| 摘要 |
A circuit 10 for filtering signals is provided comprising a first connection 12 configured to be connected to a signal line 16 carrying varying signal voltage and a second connection 14, 18. The circuit also comprises a first circuit portion 20 comprising a parallel resonant circuit having a variable capacitance 26 connected in parallel with a first inductor 24; and a second circuit portion 22 comprising a second inductor 28. The first circuit portion and the second circuit portion are connected in series between the first connection and the second connection. The circuit therefore provides an arrangement of a parallel resonant circuit in series with a series resonant circuit with the pass/block frequencies of each being tunable by varying the capacitance of the shared variable capacitor. The inductance of the first inductor may be greater than the inductance of the second inductor with the ratio of their magnitudes being 1:H2-1 with H being an integer. The filter circuit provides a desired signal to pass through with little or no attenuation while interfering signals, such as a harmonic of the desired signal are heavily attenuated. In a particular embodiment the filter circuit is designed to filter out the third harmonic of the desired signal. The inductance of the first inductor may be eight times that of the inductance of the second inductor. The filter circuit may be used as part of a direct conversion receiving chain or direct conversion transmitting chain and be employed between mixing and amplifying circuitry. The filter circuit may filter out the strong third harmonic of a square wave local oscillator signal.
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