| 摘要 |
A loop filter (4) having an adapt input (26), a normal input (24) and an output (23) is disclosed. Each of the inputs (24, 26) is connectable to mutually exclusively operable current sources to operate the loop filter (4) in an adapt and a normal mode respectively for controlling a voltage controlled oscillator (14) connectable to the output. The loop filter (4) includes a circuit (27) which when operated in the normal mode is connected between the normal input (24) and the output (23) to introduce a pole (P3) to a normal mode frequency response (32) of the loop filter (4). When switched to operate in the adapt mode the first circuit (27) becomes connected in series between the output (23) and a signal ground to introduce a zero to an adapt mode frequency response of the loop filter (4).
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