Design and implementation of a fast logarithmic stepped sine for a fixed sample rate digital network analyser

A method is presented for measuring the transfer function and the harmonic distortion with a very accurate twelfth-octave stepped sine analysis between 20 Hz and 20 kHz using a digital fast-Fourier-transform- (FFT)- based analyzer. One of the major concerns in developing the algorithm was to obtain maximum measurement speed while maintaining high accuracy. The method is based on the use of interpolated FFT algorithm. This makes it possible to reduce the frequency error relative to the perfect twelfth octave frequency to less than 0.1% while keeping the needed amount of measured data quite low, thus considerably reducing the measurement time. Despite some small problems with the calibration of the channel, the accuracy is better than ±0.0004 in amplitude and 0.01° in phase in the band from 20 Hz to 2 kHz. In the 2 kHz to 20 kHz band, the phase error and the amplitude error rise to ±0.002 and ±0.2°, respectively. The dynamic range of the harmonic distortion measurement is about 75 dB. It allows visualization of the individual harmonics as well as the total harmonic distortion in the 20 kHz band for the five first harmonics