Suitable multimeter or oscilliscope for pulse waveforms

Discussion in 'Test & Measurement Forum' started by Bhope691, Aug 8, 2018 at 6:53 AM.

  1. Bhope691

    Thread Starter New Member

    Oct 24, 2016

    I am generating a pulse waveform which has a time period of between 150 - 250 us (it is not constant) and repeats every 4ms. I was trying to find a multimeter or oscilliscope that can give me the True RMS of this waveform but don't really know what I should be looking for. I have tried the ISO-TECH IDM66RT, the KEYSIGHT U1232A and the AMPROBE 37XR-A but all are giving different True RMS readings (For voltage and current). I believe it is because these are for Sinusoidal wave forms and are struggling with the pulse.

    Is there a suitable multimeter or oscilliscope that could give me a reliable True RMS reading for Voltage and Current and what would I need to look for in a multimeter?

  2. danadak

    Well-Known Member

    Mar 10, 2018
    Take a look at this -

    Look at meter specs of what CF it can handle.

    DSO scopes generally speaking will do accurate RMS measurments. You have
    to check their math capability specs to see if they can handle measurments like
    RMS. DS1054Z a popular scope that can do this.

    Regards, Dana.
    Wuerstchenhund likes this.
  3. Janis59

    Active Member

    Aug 21, 2017
    One channel? Two channel? Four channel? Standalone (expensiver)? PC extension (cheaper, and easily write a screenshots)?
    One channel, two channel, for channel versions ARM based on size of small phone about 100 USD chinese DSO in the ebay, if bit better then Chinese four channel devices of Hantek series about 200 USD.If really quality is thye goal, then Picoscope (picotech) series (those at 100 USD are rather weak but those in 4500 USD thick end are good indeed).

    1 Personally I have a Hantek 6254 (4ch, 300 MHz, 1Gs/s), Hantek 6204 (200 MHz), open source nano size one channel 50 MHz device (dont remember the exact name) and Picotech 6404D reserved for more serious tasks - 1 GHz 5Gs/s. If one is not trying to see something in WiFi frequency, its rather good gentleman complect.

    Of course, there are plethora of other firm products, but for amateur work these has a ultimate price against parameters ratio, however some heavyweight brands may be much more prestige.

    Contrary is if the aim is GHz technique, Then the choices are picoscope or Rhode&Schwarz, but then You`ll need to sell the family residence or better two of, to pay the bill :)
    Last edited: Aug 8, 2018 at 4:17 PM
  4. Janis59

    Active Member

    Aug 21, 2017
    RE: Danadak RE:""DSO scopes generally speaking will do accurate RMS measurments.""

    For fast and well reapeating signals yes, of course. But my this month most shocking revelation ios that for slow long non-repeatable signals happens the effect of "far Future make the hard impact on Past, that small change in future capitally changes the whole past.". And then - more than logic, the RMS value is changed along the way. I wrote the claim for this to the Hantek, but they play the strauss police, take head in sand and play the "bad wire between us" game. Thus I dedicate - sure guilty.
    However its not so drastic as sounds, my dataserie longs 1000 seconds and has at least few thousands of sharp microsecond scale peaks I want to look in-deep. In normal work regimes such defect is not observable.
  5. Wuerstchenhund


    Aug 31, 2017
    OK, well to realistically capture this signal you need around 60kHz BW which for a scope is nothing, however this may well exceed the capabilities of most multimeters (which isn't really the best tool for measuring pulse waveforms anyways).

    Forget multimeters, just use a scope. As to which one, well pretty much any DSO made by the big brands (HP/Agilent/Keysight, LeCroy, Rohde & Schwarz, Tektronix) will do, as should any from the better B-brands (Rigol, Siglent). Forget about Hantek or OWON, they are crap.

    A good choice would be a Rigol DS1054z or the Siglent SDS1000X-E Series, which both are pretty much the cheapest scopes that are decent instruments and not just a pile of dung. But without knowing your budget and requirements (you're hardly going to buy a scope just to measure this signal, or are you?) giving a recommendation is difficult.

    If it has to be cheap, look for a second hand HP 54645A or 54645D digital scope. They can often be found below $200 and still make nice beginners' scopes.
  6. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
    By definition, you really can't because it's not "continuously" periodic if that's how I interpret the above.

    You might be able to compute "energy" in units like Watt-seconds for a given time range. e.g. T11-T120 (11-120) seconds
  7. Wuerstchenhund


    Aug 31, 2017
    These aren't scopes, these are toys. If you think that these $100 ARM scope kits will give you a reliable test instrument then you're dreaming.

    Sorry but Hantek is crap. They are cheap but most of their scopes are outdated designs, they are slow, often come with miserably small sample memory sizes, and loads of bugs in their firmware (and they rarely get fixed).

    If you really can't afford to spend more than $200 then I'd rather go with a good second hand scope like the HP 54645A or 54645D ('A' is scope only, 'D' is the MSO variant with logic analyzer). It offers all relevant measurements, has a very high update rate and 1Mpts of memory which is the bottom end of what I'd recommend these days. With some luck $250 might even buy an Agilent 54622A/D which adds triggers for a few serial standards.

    PicoScopes are limited in BW (their realtime scopes go to 1GHz only) and samplerate (5GSa/s max), they lack active probe interfaces and the faster ones only have low impedance inputs. And you need to have a PC or laptop on your bench and operate the scope with keyboard and mouse.

    PicoScopes are great for what they do, we use a bunch of them in automated setups but even their biggest fans wouldn't recommend them as a replacement for a proper bench scope.

    It's not about prestige, it's about performance, reliability, and a support you can depend on.

    And there's a reason the majority of scopes out there are from Keysight (former Agilent which was former HP), and that many of the high end and ultra high end scopes come from LeCroy. Both offer the best performance in their class, and the support is top notch.

    For hobbyists the best options are the Rigol DS1054z (a 4ch scope for $400) and the Siglent SDS1000X-E Series. Both scopes offer serious performance and features that make them tools not toys, and they are way above anything made by Hantek without costing a lot more.

    Besides, there's still the 2nd hand market, with lots of opportunities to get a real high end scope for rock-bottom prices, if you know what to look out for.

    If you need a 1GHz scope then at the moment the LeCroy WaveSurfer 3104z and Keysight DSOX3104T are pretty much the best 8bit scopes in that class. There's also the Tektronix MDO Series but like most Tek scopes it's painfully slow and has a horrible user interface.

    If you need more than 1GHz but not more than 8GHz then the LeCroy WavePro HD and Keysight DSO-S are both the best choices, although the R&S RTO2000 or R&S RTP aren't bad scope either. Tek has the new MSO5 and MSO6 Series but it still suffers from typical Tek problems (horrible UI, slow/locks up when doing stuff), plus both are still full of bugs.

    If you need more than 8Ghz but then there's no R&S, and it's pretty much down to Keysight, LeCroy and Tektronix (although the Tek DPO70kSX can't keep up with LeCroy's and Keysight's offerings).

    If you need up to 100GHz then there's only LeCroy's LabMaster 10zi-A (100GHz with up to 40 channels) or since recently Keysight's UXR (110Ghz, up to 4 channels).

    If your RMS measurements are incorrect then there's something wrong with either your scope or your measurement setup. And in this case I doubt the problem is even with Hantek.

    You stated you have a Hantek 6204? According to the manufacturer that scope has a measly 64k sample buffer. Which means that at 1GSa/s it can only catch 64us until the buffer is full, and that means that it has to drop the sample rate dramatically at longer time bases.

    Using your 1000 seconds, 64kpts allows for a sample rate of 64 samples/s, which would be sufficient to sample a signal which has no frequency components beyond 30Hz. There's no way in hell you can capture a 1000s period and have this scope appropriately resolve "microsecond scale peaks" at the same time. Which should explain why your RMS measurements are off.

    There are ways to deal with this even on a low memory scope like your Hantek, i.e. multiple acuisitions. On a better scope there are even features like segmented memory which are made for such applications. But the main requirement is that the user not only knows his scope but actually understands the principle behind Nyquist sampling and its limitations.
    Last edited: Aug 10, 2018 at 4:38 AM
  8. Wuerstchenhund


    Aug 31, 2017
    Indeed, because of the pulse width variation it's not periodic.

    But that doesn't prevent him to measure momentaneous RMS, i.e. in a single acquisition. Using statistics he could even measure the RMS range based on the pulse width.

    He could also simply calculate the AC RMS or AC+DC RMS:

    (Image courtesy by Keysight Technologies)
    Last edited: Aug 9, 2018 at 8:57 AM
  9. Janis59

    Active Member

    Aug 21, 2017
    RE:""There's no way in hell you can capture a 1000s period and have this scope appropriately resolve "microsecond scale peaks" at the same time.""
    1) I never had even idea it ought be done. Always the screen resolution is sth fully OK, and if the higher resolution is needed then logically the sweep time must be shortened.
    2) And its sure that processes what happens in random moments and their time-span is only 1...30 microseconds once a few seconds are stably fixed on screenshots, like sharp form-less needles, thus means the hypothesis about 30Hz BW is fully wrong. I would be scared from Your text if not in the last week I would not done about few hundreds such screenshots successfuly - with satisfacting resolution. However You are right that "Zoom" functionality here is just purely dogs sh** - the clear sign of memory weakness, what never I have noted for Picotech machines.
    How far I understood from the Manual, those shocking short memory is just used intensively to flood the results to computer, where RAM restrictions are hardly less.
    And, the battery feed laptop bound system is only worth to discuss, because all in-build screen type needs a network cable. It means them are not capable to measure anything at kilovolt to hundred kilovolt above the earth. Just isolation transformer in such situations is bad help because of inherited capacitances.
    And last argument. Such high-risk obstacles are time by time leading to thunder via scope. Thats much better to have a minimally fast scope for 50 USD let it only be capable to make a task, not a powerful machine for 400 USD if one day may lead to few of it must be thrown in junk.
    But indeed, I have yet not opened it to revise the circuitry, thus I cannot comment HOW they get it.

    P.S. Thanks for idea about LeCroy. It sounds so much French that I didnt took it seriously. Mea culpa. When in future I shall have a larger project money once, I shall seriously meditate about to purchase similar.
  10. Wuerstchenhund


    Aug 31, 2017
    If you chose a (much) shorter timebase then yes, capturing microsecond peaks is possible, as with a shorter timebase the sampling rate will increase.

    The thing is math and physical facts don't care if you believe in them or not. And the fact remains that with only 64k your sample rate can't go any higher than 64 Samples/s if the period you want to capture is 1000s. This is simple math.

    And 64 Samples/s are sufficient only for signals that don't have more than 30 Hz in BW, because everything higher would violate Nyquist-Shannon.

    Which as I said works fine if you shorten the timebase to something very narrow.

    If your timebase however was set to something like 1000s then the captured signal won't have a lot to do with reality.

    I didn't say anything about zoom, and sample memory size is not just important for zoom. It's an important marker for the overall performance of your scope and a deciding factor as by how much your sample rate (and thus usable BW) drops in longer time base settings.

    This isn't physically possible. Your Hantek scope is an 8bit scope, which means when sampling at 1GSa/s it produces 1GB/s of data. But the interface is only USB 2.0, which even if it supports the fastest mode (HiSpeed) is limited to only 35MB/s (or in other words, the interface would become saturated at sampling speeds higher than 35MSa/s). And this doesn't even take into account any overhead.

    Well, many bench scopes support USB connection in addition to Ethernet (and some even have only USB), but you're right, a bench scope, unless battery powered (rare) will always be earth-bound, which is a problem is a big problem if you have to measure high voltage mains. That's one of the areas where USB scopes shine, especially as these applications are generally low-frequency anyways.

    Absolutely, and if you carry the scope around a lot there's always the danger of damage.

    But high voltage mains is a special case that has distinctive requirements from general EE, which usually focuses around a bench in a lab where either voltages are low or safety facilities are in place to protect the user.

    LeCroy (an American company) is often overlooked, even when they invented the modern digital scope and have been pretty much at the forefront of technology for most of its existence (some of the scopes they make you can't get anywhere else). Most people think "Tektronix" when they think about scopes because back in the days Tektronix made the best analog scopes you could buy. Unfortunately Tek never had a hand for digital scopes, and most of the DSOs they made were between 'mediocre' and 'pile of dung'.

    But if your application is high voltage mains then you won't find anything in LeCroy's portfolio, as they make lab scopes only. Instead, have a look at Keysight, they have great USB scopes (the existing U2701A/U2702A which come with 32Mpts of memory and the new Streamline series with 4Mpts and 5GSa/s) as well as very nice handheld scopes (U1600 Series).

    If Keysight doesn't fit your budget then have a look at Siglent, they have the SHS800 and SHS1000 Series of handheld scopes. The SHS1000 is classed at 1000V CAT II and 600V CAT III, and both sell at reasonable prices.
    Last edited: Aug 9, 2018 at 2:32 PM
  11. Janis59

    Active Member

    Aug 21, 2017
    Okay, thanks for idea about Siglent.
    Actually, I had to buy those Hantek because in commandment I forgot the scope home, some 1200km behind, thus I looked for something cheap and be delivered in the same day. So for this moment I cant say anything bad, except the absent customer service and small memory. But sure, next time when one my bag will stay in dark at garage I shall buy Siglent :)

    Actually, That commandment happened like horror movie. After driving the 3000 km until Germany in the middle highway the motor clapped. One year after timing belt change it cracked (probably hidden Chinese) thus all the valve group went dead. But inspite of my arrogance of not obtaining the ADAC card before the trip, the very God kept me by his strong wings - at 100 miles per hour I rolled by inertia about 5 kilometers, then about 3km slower down the hill and there was my saving place, the Rastplatz. So at least I slipped away from few thousand large penalty. There (with no less adventures) I managed to buy another car (this moment Germany has laid new anti-diesel law, thus all diesels became for just laughful prices) and trailer. To leave the stranded car at rastplatz means the cameras will tell em who is owner even if I screw off the number plates so I shall pay the five digit penalty unavoidable. Therefore I drove the new car, wagon and old car back to home, legalized the new in homeland numbers and with only one day too late was in that Poland lab where I must work two months for the current exploration project at collaboration agreement with my native University. Every man would be able forget the most important bag into his garage after driving about 10 000 km in the 7 days. I would not believe it is possible at all before I won.

    P.P.S. Just looked about those scopes price 1100 USD plus taxes 250 plus forwarding 150 - and for this money ONLY 100 MHz!!!. The same time four channel Hantek about 130 and no other payments, frequency 300 MHz. Im not sure I even can utilize the SHS1000 series for my job current task, if sometimes I need to build a slow process voltage and amperage curves by time and print them in good size, while to see on them rapid peaks how many time they occured. And sometimes get deeper in those peaks what has structure until sub-microsecond scale. This moment I just try to ""write"" a data movie, such option Hantek gives. And then hope that few peaks will happen on it (as shorter timebase as shorter the movie). When let that holy crap be viewed in slo-mo, then cach a Print Screen (sad those China boys not made a way to roll it manually, frame by frame). Thus, with pain in the a** but it is possible to get into computer those explorable waveforms of deep physics. Hope soon will happen laid out the article about it. Hope You will not be those Peer Reviewer of article saying that Hantek is bull**** therefore all my article is the same kind of :)
    Last edited: Aug 10, 2018 at 1:42 PM
  12. ebp

    Well-Known Member

    Feb 8, 2018
    While the fanbois do battle over brands, let's focus on the actual requirements.
    Can you tell us some more about the nature of the pulses:
    1. are they rectangular or some other shape?
    2. is the shape consistent?
    3. do you need to the RMS value based on a fixed pulse width or a sort of "average" RMS over a period where the pulse width is varying?
    4. is the peak amplitude known?
    What degree of precision and accuracy to you need.

    With the right set of circumstances, you can determine the RMS value by reading the DC average value with a cheap DMM and doing some simple arithmetic. Lots of circumstances would make this impossible, but for some it is quite easy.