Siglent SDS1104X-E Quick Cal Question

[TomKatt]

Long time EEVblog viewer, new poster so pls be kind ;-)

Recently procured a Siglent SDS1104X-E to supplement my old reliable HP 180A CRO and couldn't help myself in immediately upgrading it to 200 MHz BW with all the other goodies (which was so simple to do one wonders if this isn't almost expected by Siglent?).  While compensating the probes I observed what I believe is some kind of calibration anomaly - Auto Set shows the Siglent calibration square wave as dead-on 1 KHz, but the P-P voltage is calculated as 3.10 volts and the channel data on the right of the screen also indicated a -1.48V offset...  Looking through the documentation I can find no spec listed for the calibration waveform, but I would 'assume' it should be 3.00 volts even.  Performing a Self Cal after the unit is warmed up does not change anything.  I also see that even in the user manual the calibration wave illustration shows a CH1 DC offset of -1.46 volts where I would expect an even 1.50 for the 3V square wave, so it seems that the manual example is likewise off from the expected 3V measurement.

Anyway...  that aside my actual question is what does the Quick Cal setting do?  The manual doesn't mention how that works or what it does, and as an ON/OFF setting rather than a 'RUN' type argument it seems to infer some ongoing operation...   I searched the forums here but likewise did not find anything that describes how the Quick Cal setting works.  Would someone be kind enough to describe the Quick-Cal setting and the best use of it vs Self Cal?

Thanks in advance!

edit - should have mentioned that my 3.10V measurement was after setting probe compensation at 10X to a proper, sharp square wave, so I do not believe the 3.10V measurement was a result of probe overshoot.

[bdunham7]

Quick Cal is in your manual on p. 181.  If the setting is enabled, it will perform an abbreviated Self Cal whenever the temperature changes by more than a few degrees to minimize offset drift in a non-temp-controlled environment.  It might do the procedure during instrument warmup as well, I'm not sure.  Neither quick or self calibration procedures have anything to so with the probe calibrator output and there's no expectation that you would see exactly 3 volts or 1.5 volts offset as vertical accuracy has nothing to do with probe compensation.

[TomKatt]

Thanks for the reply!

So, does that mean the scope has a built in thermal sensor that monitors the environmental temperature?  And wouldn't periodic calibration routines interfere with live measurements?  Strange feature.

I understand the calibration process would not directly affect the probe calibration signal, but it would affect the resulting measurement display if it was off.  My old CRO has two calibration voltage levels that are dead on - I guess I expected similar operation on the new fangled equipment.

[Fungus]

(which was so simple to do one wonders if this isn't almost expected by Siglent?).

Of course it is. Everybody would buy Rigols if they didn't allow it.

Looking through the documentation I can find no spec listed for the calibration waveform, but I would 'assume' it should be 3.00 volts even.

I wouldn't assume anything.

It's probably based on 3.3V internally (they're not going to put in a special voltage regulator just for that signal). What does your HP say?

[Fungus]

wouldn't periodic calibration routines interfere with live measurements?

Apparently it does: 

https://www.eevblog.com/forum/testgear/unlocking-siglent-sds1104x-e-step-by-step/msg3444072/#msg3444072

You don't have to turn it on though.

I wouldn't rely on an oscilloscope for voltage readings better than about 3-4% error. Look up the spec for vertical gain accuracy...

[bdunham7]

So, does that mean the scope has a built in thermal sensor that monitors the environmental temperature?  And wouldn't periodic calibration routines interfere with live measurements?  Strange feature.

Yes, but I think it is very, um...quick?  I've not noticed it interrupting anything and if it did, you can just deselect it.

I understand the calibration process would not directly affect the probe calibration signal, but it would affect the resulting measurement display if it was off.  My old CRO has two calibration voltage levels that are dead on - I guess I expected similar operation on the new fangled equipment.

The offsets that the self-cal procedures correct for are usually so small that you wouldn't notice them at any setting above 5mV/div, so on a 3 volt signal I'd expect no discernable difference.  If your old CRO had two calibration voltages then perhaps they intended additional uses for it, but on the Siglent it is only for probe compensation and the exact levels make no difference.  Offset and gain are done by the scope without external inputs.

[Fungus]

So, does that mean the scope has a built in thermal sensor that monitors the environmental temperature?  And wouldn't periodic calibration routines interfere with live measurements?  Strange feature.

Yes, but I think it is very, um...quick?

it sounds like you're only supposed to turn it on in places where ambient temperature goes up and down a lot.

I've not noticed it interrupting anything and if it did, you can just deselect it.

Maybe your ambient temperature isn't changing much so it doesn't ever do anything. 

[bdunham7]

Maybe your ambient temperature isn't changing much so it doesn't ever do anything. 

AFAIK it runs at power on and during the instrument warmup period as well.

[Fungus]

My old CRO has two calibration voltage levels that are dead on - I guess I expected similar operation on the new fangled equipment.

If your old CRO had two calibration voltages then perhaps they intended additional uses for it
[/quote]

CROs don't have self-cal so the signals had to be accurate so you can use them to check the vertical gain as well as compensate the probes.

[TomKatt]

The offsets that the self-cal procedures correct for are usually so small that you wouldn't notice them at any setting above 5mV/div, so on a 3 volt signal I'd expect no discernable difference.  If your old CRO had two calibration voltages then perhaps they intended additional uses for it, but on the Siglent it is only for probe compensation and the exact levels make no difference.  Offset and gain are done by the scope without external inputs.

I guess that makes sense - I come from the old CRO world and some habits (and assumptions) take time to break :-)  I know that scopes aren't really intended for vertical accuracy, but as I mentioned it struck me as odd compared to how my old CRO operates with 250mV + 10V calibration signals that are spot on.  Obviously voltage accuracy is not really required to adjust probe frequency compensation.

Thanks everyone for the feedback.

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[Performa01]

Yes, times have changed since the HP 180A.

The internal calibration source of a modern DSO like the Siglent SDS1000X-E is the reference for the offset DAC, which has far better resolution than just 8 bits.

The old (and partially outdated) review in this thread

https://www.eevblog.com/forum/testgear/siglent-sds1104x-e-in-depth-review/

demonstrates and discusses all the important basics of this remarkable little scope.

You can find a verification of the DC accuracy on page 51 of the document, which was better than 2% over the entire gain range (500 µV/div to 10 V/div) and better than 1% for 1 mV/div to 500 mV/div.

There's also a demonstration measuring 205 V DC to less than 0.08% error, making use of the very accurate channel offset.

Not everything what cannot exist in old obsolete CROs has to be a "weird feature".
In contrast to some much more expensive instruments, you can turn Quick Cal off in the SDS1000X-E - if you really think you cannot afford three seconds of interruption for an accurate offset during warmup. Once warmup is completed, you'll most likely never see this again, unless your room temperature changes by more than maybe 5 K.

[TomKatt]

Your review was actually one of the primary influences in my purchase decision!  Thanks for that extremely in-depth work!

[rf-loop]

...you can turn Quick Cal off in the SDS1000X-E - if you really think you cannot afford three seconds of interruption for an accurate offset during warmup. Once warmup is completed, you'll most likely never see this again, unless your room temperature changes by more than maybe 5 K.

I will add tiny bit more...

@TomKatt:
Example if want use slow roll mode inside first 30minute after power on, in some cases it is mandatory to turn Quick-Cal:OFF.
If Quick-Cal:ON it resets rolling every 15s if any active channel (1x) sensitivity is between 500uV - 2.0mV/div (of course same is 10x 5mV - 20mV/div.)
Same also if need do other long time acquisitions in this warming period or situations where temp change may trig Quick-Cal process. For avoid these acq.resets just Quick-Cal:OFF.
Fortunately and wisely, this has been done so that it can be shut down when need.


Cal Out: Only designed purpose for front panel Cal out is for adjust probe "LF compensation"  (adjusting square top/bottom as straight as possible. It is not designed for level cal or time cal etc as can be or is in some old timers.)  Also it is not at all suitable source for internal adjustments in every channel front end, it need external generator)

[TomKatt]

Just a quick follow up - I tested the calibration signal terminals with my Greenlee 820A (aka Brymen 827s) True RMS meter, which showed a voltage of 1.498 with a duty cycle 0f 50.02%.  That works out to 2.996V P-P.  My HP 180A CRO shows the Siglent calibration signal as pretty close to 3V even, perhaps just shy of the center of the division line (analog scopes rely on a bit of tongue at the right angle) , which seems to agree with the meter reading (and always has in the past).  I note the Siglent measurement occasionally flips back and forth between 3.10 and 3.08 (but never 3.09?)...

In fairness, none of my equipment is officially calibrated (other than the Siglent being new with factory Cal certificate), and my HP 180A is 55 years old (and still running like a champ - they don't make gear like that anymore ).  Also, the Siglent measurement of 3.10V is pretty much within it's advertised spec of +/- 3.0% if I average the 3.08 - 3.10 readings to 3.09V.  I also 'upgraded' the scope to 200 MHz BW (status now displays model as SDS1204X-E), which although I do not believe affects the internal calibration, I have no real idea what ramifications that modification may have.

Again, I understand that vertical accuracy is not necessarily the primary purpose of a scope as a tool, but I guess I expected "perfection" from a virtually brand new device pretty much out of the box, especially at lower voltage and frequency.  But, having fired up my old CRO for comparison purposes, I was instantly reminded how advanced technology has become    I'd be hard pressed to measure the multitude of parameters the Siglent offers with a simple button press on an old CRO.

Thanks again for all of the informative comments - I will say that Siglent's description of the Quick-Cal feature leaves much to be desired.

[bdunham7]

Also, the Siglent measurement of 3.10V is pretty much within it's advertised spec of +/- 3.0% if I average the 3.08 - 3.10 readings to 3.09V.  I also 'upgraded' the scope to 200 MHz BW (status now displays model as SDS1204X-E), which although I do not believe affects the internal calibration, I have no real idea what ramifications that modification may have.

Again, I understand that vertical accuracy is not necessarily the primary purpose of a scope as a tool, but I guess I expected "perfection" from a virtually brand new device pretty much out of the box, especially at lower voltage and frequency. 

Well first, you have to read all the specs for DC gain, not just the headline 3% figure.  However, what you are seeing is something else and interpreting your scope measurements takes some thinking.  Possible causes for unexpectedly high errors here include not using enough of the ADC range and not accounting for noise.

Try this: Just to be sure all the settings are correct, start by pressing the 'Default' button and then setting up the scope from there. Using one of the probes that came with the scope, set to 1X, connect CH1 to the calibrator output and set the scope for 500µs/div, 500mV/div and a CH1 offset (position) of -1.500V.  Perhaps that is what you are already doing.    Now select four measurements, Pk-Pk, Amplitude, StDev and RMS.  Note all four measurements, then from the CH1 menu, press the button under 'BW Full' so that it reads 'BW 20M'.  Note any changes.

I will say that Siglent's description of the Quick-Cal feature leaves much to be desired.

The days of detailed manuals hundreds of pages long explaining every technical detail of a complete schematic and parts list is loooooong gone!  But yeah, they could have written two paragraphs.

[Performa01]

Like the majority of digital oscilloscopes, the SDS1000X-E has an 8-bit acquisition system.

One division on the display happens to be equivalent to 25 LSB of the ADC.

So what vertical gain did you use to measure the 3 Vpp? It probably was at the optimum of 500 mV/div, then one LSB is equivalent to 20 mV. This would explain why you can see non-integrative measurements like peak-peak only in steps of 20 mV.

Why it measures 3.08 V or 3.10 V and not closer to 3.00 V (+/- 0.02 V), as suggested by your other measurements, that is another question. But we'll try to find an answer...

I pretty much expect that it would measure more accuratey with a direct coax connection or x1 probes.

Have you noticed that the measurement error on a x10 probe depends on the accuracy of both the probe and scope input impedance?
For most passive high impedance probes, accuracy of the input impedance is not specified. Your guess about the worst case error is as good as mine.
Then the input impedance of the scope itself. The specification says 2% - and that is the minimum additional error margin for such measurements.

Out of curiosity I've checked an old PP510 probe with the 12-bit SDS2000X HD. For this instrument, the input impedance accuracy is still only 2%, but at least the DC accuracy is specified as 0.5%, so it can easily compete with the average 3.5 digit DMM.

in x1 mode, we get 3.018 V amplitude; so the voltage of the calibrator output is only off by 0.6%. Even though its tolerance is not specified, this output is clearly produced by some 3.0 V logic circuit and modern voltage regulators are just accurate like that.

In x10 mode, we get 301.25 mV amplitude, so one might come to the wrong conclusion that the error is now even lower at only 0.417%. No, it is not, because the true calibration signal amplitude is not 3.00 V! Yet the error because of tolerances in the impedances of both probe and scope do not appear to be nearly as bad as expected.

Have a look at the attached screenshots for more information on this test.

In any case, this little experiment should tell us several things:

• For calculating the total error margin, we need to take the probes into account
• Passive high impedance probes often do not have any specifications that allow the estimation of the additional error margin, but the tolerance of the scope input impedance is already high enough to rule out the guarantee for precise measurements with any probe other than x1
• Measurements statistics are your friend whenever accuracy is important. Always use the statistic mean values over a number of measurements
• Most important: select the right measurement for the task! Peak-Peak is not going to give the right answer to the question about the amplitude of a square wave, because it will include overshots, ringing and noise

[TomKatt]

Point taken - I will test that out later this afternoon.

The 3.10 measurement was derived from an original default state with a compensated probe set to 10X connected to the calibration terminals and then pressing the AutoSet button, which set the scope to  500mV /d @ 200us /d.  Pressing Measure brought up the PP volt parameter at the bottom, which indicated 3.10 - 3.08.  The scope had been running in excess of 30 minutes prior to testing.  Basically, I followed the Probe Compensation procedure as described in the operation manual.

But I should test other amplitude and X1 settings to see if that influences the result.

Thanks both for the wisdom and insight.

Edit - although the SDS1104X-E may be among the less expensive scopes in it's class, I suspect that it is likely more accurate than any of the gear I own.  Which is what kind of surprised me and I will not be shocked to discover my interpretation was all PEBKAC.

Dbl Edit - I should probably also use the probe ground spring instead of the alligator clip "antenna"...

[bdunham7]

The 3.10 measurement was derived from an original default state with a compensated probe set to 10X connected to the calibration terminals and then pressing the AutoSet button, which set the scope to  500mV /d @ 200us /d.  Pressing Measure brought up the PP volt parameter at the bottom, which indicated 3.10 - 3.08.  The scope had been running in excess of 30 minutes prior to testing.  Basically, I followed the Probe Compensation procedure as described in the operation manual.
Edit - although the SDS1104X-E may be among the less expensive scopes in it's class, I suspect that it is likely more accurate than any of the gear I own. 

Using the 10X probe is OK except it does give you one more potential source of a scaled error and results in more overall noise.  But I suspect that the vast majority of your 'error' is simply that the Pk-Pk value includes overshoot plus any noise within a BW of 200MHz, along with the fact that the minimum resolution (ADC step) is 20mV.  This is why you don't see 3.09V.  There are 500,000 samples in each of those 1cm or 1div lines at the top and bottom of the square wave.  Try zooming in on one down to 5ns/div or so.  As far as accuracy it depends on what you are measuring.  For DC, your DMM will be leaps and bounds more accurate.  For duty cycle, pulsewidth, time, etc, the scope is virtually perfect for most applications.

I should probably also use the probe ground spring instead of the alligator clip "antenna"...

Not really, not for this.  In fact, when you are hooked to the calibrator, you don't need any ground connection at all.  But in general, no matter what you do noise will be with you always--so it is better to first learn how to deal with it using scope settings. 

[Performa01]

Even though noise shouldn't be a major issue at 50 mV/div (=500 mV/div @ x10), it still affects measurements once we get in the realm of <1% error.

My former test was in normal acquisition mode and with no bandwidth limit, i.e. the full 570 MHz -3 dB bandwidth of the SDS2504X HD.

Now I have repeated it with ERES 2.0 acquisition mode, which provides additional four bits of resolution and a 20 MHz bandwidth limit.

Lo and behold, the amplitude is now almost spot on at 3.00944 V! This is just 0.32% higher than the nominal 3 V, for which no accuracy specification exists. The peak-peak measurement at 3.027 V is still off by about 0.6% - but this is certainly very dependent on the quality of the state of the probe compensation.

In x10 mode, we can see a lower voltage again, hinting on a probe factor slightly less than 10. The measurement of the 299.614 mV amplitude hints on a probe error of about 0.44% and the peak-peak voltage is way off at 302.557 V, which corresponds to an error of almost 1%!

[bdunham7]

Even though noise shouldn't be a major issue at 50 mV/div (=500 mV/div @ x10), it still affects measurements once we get in the realm of <1% error.

After the OP has a chance to run his own test, I'll post a screenshot of me doing it on my scope.  At the settings discussed, a 0.1V error is 5 LSB combined on Pk-Pk readings.  Actual noise from all sources would only need to be 2-3 LSB peak to account for what he is seeing.  Also, it does look like Siglent has intentionally made the calibrator output pretty much exactly 0-3V.

[TomKatt]

Also, it does look like Siglent has intentionally made the calibrator output pretty much exactly 0-3V.

I always thought that's why they were called 'Calibration' terminals as opposed to 'Compensation' terminals...  It seems reasonable to assume (to me) that both amplitude and frequency would be calibrated.  I'd be surprised if any Tek or Keysight scope did not specify their calibration point parameters.

Edit - N/M, wrong again.  Apparently nobody specifies voltage any longer, they just specify frequency and 'approximate' voltage.  Older CRO documentation specifies amplitude, but modern gear is all listed as ~ (Tek 465 even species available calibration signal current as 30ma +/- 2% based on a +/- 1% 300mv amplitude at an approximate 1KHz frequency)

I'll take some screenshots of my tests later this afternoon for comparison.

Thanks again!

[tautech]

Also, it does look like Siglent has intentionally made the calibrator output pretty much exactly 0-3V.

I always thought that's why they were called 'Calibration' terminals as opposed to 'Compensation' terminals...  It seems reasonable to assume (to me) that both amplitude and frequency would be calibrated.  I'd be surprised if any Tek or Keysight scope did not specify their calibration point parameters.

Edit - N/M, wrong again.  Apparently nobody specifies voltage any longer, they just specify frequency and 'approximate' voltage.  Older CRO documentation specifies amplitude, but modern gear is all listed as ~ (Tek 465 even species available calibration signal current as 30ma +/- 2% based on a +/- 1% 300mv amplitude at an approximate 1KHz frequency)

I'll take some screenshots of my tests later this afternoon for comparison.

Thanks again!

While this may be so the old CRO probe compensation outputs were notoriously all over the place especially when these scopes had a bit of age on them. 
Without means for adjustment a probe compensation output can only be described as suitable for compensating probes and for a sanity check.
However for some of the ooooold Teks the output bar also served as calibration output for current probes for which they indeed were very useful however I'm unaware of any modern scope that provides a current probe calibration/check output and instead many now use fixtures:
https://siglentna.com/product/power-analysis-deskew-fixture/

As far as how measurements perform in the modern DSO it is certainly how you drive them and another data point for you to consider is my old SN#0012 SDS1104X-E, just cold booted, unknown when Self Cal was last run, Quick Cal = Off and using BNC Croc clip leads to the probe compensation terminal and some Stats running for good measure.

Now tell me how you might measure more precisely with a CRO considering the parallax error they can introduce ?

[TomKatt]

Now tell me how you might measure more precisely with a CRO considering the parallax error they can introduce ?

Please do not misunderstand me - I fully appreciate modern DSO scopes and believe their abilities far exceed CRO's in virtually every respect.  If anything, I guess that's why I got confused on this topic expecting a 3.00V measurement to display on the screen.   Believe me I know there is no "precision" to my CRO because everything is based on visual estimation.

Anyways....  I ran a few more tests yesterday and I am now convinced the comments about sampling and noise affecting the readings are correct and that the scope is reporting what it is seeing pretty accurately.  I see now that the Pk-Pk parameter is not the best one to convey the signal - when I looked at ALL the measurements I see that the Amplitude parameter displayed exactly 3.00V with 10X probe and 3.02V with the 1X probe.  And despite limiting the BW to 20MHz, I can see that the sensitivity of the scope is seeing noise that affects the measurements.

Thanks again for helping an old dog learn a few new tricks.

Edit - If the new Siglent only displayed waveforms like my CRO (no digital measurements), I would have looked at that signal and thought "yup - 3V / 1KHz square wave as expected" in my CRO mindset.  The actual measurement differing from that threw me off a bit.

[bdunham7]

I let my scope warm up for at least half an hour and then ran the Self-Cal.  This is what I got using only a 1X probe.  Comparing the pictures you can see ever so slightly more noise on the BW full setting.  I included StDev and RMS as another demonstration of how measurements on the scope may not mean exactly what you would think at first.

[TomKatt]

Forgot to mention that my results also allowed >30 min warmup followed by a Self Cal. 

My results seem virtually identical to yours.  Good to know.

Thanks for the follow up