Hi Peter,
I was also unable to find any specific documentation for the blocks under Simulink Extras. We can still although leverage -
'Right click on the block > Masks > Look under the Mask' option to know about the behaviour of of the block.
This block is actually an S-Function written in MATLAB Code:
% View Averaging Spectrum Analyzer Code%
edit msfuntf
The code is documentated as follows and provides information about the block:
%MSFUNTF an MATLAB S-function which performs transfer function analysis using ffts.
% This MATLAB file is designed to be used in a Simulink S-function block.
% It stores up a buffer of input and output points of the system
% then plots the frequency response of the system based on this information.
%
% The input arguments are:
% npts: number of points to use in the fft (e.g. 128)
% HowOften: how often to plot the ffts (e.g. 64)
% offset: sample time offset (usually zeros)
% ts: how often to sample points (secs)
% averaging: whether to average the transfer function or not
%
% The spectrum analyzer displays three plots: the time history,
% the phase and magnitude of the transfer function.
For more information, you also refer to this similar matlab answer thread - https://www.mathworks.com/matlabcentral/answers/36280-using-simulink-s-averaging-spectrum-analyzer
