RTA harmonic distortion frequency range

trobbins

Member
VIP Supporter
Thread Starter
Joined
Nov 26, 2017
Messages
121
Location
Australia
I've used the signal generator and RTA with distortion panel to assess harmonic distortions of equipment, and really appreciate the frequency analysis down to 2Hz for assessment of certain circuits.

I just noticed that a valid analysis and presentation of harmonic distortion went down to a fundamental frequency of 8Hz for a simple external sine oscillator circuit, but responded with N/A below that frequency, for the max 131072 FFT length. Would that be consistent with the programming of REW and FFT limits when there is no internal generator fundamental frequency to lock to?
 

John Mulcahy

REW Author
Joined
Apr 3, 2017
Messages
7,212
If REW's generator is not being used the peak must be at least 5 FFT bins from the band edges. At a 48k sample rate that would allow operation down to 2 Hz with a 128k FFT, unless the spectrum of the input is such that the peak occurs lower than the centre frequency. The window choice also has an effect as higher order windows (such as BH-7) have a greater spreading effect. If using the REW generator the best option is probably to select the option to lock the generator frequency to the FFT length and use a rectangular window.
 
Top Bottom