The Basics of Making Accurate Measurements

For accurate measurements, you should use the right settings when acquiring data with an oscilloscope. Knowing the characteristics of the signal in consideration helps you to choose the correct settings. Such characteristics include the following:

1. Find out the peak-to-peak value: This parameter, in units of volts, reflects the maximum change in signal voltage. If V is the signal voltage at any given time, then V pk-to-pk = V max -V min. The peak-to-peak value affects the vertical sensitivity or gain of the input amplifier. If you do not know the peak-to-peak value, start with the smallest gain (maximum input range), and increase it until the waveform is digitized using the maximum dynamic range without clipping the signal. Figure B-7 shows that a gain of 5 is the best setting to digitize a 300 mV, 1 MHz sine wave without clipping the signal.


2. Be aware of source impedance: Most digitizers and digital storage oscilloscopes (DSOs) have a 1 Mega Ohm input resistance in the passband. If the source impedance is large, the signal will be attenuated at the amplifier input and the measurement will be inaccurate. If the source impedance is unknown but suspected to be high, change the attenuation ratio on your probe and acquire data. In addition to the input resistance, all digitizers, DSOs, and probes present some input capacitance in parallel with the resistance. This capacitance can interfere with your measurement in much the same way as the resistance does.

3. Consider input frequency: If your sample rate is less than twice the highest frequency component at the input, the frequency components above half your sample rate will alias in the passband at lower frequencies, indistinguishable from other frequencies in the passband. If the signal’s highest frequency is unknown, you should start with the digitizer’s maximum sample rate to prevent aliasing and reduce the digitizer’s sample rate until the display shows either enough cycles of the waveform or the information you need.

4. Consider the general signal shape: Some signals are easy to capture by ordinary triggering methods. A few iterations on the trigger level finally render a steady display. This method works for sinusoidal, triangular, square, and saw tooth waves. Some of the more elusive waveforms, such as irregular pulse trains, runt pulses, and transients, may be more difficult to capture. Figure B-8 shows an example of a difficult pulse-train trigger.


Ideally, the trigger event should occur at condition one, but sometimes the instrument may trigger on condition two because the signal crosses the trigger level. You can solve this problem without using complicated signal processing techniques by using trigger hold-off, which lets you specify a time from the trigger event to ignore additional triggers that fall within that time. With an appropriate hold-off value, the waveform in Figure B-8 can be properly captured by discarding conditions two and four.

5. Consider input coupling: For most oscilloscopes, you can configure the input channels to be DC coupled or AC coupled. DC coupling allows DC and low-frequency components of a signal to pass through without attenuation. In contrast, AC coupling removes DC offsets and attenuates low frequency components of a signal. This feature can be exploited to zoom in on AC signals with large DC offsets, such as switching noise on a 12 V power supply.

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