Apparatus and method for tuning center frequency of a filter

A gm replica circuit comprising:

• a pair of input transistors comprising a first input transistor and a second input transistor, the pair of input transistors receiving a reference voltage, the first input transistor coupled to a reference current;
• a current mirror coupled to the pair of input transistors;
• and
• a gm setting device, the gm setting device having three ends, a control end of the three ends coupled to the second input transistor, the other two ends respectively connected to the first transistor and the second input transistor.

A tuning method for tuning a filter, the filter comprising a voltage-controlled oscillator (VCO) circuit, the tuning method comprising:

• controlling the filter to enter a tuning mode by disabling the entire filter except for the VCO and allowing the VCO to generate an oscillation signal according to a driving signal;
• comparing a frequency of the oscillation signal outputted by the VCO with a reference frequency to generate a comparison result;
• adjusting the driving signal according to the comparison result in the tuning mode;
• and
• controlling the filter to enter a normal mode from the tuning mode by enabling the entire filter to operate according to the driving signal when the frequency of the oscillation signal is substantially equal to the reference frequency.