Why can I use a bandpass filter for my loop filter?
A PLL uses a VCO to steer its output frequency in order to obtain a phase locked condition with a reference signal. The VCO generates a frequency proportional to the DC level applied to its control input. In a PLL, the VCO is essentially controlled by the DC level at the output of the loop filter. A bandpass filter, by definition, does not pass DC. Hence, if a band pass loop filter is used there is no DC signal available to control the VCO.
