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Which oscilloscope and/or logic analyser?

Discussion in 'General Electronics Chat' started by earckens, Aug 31, 2016.

  1. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    What do you mean for 'all round use'? - to take that literally there's no actually such thing.

    For 'all round use' pretty well any 'proper' scope, either analogue or digital, would be perfectly suitable - but no matter what the specification there's going to be the odd occasion where it's not suitable.

    But as I've said, higher spec scopes are only slightly more useful than far cheaper ones.
     
  2. spec

    spec Well-Known Member Most Helpful Member

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    OK :)

    spec
     
  3. case-sensitive

    case-sensitive Member

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    in analogue audio, if the amplitude halves every octave, say, you need some decent signal at several MHz to become audible below 20kHz. There are some filters and this and that at the amp input, I played around a bit, they act as a lowpass.
    Reg specs, I have the sneaky suspicion, there is just 1 circuit for all and then a few parts involved to get different ratings. There was a table at some supplier with say 20, 50 and 70 MHz scopes have same sampling rate and then the 100MHz has double that. Bit like in the days the Yamaha/Traxdata 2 speed burner, remove a resistor and its 4 speed. Or some recent Flir IR camera, firmware hack and you get 4 times as many pixels as the sensor was the same.
     
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  4. dave

    Dave New Member

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  5. spec

    spec Well-Known Member Most Helpful Member

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    I'm not sure what you are saying but I assume that a scope would be used for fault-finding/design proving rather than just looking at nice sine waves up to 20KHz produced by perfectly working audio amplifiers.

    You obviously cannot hear anything over 20Khz- for most people with good hearing it is around 12Khz to 15KHz- but you can hear the difference between an amplifier with a response flat to 30KHz and an amplifier that is just flat to 15KHz.

    Of course, you cant hear anything over 20Khz, but you can hear the effects of high frequencies. For example an amplifier may be oscillating at a high frequency at a low amplitude. This would not be apparent on a scope with a low frequency response- the amplifier output would just look like DC. You can get amplifiers that burst into oscillations at certain parts of an audio frequency sine wave. You will not hear this directly but you will hear the effects- normally the amp does not sound right but you can't quite put your finger on it.

    You can also get amplifiers picking up an electromagnetic RF carrier.

    Certain amplifier/speaker combinations will cause spurious oscillations which would not be visible on an LF scope. Exposure amplifiers were particularly vulnerable at one time.

    I have seen many amps that keep blowing output transistors and drivers and this has been traced to frequency instability- in one case the cause was a 47p capacitor that had a dry joint.

    On a guitar amplifier, later high frequency 2N3055 transistors had been fitted as a repair and every few months the transistors would blow. That was traced to HF oscillations under certain conditions.

    And so on...

    And there is much more to electronics than just audio amplifiers.:)

    spec
     
    Last edited: Jan 26, 2017
  6. spec

    spec Well-Known Member Most Helpful Member

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    Just another point, I worked for a big company that had a huge range of scopes that we could sign out and, as I have implied previously, provided the scope was fundamentally sound the biggest advantage was to have a wide bandwidth. Just ask this question, why are wide band scopes made and why are people prepared to pay such a high price for them.:)

    spec
     
  7. spec

    spec Well-Known Member Most Helpful Member

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    Just a little anecdote, on another forum someone was complaining that their scope was displaying an awfully distorted TTL square wave. After much discussion it was decided that adjusting the probe compensation was the answer, because this produced a much better looking square wave and he was happy.

    Until it was pointed out that all he was doing was beautifying the appearance on the scope screen, but the severe ground and top bounce was real. It was later discovered that the layout was the problem. The ground bounce was in fact so large that it was in danger of multiple clocking.

    A low frequency scope would have the same effect of masking the core problem.

    spec
     
  8. ChrisP58

    ChrisP58 Well-Known Member

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    spec. I agree wholeheartedly with the points you make in your last few posts.

    As a diagnostic tool, an o'scopes value is showing you, not what you expect to see at it's fundamental frequency, but what shouldn't be there, at whatever frequency it manifests itself.

    And the higher the bandwidth, the more problems that you'll be able to see. But if you can't see them, you can't fix them.

    But everyone needs to decide what their budget allows. If you can't afford a high power microscope, then start with a hand held magnifying glass. It will still show you more than you would see with the naked eye.
     
  9. case-sensitive

    case-sensitive Member

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    1st iteration with my amps they made some sort of whale song, started screetching, went louder, than stopped and started again. This was just a small capacitor in the feedback loop, changing it from foil to tantalum stopped it.
    I guess so, some crazy feedback would overload the amp, those whale songs actualle went quite loud, could easily blow up the driver. Especially when you have an active amp, where in an ideal world each driver gets its own portion, but the amp decides to put way off frequencies in.
     
  10. spec

    spec Well-Known Member Most Helpful Member

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    Hi Chris,
    And I agree whole wholeheartedly with your statement.:)
    In fact when I started in electronics a scope was way beyond my means and even the most humble scope would have been a treasure (I spent 30 years making my own scopes). A digital storage scope, whatever bandwidth, for a few bucks would have seemed like science fiction.:wideyed:

    I then bought a faulty 20MHz Gould scope for £5UK and that was a treasure.

    But, if I have it right, this thread is about the important characteristics of a scope not so much about what you can afford. My feeling is that it is best to decide what the important characteristics of an item are, not just scopes, so that you can buy the best item, for your purposes, that your budget allows. All I am saying is that the scope bandwidth is one of the most important factors.

    For example, when I recently bought a scope, for the same money, I could have got a scope with all sorts of bells and whistles but a medium bandwidth, but instead I went for a simple dual beam scope with a higher bandwidth.

    spec
     
  11. spec

    spec Well-Known Member Most Helpful Member

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    :D The most difficult thing when building any sort of feedback system is frequency stability, audio amplifiers especially, but also humble stabilized power supplies, even three terminal regulators.

    Not only can you get frequency instability in the main amplifier loop, but you can often get frequency instability in the sub amplifiers, for example each half of the output stage, especially with a Szikalai configuration.

    And the VBE multiplier, common in most class AB audio amplifiers, can get up to all sorts of antics.

    Then there is the voltage amplifier stage (VAS), which experiences huge voltage swings that alter the transistor characteristics, especially frequency response, throughout the voltage range.

    I will stop now- afraid audio amps, power supplies and scopes are are all drugs with me.:)

    spec
     
    Last edited: Jan 26, 2017
  12. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Why do they make cars that do 200mph when the speed limit is 70 :D

    They make wideband scopes to cope with the rare occasions they might be needed - and certainly in some industries they are needed much of the time. But in the VAST majority of cases, and certainly for hobby use, a much lower spec is all you're ever likely to need.
     
  13. spec

    spec Well-Known Member Most Helpful Member

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    I would not argue with that Nigel. :)

    spec
     
  14. dssence

    dssence New Member

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    Hello my friends I'm from Argentina, I'm mostly doing chip level repairs, and also some embedded projects. I'm about to buy an oscilloscope. I've been struggling getting an affordable 100mhz 4 channels scope but there's many crappy chinese on the market.
    Sadly in my country I can only afford buying the following. Yes I know you may start telling about other brands.. but I can only find the followings

    - UNI-T UPO2104CS Supposedly has Ultra phosphor technology but support on the site it's crap no manual no youtube reviews..

    - OWON Latest model XDS3104E has a local distributor in my country covering warranty if anything happens. So this is my way to go.

    But I would like to ask you what do you think about the following this scope has

    an 8-bit ADC , and the advertise as having multi- gradient it's a way of saying they have some ultra phosphor tecnology .

    http://owontme.com/products_owon_xds3000-e_series_4ch_12|14bit_touchscreen_digital_oscilloscope

    Also bus decoding its optional on the Owon and on UNI-T it's being implemented although unit is ****

    This is what I'm talking about when I mean about intensity gradient

    [​IMG]

    The guy from OWON support answered on email the following

    This is the video the guy sent where he " SUPPOSEDLY" shows this intensity gradient or ultra phosphor , do you guys see the intensity grading ? I don't I just see red solid waveform

    https://drive.google.com/file/d/0B7WSDcV1o91TXzcyNm5nUUItaEMtWVZDRzl1Z2RpQWZscTUw/view?usp=sharing
     

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