Meteorological Instrument Performance Characteristics

Dynamic Response » Response to Specific Input Functions » Consequences for Making Measurements at High Frequency

If good time response is needed for a particular measurement, much attention must be devoted to minimizing the effects of time lags and phase shifts. Time lags introduced by delays in signal lines and instrument components are often difficult to minimize. Another concern when sampling time-series measurements is choosing an appropriate sample rate. If a system samples at a frequency f, it is not possible to detect sine-wave components with a frequency faster than f/2, called the Nyquist frequency.

A graph showing aliasing of an f=0.9 sine wave by an f=0.1 sine wave by sampling at a period of T=1.0.

The black circles on the figure indicate the frequency of sampling, f. The signal (red line) is varying at a faster rate and can’t be fully captured. Image from Creative Commons.

Furthermore, higher-frequency components can be “aliased” to appear as contaminating contributions at lower resolved frequencies. To avoid this contamination, it is best to remove components above the Nyquist frequency by filtering (using filters with better cutoff characteristics than the dynamic systems illustrated in this lesson). General guidance is to sample fast enough to give a Nyquist frequency significantly above the highest frequency of interest and then filter at or below the sample frequency to eliminate higher-frequency components that might influence the resolved frequency range. Study of the frequency content of signals often is done using spectral analysis, either with appropriate equipment or numerically. This topic is beyond the scope of this lesson, but in some studies it can be crucial to understanding the spectral content of measurements.