What is the best way to measure frequency with a digital oscilloscope?
Digital Storage Oscilloscopes (DSOs) or Digital Real-Time oscilloscopes (DRTs) all measure frequency by calculating it from the waveform period (1/period = frequency). The period is found by determining the mid-point between the maximum and minimum waveform values, and measuring the time from the first mid-point in the waveform data to the third mid-point.
All digital oscilloscopes have a horizontal resolution limit. The limit is determined by the maximum record length available. The record length is the number of horizontal points in the digitized waveform. One over the record length (1/record length) is the resolution limit for period and frequency measurements.
To make the most accurate period and frequency measurement, make sure that you are using all the resolution available. Use the longest possible record length, then select the horizontal scale that uses the most record length for the period and frequency measurements. Use signal averaging or HiRes mode to remove noise and use the greatest amplitude possible. This helps the oscilloscope accurately determine the mid-point values and positions, and the best waveform period and frequency measurements.
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