Frequency Domain Measurement Fundamentals

Return to Fundamentals of High-Speed Digitizers

This tutorial recommends tips and techniques for using National Instruments high-speed digitizers to build the most effective data sampling system possible. In this tutorial, you will learn fundamental information about the underlying theory of sampling with a high-speed digitizer and various methods to optimize the performance of your data sampling. This section of the tutorial covers the topics below.

Flatness

Flatness, or passband flatness, is a term expressed in dB to specify the limits within which the amplitude of a signal varies across a given frequency range. Flatness is important when measuring signals that have a wide range of frequency components. As the frequency of any incoming waveform rises, the measured amplitude slowly falls toward the -3 dB cutoff point of the bandwidth, thus the higher frequency components of the signal are attenuated more than the lower frequency components, changing the overall signal. Flatness describes how well the analog front end maintains the amplitude of an incoming signal when it passes signals of different frequencies to the analog-to-digital converter (ADC). The front end is all components before the ADC, such as the amplifier and filter. A maximally flat front end passes all frequencies with the same amount of attenuation, so the measured signal looks like the input signal. Figure 1 represents a digitizer with excellent flatness. The board acquires essentially the same amplitude for all signals before 20 MHz, and only slight attenuation before 55 MHz, then rapidly drops down to the -3 dB point.


Figure 2 represents a digitizer with average flatness. Note that the measured signal loses amplitude as its frequency increases, shown by the downward slope of the line.

Spurious Free Dynamic Range (SFDR)

Spurious free dynamic range (SFDR) is the usable dynamic range before spurious noise interferes with or distorts the fundamental signal. SFDR is the measure of the ratio in amplitude between the fundamental signal and the largest harmonically or non-harmonically related spur from DC to the full Nyquist bandwidth (half the sampling rate). A spur is any frequency bin on a spectrum analyzer, or from a Fourier transform, of the analog signal. SFDR is expressed in dBc (with respect to the carrier frequency amplitude) or dBFS (with respect to the full scale of the ADC).

Signal to Noise Ratio (SNR)

Signal to Noise Ratio (SNR) is the ratio of the RMS of the input signal level to the RMS noise level, expressed in dB. The bandwidth of the noise level should be specified. The larger the number, the better you can differentiate your signal from the noise.

See Also:
Signal-to-Noise Ratio Explanation

Total Harmonic Distortion (THD)

The total harmonic distortion (THD) of a signal is the ratio of the sum of the powers of the first five harmonics above the measured fundamental frequency to the power of the fundamental frequency. THD can be expressed in dB, dBc (with respect to the carrier frequency amplitude), or percent. Measurements for calculating the THD are made at the output of a device under specified conditions. THD is defined by the following equation:
Where H is the amplitude of each harmonic and F is the amplitude of the fundamental frequency.

See Also:
Harmonic Distortion Analysis in LabVIEW 6i

Signal to Noise and Distortion (SINAD)

Also known as THD+N or signal to THD plus noise.

Signal-to-noise-and-distortion (SINAD) is the ratio of the RMS signal amplitude (set to 1 dB below full scale) to the RMS sum of all other spectral components, including the harmonics but excluding DC. SINAD is usually expressed in dB.

Comparison Between Instruments

When comparing one instrument to another it is not enough to say the SFDR, SINAD, THD, or SNR is a specific dB value. It is important when comparing specifications to know what the fundamental input frequency is when the specification was derived. For example, an SFDR specification derived from one product at 1 MHz cannot be compared to another SFDR specification derived from another product at 10 MHz.

In the case of SNR and SINAD, both of which are related to the noise level/floor, it is also important to know the specified bandwidth over which the specification was derived. For example, the NI 5122 SFDR is specified as -75 dBc at 10 MHz. However, when the input frequency is lowered to 1 MHz, we typically see the SFDR increase to -85 dBc or greater.
Related Links:
Fundamentals of High-Speed Digitizers
The Fundamentals of FFT-Based Signal Analysis and Measurement in LabVIEW and LabWindows/CVI
Reduce Errors Into Your High-Speed Data Acquisition Applications
Data Acquisition Specifications -- a Glossary

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