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In this way, when the meter added 40 dB of gain, the oscilloscope’s voltage probe menu was set to 1/100, allowing the oscilloscope’s automated measurement system to reflect the signal at the input of the meter.
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The meter’s range switch had 10 dB steps, but was switched in 20 dB (×10) steps because this allowed the probe calibration menu on the oscilloscope to be adjusted by a matching factor of 10. This test set-up exploits the digital oscilloscope’s averaging function to measure synchronous signals below the AC voltmeter’s self-noise. As an example, mains hum buried in noise can easily be detected and measured by triggering the oscilloscope from AC line, then invoking averaging.Ī crosstalk test might inject a square wave into one audio channel from a signal generator, then look for crosstalk on the power supply or another audio channel, and trigger the oscilloscope from the generator’s trigger output.įigure 4.29. The reduction in noise is proportional to √ n so averaging over 512 traces reduces noise by 27 dB, but at the expense of increasing the time required for the display to stabilise.Īveraging is a very powerful method of extracting signals seemingly buried in noise, but when the repetitive element is obscured by noise, external triggering is required. Averaging is typically adjusted in binary logarithmic steps (2, 4, 8, etc.), perhaps from two to 512 traces. Provided that the oscilloscope is triggered from the repetitive signal, averaging across traces (usually termed waveforms) retains the repetitive element but random noise averages towards zero. The oscilloscope displays a repetitive waveform, so each sweep across the screen draws a trace very similar to the one before it. Morgan Jones, in Building Valve Amplifiers (Second Edition), 2014 Averaging and noiseīecause a digital oscilloscope effectively stores the data resulting from each sweep as a data base, it can explicitly perform mathematical calculations such as averaging. Measurement accuracy depends on how accurately the distance between two cycles is read, and it is difficult to reduce the error level below ±5% of the reading. The cycle time is therefore 25 ms, and hence the frequency is 1000/25 (i.e., 40 Hz). For example, suppose the distance between two cycles is 2.5 divisions when the internal time base is set at 10 ms/div. Calculation of the unknown frequency from this measured time interval is relatively simple. Measurement accuracy by this method is limited, but it can be optimized by measuring between points in the cycle where the slope of the waveform is steep, generally where it is crossing through from the negative to the positive part of the cycle. First, the internal time base can be adjusted until the distance between two successive cycles of the measured signal can be read against the calibrated graticule on the screen. When this direct facility is unavailable (in some digital oscilloscopes and all analog ones), two alternative ways of using the instrument to measure frequency are available. Many digital oscilloscopes (particularly the more expensive ones) have a push button on the front panel that causes the instrument to compute and display the frequency of the input signal automatically as a numeric value. Morris, Reza Langari, in Measurement and Instrumentation (Third Edition), 2021 6.8.3 Measurement using an oscilloscope