Compact transformer-based notch filter
A notch filter is coupled to a first input node and a second input node, and is configured to present a capacitive load to a differential signal provided to the first and second input nodes, and to present a series-resonant inductive-capacitive load to a common-mode signal provided to the first and second input nodes. The notch filter includes a transformer and a capacitor bank. The transformer includes a first winding having a positive-polarity terminal coupled to the first input node and a second winding having a positive-polarity terminal coupled to the second input node. The capacitor bank includes a first capacitor coupled between a negative-polarity terminal of the first winding and a bias node, and a second capacitor coupled between a negative-polarity terminal of the second winding and the bias node. The first and second capacitors may be variable capacitors.
1 . A circuit comprising:
a first input node and a second input node configured to receive a differential signal, the differential signal including a differential component and a common-mode component; and
a notch filter coupled to the first input node and the second input node, the notch filter configured to:
present, across the first and second input nodes, a capacitive load to the differential component of the differential signal, and
present, across the first and second input nodes, a series-resonant inductive-capacitive load to the common-mode component of the differential signal.
2 . The circuit of claim 1 , wherein the notch filter comprises:
a transformer including:
a first winding having a positive-polarity terminal coupled to the first input node, and
a second winding having a positive-polarity terminal coupled to the second input node; and
a capacitor bank including:
a first capacitor coupled between a negative-polarity terminal of the first winding and a bias node, and
a second capacitor coupled between a negative-polarity terminal of the second winding and the bias node.
3 . The circuit of claim 2 , wherein a self-inductance of the first winding is substantially equal to a self-inductance of the second winding.
4 . The circuit of claim 2 , wherein a self-inductance of the first winding is substantially equal to a mutual-inductance between the first winding and the second winding.
5 . The circuit of claim 2 , wherein each of the first capacitor and the second capacitor are variable capacitors having a capacitance determined by a value of a capacitance control signal.
6 . The circuit of claim 5 , wherein for each of a plurality of values of the capacitance control signal, a capacitance of the first capacitor corresponding to that value is equal to a capacitance of the second capacitor corresponding to that value.
7 . The circuit of claim 5 , wherein the capacitance control signal is a digital signal having one or more bits or an analog signal.
8 . The circuit of claim 2 , wherein a resonant frequency of the series-resonant inductive-capacitive load presented to the common-mode component of the differential signal is determined according to a self-inductance of the first winding, a mutual inductance between the first winding and the second winding, and a capacitance of the first capacitor.
9 . The circuit of claim 2 , wherein an area of overlap between an area including the first winding and an area including the second winding includes two-thirds or more of the first winding and two-thirds or more of the second winding.
10 . The circuit of claim 2 , wherein a first current corresponding to the differential component of the differential signal flows in a clockwise direction in the first winding and a second current corresponding to the differential component of the differential signal flows in a counter-clockwise direction in the second winding.
11 . The circuit of claim 2 , wherein a first current corresponding to the common-mode component of the differential signal flows in a first direction in the first winding and a second current corresponding to the common-mode component of the differential signal flows in the first direction in the second winding, where the first direction is clockwise or counter-clockwise.
12 . The circuit of claim 2 , wherein the bias node is coupled to DC voltage or ground.
13 . The circuit of claim 1 , further comprising:
an output transformer including a primary winding and a secondary winding,
wherein a first terminal of the primary winding is coupled to the first input node and a second terminal of the primary winding is coupled to the second input node.