Nyquist Sampling Rate
To digitise an analogue avionics signal without aliasing, the sampling rate must be at least twice the highest frequency present.
Formula
Variables
Example
Audio to 3.4 kHz: f_s ≥ 6.8 kHz (voice comms sample at 8 kHz)
How it works
When an analogue signal is turned into digital samples, the samples must be taken fast enough to capture the fastest change in the waveform. If the sampling rate is below twice the highest frequency present, higher-frequency content can masquerade as lower frequencies in the digital signal, a problem called aliasing. The Nyquist limit is the minimum safe rate that prevents that overlap for an ideal band-limited signal.
When to use it
Use this when deciding how fast an avionics data converter or recorder must sample a sensor, comms, or navigation signal before digitising it. It is also used when checking whether an existing sampling system is fast enough to avoid aliasing in analysis or signal processing.
Worked example, step by step
A vibration sensor on a turbine mount contains frequency components up to 12 kHz. Given: f_max = 12 kHz Nyquist requirement: f_s ≥ 2 f_max Substitute: f_s ≥ 2 × 12 kHz Calculate: f_s ≥ 24 kHz So the sampling rate must be at least 24 kHz; in practice, a higher rate may be chosen to allow for filter roll-off and extra margin.
Common mistakes to avoid
- Using the average frequency instead of the highest frequency present
- Forgetting that the signal must also be properly low-pass filtered before sampling
- Choosing a sampling rate exactly at the limit without allowing margin for real-world filter imperfections
Did You Know?
Digital flight-data recorders and fly-by-wire buses sample well above Nyquist and add anti-alias filters, so high-frequency noise can never masquerade as a valid signal.
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