Active filter in PLL circuit
A low pass filter can be built in a chip by reducing the value of circuit element forming a low pass filter in a PLL circuit, especially by reducing the value of a electrostatic capacity. An active filter used in a PLL circuit having two charge pump circuits in a subsequent stage of a phase comparator includes a first circuit component connected between the output of one charge pump circuit and a ground, a second circuit component between the output of another charge pump circuit and the ground, and a voltage adder for adding up voltages between both ends of each of the first and second circuit components.
1. An active filter receiving output from a first charge pump circuit and a second charge pump circuit, comprising:
a first circuit component connected between output of the first charge pump circuit and a ground;
a second circuit component connected between output of the second charge pump circuit and the ground;
a voltage addition unit adding up voltage between both ends of the first circuit component and voltage between both ends of the second circuit component,
an additional filter at a subsequent stage of the active filter, and the additional filter comprises:
resistance, one terminal of which is connected to an output of the voltage addition unit;
a capacity connected between another terminal of the resistance and the ground, wherein:
the additional filter uses voltage between both ends of the capacity as output voltage; and
a selector circuit selecting said second charge pump circuit from two charge pump circuits each of which has a different current ratio of output current relative to current from the first charge pump circuit, the active filter receives output from the first charge pump circuit and the second charge pump circuit.
2. The active filter according to claim 1 , wherein
the active filter further comprises either a parallel connection of a resistance and a capacitor connected between an output of a charge pump circuit coupled thereto and ground, or a capacitor connected between a charge pump circuit coupled to thereto and ground.
3. The active filter according to claim 1 , wherein
each of the first charge pump circuit and the second charge pump circuit is coupled to a phase comparator and each of the first charge pump circuit and the second charge pump circuit outputs a current different from each other, wherein
the first charge pump circuit and the second charge pump circuit each provides an output to the active filter coupled thereto on the basis of a period corresponding to a result of the phase comparator connected to the first charge pump circuit and the second charge pump circuit.
4. A semiconductor integrated circuit comprising the active filter according to claim 1 , and another circuit which are formed on a single chip.