One on the reasons that the Modfactor cannot mimick some phasers is that it is lacking a control to set the center of the sweep. A lot of phasers has an internal trimpot to set this. Others make it user tweakable and call it "Manual". From the analog perspective, it is a DC offset (or bias) summed with the LFO signal to move the center of the sweep. By restricting the depth,and using this knob the notches can be set to sweep only in certain ranges: highs only, mids only or lows only, etc.
