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oscilloscope bandwidth question

Discussion in 'Electronic Basics' started by [email protected], Jan 10, 2006.

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  1. Guest

    Hi,
    I am looking at buying a cheap scope, thinking og the Velleman hps10
    or hps40.
    The specs for the hps40 says 40 MS/s sampling rate ans 12 MHz analogue
    bandwidth.
    What exactly does that mean?
    I though a 40 MS rate would give a 20 MHz BW, at least for periodical
    signals.
    I am a PIC novice and wanted the meter to monitor logic levels and
    serial data transmission, is that a bad idea?
    Thanks
    Soren
     
  2. Dan Hollands

    Dan Hollands Guest

    Show be fine for your application

    20MHz is the theoretical bandwidth but the 12 MHz is more realistic for
    seeing something you recognize
     
  3. Guest

    Thanks for that!

    Cheers
    Soren
     
  4. Phil Allison

    Phil Allison Guest

    <

    ** The spec in the owners manual ( Velleman site ) actually says:

    " Maximum sampling rate: 40 MS/s "

    Means the sampling rate depends on the time base setting.

    This is quite unlike analog scopes where the vertical bandwidth is *
    independent * of the time base setting.

    At low speeds for viewing low frequency waveforms, the sampling rate of the
    Velleman scope may be only a few thousand per second - which in effect
    reduces the vertical bandwidth to a few kHz.

    Then there are all the problems with aliasing.




    ........ Phil
     
  5. That is only to avoid aliasing on the ADC.
    The sample rate really has nothing to do with sampling repetitive
    signals on a Digital scope like this.
    A DSO could have a 40MS/s sample rate but have a 1GHz bandwidth. It
    does this by sampling the signal over many many cycles, and this is
    what the Velleman might do at high bandwidths. This is known as
    "repetitive sampling" mode.
    This mode is useless for "single shot" applications like capturing the
    serial data transmission as you want. In this case you want "real-time"
    single shot mode. In the case of the Velleman at maximum sample rate it
    will take 40MS/s. If you viewed a 10MHz waveform in this mode you would
    get *4* sample points on the screen which is obviously fairly useless
    to you. As a rule of thumb, in real-time mode you'll want at least 10
    samples per cycle to get an idea of what your waveform is like. That
    means your 40MS/s 12MHz bandwidth scope will have a useful "real-time"
    bandwidth of 4MHz displaying 10 point per cycle. I'd personally round
    it down to about 1MHz.

    Also, the sample memory in the Velleman is likely to be small (256-512
    samples maybe?). This doesn't give you much detail in your single shot
    waveform capture.

    The 12MHz quoted is the bandwidth of the analog input amplifiers, it
    has nothing to do with the sample rate.
    It should be OK for that, so long as you don't want to see more than
    say a dozen bytes of serial data at once. To see more you need a DSO
    with a much bigger sample memory.

    Dave :)
     
  6. Guest

    You will do much better looking on ebay, loads of old tek scopes go for
    peanuts. You'll also end up wanting a logic analiser at some point or
    at least storage of some sort.
     
  7. Yukio YANO

    Yukio YANO Guest

    It would seem to me that a soundcard oscilloscope would be suitable for
    your application. The price would vary from 0/~100 dollars. Google
    "Sound Card oscilloscope" to download suitable software and interface
    hardware. BW (bandwidth) is not a real problem in your application.

    Yukio YANO
     
  8. redbelly

    redbelly Guest

    I'll just add, for the benefit of the OP, that the "analog bandwidth"
    is the frequency at which a sine wave is reduced to about 70% of its
    actual amplitude. (More precisely the factor is the square root of 1/2)

    Mark
     
  9. Guest

    Thanks for that to you all!
    I had newer heard of repetitive sampling mode.
    In the mean time i bought the oscilloscope (almost ½ price in the US
    cmp. to Oz) and for the price and aplication i am happy with it. One
    thing that does puzzle my is why they did not put a little more memory
    in. as you say David, the stoage is a few hundred points. It seems
    like a cheap feature to build in a little extra mem and greatly amplify
    the use of the scope, but i guess Velleman has their resons.

    Thanks!
     
  10. The reason for the lack of memory is most likely a combination of
    several factors:
    1) They are building to a low price point, so a few dollars extra
    manufacturing cost here and there can make a big difference to the
    final resale price.

    2) The screen is only small, so it kinda makes sense just to have
    enough memory to fill the resolution of the screen. For the intended
    target market (service people who just want to see a basic waveform on
    the screen) that is probably good enough. It also means you don't have
    to develop the pan & zoom software functionality required, thus getting
    them to market quicker etc.

    3) The design architecture is probably based on a low end FPGA and they
    would have used the internal SRAM to store the data, eliminating the
    need for high speed external memory. Most low end FPGA's have very
    limited SRAM.

    Dave :)
     
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