Re: vco phase lag
svensl,
A VCO is a free-running oscilator whose output frequency is controlled by a DC voltage. As such, phase has no meaning, because there is no reference with which to compare the phase. However, in its most common application, a phase locked loop, the VCO integrates the phase difference between the reference input and the output. If you draw a diagram of two square waves that have 90 degrees phase shift between them, you will see that this is the condition under which the
difference waveform is perfectly symmetrical, and, therefore has a zero integral (long term). It is this integral that is used to adjust the frequency of the local oscillator in the VCO. As a result, if the VCO is under equilibrium conditions, its output will be shifted 90 degrees with respect to the reference.
regards,
Kral