I should add the voltage reference is an equally innovative device for checking meter calibration. I understand that between +5v and -5v there would be no zero crossing were it not for circuit impedance. And I understand that with digital sampling the zero crossing window is very very small. But my cheap averaging meters read 11% high and my RMS meters read very close to 5 volts. (I am a meter junkie) I don’t envy your job of reconciling digital sampling with AC theory.