The VCO is part of the full circuit that measures the instantaneous frequency. The other main part is a PLL, a phase locked loop. Both of these are very hard to design and implement properly.
The purpose of the function generator is to take the signal from the instantaneous frequency measurer and convert it to the proper control voltage for the plate resistor, based on the shape of the headphone impedance curve. Remember the headphone impedance is a function of frequency, and the function generator takes the frequency in and outputs what the headphone impedance is at that frequency. Then that signal is used to control the plate resistance.
The band analyzer is less like a spectrum analyzer and more like a band equalizer. It's a few selected bands each filtered out independently. A spectrum analyzer uses one of several derivatives of the Fast Fourier Transform to analyze a time domain signal. Very different process, and in my opinion, the wrong math for what you want to do.
The purpose of the function generator is to take the signal from the instantaneous frequency measurer and convert it to the proper control voltage for the plate resistor, based on the shape of the headphone impedance curve. Remember the headphone impedance is a function of frequency, and the function generator takes the frequency in and outputs what the headphone impedance is at that frequency. Then that signal is used to control the plate resistance.
The band analyzer is less like a spectrum analyzer and more like a band equalizer. It's a few selected bands each filtered out independently. A spectrum analyzer uses one of several derivatives of the Fast Fourier Transform to analyze a time domain signal. Very different process, and in my opinion, the wrong math for what you want to do.