PC Oscilloscope

Discussion in 'General Electronics Chat' started by CVMichael, Feb 24, 2010.

  1. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
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    I want to buy a PC Oscilloscope, and I found this one: USB BitScope 325, and I like it a lot, especially because it has 2 analog and 8 digital imputs, and that is very usefull to me becuase I mostly work with microcontrollers. The price is also in the acceptable range for me with this Oscilloscope.

    The only thing I don't like about it is that it only 100MHz, and I would like to measure signals up to 100MHz, therefore according to the Nyquist–Shannon sampling theorem, I would need double the frequency, i.e. 200MHz.

    But even worse... I looked up for PC Oscilloscope in YouTube, and I found this guy: http://www.youtube.com/watch?v=Ev121xAt_k4 who says that the BitScope can actually only measure up to 4MHz ? :confused: I don't quite get that....

    Anyways, I need to find a faster PC Oscilloscope with digital imputs also, and I can only find PC Oscilloscope with analog inputs... (at 200 MHz or higher)

    Can anyone help me with this please ?

    Thank You,
    Michael
     
  2. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
    416
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    No one has experience with PC Oscilloscopes to give me some insights ???
     
  3. ChrisChemist116

    Active Member

    Mar 13, 2009
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    1
    Hi, first of all maybe i am not the right person to answer this question as my expertise in electronics is not more sophisticated than a regular hobbyist.

    But i can tell you one thing. What you need to do is stay to the facts as provided by the manufacturer of the oscilloscope. There are many people out there in youtube providing information based on their own experience which maybe isn't so good.

    Technically speaking the guy in the video is telling the 4Mhz value is to the single shot acquisition that has Bitscope 100. However in the first row of his table it tells ABW which i believe means analog bandwidth which has 100 Mhz value written.
    I dont know if both have the same interpretation but i believe its different.

    I found explanation given here about that is useful, maybe it can be for you too: http://www.electronics-manufacturers.com/info/data-acquisition/digital-oscilloscope.html
    http://ecmweb.com/mag/electric_understanding_sample_rate/

    I also work with microcontrollers, but i dont need to use oscilloscopes (maybe i dont have enough credit yet to buy one). :)

    There are many electronic technicians out there and as well electronics engineers in this forum which may kindly assist you.

    I hope my comment may had been useful to you.
     
  4. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
    416
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    Hi ChrisChemist116, thanks for your reply, and the links were very usefull.

    I almost placed an order for the USB BitScope 325, but last minute I thought to ask them a question. I e-mailed them, but I did not get a reply yet. Also I e-mail them a few weeks ago with another question, and I did not get a reply.

    This is getting me worried... is there anyone there ??
    The total order is about $1000 (with the 2 analog probes, and the logic probe), and if I have any problems, then I won't get any support from them ?

    I don't know what to do... I like the BitScope, but if they are not responsive, then I don't want to place that order...
     
  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
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    One technique used in digital 'scopes to capture fast signals is to get just one sample following each trigger interval.

    Imagine looking at a 10MHz digital bus pattern. The (4MHz) scope cannot sample fast enough to grab the waveform cleanly in one go, so it just does one sample after each trigger cycle, but with each sample delayed an extra 10nS.

    As long as the waveform being sampled is repetitive and the scope trigger is getting a proper lock, it makes no difference if the 10ns, 20ns, 30ns, 40ns.... samples were all taken in one burst or on consecutive trigger cycles a few uS apart.

    The problem with that method is that if the signal is continuously changing, each sample is of a different thing and all you get is 'noise' as far as the display goes.

    What you buy is entirely up to you and what you want to do with the gear, but be aware of the difference in operation with the two different sampling speeds of any particular unit.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Hello CVMichael,
    The Nyquist-Shannon sampling theorem works great for a pure sinewave, but when you are looking at square waves, you need to keep in mind that a "perfect" square wave is comprised of the fundamental frequency, and all of the odd harmonics of the fundamental frequency. This implies in order to realize a perfect square wave, you would need unlimited bandwidth. But, reality sets in, and you have to compromise somewhere. That's where the /10 "rule of thumb" comes in.

    Take a look at the animation in this Wiki entry, about halfway down:
    http://en.wikipedia.org/wiki/Square_wave

    So, if you really need to look at a 100MHz square wave on a 'scope, you really need much higher than 200MHz bandwidth; more like 1GHz. Such 'scopes are mighty expensive.

    I picked up a couple of 60MHz analog scopes on an auction website a few years back. The old analog scopes are great for analog stuff; not so great for digital. Looking at a 50MHz crystal oscillator output gives me a display that looks much more like a sine wave than a square clock; that's due to the bandwidth limitations of the 'scope. You basically have to understand why the display isn't a perfect square wave in order to have a chance at interpreting what the circuit output signal really is.
     
  7. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
    416
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    But a square wave is digital, so I won't use the analog probe, I will use one of the logic channels to measure the signal, so then I will be able to measure higher frequencies, right ?

    Either way, as you said faster scopes are way expensive, so this means I will have to settle with the lower speeds, and clock down whatever I am testing, and after testing I will set the normal clock speed to the device.

    So I guess I don't really have a choice... this is as much as I can afford.

    But I am still undecided whether I should place that order because they still did not respond to my e-mails.
     
  8. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    I would recommend that you find some local electronic enthusiasts and get some experience with a variety of scopes before plunking down your money on one you've only read specs on. The reason is that you need some practical experience to guide your purchase -- and have realistic performance expectations.

    The "divide the bandwidth by 10" is a good rule of thumb for looking at a scope's capabilities. Even then, it may not be good enough for fast risetime signals if you're interested in timing details. Thus, a 100 MHz scope will let you do a pretty good job of looking at a variety of non-sinusoidal signals in the 1-10 MHz region. A lot of the interesting information in nature is non-sinusoidal. :p

    As SgtWookie said, if you understand the limitations, you can improve your understanding of a particular display. For example, I have a Phillips 25 MHz analog scope I bought in the 1970's and it is a good design. The 3 dB sine wave bandwidth is slightly conservative and is closer to 30 MHz than 25 MHz. Yet this scope triggers on and can make useful measurements on sine waves in excess of 60 MHz -- if one understands the limitations and characterizes the amplitude response. The two digital scopes I have (60 and 100 MHz) don't perform nearly as well beyond their bandwidths (well, I don't have a signal generator to properly test the 100 MHz scope, so I don't have comparative data for it).
     
  9. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Beyond 25Mhz Not only does your scope have to be up to it but also your connection.

    Poking around with a 'probe' is likely to produce highly misleading results.

    Beyond 100mHz you need special test jigs.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    I'd take Someonesdad up on his advice, and find a local electronics enthusiast group. Ham radio would be good to get involved in.

    I am not keen on the newer digital scopes.

    You still need lots of bandwidth to monitor digital signals. Adding a load to a digital signal line can "upset the apple cart" so to speak, and cause errors.
     
  11. BMorse

    Senior Member

    Sep 26, 2009
    2,675
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    check this one out, it is the one I use, upto 500Mhz....

    http://www.pctestinstruments.com/

    Customer support is also great, met some of the people at ESC in silicon valley last year......
     
  12. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
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    That one looks really good, but all inputs are digital, there are no analog iputs...
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    If you want to look at analog signals, get an old analog scope.

    If you want to look at digital signals, you could do far worse than BMorse' suggestion.
     
  14. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
    416
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    Hi BMorse,

    From what I see it does not come with any XKM grabbers. But the thing that is confusing me is that the Logic analyzer has 34 channels, but in the choises I have 20 or 38... don't I need 34 XKM grabbers, one for each channel ????
     
  15. BMorse

    Senior Member

    Sep 26, 2009
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    Actually mine came with 12 of the xkm grabbers, I did not buy this either I got it for free during the Microchip Design Challenge earlier last year (www.MyPic32.com).... so I don't actually know what all comes standard with it.....

    B. Morse
     
  16. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
    416
    17
    Just letting you know that I placed the order for LA1034 LOGICPORT LOGIC ANALYZER, so I should get it by the end of this week (hopefully). They did not send me the shipping & tracking information yet...

    I was thinking... the Logicport has 34 digital inputs, maybe I can "extend" the logic port by adding a small ADC and connect 8 of the input probes to the output of the ADC, and the input to the ADC to whatever I want to measure as analog...
    The only problem with that is that on the computer I will have to figure out how to convert the 8 bits to show the oscilloscope. I wonder if the software for the Logicport is open source ? (by looking at their web-site it seems it's not open source... :( )
    But still, worst case scenario, I can save watever I recorded in an CSV (I really hope it gives that option), then I can convert the bits to view the data in a graph... but that won't be "live"...

    Anyways.... Do you think it's a good idea ?

    Also, do you know of any ADC chips that can go up to 500MHz ? I am only familiar with ADC0808 wich goes up to 640KHz, quite far from what I would need in this case...
     
    Last edited: Mar 1, 2010
  17. BMorse

    Senior Member

    Sep 26, 2009
    2,675
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    You can actually download the software now and run it in demo mode (It will look for the device and automatically run in demo if it doesn't find it...) to see some of the samples they have, they have a sample on there where they were connected to an ADC..... pretty cool tool for debugging all sorts of communication protocols, I like how it has interpreters for I2C, SPI , RS232, etc.. It is really helpful when debugging a mcu based project to see if data being sent is right....

    B. Morse
     
  18. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
    416
    17
    Hi BMorse,

    I received it !! :D The fastest delivery ever, from Phoenix USA to Toronto Canada in less than 24 hours :)

    It took me a while to figure out the basics on how to even start getting any data.

    But there is one thing (problem) that I don't understand: It does not seem to work when I set the Sample Rate at 500MHz :confused: it works at any other sample rate, all the way up to 200MHz, but not 500MHz....

    I simply don't get any data at 500MHz, am I doing something wrong ? (please see screenshots attached)
     
  19. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Is your PC processor up to the task?
     
  20. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
    416
    17
    I have a dual core 3.2GHz, 2GB RAM, so I would think that's a "Yes"

    I even tried to overclock it to 3.5GHz, and I still did not get any results at 500MHz sampling rate
     
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