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    Scope buyers guide 'The Technical Perspective'



    This article aims to assist you in selecting a scope for diagnostic purposes by explaining how a digital scope works.







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    Digital Dual channel scope (AVL DISCOPE 200)



    ..Scope technology

    A Digital scope will divide the time line in equal sections (sampling rate). At the end of every time frame a (voltage) measurement will be made. The measurement heights (voltage levels) of each measurement will be displayed on a screen with the appropriate time increments in between.

    The displayed 'dots' are now connected by the scope software ('connect the dots'). Any voltage fluctuations between the 'dots' are not displayed by most scopes.
    Some scopes will still measure between the dots and display either an average or a maximum value. These scopes are rare and this function is usually added to compensate for the inability to measure and display fast.

    In the pattern below have I drawn black 'dots' over the existing scope pattern with time increments equal to a 12.5 kS/sec sampling rate. The dots are connected by a red line.

    2) The same pattern as in 1) but with a 12.5 kS/sec pattern overlay. Note how much is lost of the signal.

    In the following pattern have I left each second 'dot' out to simulate a 6.25 kS/sec sampling rate on the same signal:

    3) The same pattern as in 1) but now with a 6.25 kS/sec pattern overlay. Note how much is lost of the signal, it is totally now unusable.

    In the following pattern have I taken one more time each second dot out to simulate a 3.2kS/sec sampling rate. On the left of the pattern have I drawn a series of green dots indicating the actual sampling rate of the ATS scope at that time (50kS/sec). The ATS can be switched to 50MS/sec (50 Mega Samples / second). This effectively means that between each green dot the ATS can still measure and display 1000 dot's.

    Needless to say that the sampling rate determines the scope's accuracy

    Same pattern again, but now with the 50kS/sec ATS measuring plotting points and a 3.2kS/sec overlay. The 3.2kS/sec overlay does not look like anything. The same will happen with the 12.5kS/sec setting when the engine speed is double (6000 RPM)

    So why is this pattern not measured at the maximum speed setting?...



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