Horizontal-axis measurements of an oscilloscope consist of analyzing the applied signal’s horizontal time axis and include measurements such as Rise Time, Frequency and Period. Although the result is usually expressed in time, it can also be in radians, ratio or in Hertz.
The resolution of the horizontal-axis is restricted by the onboard clock’s sample rate, and just like the vertical axis, the accuracy of the horizontal axis can be reduced by low- and high-frequency errors.
High-frequency errors are comprised of phase noise or clock jitter. These errors, however, are minute when compared to the errors caused by the clocks normally used in most oscilloscopes. Incorrect readings in horizontal-axis measurements are often attributed to phase-noise or clock jitter, when it is actually the lack of vertical-axis noise or accuracy that is causing the problem.
Low-frequency errors consist of drift associated with aging, temperature and others. Annual factory calibrations must be done to ensure the clock’s accuracy over a long period of time.
Most horizontal-axis measurements are straight forward. The Positive Pulse Width determines the period between the first rising edge and the first falling edge, while the Negative Pulse Width function does the exact opposite. The Period determines the average time for a cycle to complete utilizing the waveform in the capture window. The Frequency, which is the inverse of the period, is expressed in Hertz. The Positive and Negative Duty Cycles are determined by taking the ratio of their respective Pulse Width to the Period. The Time of Minimum and Time of Maximum functions allow users to retrieve the time of the first minimum and maximum levels of measurements.
Edge Measurements, which is a subset of horizontal-axis measurements, are made in relation to the user-selectable Reference Low (REF LOW), Reference Middle (REF MID), and Reference High (REF HIGH). Although REF LOW, REF MID and REF HIGH are set by default at 10 percent, 50 percent and 90 percent Amplitude, respectively, they can still be adjusted to meet the needs of the applications or input with respect to absolute voltages.
When taking edge measurements, users may encounter problems such as inaccurate crossings caused by noise on the vertical axis. The noisy signal generates a shorter than expected Rise Time Measurement and false crossings. These problems can be avoided by averaging and oversampling or by utilizing the Smooth function prior to making the measurements.
Just like the distinction made between relative and absolute voltage measurements in the vertical-axis segment, there are relative and absolute time measurements also. The Period of a waveform, for instance, compares two points on the similar waveform. Thus, users need not relate it to absolute time or real-world since it is considered a relative time measurement.
Absolute time is important when measuring the Time of Maximum, which determines the first maximum voltage level in association with the start of the acquisition.
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