I'd support you on that, but a guess as well.Don't know for a fact, but I would guess that Phase Width Modulation would be equated to having SCR controls operated at a line frequency. By controlling the phase relationship of the SCR triggers to the line, the output is controlled. Again, just a guess, but I think it sounds reasonable. How about other opinions.
Controlling a Servo
Servos are controlled with a 5vdc positive-going variable pulse width that repeats every 20mS. The pulse length determines the servo output shaft position but the 20mS frame rate is not position critical. The frame rate does need to repeat at least every 20mS or it will lose power or even stutter. The servo was designed for pulse widths that vary from 1.0mS to 2.0mS, where 1.5mS is centered. In the normal range the servo will move +/- 45 degrees from the centered (neutral) position.
Most servos can be positioned to around +/- 90 degrees from neutral by expanding the range to 0.75mS to 2.25mS. However, care must be taken to avoid commanding a servo beyond its capabilities. Personal experience with Hitec servos reveals all of the standard size servos can achieve a full 180 degree range. Micro and special purpose servos are often limited to less than 180 degrees.