Oops, the envelope (green) is far from the constant.
This is an x-y plot of the side tone and its delayed version. The delay is four sample intervals, which should relate to 90 degrees phase difference with the sampling frequency of 8kHz. Oh, yes! I forgot to set the side-tone frequency to 500Hz, or more exactly, I set the frequency all right through the computer I/F, but when I touch one of the control knobs on my rig, say the AF volume, the rig automatically read the setting of other volumes, and the side-tone frequency becomes 600Hz, which is my preferred tone.