The oscilloscope’s frequency domain measurements

The oscilloscope’s frequency domain measurements translate a time-domain waveform with a FFT (fast Fourier transform) and measures the frequency domain’s distortion and noise characteristics. Frequency domain measurements offer magnitude and phase characteristics over frequency.

Utilizing the FFT to transform a signal quickly into its frequency components is powerful since it reveals the characteristics of signal that cannot be viewed in the time-domain.

Similar to horizontal- and vertical-axis measurements, the FFT’s accuracy can be enhanced by analyzing longer waveforms. Considering the calculation’s nature, the resolution is limited to half of the onboard processor’s resolution.

When a signal is converted to the frequency-domain, it allows the oscilloscope to perform five measurements, which assume that the input signal comes in the form of a perfect single-frequency sine wave, while the rest of the frequency components are noise or harmonics. Except for ENOB (bits), all are expressed in decibels relative to carrier (dBc). The only negative value is THD.

The Signal-to-Noise Ratio (SNR) refers to the ratio of the fundamental frequency’s RMS amplitude to all non-harmonic noise sources’ RMS amplitude. The first nine harmonics are not considered by SNR as noise. SNR is usually used when the narrow-band around the fundamental frequency is the only concern and the harmonics will not affect the system under test.

A concern when utilizing active components such as mixers and amplifiers, the Total Harmonic Distortion (THD) refers to the ratio of the total of the first nine harmonics’ RMS amplitude to the fundamental’s RMS amplitude.

The Spurious-Free Dynamic Range (SFDR) refers to the ratio of the fundamental’s RMS amplitude to the largest spurious signal’s RMS amplitude. The spurious signal can be noise or harmonic frequency component. SFDR is commonly utilized when there is a dominant spurious signal relevant to the other distortion and noise components.

Generally utilized in broad-band applications where all noise and harmonics affect the signal, the Signal-to-Noise and Distortion (SINAD) refers to the ratio of the fundamental’s RMS amplitude to the total of all distortion and noise sources’ RMS amplitude, which is equal to the sum of the THD and SNR.

SINAD can also be expressed in Effective Number of Bits (ENOB), which offers a way of measuring the input signal dynamic range as if the signal was converted using the ADC.

A high-speed Analog to Digital Converters’ (ADCs) specification and test procedures are usually expressed in terms of frequency domain measurements. Some oscilloscopes offer frequency measurements that can be utilized to imitate a more expensive spectrum analyzer to complete the tests. One of the often used tests is a multi-tone or two-tone distortion test. It is completed due to the intermodulation distortion produced by the ADC sampling of a signal comprised of two or more sine wave. When the FFT is created, measurements such as SINAD and THD can be utilized to characterize the ADC’s performance.