem when the probe tip is shorted to the ground clip; the loop created is large enough to pick up a lot of EMI.  Grounding the probe tip with a coaxial ground should produce a clean trace.

The only thing that properly flatlines the trace is setting the scope input coupling to ground.

Ground coupling does not actually ground the input on these oscilloscopes; instead it disconnects a later stage of amplification so the display does not represent the actual input noise.

If you zoom out to a more normal timescale it'll just appear as the classic fuzziness which is inevitable when dealing with a 200 MHz scope collecting 200 MHz BW worth of noise.  I'll admit though, it does look a bit more than I'd have expected.

The fuzziness is inevitable but that much?

We can measure the amount of noise to make a quantitative comparison.  The photograph showing the shorting cap and 500uV/div has about 0.7mV peak-to-peak noise.  Dividing by 5 produces a good estimate of RMS noise so about 140uV RMS noise.  My 200 MHz analog oscilloscopes under the same conditions have an accurately measured RMS noise of about 20uV.  A 7 times difference could be reasonable for a poor design.

One thing that is disappointing, though, is that Rigol don't seem to specify a noise value or figure in their datasheet. Not sure if it's standard practice to omit such a figure from the specs, but it certainly makes it harder to defend the performance of a product when they don't make a specific claim.

Any good marketing department knows not to advertise things which show their product in a poor light.  Rigol has a history of misrepresenting things.

Noise is an inevitability of using wide bandwidths, although my attempts to calculate a figure have given an unexpectedly high result. A plain old resistor, such as the termination resistor in the scope, of 1 megaohm at 20 degrees C produces 1.8mV RMS white noise when observing over a 200 MHz bandwidth. This is over 3 divisions on your scale, RMS, which would mean that your scope is performing better than a theoretically perfect scope, which is clearly wrong. I'm not sure how the 16pF capacitance of the frontend factors in, or whether I'm missing something else?

With an open input, the 16pF input capacitance has an overwhelming effect on the 1 megohm input resistance limiting the bandwidth to about 10 kHz.  With the input shorted, noise is dominated by the high impedance input buffer.

TLDR; I suspect you should watch the EEVBlog episode debunking the claim that digital scopes are noisier than analog. It does a good job of conveying that noise is an intrinsic part of life, not some design mistake that can be arbitrarily quashed. Clearly there is some sort of design imperfection in the Rigol scope, but still cruicially important to understand what a theoretically perfect scope looks like -- because it's not dead flat, ever.

Unfortunately Dave did not make any actual noise measurements although this would have been difficult on the analog oscilloscope he showed (Tektronix 2225?) because its input noise is about on the same level as its trace width.

There is a difference in how the noise is displayed which makes DSOs look worse but many DSOs are noisier than the analog oscilloscopes they replaced.

But i can't imagine a company would design a scope such that the highest couple of gain settings show up significant noise in their amplifier.

The high gain settings are 100% useful on more sensible timescales, when using acquisition averaging, or when using the bandwidth limit. So your expectation is unreasonable here.

Sample rate and time/div should have practically no effect on noise except changing the low frequency cutoff.

The highest gain settings are more useful when bandwidth limiting, averaging, or high resolution mode are used.  It was not uncommon for analog oscilloscopes to deliberately limit input bandwidth at higher vertical sensitivities.

[A Hellene]

- Everything that has been stated about EMI is more than valid.
- Even the suggestive hint about the FM radio band interference intercepted by the pick-up head being formed by the ~100nH inductor of the probe ground lead when it is attatched to the probe tip is also valid.
- Not to mention that I, myself, was really hesitant to scare our fellow EEVBlogger by my mention of a probable hardware (or poor design) fault of his electronic test equipment device.

BUT,

..what kind of everyday electromagnetic interference (meaning that we are not residing within the Van Allen Belts area!) can find its way through a BNC Shorting Cap, like the one depicted by the OP in the following picture?




-George