The following equation defines signal in noise and distortion (SINAD).
The SVT SINAD VI identifies the fundamental tone and removes the fundamental tone from the input signal to measure the power in the harmonics and noise. The SVT SINAD VI identifies the tone with the highest amplitude as the fundamental tone unless you wire a value to the expected fundamental frequency [Hz] terminal.
You can specify a frequency range to refine the measurement. The measured power in the total signal is distributed across the entire measurement bandwidth as a result of the noise in the measured signal. You can limit the measurement to a particular frequency range, exclude low frequency noise, or exclude high frequency noise by selecting an appropriate frequency range. For example, if you want to specifically exclude DC energy from the measurement, set the start frequency to at least 8(df), where df is the frequency resolution of the FFT that the SVT SINAD VI uses. If you see frequency components of interest below 8(df) that are not DC, reduce df by increasing the measurement duration. Use the following equation to determine the relationship between df and the measurement duration, where T is the measurement duration:
The following example shows how you can effectively remove DC energy from a measurement and still measure SINAD with low-frequency test signals. The following block diagram is a modified version of the SVXMPL_One Shot Gain and Distortion (Traditional DAQ) example VI, located in the labview\examples\Sound and Vibration\Audio Measurements directory.
The previous block diagram uses the same single-tone excitation signal and finite acquisition used in the example VI. However, the SVT SINAD VI replaces the SVT Gain VI and SVT THD and Harmonic Components VI that the example VI uses. The added frequency range control enables you to control the bandwidth of the SINAD measurement.
You can use the VIs in the previous block diagram to measure the SINAD for the left channel of a device like a two-channel audio equalizer. In the example, the left channel level settings are set to 0 dB for all available octave bands. The measurement result displayed in the following front panel has a 1 s integration time that results in a frequency resolution of 1 Hz. The start and stop frequencies are set to 20 Hz and 20 kHz, respectively. The tone frequency used in the test is 400 Hz. With these settings, the SINAD measurement includes the entire audio band and prevents DC energy from giving an inaccurately low result.
The reported SINAD for the left channel of the audio equalizer is SINAD=73.4 dB, 20 Hz–20 kHz, test amplitude 2 Vpk, test frequency 400 Hz, and unity gain.